Minimally Invasive Transforaminal Lumbar Interbody Fusion Using Banana-Shaped and Straight Cages: Radiological and Clinical Results from a Prospective Randomized Clinical Trial

Minimally Invasive Transforaminal Lumbar Interbody Fusion Using Banana-Shaped and Straight Cages:... Abstract BACKGROUND In minimally invasive transforaminal lumbar interbody fusion (MIS-TLIF), cage type and position play important roles in fusion achievement and sagittal alignment correction. However, no prospective randomized comparison of the results using different types of cage has been reported to date. OBJECTIVE To compare the radiological and clinical outcomes of unilateral MIS-TLIF using 2 types of cage. METHODS All candidates for single-level MIS-TLIF were randomized into banana-shaped cage and straight-cage groups. Plain radiographs and computed tomography scans were used for assessment of cage positions, fusion status, disc height, segmental lordotic angle, cage subsidence, and pelvic parameters. Clinical outcome was assessed using visual analog scale and Oswestry Disability Index scores. RESULTS Forty-four and 40 consecutive patients were operated on using banana-shaped and straight cages, respectively. Cage position was more anterior and lateral in the straight-cage group and more medial and posterior in the banana-shaped cage group. Solid fusion was achieved in 95.2% and 96.6% of the 2 groups, respectively, at 12 mo. The change in disc height and segmental lordotic angle postoperatively was significantly greater in the banana-shaped cage group. The incidence of subsidence during follow-up was significantly higher in the banana-shaped cage group (P < .04). Clinically, the visual analog scale and Oswestry Disability Index scores decreased significantly after surgery in both groups, with no significant difference between the groups. CONCLUSION Our preliminary outcomes suggest that the subsidence rate may be higher using banana-shaped cages in MIS-TLIF, possibly due to their more medial final position. Banana-shaped cage, Lumbar disc disease, Lumbar interbody fusion, Minimally invasive spine surgery, Minimally invasive transforaminal lumbar interbody fusion, Straight cage, Subsidence ABBREVIATIONS ABBREVIATIONS CT computed tomography DH disc height LLA lumbar lordotic angle MIS-TLIF minimally invasive transforaminal lumbar interbody fusion ODI Oswestry Disability Index PLIF posterior lumbar interbody fusion SLA segmental lordotic angle VAS visual analog scale In recent years, minimally invasive transforaminal lumbar interbody fusion (MIS-TLIF) has gained popularity with the advancement of techniques and instruments specialized for minimally invasive spine surgery, such as tubular retractors and percutaneous pedicle screw fixation methods.1-6 MIS-TLIF is still one of the more popular surgical procedures because most surgeons are more familiar with the posterior approach, the reduced neural tissue retraction compared with posterior lumbar interbody fusion (PLIF), and the reduced trauma to back muscles and bony structures compared with conventional open TLIF or PLIF.7-10 In MIS-TLIF, the ideal interbody cage should not only restore disc height (DH) and indirectly decompress nerve roots but also induce segmental or lumbar lordosis.11-13 The contact area and location between the cage and the endplates influence the occurrence of endplate damage or subsidence. The most popular types of cage in use today are banana-shaped and straight cages, but to the best of our knowledge, no prospectively designed study has compared the surgical and clinical outcomes of MIS-TLIF using the 2 cage types. METHODS Study Design The institutional review board at our hospital approved this study and standardized work-up protocol. Written consent was acquired from all patients prior to enrollment in this study. The registration number is KC12OISI0498. All operations were done by a single senior surgeon. The eligibility criteria were: (1) unstable single level low-grade (Meyerding grade I or II) isthmic or degenerative spondylolisthesis; (2) foraminal stenosis with central stenosis, degenerative disc disease, or recurred disc herniation; (3) persistence of symptoms that correlated with the radiological findings despite conservative treatment for a minimum of 6 wk; and (4) a minimum 12-mo follow-up period. Patients with metabolic bone disease, infection, spinal trauma, tumors, and multilevel fusion were excluded from the study, as were patients with possibility of secondary gain. In order to determine the minimum total sample size, we tested the hypothesis that increase of  DH is >2° greater in banana-shaped cage group,14 with null hypothesis being no difference between groups. A minimum total sample size of 84 was determined using G*power 3.1 (test type = t-test, effect size d = 0.8, α error probability = 0.05, power level [1–β error probability] = 0.95). All eligible subjects were randomized into 2 groups: the banana-shaped cage and straight-cage groups. Equal numbers of paper strips on which “banana-shaped cage” or “straight cage” was printed were randomly inserted into envelopes. On the morning of the operation, a clinical research assistant picked one envelope at random, and the patient underwent surgery using the type of cage indicated on the paper inside the envelope. Preoperative radiological evaluation consisted of standing anteroposterior and lateral radiographs, standing whole spine radiograph, magnetic resonance imaging, and computed tomography (CT) of the lumbar spine in all patients. Postoperative radiographs were taken immediately after the operation, at the 6- and 12-mo follow-up visits, and annually thereafter. A CT scan of the lumbar spine was taken at the 6- and 12-mo follow-up visits and annually thereafter. Visual analog scale (VAS) and Oswestry Disability Index (ODI) scores were checked preoperatively, immediately postoperatively, and at 6- and 12-mo visits. Surgical Technique All operations were done by the corresponding author. All MIS-TLIF procedures were done via a unilateral approach. Under fluoroscopic guidance, a 2- to 3-cm paraspinal skin incision is made on the side of operation, and incision is made on the lumbodorsal fascia between the longissimus and multifidus muscles and sequentially widened using tubular dilators (Insight Access Retractor System, Synthes Spine, Raynham, Massachusetts), and a 24-mm working channel is docked. Under microscope visualization, a total facetectomy and partial laminectomy are performed, the ligamentum flavum is resected, and nerve root is gently retracted medially. Complete discectomy is performed, and the caudal and cranial endplates are gently scraped with angled ring curettes. Bilateral decompression, when indicated, was done through the unilateral laminofacetectomy site. Morselized bone fragments obtained from laminofacetectomy are then packed into the anterior portion of the disc space. Next, either a banana-shaped cage (Crescent, Medtronic Sofamor Danek, Memphis, Tennessee) or a straight cage (Opal, Synthes Spine, Raynham, Massachusetts; Figure 1) filled with morselized bone fragments is inserted into the disc space. FIGURE 1. View largeDownload slide Medtronic Crescent™ cage (left), which has a banana-shaped profile, and Synthes Spine Opal™ spacer (right), which has a straight profile. Both cages are bullet-nosed for ease of insertion into the disc space. All numerical data are from product brochures and manuals supplied by the manufacturers. Images of cages are reproduced with permission from the respective manufacturers. FIGURE 1. View largeDownload slide Medtronic Crescent™ cage (left), which has a banana-shaped profile, and Synthes Spine Opal™ spacer (right), which has a straight profile. Both cages are bullet-nosed for ease of insertion into the disc space. All numerical data are from product brochures and manuals supplied by the manufacturers. Images of cages are reproduced with permission from the respective manufacturers. For insertion of the banana-shaped cage, the cage is introduced from the lateral side and pushed as anteriorly as possible. Then, the connector of the cage holder is loosened, which allows for transverse rotation of the cage and tamping of the cage further anteriorly (Figure 2). FIGURE 2. View largeDownload slide Technique for insertion and positioning of a banana-shaped cage. Image courtesy of Medtronic Inc. FIGURE 2. View largeDownload slide Technique for insertion and positioning of a banana-shaped cage. Image courtesy of Medtronic Inc. For insertion of the straight cage, the cage is introduced from the lateral side and inserted towards the anterior annulus of the contralateral side in a diagonal trajectory, as anteriorly as possible (Figure 3). FIGURE 3. View largeDownload slide Technique for insertion and positioning of a straight cage. Image courtesy of DePuy Synthes Spine. FIGURE 3. View largeDownload slide Technique for insertion and positioning of a straight cage. Image courtesy of DePuy Synthes Spine. Percutaneous pedicle screws (Sextant, Medtronic Sofamor Danek, Memphis, Tennessee; or Viper 2, Synthes Spine, Raynham, Massachusetts) are inserted under fluoroscopic guidance, and rods of an adequate size are fitted. The wounds are copiously irrigated, drainage catheters are placed, and the wounds are sutured layer by layer. Radiological Assessment DH, segmental lordotic angle (SLA), lumbar lordotic angle (LLA), and pelvic parameters were measured using plain radiographs. DH was defined as the distance between the center of the superior and inferior endplates of the index level. SLA was measured as the Cobb angle between lines parallel to the upper endplate of the cranial vertebral body and lower endplate of the caudal vertebral body of the index disc level. LLA was measured as the Cobb angle between the lines parallel to superior endplate of L1 and the upper endplate of the sacrum (Figure 4). Regarding pelvic parameters, the pelvic incidence was measured as the angle between the line perpendicular to the sacral plate at its midpoint and the line connecting this point to the axis of the femoral heads. For the sacral slope, the angle formed by the upper endplate of S1 and a horizontal line was measured. For the pelvic tilt, the angle formed by the line connecting the midpoint of the S1 upper endplate and the center of the femoral heads and the vertical line was measured (Figure 5). FIGURE 4. View largeDownload slide A: Segmental lordotic angle (SLA), B: SLA of level L5-S1, and y: lumbar lordotic angle (LLA). FIGURE 4. View largeDownload slide A: Segmental lordotic angle (SLA), B: SLA of level L5-S1, and y: lumbar lordotic angle (LLA). FIGURE 5. View largeDownload slide a: Disc height (DH), b: disc angle, c: sacral slope (SS), d: pelvic incidence (PI), and e: pelvic tilt (PT). FIGURE 5. View largeDownload slide a: Disc height (DH), b: disc angle, c: sacral slope (SS), d: pelvic incidence (PI), and e: pelvic tilt (PT). The modified Bridwell fusion criteria15,16 (Table 1) for the lumbar spine were used to assess fusion on CT scans of the lumbar spine obtained at 6 and 12 mo after the operation. Grades I and II were considered satisfactory fusion. The position of the cage on the axial CT scan image was analyzed using a 3 × 3 grid system. The axial image of the vertebral body was divided into 0 segments by overlaying a 3 × 3 grid onto the image, and the area of the grid occupied by the interbody cage was recorded (Figure 6). Any cage subsidence, defined as >2 mm migration of the interbody cage into the adjacent vertebral bodies,17-19 was noted when identified during postoperative follow-up examinations. FIGURE 6. View largeDownload slide Position of the cage assessed using a 3 × 3 grid system. The vertebral body on axial CT image was divided into 9 segments, and the area covered by the interbody cage was determined. In this case, the cage covers the anterior, middle, posterior, and lateral portions of the body (A: anterior, M: middle, L: lateral, P: posterior). FIGURE 6. View largeDownload slide Position of the cage assessed using a 3 × 3 grid system. The vertebral body on axial CT image was divided into 9 segments, and the area covered by the interbody cage was determined. In this case, the cage covers the anterior, middle, posterior, and lateral portions of the body (A: anterior, M: middle, L: lateral, P: posterior). TABLE 1. Modified Bridwell Fusion Criteria Grade I  Fused with remodeling and trabeculae present  Grade II  Graft intact, not fully remodeled and incorporated, but no lucency present  Grade III  Graft intact, potential lucency present at top and bottom of the graft  Grade IV  Fusion absent with collapse/resorption of the graft  Grade I  Fused with remodeling and trabeculae present  Grade II  Graft intact, not fully remodeled and incorporated, but no lucency present  Grade III  Graft intact, potential lucency present at top and bottom of the graft  Grade IV  Fusion absent with collapse/resorption of the graft  View Large All measurements were performed twice by a spine fellow using images stored in a picture archiving and communication system (Maroview, Marosis Co., Seoul, Korea). Clinical Assessment Clinical and functional outcomes were measured using a VAS for back pain and leg pain and the ODI. All clinical and functional assessments were conducted by one clinical research assistant. Statistical Analysis Statistical analysis was performed using Student's t-test. The test was used to compare the demographic parameters, lumbosacral and pelvic parameters, and perioperative parameters between the 2 groups. Statistical significance was defined as P < .05. All analyses were performed using SPSS version 21.0 (IBM Corporation, Armonk, NY). RESULTS Ninety patients (34 males and 56 females) were operated between March 2014 and June 2015, and 6 patients were lost to follow-up before 12 mo (dropout rate, 6.7%). Forty-four and 40 patients were randomly allocated to the banana-shaped cage group and straight-cage group, respectively. There were a total of 84 operated levels. The mean age of patients was 62.9 ± 7.2 (53-78) yr in the banana-shaped cage group and 65.4 ± 6.2 (59-82) yr in the straight-cage group. Bilateral decompression through a unilateral facetectomy site was performed in 39.2% (33/84). The demographic data of the included patients are summarized in Table 2. TABLE 2. Demographic Characteristics of the Enrolled Patients   Banana-shaped cage group  Straight-cage group  P value  Characteristic        Age  62.9 ± 7.2 (range 53-78)  65.4 ± 6.2 (range 59-82)  .09  Sex  Male: 15 (34.1%) Female: 29 (65.9%)  Male: 16 (40%) Female: 24 (60%)    BMI (kg/m2)  26.5 ± 7.1 (17.0-45.9)  25.0 ± 2.8 (13.9-40.8)  .21  Bone density (T-score)  –1.6 ± 0.8 (-4.0 to 1.5)  –1.4 ± 0.7 (-4.0 to 1.5)  .23  Primary diagnosis        Spinal stenosis without spondylolisthesis  37 (84.1%)  33 (82.5%)  .85  Isthmic spondylolisthesis  5 (11.4%)  5 (12.5%)  .88  Degenerative spondylolisthesis  13 (29.5%)  13 (32.5%)  .77  Operated level        L4-5  32 (72.7%)  30 (75%)  .81  L5-S1  12 (27.3%)  10 (25%)  .81  Bilateral decompression through unilateral facetectomy  17(38.6%)  13 (32.5%)  .56    Banana-shaped cage group  Straight-cage group  P value  Characteristic        Age  62.9 ± 7.2 (range 53-78)  65.4 ± 6.2 (range 59-82)  .09  Sex  Male: 15 (34.1%) Female: 29 (65.9%)  Male: 16 (40%) Female: 24 (60%)    BMI (kg/m2)  26.5 ± 7.1 (17.0-45.9)  25.0 ± 2.8 (13.9-40.8)  .21  Bone density (T-score)  –1.6 ± 0.8 (-4.0 to 1.5)  –1.4 ± 0.7 (-4.0 to 1.5)  .23  Primary diagnosis        Spinal stenosis without spondylolisthesis  37 (84.1%)  33 (82.5%)  .85  Isthmic spondylolisthesis  5 (11.4%)  5 (12.5%)  .88  Degenerative spondylolisthesis  13 (29.5%)  13 (32.5%)  .77  Operated level        L4-5  32 (72.7%)  30 (75%)  .81  L5-S1  12 (27.3%)  10 (25%)  .81  Bilateral decompression through unilateral facetectomy  17(38.6%)  13 (32.5%)  .56  View Large Mean operating time was 2.4 ± 0.5 (1.5-3.75) h from skin incision to percutaneous screw fixation and final wound closure in the banana-shaped cage group and 2.5 ± 0.4 (1.5-3.5) h in the straight-cage group (P = .32). Mean blood losses were 292.9 ± 180.1 (150-1200) and 288.9 ± 170.3 (200-1000) mL, respectively (P = .93). Mean hospital stay postoperatively was 5.5 ± 1.4 (5-12) and 5.7 ± 1.7 (5-14) d, respectively (P = .56; Table 3). There were no revision cases in either group. TABLE 3. Perioperative Data and Cage Properties   Banana-shaped cage group  Straight-cage group  P value  Operating time (h)  2.4 ± 0.5 (1.5-3.75)  2.5 ± 0.4 (1.5-3.5)  .32  Mean blood loss (mL)  292.9 ± 180.1 (150-1200)  288.9 ± 170.3 (200-1000)  .93  Mean hospital stay (days)  6.5 ± 1.4 (5-12)  6.7 ± 1.7 (5-14)  .56  Height of cage used (mm)  10.13 ± 1.1  9.82 ± 0.93  .17    Banana-shaped cage group  Straight-cage group  P value  Operating time (h)  2.4 ± 0.5 (1.5-3.75)  2.5 ± 0.4 (1.5-3.5)  .32  Mean blood loss (mL)  292.9 ± 180.1 (150-1200)  288.9 ± 170.3 (200-1000)  .93  Mean hospital stay (days)  6.5 ± 1.4 (5-12)  6.7 ± 1.7 (5-14)  .56  Height of cage used (mm)  10.13 ± 1.1  9.82 ± 0.93  .17  View Large Mean DH (mm) increased from 7.43 ± 2.97 to 11.11 ± 2.24 postoperatively (P < .05) and decreased to 9.67 (P = .20) at 6 mo and 8.18 at 12 mo (P = .54) in the banana-shaped cage group, while in the straight-cage group it increased from 8.43 to 9.88 postoperatively (P < .05) and decreased to 9.32 at 6 mo (P = .09) and 8.72 at 12 mo (P = .27). The difference in the change in DH was significantly higher in the banana-shaped cage group postoperatively (P = .011) but was not significant at 6 and 12 mo (P = .15). Mean SLA (°) increased from 12.14 to 16.34 postoperatively (P < .05) and decreased to 12.55 (P = .79) at 6 mo in the banana-shaped cage group, while in the straight-cage group it increased from 14.65 to 18.22 postoperatively (P = .032) and decreased to 16.79 at 6 mo (P = .22). The increase in SLA was significantly higher in the banana-shaped cage group postoperatively but not at 6 and 12 mo. Mean LLA (°) was not significantly increased during the follow-up periods compared to preoperative data in either of the groups, and there was no significant difference in the change in LLA between the groups (Table 4). Mean DH, SLA, and LLA at various stages are summarized in Figures 7-9. FIGURE 7. View largeDownload slide Changes in disc height (DH). FIGURE 7. View largeDownload slide Changes in disc height (DH). FIGURE 8. View largeDownload slide Changes in SLA. FIGURE 8. View largeDownload slide Changes in SLA. FIGURE 9. View largeDownload slide Changes in LLA. FIGURE 9. View largeDownload slide Changes in LLA. TABLE 4. Changes in Sagittal Radiographic Parameters     Banana-shaped cage group  Straight-cage group  P value  DH  Preop to postop  3.68 ± 1.13  2.32 ± 0.94  <.0001    Preop to 6 mo  2.44 ± 1.29  1.49 ± 0.86  .0006    Preop to 12 mo  2.25 ± 0.31  1.34 ± 0.79  <.0001  SLA  Preop to postop  4.62 ± 2.53  3.33 ± 3.01  .036    Preop to 6 mo  2.92 ± 3.95  2.19 ± 3.76  .38    Preop to 12 mo  0.05 ± 2.85  1 ± 4.16  .22  LLA  Preop to postop  4.6 ± 7.14  2.5 ± 7.34  .18    Preop to 6 mo  0.8 ± 7.52  1.1 ± 6.35  .84    Preop to 12 mo  − 0.02 ± 7.89  0.5 ± 6.26  .74      Banana-shaped cage group  Straight-cage group  P value  DH  Preop to postop  3.68 ± 1.13  2.32 ± 0.94  <.0001    Preop to 6 mo  2.44 ± 1.29  1.49 ± 0.86  .0006    Preop to 12 mo  2.25 ± 0.31  1.34 ± 0.79  <.0001  SLA  Preop to postop  4.62 ± 2.53  3.33 ± 3.01  .036    Preop to 6 mo  2.92 ± 3.95  2.19 ± 3.76  .38    Preop to 12 mo  0.05 ± 2.85  1 ± 4.16  .22  LLA  Preop to postop  4.6 ± 7.14  2.5 ± 7.34  .18    Preop to 6 mo  0.8 ± 7.52  1.1 ± 6.35  .84    Preop to 12 mo  − 0.02 ± 7.89  0.5 ± 6.26  .74  View Large For pelvic parameters, while pelvic incidence and pelvic tilt decreased and sacral slope increased postoperatively in both groups, there was no significant difference between the 2 groups (Table 5). Radiographic evidence of fusion was observed in 61.6% and 63.6% at 6 mo (P = .85) and 95.2% and 96.6% at 12 mo (P = .75) in the banana-shaped cage group and straight-cage group, respectively, with no significant difference between the groups. Cage subsidence was observed in 14 patients in the banana-shaped cage group (31.8%) and in 7 patients (17.5%) in the straight-cage group (P = .13; Table 6). TABLE 5. Pelvic Parameters   Banana-shaped cage group  Straight-cage group  P value  PI preoperatively  57.17 ± 6.65  55.73 ± 9.67  .11  PI postoperatively  56.78 ± 8.27  54.25 ± 8.83  .80  SS preoperatively  31.36 ± 5.86  32.12 ± 6.91  .59  SS postoperatively  31.94 ± 4.48  32.22 ± 8.13  .84  PT preoperatively  25.81 ± 7.25  23.9 ± 9.86  .31  PT postoperatively  25.23 ± 6.79  23.43 ± 8.82  .30    Banana-shaped cage group  Straight-cage group  P value  PI preoperatively  57.17 ± 6.65  55.73 ± 9.67  .11  PI postoperatively  56.78 ± 8.27  54.25 ± 8.83  .80  SS preoperatively  31.36 ± 5.86  32.12 ± 6.91  .59  SS postoperatively  31.94 ± 4.48  32.22 ± 8.13  .84  PT preoperatively  25.81 ± 7.25  23.9 ± 9.86  .31  PT postoperatively  25.23 ± 6.79  23.43 ± 8.82  .30  View Large TABLE 6. Fusion and Subsidence Rates     Banana-shaped cage group  Straight-cage group  P value  Fusion rates (%)  6 mo  61.6  63.6  .85    12 mo  95.2  96.6  .75  Subsidence  Rate (%)  31.8  17.5  .13    Mean (mm)  3.6 ± 0.89 (2-5)  3.0 ± 1.53 (2-6)  <.05      Banana-shaped cage group  Straight-cage group  P value  Fusion rates (%)  6 mo  61.6  63.6  .85    12 mo  95.2  96.6  .75  Subsidence  Rate (%)  31.8  17.5  .13    Mean (mm)  3.6 ± 0.89 (2-5)  3.0 ± 1.53 (2-6)  <.05  View Large Cages tended to be positioned more medially and posteriorly in the banana-shaped cage group, while the location of the straight cage tended to be distributed more evenly among the anterior, posterior, and lateral portions of the body (Table 7). TABLE 7. Cage Positions   Banana-shaped cage  Straight cage  P value  Anterior  29.5% (13)  62.5% (25)  .0026  Posterior  36.4% (16)  55.5% (22)  .08  Middle  100% (44)  100% (40)  –  Lateral  34.1% (15)  38.6% (17)  .67    Banana-shaped cage  Straight cage  P value  Anterior  29.5% (13)  62.5% (25)  .0026  Posterior  36.4% (16)  55.5% (22)  .08  Middle  100% (44)  100% (40)  –  Lateral  34.1% (15)  38.6% (17)  .67  View Large Patients were ambulated 6 to 8 h postoperatively to assess their postoperative day functional outcomes. The mean VAS scores for back pain and leg pain decreased significantly in both groups. Mean ODI scores improved significantly postoperatively and were maintained throughout the follow-up period. No significant difference was observed between the groups (Figures 10A and 10B). FIGURE 10. View largeDownload slide A, VAS scores. B, ODI scores. FIGURE 10. View largeDownload slide A, VAS scores. B, ODI scores. There were no cases of perioperative and postoperative complications requiring revision surgery, such as postoperative hematoma, malpositioned screws with violation of neural elements, or postoperative infection in either group. DISCUSSION MIS-TLIF has gained in popularity due to its advantages of smaller incisions, reduced paraspinal muscle trauma, decreased intraoperative blood loss, shorter hospital stays, and decreased rates of operative site infection, resulting in lower postoperative morbidity and expedite postoperative recovery.5,8,9,20-23 L4-5 and L5-S1 are most frequently treated in single-level lumbar fusion surgery,24-26 and they also account for a large portion of lumbar lordosis.26 The stability of the fused segment is dependent largely on the location, position, and surface area of the interbody cage.27,28 The rates of reoperation, pseudoarthrosis, development of adjacent segment pathology, and clinical recurrence of symptoms are influenced by the stability of the fusion segment.19,29 The interbody cage is more important in MIS-TLIF, which features a narrower working field, asymmetric approach, and less area for posterolateral fusion.10,30 Few biomechanical or retrospective studies that have evaluated the properties of the various types of interbody cage and their effect on the fusion rate and sagittal alignment correction have reported conflicting results. Cho et al. reported no difference in subsidence rate and construct stability between banana-shaped and straight cages,31 while others reported that the subsidence rate differed according to cage type.14,32,33 The increase in DH was significantly higher in the banana-shaped cage group in all postoperative and follow-up radiographs. Likewise, improvement in SLA was significantly higher in the banana-shaped cage group postoperatively, and while the improvement was also higher in the subsequent follow-up radiographs, no statistical significance was reached. The higher DH and SLA restoration in the banana-shaped cage group may be accounted for by the fact that the banana-shaped cages were taller than the straight cages (10.13 ± 1.1 vs 9.82 ± 0.93 mm, P = .17) and that the Crescent cage has 6 degrees of inherent lordotic angle. The superiority of banana-shaped cages in DH and SLA restoration is concordant with previous reports.14,34 That improvement in SLA was not significantly superior in 1 group at the 6- and 12-mo follow-ups suggest that cage migration or subsidence occurred between the immediate postoperative evaluation and the 6-mo follow-up. Kim et al14 reported that a banana-shaped cage or curvilinear cage is superior for creation of LLA and attributed this to its more anterior position. In our study, there was no significant difference in restoration or creation of LLA between the 2 groups. Changes in pelvic parameters also did not differ significantly between the 2 groups. In a meta-analysis of fusion rates of MIS-TLIF, Wu et al35 reported the fusion rate to be 94.8% to 95%. Our assessment yielded similar results. While fusion rates of the 2 groups were not significantly different, the cage subsidence rate was higher in the banana-shaped cage group. According to Closkey et al,36 to achieve successful fusion, the surface area of the cage in contact with the endplate should be at least 30% of the total surface area of the endplate. The banana-shaped cage used in our study has a similar surface area (135-180 mm2) to the straight cage (133-175 mm2). With regard to the strength of different parts of the vertebral endplate, several studies have indicated that the rim of the apophyseal ring has a higher density than the middle portion of the endplate and is more resistant to compressive load.37-39 Several in Vitro biomechanical studies also report that the posterior and lateral parts of the endplate are the strongest and most resistant to compressive load.38,40 In view of these reports, we assessed the relative positions of 2 types of cage on axial CT images. Contrary to our expectations, straight cages had significantly better coverage of the anterior portions of the endplates as well as posterior portions, with a P value approaching significance. Coverage of the lateral position was slightly higher in the banana-shaped cage group, albeit not significantly so. These results imply that banana-shaped cages tended to be placed mostly centrally (medial position) and laterally, while straight cages tended to cover broader regions of the endplates spanning the anterior and posterior portions more evenly. Because the middle part was the weakest part of the endplate, these results might explain why the endplate violation and subsidence rate was higher in the banana-shaped cage group. Abbushi et al28 also reported a higher migration rate when cages were positioned mediomedially. Ideally, according to the recommendations in the manual supplied by the manufacturer and several reports,34,41 banana-shaped cages should be repositioned once inserted into the disc space to be as anterior as possible, but this was not achieved in a large number of the cases because of the narrowed disc space, and possibly because of packed morselized bone in the anterior portion of the disc space prior to cage insertion. Bone chips may have hindered pushing of the cage to the furthest anterior portion of the disc space, especially when inserting banana-shaped cages, which by design are meant to slide along the posterior wall of the anterior annulus during insertion. We suspect that, as a result of these shortcomings, the cages ended up in the central portion of the vertebral endplates. Straight cages by design do not require rotational repositioning and additional manipulation once inside the disc space, which could have contributed to the lower rates of endplate violation. Positioned diagonally across the disc space, they tended to cover a broader area of the endplate, covering anterior and lateral zones where resistance to compressive load is highest. From these observations, when using the banana-shaped cage, the authors would suggest not overfilling the anterior portion of the disc space so as to not hinder the tamping of the cage to the anterior portion of the disc. Inserting morselized bone prior to cage insertion under the microscope may help determine the adequate amount of bone that should be inserted. Other factors should be taken into consideration. Excessive and overzealous curettage during disc space preparation with shavers and curettes can lead to endplate damage and cage subsidence,42,43 and different surgeons have different standards for sufficient endplate curettage. In our study, a single surgeon performed all operations, and disc space was prepared to a similar degree in all cases in a consciously meticulous manner. Patient-related factors, including age, obesity, and osteoporosis can also affect subsidence rates.43 However, there were no significant differences in these factors between the 2 groups in our study. While VAS and ODI scores decreased significantly in both groups in the postoperative period, no significant difference was found between the groups, suggesting that, despite differences in degrees of sagittal correction and subsidence rates, clinical outcomes were not affected in the first 12 mo postoperatively. Limitations Our study had a number of limitations. First, the number of enrolled patients was small. Second, all operations were done by a single experienced surgeon, and the results might have been different if a cohort of surgeons with varying degrees of experience had performed the procedures. Third, the patients were not stratified according to the level of operation. As the properties of L4-5 and L5-S1 are different in terms of their contribution to sagittal alignment and the dimensions of the disc space, the different numbers of each level operated on could have resulted in bias. Fourth, the minimum follow-up period of 12 mo might have been insufficient to obtain convincing results with regard to the fusion and long-term subsidence rates. As this is an ongoing prospective study, the long-term follow-up results will likely offer more insight into the effects of the 2 cage types. CONCLUSION In this randomized clinical trial, we compared the radiological and clinical outcomes of MIS-TLIF performed using banana-shaped cages and straight cages. While fusion rates were similar in the 2 groups, the banana-shaped cage was significantly superior to the straight cage in terms of DH and SLA restoration postoperatively, although we found no evidence that the significance is maintained during the follow-up period. This may be attributable to the more central position of the banana-shaped cage, which may have influenced the subsidence rate. Disclosures The authors declare that there is no conflict of interest regarding the publication of this paper. The authors acknowledge no funding or grants have been received from any sources. The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article. REFERENCES 1. Schwender JD, Holly LT, Rouben DP, Foley KT. Minimally invasive transforaminal lumbar interbody fusion (TLIF): technical feasibility and initial results. J Spinal Disord Tech . 2005; 18( suppl): S1- S6. Google Scholar CrossRef Search ADS PubMed  2. Fan S, Hu Z, Zhao F, Zhao X, Huang Y, Fang X. Multifidus muscle changes and clinical effects of one-level posterior lumbar interbody fusion: minimally invasive procedure versus conventional open approach. Eur Spine J . 2010; 19( 2): 316- 324. Google Scholar CrossRef Search ADS PubMed  3. Dhall SS, Wang MY, Mummaneni PV. Clinical and radiographic comparison of mini-open transforaminal lumbar interbody fusion with open transforaminal lumbar interbody fusion in 42 patients with long-term follow-up. J Neurosurg Spine . 2008; 9( 6): 560- 565. Google Scholar CrossRef Search ADS PubMed  4. Park Y, Ha JW, Lee YT, Sung NY. Minimally invasive transforaminal lumbar interbody fusion for spondylolisthesis and degenerative spondylosis: 5-year results. Clin Orthop Relat Res . 2014; 472( 6): 1813- 1823. Google Scholar CrossRef Search ADS PubMed  5. Seng C, Siddiqui MA, Wong KP et al.   Five-year outcomes of minimally invasive versus open transforaminal lumbar interbody fusion: a matched-pair comparison study. Spine (Phila Pa 1976) . 2013; 38( 23): 2049- 2055. Google Scholar CrossRef Search ADS PubMed  6. Pelton MA, Phillips FM, Singh K. A comparison of perioperative costs and outcomes in patients with and without workers' compensation claims treated with minimally invasive or open transforaminal lumbar interbody fusion. Spine . 2012; 37( 22): 1914- 1919. Google Scholar CrossRef Search ADS PubMed  7. Lee KH, Yue WM, Yeo W, Soeharno H, Tan SB. Clinical and radiological outcomes of open versus minimally invasive transforaminal lumbar interbody fusion. Eur Spine J . 2012; 21( 11): 2265- 2270. Google Scholar CrossRef Search ADS PubMed  8. Karikari IO, Isaacs RE. Minimally invasive transforaminal lumbar interbody fusion: a review of techniques and outcomes. Spine . 2010; 35( 26 suppl): S294- S301. Google Scholar CrossRef Search ADS PubMed  9. Shunwu F, Xing Z, Fengdong Z, Xiangqian F. Minimally invasive transforaminal lumbar interbody fusion for the treatment of degenerative lumbar diseases. Spine . 2010; 35( 17): 1615- 1620. Google Scholar CrossRef Search ADS PubMed  10. Wong AP, Smith ZA, Stadler JA 3rd et al.   Minimally invasive transforaminal lumbar interbody fusion (MI-TLIF): surgical technique, long-term 4-year prospective outcomes, and complications compared with an open TLIF cohort. Neurosurg Clin N Am . 2014; 25( 2): 279- 304. Google Scholar CrossRef Search ADS PubMed  11. Lazennec JY, Ramare S, Arafati N et al.   Sagittal alignment in lumbosacral fusion: relations between radiological parameters and pain. Eur Spine J . 2000; 9( 1): 47- 55. Google Scholar CrossRef Search ADS PubMed  12. Uribe JS, Myhre SL, Youssef JA. Preservation or restoration of segmental and regional spinal lordosis using minimally invasive interbody fusion techniques in degenerative lumbar conditions: a literature review. Spine . 2016; 41( suppl 8): S50- S58. Google Scholar PubMed  13. Jagannathan J, Sansur CA, Shaffrey CI. Iatrogenic spinal deformity. Neurosurgery . 2008; 63( 3 suppl): 104- 116. Google Scholar CrossRef Search ADS PubMed  14. Kim JT, Shin MH, Lee HJ, Choi DY. Restoration of lumbopelvic sagittal alignment and its maintenance following transforaminal lumbar interbody fusion (TLIF): comparison between straight type versus curvilinear type cage. Eur Spine J . 2015; 24( 11): 2588- 2596. Google Scholar CrossRef Search ADS PubMed  15. Bridwell KH, Lenke LG, McEnery KW, Baldus C, Blanke K. Anterior fresh frozen structural allografts in the thoracic and lumbar spine. Do they work if combined with posterior fusion and instrumentation in adult patients with kyphosis or anterior column defects? Spine . 1995; 20( 12): 1410- 1418. Google Scholar CrossRef Search ADS PubMed  16. Bridwell KH, O’Brien MF, Lenke LG, Baldus C, Blanke K. Posterior spinal fusion supplemented with only allograft bone in paralytic scoliosis. Does it work? Spine . 1994; 19( 23): 2658- 2666. Google Scholar CrossRef Search ADS PubMed  17. Kim MC, Chung HT, Cho JL, Kim DJ, Chung NS. Subsidence of polyetheretherketone cage after minimally invasive transforaminal lumbar interbody fusion. J Spine Disord Tech . 2013; 26( 2): 87- 92. Google Scholar CrossRef Search ADS   18. Beutler WJ, Peppelman WC Jr. Anterior lumbar fusion with paired BAK standard and paired BAK Proximity cages: subsidence incidence, subsidence factors, and clinical outcome. Spine J . 2003; 3( 4): 289- 293. Google Scholar CrossRef Search ADS PubMed  19. Chen L, Yang H, Tang T. Cage migration in spondylolisthesis treated with posterior lumbar interbody fusion using BAK cages. Spine . 2005; 30( 19): 2171- 2175. Google Scholar CrossRef Search ADS PubMed  20. Lee JH, Na KH, Kim JH, Jeong HY, Chang DG. Is pelvic incidence a constant, as everyone knows? Changes of pelvic incidence in surgically corrected adult sagittal deformity. Eur Spine J . 2016; 25( 11): 3707- 3714. Google Scholar CrossRef Search ADS PubMed  21. Adogwa O, Parker SL, Bydon A, Cheng J, McGirt MJ. Comparative effectiveness of minimally invasive versus open transforaminal lumbar interbody fusion: 2-year assessment of narcotic use, return to work, disability, and quality of life. J Spinal Disord Tech . 2011; 24( 8): 479- 484. Google Scholar PubMed  22. Park Y, Ha JW. Comparison of one-level posterior lumbar interbody fusion performed with a minimally invasive approach or a traditional open approach. Spine . 2007; 32( 5): 537- 543. Google Scholar CrossRef Search ADS PubMed  23. Peng CW, Yue WM, Poh SY, Yeo W, Tan SB. Clinical and radiological outcomes of minimally invasive versus open transforaminal lumbar interbody fusion. Spine . 2009; 34( 13): 1385- 1389. Google Scholar CrossRef Search ADS PubMed  24. Han SH, Hyun SJ, Jahng TA, Kim KJ. A Comparative radiographic analysis of fusion rate between L4-5 and L5-S1 in a single level posterior lumbar interbody fusion. Korean J Spine . 2015; 12( 2): 60- 67. Google Scholar CrossRef Search ADS PubMed  25. Okoro T, Sell P. A short report comparing outcomes between L4/L5 and L5/S1 single-level discectomy surgery. J Spine Disord Tech . 2010; 23( 1): 40- 42. Google Scholar CrossRef Search ADS   26. Kim JY, Park JY, Kim KH et al.   Minimally Invasive transforaminal lumbar interbody fusion for spondylolisthesis: comparison between isthmic and degenerative spondylolisthesis. World Neurosurg . 2015; 84( 5): 1284- 1293. Google Scholar CrossRef Search ADS PubMed  27. Zhao J, Hou T, Wang X, Ma S. Posterior lumbar interbody fusion using one diagonal fusion cage with transpedicular screw/rod fixation. Eur Spine J . 2003; 12( 2): 173- 177. Google Scholar PubMed  28. Abbushi A, Cabraja M, Thomale UW, Woiciechowsky C, Kroppenstedt SN. The influence of cage positioning and cage type on cage migration and fusion rates in patients with monosegmental posterior lumbar interbody fusion and posterior fixation. Eur Spine J . 2009; 18( 11): 1621- 1628. Google Scholar CrossRef Search ADS PubMed  29. Tan JS, Bailey CS, Dvorak MF, Fisher CG, Oxland TR. Interbody device shape and size are important to strengthen the vertebra-implant interface. Spine . 2005; 30( 6): 638- 644. Google Scholar CrossRef Search ADS PubMed  30. Gu G, Zhang H, Fan G et al.   Comparison of minimally invasive versus open transforaminal lumbar interbody fusion in two-level degenerative lumbar disease. Int Orthop . 2014; 38( 4): 817- 824. Google Scholar CrossRef Search ADS PubMed  31. Cho W, Wu C, Mehbod AA, Transfeldt EE. Comparison of cage designs for transforaminal lumbar interbody fusion: a biomechanical study. Clin Biomech (Bristol, Avon) . 2008; 23( 8): 979- 985. Google Scholar CrossRef Search ADS PubMed  32. Zhao FD, Yang W, Shan Z et al.   Cage migration after transforaminal lumbar interbody fusion and factors related to it. Orthop Surg . 2012; 4( 4): 227- 232. Google Scholar CrossRef Search ADS PubMed  33. Bakhsheshian J, Khanna R, Choy W et al.   Incidence of graft extrusion following minimally invasive transforaminal lumbar interbody fusion. J Clin Neurosci . 2016; 24: 88- 93. Google Scholar CrossRef Search ADS PubMed  34. Lindley TE, Viljoen SV, Dahdaleh NS. Effect of steerable cage placement during minimally invasive transforaminal lumbar interbody fusion on lumbar lordosis. J Clin Neurosci . 2014; 21( 3): 441- 444. Google Scholar CrossRef Search ADS PubMed  35. Wu RH, Fraser JF, Hartl R. Minimal access versus open transforaminal lumbar interbody fusion: meta-analysis of fusion rates. Spine . 2010; 35( 26): 2273- 2281. Google Scholar CrossRef Search ADS PubMed  36. Closkey RF, Parsons JR, Lee CK, Blacksin MF, Zimmerman MC. Mechanics of interbody spinal fusion. Analysis of critical bone graft area. Spine . 1993; 18( 8): 1011- 1015. Google Scholar CrossRef Search ADS PubMed  37. Abe K, Orita S, Mannoji C et al.   Perioperative complications in 155 patients who underwent oblique lateral interbody fusion surgery: perspectives and indications from a retrospective, multicenter survey. Spine . 2017; 42( 1): 55- 62. Google Scholar CrossRef Search ADS PubMed  38. Grant JP, Oxland TR, Dvorak MF. Mapping the structural properties of the lumbosacral vertebral endplates. Spine . 2001; 26( 8): 889- 896. Google Scholar CrossRef Search ADS PubMed  39. Lowe TG, Hashim S, Wilson LA et al.   A biomechanical study of regional endplate strength and cage morphology as it relates to structural interbody support. Spine . 2004; 29( 21): 2389- 2394. Google Scholar CrossRef Search ADS PubMed  40. Labrom RD, Tan JS, Reilly CW, Tredwell SJ, Fisher CG, Oxland TR. The effect of interbody cage positioning on lumbosacral vertebral endplate failure in compression. Spine . 2005; 30( 19): E556- E561. Google Scholar CrossRef Search ADS PubMed  41. Wang SJ, Han YC, Pan FM, Ma B, Tan J. Single transverse-orientation cage via MIS-TLIF approach for the treatment of degenerative lumbar disease: a technical note. Int J Clin Exp Med . 2015; 8( 8): 14154- 14160. Google Scholar PubMed  42. Kuslich SD, Ulstrom CL, Griffith SL, Ahern JW, Dowdle JD. The Bagby and Kuslich method of lumbar interbody fusion. History, techniques, and 2-year follow-up results of a United States prospective, multicenter trial. Spine . 1998; 23( 11): 1267- 1278; discussion 1279. Google Scholar CrossRef Search ADS PubMed  43. Lim TH, Kwon H, Jeon CH et al.   Effect of endplate conditions and bone mineral density on the compressive strength of the graft-endplate interface in anterior cervical spine fusion. Spine . 2001; 26( 8): 951- 956. Google Scholar CrossRef Search ADS PubMed  COMMENTS In their manuscript, the authors present the results of a randomized prospective trial comparing the clinical and radiographic outcomes of 2 different cage types (banana-shaped vs straight-cages) used in MIS-TLIF. In total, 84 patients were included in this study. The authors found a significantly higher increase in DH and segmental lordotic angle immediately postoperatively. However, upon follow-up these differences were lost. Clinical outcome and fusion rates were similar for both groups. However, banana-shaped cages were associated with a significantly higher rate of cage subsidence which may be caused by the more medial position of the banana-shaped cages. The study is interesting in several regards. Firstly, it sets out to investigate unanswered questions regarding the operative technique (MIS-TLIF) that have not been thoroughly answered yet. In daily practice, it can be very helpful for a surgeon to know which benefits different cage types offer. Additionally, the study is designed as a prospective randomized trial, which adds impact to the results of this study. However, the study has some limitations that are discussed by the authors as well. The patient numbers are rather low and the follow-up period is short. As there appears to be a significantly higher subsidence rate with banana-shaped cages, it would be interesting to see whether this affects the long-term outcome. Based on the results presented in the manuscript, surgeons are free to choose either cage based on their preference. But even if its effect on the clinical and radiographic outcome remains unclear, it seems reasonable to try to place banana-shaped cages as anteriorly as possible in order to lower the risk of cage subsidence. Roger Hartl New York, New York The authors have presented a timely and thoughtful randomized clinical trial asking the question whether or not there is a difference (both clinical and radiographic) between minimally invasive transforaminal lumbar interbody fusions performed with a straight cage or banana-shaped cage. A rigorous radiographic evaluation was completed preoperatively and postoperatively in patients undergoing an MIS-TLIF, to include position of the interbody, fusion status, disc height, segmental lordotic angle, subsidence, and spinopelvic parameters, along with validated clinical outcome scales. Reasonably sized cohorts were randomized into 2 groups and then followed for 12 months. Fusion rates were nearly equivalent in the 2 groups (95.2% vs 96.6%), indicating that to achieve an arthrodesis, the type of interbody that is selected is meaningless. Instead it is meticulous preparation of the endplates that will reliably achieve an arthrodesis. After all, Cloward reliably achieved interbody fusions without instrumentation.1 The authors made several interesting observations: disc height and segmental lordotic angle was greater in the banana-shaped cage group in the immediate postoperative period than in the straight cage group. However, the incidence of subsidence during follow-up was significantly higher in the banana-shaped group resulting in the loss of lordosis and disc height achieved through the operation. Such an observation also reinforces the old adage that nothing ruins a perfectly good operation like follow-up. There are 2 especially important findings in this prospective randomized study. The first is the issue of segmental lordosis. The authors found that the mean segmental lordosis angle increased in both groups, but the increase was greater in the banana-shaped cage than the straight cage. Such an observation is important because other authors have reported the exact opposite with a loss of lordosis in transforaminal approaches, leading some to suggest that the TLIF is a kyphosing operation.2 The data presented by these authors clearly refutes this and would suggest the capacity to achieve lordosis in a TLIF is once again technique related. The second observation is the location of the interbody spacer. I share the authors' surprise to learn that contrary to the intuitive assumption, the banana-shaped interbody would have better coverage of the anterior portion of the endplates, this study demonstrated the exact opposite. While I do not refute the rigorous radiographic analysis performed by the authors, I would suggest that a conclusion that straight cages have better anterior coverage than banana cages applies only for this study and should not be broadly applied. In this reviewer's experience, broader coverage of the anterior aspect of the endplate is actually better achieved with banana-shaped interbody than straight cages. The evolution of the interbodies and their use is especially germane in this circumstance. Straight cages were designed based on Cloward's PLIF operation, where straight cages were place bilaterally with bilateral access to the disc space. Unilateral access to the disc space introduced by Harms spawned the development of a geometry that could achieve coverage across the disc space from that unilateral approach.3 The straight cages used in transforaminal approaches were never intended to be placed obliquely through a transforaminal corridor. However, the familiarity with that configuration fit naturally in the hands of those surgeons who migrated from PLIFs to TLIFs and the use of straight interbodies in TLIF has become commonplace. As the authors mention, rotating an interbody into the anterior half and center of the disc space is more technically demanding than securing a straight interbody obliquely across the disc space. It is likely more the consequence of technique than geometry that explains the authors' radiographic observation that straight cages had better coverage of the anterior and posterior portions of the endplates, while the coverage of the lateral aspects of the endplates was slightly higher with the banana-shaped geometry. The authors found banana-shaped interbodies are placed mostly central and lateral, while straight interbodies cover the anterior and posterior portion more evenly. It is the position of the interbody within the disc space that led directly to the authors next observation of subsidence, which they found more likely in the banana-shaped interbody than the straight caged interbody. While the authors do not specify the dimensions of the banana-shaped interbodies used, they do report a range: 25-36 mm. It would stand to reason that the smaller the interbody the less likely it would be that the interbody would reach the apophyseal ring. Without the support of the apophyseal ring the result was subsidence. The authors specifically discuss this in their discussion, citing studies that the posterior and lateral aspects of the vertebral body endplate are the most resistant to a compressive load. They obviously recognize the technique component that contributes to subsidence. Specifically, the authors mention that over filling the anterior aspect of the disc space with graft material, will prevent the interbody from reaching the anterior aspect of the disc space. The authors acknowledge that the technique guide along with various operative technique papers have emphasized the importance of placing the interbody in the anterior aspect of this disc space. Another option, not currently available, is a graft with dimensions that when rotated through a transforaminal corridor reaches the apophyseal ring. Perhaps with the long-term radiographic results of the banana-cage demonstrated with this study, the anatomical basis for longer banana-shaped interbodies that can span from apophyseal ring to apophyseal ring may be further explored. Regardless, the authors are to be commended on orchestrating a prospective, randomized clinical trial attempting to answer the question that undoubtedly countless of surgeons have asked who feel strongly about the geometry of interbody that they use. It is increasingly difficult to accomplish these types of studies in the current climate of medicine (A CT scan of the lumbar spine in an asymptomatic patient is no longer available to most clinicians in the United States.) While the results of this study actually demonstrate a remarkable parity in the 2 geometries (straight and banana) in the long-term, the question that needs to be asked after reviewing the long-term radiographic analysis is how to capture the short-term radiographic benefits of banana-shaped cages for the long-term. We can only speculate as to the answer to that question, but based on the data provided by this manuscript, one can safely venture that it would be a combination of geometry, dimensions, and surgical technique. Perhaps it will be these modifications that will lead to long-term restoration of disc height, preservation of segmental lordosis and reliably achieve an arthrodesis in the MIS TLIF patients of tomorrow. Luis Manuel Tumialan Phoenix, Arizona 1. Cloward RB. The treatment of ruptured lumbar intervertebral discs by vertebral body fusion. I. Indications, operative technique, after care. Journal of neurosurgery. Mar 1953;10(2):154-168. 2. Hsieh PC, Koski TR, O'Shaughnessy BA, et al. Anterior lumbar interbody fusion in comparison with transforaminal lumbar interbody fusion: implications for the restoration of foraminal height, local disc angle, lumbar lordosis, and sagittal balance. Journal of neurosurgery. Spine. Oct 2007;7(4):379-386. 3. Harms J, Jeszenszky D. The Unilateral, Transforaminal Approach for Posterior Lumbar Interbody Fusion. Orthopaedics and Traumatology. 1998;6(2):88-99. Copyright © 2017 by the Congress of Neurological Surgeons http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Neurosurgery Oxford University Press

Minimally Invasive Transforaminal Lumbar Interbody Fusion Using Banana-Shaped and Straight Cages: Radiological and Clinical Results from a Prospective Randomized Clinical Trial

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Congress of Neurological Surgeons
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Copyright © 2017 by the Congress of Neurological Surgeons
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0148-396X
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1524-4040
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10.1093/neuros/nyx212
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Abstract

Abstract BACKGROUND In minimally invasive transforaminal lumbar interbody fusion (MIS-TLIF), cage type and position play important roles in fusion achievement and sagittal alignment correction. However, no prospective randomized comparison of the results using different types of cage has been reported to date. OBJECTIVE To compare the radiological and clinical outcomes of unilateral MIS-TLIF using 2 types of cage. METHODS All candidates for single-level MIS-TLIF were randomized into banana-shaped cage and straight-cage groups. Plain radiographs and computed tomography scans were used for assessment of cage positions, fusion status, disc height, segmental lordotic angle, cage subsidence, and pelvic parameters. Clinical outcome was assessed using visual analog scale and Oswestry Disability Index scores. RESULTS Forty-four and 40 consecutive patients were operated on using banana-shaped and straight cages, respectively. Cage position was more anterior and lateral in the straight-cage group and more medial and posterior in the banana-shaped cage group. Solid fusion was achieved in 95.2% and 96.6% of the 2 groups, respectively, at 12 mo. The change in disc height and segmental lordotic angle postoperatively was significantly greater in the banana-shaped cage group. The incidence of subsidence during follow-up was significantly higher in the banana-shaped cage group (P < .04). Clinically, the visual analog scale and Oswestry Disability Index scores decreased significantly after surgery in both groups, with no significant difference between the groups. CONCLUSION Our preliminary outcomes suggest that the subsidence rate may be higher using banana-shaped cages in MIS-TLIF, possibly due to their more medial final position. Banana-shaped cage, Lumbar disc disease, Lumbar interbody fusion, Minimally invasive spine surgery, Minimally invasive transforaminal lumbar interbody fusion, Straight cage, Subsidence ABBREVIATIONS ABBREVIATIONS CT computed tomography DH disc height LLA lumbar lordotic angle MIS-TLIF minimally invasive transforaminal lumbar interbody fusion ODI Oswestry Disability Index PLIF posterior lumbar interbody fusion SLA segmental lordotic angle VAS visual analog scale In recent years, minimally invasive transforaminal lumbar interbody fusion (MIS-TLIF) has gained popularity with the advancement of techniques and instruments specialized for minimally invasive spine surgery, such as tubular retractors and percutaneous pedicle screw fixation methods.1-6 MIS-TLIF is still one of the more popular surgical procedures because most surgeons are more familiar with the posterior approach, the reduced neural tissue retraction compared with posterior lumbar interbody fusion (PLIF), and the reduced trauma to back muscles and bony structures compared with conventional open TLIF or PLIF.7-10 In MIS-TLIF, the ideal interbody cage should not only restore disc height (DH) and indirectly decompress nerve roots but also induce segmental or lumbar lordosis.11-13 The contact area and location between the cage and the endplates influence the occurrence of endplate damage or subsidence. The most popular types of cage in use today are banana-shaped and straight cages, but to the best of our knowledge, no prospectively designed study has compared the surgical and clinical outcomes of MIS-TLIF using the 2 cage types. METHODS Study Design The institutional review board at our hospital approved this study and standardized work-up protocol. Written consent was acquired from all patients prior to enrollment in this study. The registration number is KC12OISI0498. All operations were done by a single senior surgeon. The eligibility criteria were: (1) unstable single level low-grade (Meyerding grade I or II) isthmic or degenerative spondylolisthesis; (2) foraminal stenosis with central stenosis, degenerative disc disease, or recurred disc herniation; (3) persistence of symptoms that correlated with the radiological findings despite conservative treatment for a minimum of 6 wk; and (4) a minimum 12-mo follow-up period. Patients with metabolic bone disease, infection, spinal trauma, tumors, and multilevel fusion were excluded from the study, as were patients with possibility of secondary gain. In order to determine the minimum total sample size, we tested the hypothesis that increase of  DH is >2° greater in banana-shaped cage group,14 with null hypothesis being no difference between groups. A minimum total sample size of 84 was determined using G*power 3.1 (test type = t-test, effect size d = 0.8, α error probability = 0.05, power level [1–β error probability] = 0.95). All eligible subjects were randomized into 2 groups: the banana-shaped cage and straight-cage groups. Equal numbers of paper strips on which “banana-shaped cage” or “straight cage” was printed were randomly inserted into envelopes. On the morning of the operation, a clinical research assistant picked one envelope at random, and the patient underwent surgery using the type of cage indicated on the paper inside the envelope. Preoperative radiological evaluation consisted of standing anteroposterior and lateral radiographs, standing whole spine radiograph, magnetic resonance imaging, and computed tomography (CT) of the lumbar spine in all patients. Postoperative radiographs were taken immediately after the operation, at the 6- and 12-mo follow-up visits, and annually thereafter. A CT scan of the lumbar spine was taken at the 6- and 12-mo follow-up visits and annually thereafter. Visual analog scale (VAS) and Oswestry Disability Index (ODI) scores were checked preoperatively, immediately postoperatively, and at 6- and 12-mo visits. Surgical Technique All operations were done by the corresponding author. All MIS-TLIF procedures were done via a unilateral approach. Under fluoroscopic guidance, a 2- to 3-cm paraspinal skin incision is made on the side of operation, and incision is made on the lumbodorsal fascia between the longissimus and multifidus muscles and sequentially widened using tubular dilators (Insight Access Retractor System, Synthes Spine, Raynham, Massachusetts), and a 24-mm working channel is docked. Under microscope visualization, a total facetectomy and partial laminectomy are performed, the ligamentum flavum is resected, and nerve root is gently retracted medially. Complete discectomy is performed, and the caudal and cranial endplates are gently scraped with angled ring curettes. Bilateral decompression, when indicated, was done through the unilateral laminofacetectomy site. Morselized bone fragments obtained from laminofacetectomy are then packed into the anterior portion of the disc space. Next, either a banana-shaped cage (Crescent, Medtronic Sofamor Danek, Memphis, Tennessee) or a straight cage (Opal, Synthes Spine, Raynham, Massachusetts; Figure 1) filled with morselized bone fragments is inserted into the disc space. FIGURE 1. View largeDownload slide Medtronic Crescent™ cage (left), which has a banana-shaped profile, and Synthes Spine Opal™ spacer (right), which has a straight profile. Both cages are bullet-nosed for ease of insertion into the disc space. All numerical data are from product brochures and manuals supplied by the manufacturers. Images of cages are reproduced with permission from the respective manufacturers. FIGURE 1. View largeDownload slide Medtronic Crescent™ cage (left), which has a banana-shaped profile, and Synthes Spine Opal™ spacer (right), which has a straight profile. Both cages are bullet-nosed for ease of insertion into the disc space. All numerical data are from product brochures and manuals supplied by the manufacturers. Images of cages are reproduced with permission from the respective manufacturers. For insertion of the banana-shaped cage, the cage is introduced from the lateral side and pushed as anteriorly as possible. Then, the connector of the cage holder is loosened, which allows for transverse rotation of the cage and tamping of the cage further anteriorly (Figure 2). FIGURE 2. View largeDownload slide Technique for insertion and positioning of a banana-shaped cage. Image courtesy of Medtronic Inc. FIGURE 2. View largeDownload slide Technique for insertion and positioning of a banana-shaped cage. Image courtesy of Medtronic Inc. For insertion of the straight cage, the cage is introduced from the lateral side and inserted towards the anterior annulus of the contralateral side in a diagonal trajectory, as anteriorly as possible (Figure 3). FIGURE 3. View largeDownload slide Technique for insertion and positioning of a straight cage. Image courtesy of DePuy Synthes Spine. FIGURE 3. View largeDownload slide Technique for insertion and positioning of a straight cage. Image courtesy of DePuy Synthes Spine. Percutaneous pedicle screws (Sextant, Medtronic Sofamor Danek, Memphis, Tennessee; or Viper 2, Synthes Spine, Raynham, Massachusetts) are inserted under fluoroscopic guidance, and rods of an adequate size are fitted. The wounds are copiously irrigated, drainage catheters are placed, and the wounds are sutured layer by layer. Radiological Assessment DH, segmental lordotic angle (SLA), lumbar lordotic angle (LLA), and pelvic parameters were measured using plain radiographs. DH was defined as the distance between the center of the superior and inferior endplates of the index level. SLA was measured as the Cobb angle between lines parallel to the upper endplate of the cranial vertebral body and lower endplate of the caudal vertebral body of the index disc level. LLA was measured as the Cobb angle between the lines parallel to superior endplate of L1 and the upper endplate of the sacrum (Figure 4). Regarding pelvic parameters, the pelvic incidence was measured as the angle between the line perpendicular to the sacral plate at its midpoint and the line connecting this point to the axis of the femoral heads. For the sacral slope, the angle formed by the upper endplate of S1 and a horizontal line was measured. For the pelvic tilt, the angle formed by the line connecting the midpoint of the S1 upper endplate and the center of the femoral heads and the vertical line was measured (Figure 5). FIGURE 4. View largeDownload slide A: Segmental lordotic angle (SLA), B: SLA of level L5-S1, and y: lumbar lordotic angle (LLA). FIGURE 4. View largeDownload slide A: Segmental lordotic angle (SLA), B: SLA of level L5-S1, and y: lumbar lordotic angle (LLA). FIGURE 5. View largeDownload slide a: Disc height (DH), b: disc angle, c: sacral slope (SS), d: pelvic incidence (PI), and e: pelvic tilt (PT). FIGURE 5. View largeDownload slide a: Disc height (DH), b: disc angle, c: sacral slope (SS), d: pelvic incidence (PI), and e: pelvic tilt (PT). The modified Bridwell fusion criteria15,16 (Table 1) for the lumbar spine were used to assess fusion on CT scans of the lumbar spine obtained at 6 and 12 mo after the operation. Grades I and II were considered satisfactory fusion. The position of the cage on the axial CT scan image was analyzed using a 3 × 3 grid system. The axial image of the vertebral body was divided into 0 segments by overlaying a 3 × 3 grid onto the image, and the area of the grid occupied by the interbody cage was recorded (Figure 6). Any cage subsidence, defined as >2 mm migration of the interbody cage into the adjacent vertebral bodies,17-19 was noted when identified during postoperative follow-up examinations. FIGURE 6. View largeDownload slide Position of the cage assessed using a 3 × 3 grid system. The vertebral body on axial CT image was divided into 9 segments, and the area covered by the interbody cage was determined. In this case, the cage covers the anterior, middle, posterior, and lateral portions of the body (A: anterior, M: middle, L: lateral, P: posterior). FIGURE 6. View largeDownload slide Position of the cage assessed using a 3 × 3 grid system. The vertebral body on axial CT image was divided into 9 segments, and the area covered by the interbody cage was determined. In this case, the cage covers the anterior, middle, posterior, and lateral portions of the body (A: anterior, M: middle, L: lateral, P: posterior). TABLE 1. Modified Bridwell Fusion Criteria Grade I  Fused with remodeling and trabeculae present  Grade II  Graft intact, not fully remodeled and incorporated, but no lucency present  Grade III  Graft intact, potential lucency present at top and bottom of the graft  Grade IV  Fusion absent with collapse/resorption of the graft  Grade I  Fused with remodeling and trabeculae present  Grade II  Graft intact, not fully remodeled and incorporated, but no lucency present  Grade III  Graft intact, potential lucency present at top and bottom of the graft  Grade IV  Fusion absent with collapse/resorption of the graft  View Large All measurements were performed twice by a spine fellow using images stored in a picture archiving and communication system (Maroview, Marosis Co., Seoul, Korea). Clinical Assessment Clinical and functional outcomes were measured using a VAS for back pain and leg pain and the ODI. All clinical and functional assessments were conducted by one clinical research assistant. Statistical Analysis Statistical analysis was performed using Student's t-test. The test was used to compare the demographic parameters, lumbosacral and pelvic parameters, and perioperative parameters between the 2 groups. Statistical significance was defined as P < .05. All analyses were performed using SPSS version 21.0 (IBM Corporation, Armonk, NY). RESULTS Ninety patients (34 males and 56 females) were operated between March 2014 and June 2015, and 6 patients were lost to follow-up before 12 mo (dropout rate, 6.7%). Forty-four and 40 patients were randomly allocated to the banana-shaped cage group and straight-cage group, respectively. There were a total of 84 operated levels. The mean age of patients was 62.9 ± 7.2 (53-78) yr in the banana-shaped cage group and 65.4 ± 6.2 (59-82) yr in the straight-cage group. Bilateral decompression through a unilateral facetectomy site was performed in 39.2% (33/84). The demographic data of the included patients are summarized in Table 2. TABLE 2. Demographic Characteristics of the Enrolled Patients   Banana-shaped cage group  Straight-cage group  P value  Characteristic        Age  62.9 ± 7.2 (range 53-78)  65.4 ± 6.2 (range 59-82)  .09  Sex  Male: 15 (34.1%) Female: 29 (65.9%)  Male: 16 (40%) Female: 24 (60%)    BMI (kg/m2)  26.5 ± 7.1 (17.0-45.9)  25.0 ± 2.8 (13.9-40.8)  .21  Bone density (T-score)  –1.6 ± 0.8 (-4.0 to 1.5)  –1.4 ± 0.7 (-4.0 to 1.5)  .23  Primary diagnosis        Spinal stenosis without spondylolisthesis  37 (84.1%)  33 (82.5%)  .85  Isthmic spondylolisthesis  5 (11.4%)  5 (12.5%)  .88  Degenerative spondylolisthesis  13 (29.5%)  13 (32.5%)  .77  Operated level        L4-5  32 (72.7%)  30 (75%)  .81  L5-S1  12 (27.3%)  10 (25%)  .81  Bilateral decompression through unilateral facetectomy  17(38.6%)  13 (32.5%)  .56    Banana-shaped cage group  Straight-cage group  P value  Characteristic        Age  62.9 ± 7.2 (range 53-78)  65.4 ± 6.2 (range 59-82)  .09  Sex  Male: 15 (34.1%) Female: 29 (65.9%)  Male: 16 (40%) Female: 24 (60%)    BMI (kg/m2)  26.5 ± 7.1 (17.0-45.9)  25.0 ± 2.8 (13.9-40.8)  .21  Bone density (T-score)  –1.6 ± 0.8 (-4.0 to 1.5)  –1.4 ± 0.7 (-4.0 to 1.5)  .23  Primary diagnosis        Spinal stenosis without spondylolisthesis  37 (84.1%)  33 (82.5%)  .85  Isthmic spondylolisthesis  5 (11.4%)  5 (12.5%)  .88  Degenerative spondylolisthesis  13 (29.5%)  13 (32.5%)  .77  Operated level        L4-5  32 (72.7%)  30 (75%)  .81  L5-S1  12 (27.3%)  10 (25%)  .81  Bilateral decompression through unilateral facetectomy  17(38.6%)  13 (32.5%)  .56  View Large Mean operating time was 2.4 ± 0.5 (1.5-3.75) h from skin incision to percutaneous screw fixation and final wound closure in the banana-shaped cage group and 2.5 ± 0.4 (1.5-3.5) h in the straight-cage group (P = .32). Mean blood losses were 292.9 ± 180.1 (150-1200) and 288.9 ± 170.3 (200-1000) mL, respectively (P = .93). Mean hospital stay postoperatively was 5.5 ± 1.4 (5-12) and 5.7 ± 1.7 (5-14) d, respectively (P = .56; Table 3). There were no revision cases in either group. TABLE 3. Perioperative Data and Cage Properties   Banana-shaped cage group  Straight-cage group  P value  Operating time (h)  2.4 ± 0.5 (1.5-3.75)  2.5 ± 0.4 (1.5-3.5)  .32  Mean blood loss (mL)  292.9 ± 180.1 (150-1200)  288.9 ± 170.3 (200-1000)  .93  Mean hospital stay (days)  6.5 ± 1.4 (5-12)  6.7 ± 1.7 (5-14)  .56  Height of cage used (mm)  10.13 ± 1.1  9.82 ± 0.93  .17    Banana-shaped cage group  Straight-cage group  P value  Operating time (h)  2.4 ± 0.5 (1.5-3.75)  2.5 ± 0.4 (1.5-3.5)  .32  Mean blood loss (mL)  292.9 ± 180.1 (150-1200)  288.9 ± 170.3 (200-1000)  .93  Mean hospital stay (days)  6.5 ± 1.4 (5-12)  6.7 ± 1.7 (5-14)  .56  Height of cage used (mm)  10.13 ± 1.1  9.82 ± 0.93  .17  View Large Mean DH (mm) increased from 7.43 ± 2.97 to 11.11 ± 2.24 postoperatively (P < .05) and decreased to 9.67 (P = .20) at 6 mo and 8.18 at 12 mo (P = .54) in the banana-shaped cage group, while in the straight-cage group it increased from 8.43 to 9.88 postoperatively (P < .05) and decreased to 9.32 at 6 mo (P = .09) and 8.72 at 12 mo (P = .27). The difference in the change in DH was significantly higher in the banana-shaped cage group postoperatively (P = .011) but was not significant at 6 and 12 mo (P = .15). Mean SLA (°) increased from 12.14 to 16.34 postoperatively (P < .05) and decreased to 12.55 (P = .79) at 6 mo in the banana-shaped cage group, while in the straight-cage group it increased from 14.65 to 18.22 postoperatively (P = .032) and decreased to 16.79 at 6 mo (P = .22). The increase in SLA was significantly higher in the banana-shaped cage group postoperatively but not at 6 and 12 mo. Mean LLA (°) was not significantly increased during the follow-up periods compared to preoperative data in either of the groups, and there was no significant difference in the change in LLA between the groups (Table 4). Mean DH, SLA, and LLA at various stages are summarized in Figures 7-9. FIGURE 7. View largeDownload slide Changes in disc height (DH). FIGURE 7. View largeDownload slide Changes in disc height (DH). FIGURE 8. View largeDownload slide Changes in SLA. FIGURE 8. View largeDownload slide Changes in SLA. FIGURE 9. View largeDownload slide Changes in LLA. FIGURE 9. View largeDownload slide Changes in LLA. TABLE 4. Changes in Sagittal Radiographic Parameters     Banana-shaped cage group  Straight-cage group  P value  DH  Preop to postop  3.68 ± 1.13  2.32 ± 0.94  <.0001    Preop to 6 mo  2.44 ± 1.29  1.49 ± 0.86  .0006    Preop to 12 mo  2.25 ± 0.31  1.34 ± 0.79  <.0001  SLA  Preop to postop  4.62 ± 2.53  3.33 ± 3.01  .036    Preop to 6 mo  2.92 ± 3.95  2.19 ± 3.76  .38    Preop to 12 mo  0.05 ± 2.85  1 ± 4.16  .22  LLA  Preop to postop  4.6 ± 7.14  2.5 ± 7.34  .18    Preop to 6 mo  0.8 ± 7.52  1.1 ± 6.35  .84    Preop to 12 mo  − 0.02 ± 7.89  0.5 ± 6.26  .74      Banana-shaped cage group  Straight-cage group  P value  DH  Preop to postop  3.68 ± 1.13  2.32 ± 0.94  <.0001    Preop to 6 mo  2.44 ± 1.29  1.49 ± 0.86  .0006    Preop to 12 mo  2.25 ± 0.31  1.34 ± 0.79  <.0001  SLA  Preop to postop  4.62 ± 2.53  3.33 ± 3.01  .036    Preop to 6 mo  2.92 ± 3.95  2.19 ± 3.76  .38    Preop to 12 mo  0.05 ± 2.85  1 ± 4.16  .22  LLA  Preop to postop  4.6 ± 7.14  2.5 ± 7.34  .18    Preop to 6 mo  0.8 ± 7.52  1.1 ± 6.35  .84    Preop to 12 mo  − 0.02 ± 7.89  0.5 ± 6.26  .74  View Large For pelvic parameters, while pelvic incidence and pelvic tilt decreased and sacral slope increased postoperatively in both groups, there was no significant difference between the 2 groups (Table 5). Radiographic evidence of fusion was observed in 61.6% and 63.6% at 6 mo (P = .85) and 95.2% and 96.6% at 12 mo (P = .75) in the banana-shaped cage group and straight-cage group, respectively, with no significant difference between the groups. Cage subsidence was observed in 14 patients in the banana-shaped cage group (31.8%) and in 7 patients (17.5%) in the straight-cage group (P = .13; Table 6). TABLE 5. Pelvic Parameters   Banana-shaped cage group  Straight-cage group  P value  PI preoperatively  57.17 ± 6.65  55.73 ± 9.67  .11  PI postoperatively  56.78 ± 8.27  54.25 ± 8.83  .80  SS preoperatively  31.36 ± 5.86  32.12 ± 6.91  .59  SS postoperatively  31.94 ± 4.48  32.22 ± 8.13  .84  PT preoperatively  25.81 ± 7.25  23.9 ± 9.86  .31  PT postoperatively  25.23 ± 6.79  23.43 ± 8.82  .30    Banana-shaped cage group  Straight-cage group  P value  PI preoperatively  57.17 ± 6.65  55.73 ± 9.67  .11  PI postoperatively  56.78 ± 8.27  54.25 ± 8.83  .80  SS preoperatively  31.36 ± 5.86  32.12 ± 6.91  .59  SS postoperatively  31.94 ± 4.48  32.22 ± 8.13  .84  PT preoperatively  25.81 ± 7.25  23.9 ± 9.86  .31  PT postoperatively  25.23 ± 6.79  23.43 ± 8.82  .30  View Large TABLE 6. Fusion and Subsidence Rates     Banana-shaped cage group  Straight-cage group  P value  Fusion rates (%)  6 mo  61.6  63.6  .85    12 mo  95.2  96.6  .75  Subsidence  Rate (%)  31.8  17.5  .13    Mean (mm)  3.6 ± 0.89 (2-5)  3.0 ± 1.53 (2-6)  <.05      Banana-shaped cage group  Straight-cage group  P value  Fusion rates (%)  6 mo  61.6  63.6  .85    12 mo  95.2  96.6  .75  Subsidence  Rate (%)  31.8  17.5  .13    Mean (mm)  3.6 ± 0.89 (2-5)  3.0 ± 1.53 (2-6)  <.05  View Large Cages tended to be positioned more medially and posteriorly in the banana-shaped cage group, while the location of the straight cage tended to be distributed more evenly among the anterior, posterior, and lateral portions of the body (Table 7). TABLE 7. Cage Positions   Banana-shaped cage  Straight cage  P value  Anterior  29.5% (13)  62.5% (25)  .0026  Posterior  36.4% (16)  55.5% (22)  .08  Middle  100% (44)  100% (40)  –  Lateral  34.1% (15)  38.6% (17)  .67    Banana-shaped cage  Straight cage  P value  Anterior  29.5% (13)  62.5% (25)  .0026  Posterior  36.4% (16)  55.5% (22)  .08  Middle  100% (44)  100% (40)  –  Lateral  34.1% (15)  38.6% (17)  .67  View Large Patients were ambulated 6 to 8 h postoperatively to assess their postoperative day functional outcomes. The mean VAS scores for back pain and leg pain decreased significantly in both groups. Mean ODI scores improved significantly postoperatively and were maintained throughout the follow-up period. No significant difference was observed between the groups (Figures 10A and 10B). FIGURE 10. View largeDownload slide A, VAS scores. B, ODI scores. FIGURE 10. View largeDownload slide A, VAS scores. B, ODI scores. There were no cases of perioperative and postoperative complications requiring revision surgery, such as postoperative hematoma, malpositioned screws with violation of neural elements, or postoperative infection in either group. DISCUSSION MIS-TLIF has gained in popularity due to its advantages of smaller incisions, reduced paraspinal muscle trauma, decreased intraoperative blood loss, shorter hospital stays, and decreased rates of operative site infection, resulting in lower postoperative morbidity and expedite postoperative recovery.5,8,9,20-23 L4-5 and L5-S1 are most frequently treated in single-level lumbar fusion surgery,24-26 and they also account for a large portion of lumbar lordosis.26 The stability of the fused segment is dependent largely on the location, position, and surface area of the interbody cage.27,28 The rates of reoperation, pseudoarthrosis, development of adjacent segment pathology, and clinical recurrence of symptoms are influenced by the stability of the fusion segment.19,29 The interbody cage is more important in MIS-TLIF, which features a narrower working field, asymmetric approach, and less area for posterolateral fusion.10,30 Few biomechanical or retrospective studies that have evaluated the properties of the various types of interbody cage and their effect on the fusion rate and sagittal alignment correction have reported conflicting results. Cho et al. reported no difference in subsidence rate and construct stability between banana-shaped and straight cages,31 while others reported that the subsidence rate differed according to cage type.14,32,33 The increase in DH was significantly higher in the banana-shaped cage group in all postoperative and follow-up radiographs. Likewise, improvement in SLA was significantly higher in the banana-shaped cage group postoperatively, and while the improvement was also higher in the subsequent follow-up radiographs, no statistical significance was reached. The higher DH and SLA restoration in the banana-shaped cage group may be accounted for by the fact that the banana-shaped cages were taller than the straight cages (10.13 ± 1.1 vs 9.82 ± 0.93 mm, P = .17) and that the Crescent cage has 6 degrees of inherent lordotic angle. The superiority of banana-shaped cages in DH and SLA restoration is concordant with previous reports.14,34 That improvement in SLA was not significantly superior in 1 group at the 6- and 12-mo follow-ups suggest that cage migration or subsidence occurred between the immediate postoperative evaluation and the 6-mo follow-up. Kim et al14 reported that a banana-shaped cage or curvilinear cage is superior for creation of LLA and attributed this to its more anterior position. In our study, there was no significant difference in restoration or creation of LLA between the 2 groups. Changes in pelvic parameters also did not differ significantly between the 2 groups. In a meta-analysis of fusion rates of MIS-TLIF, Wu et al35 reported the fusion rate to be 94.8% to 95%. Our assessment yielded similar results. While fusion rates of the 2 groups were not significantly different, the cage subsidence rate was higher in the banana-shaped cage group. According to Closkey et al,36 to achieve successful fusion, the surface area of the cage in contact with the endplate should be at least 30% of the total surface area of the endplate. The banana-shaped cage used in our study has a similar surface area (135-180 mm2) to the straight cage (133-175 mm2). With regard to the strength of different parts of the vertebral endplate, several studies have indicated that the rim of the apophyseal ring has a higher density than the middle portion of the endplate and is more resistant to compressive load.37-39 Several in Vitro biomechanical studies also report that the posterior and lateral parts of the endplate are the strongest and most resistant to compressive load.38,40 In view of these reports, we assessed the relative positions of 2 types of cage on axial CT images. Contrary to our expectations, straight cages had significantly better coverage of the anterior portions of the endplates as well as posterior portions, with a P value approaching significance. Coverage of the lateral position was slightly higher in the banana-shaped cage group, albeit not significantly so. These results imply that banana-shaped cages tended to be placed mostly centrally (medial position) and laterally, while straight cages tended to cover broader regions of the endplates spanning the anterior and posterior portions more evenly. Because the middle part was the weakest part of the endplate, these results might explain why the endplate violation and subsidence rate was higher in the banana-shaped cage group. Abbushi et al28 also reported a higher migration rate when cages were positioned mediomedially. Ideally, according to the recommendations in the manual supplied by the manufacturer and several reports,34,41 banana-shaped cages should be repositioned once inserted into the disc space to be as anterior as possible, but this was not achieved in a large number of the cases because of the narrowed disc space, and possibly because of packed morselized bone in the anterior portion of the disc space prior to cage insertion. Bone chips may have hindered pushing of the cage to the furthest anterior portion of the disc space, especially when inserting banana-shaped cages, which by design are meant to slide along the posterior wall of the anterior annulus during insertion. We suspect that, as a result of these shortcomings, the cages ended up in the central portion of the vertebral endplates. Straight cages by design do not require rotational repositioning and additional manipulation once inside the disc space, which could have contributed to the lower rates of endplate violation. Positioned diagonally across the disc space, they tended to cover a broader area of the endplate, covering anterior and lateral zones where resistance to compressive load is highest. From these observations, when using the banana-shaped cage, the authors would suggest not overfilling the anterior portion of the disc space so as to not hinder the tamping of the cage to the anterior portion of the disc. Inserting morselized bone prior to cage insertion under the microscope may help determine the adequate amount of bone that should be inserted. Other factors should be taken into consideration. Excessive and overzealous curettage during disc space preparation with shavers and curettes can lead to endplate damage and cage subsidence,42,43 and different surgeons have different standards for sufficient endplate curettage. In our study, a single surgeon performed all operations, and disc space was prepared to a similar degree in all cases in a consciously meticulous manner. Patient-related factors, including age, obesity, and osteoporosis can also affect subsidence rates.43 However, there were no significant differences in these factors between the 2 groups in our study. While VAS and ODI scores decreased significantly in both groups in the postoperative period, no significant difference was found between the groups, suggesting that, despite differences in degrees of sagittal correction and subsidence rates, clinical outcomes were not affected in the first 12 mo postoperatively. Limitations Our study had a number of limitations. First, the number of enrolled patients was small. Second, all operations were done by a single experienced surgeon, and the results might have been different if a cohort of surgeons with varying degrees of experience had performed the procedures. Third, the patients were not stratified according to the level of operation. As the properties of L4-5 and L5-S1 are different in terms of their contribution to sagittal alignment and the dimensions of the disc space, the different numbers of each level operated on could have resulted in bias. Fourth, the minimum follow-up period of 12 mo might have been insufficient to obtain convincing results with regard to the fusion and long-term subsidence rates. As this is an ongoing prospective study, the long-term follow-up results will likely offer more insight into the effects of the 2 cage types. CONCLUSION In this randomized clinical trial, we compared the radiological and clinical outcomes of MIS-TLIF performed using banana-shaped cages and straight cages. While fusion rates were similar in the 2 groups, the banana-shaped cage was significantly superior to the straight cage in terms of DH and SLA restoration postoperatively, although we found no evidence that the significance is maintained during the follow-up period. This may be attributable to the more central position of the banana-shaped cage, which may have influenced the subsidence rate. Disclosures The authors declare that there is no conflict of interest regarding the publication of this paper. The authors acknowledge no funding or grants have been received from any sources. The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article. REFERENCES 1. Schwender JD, Holly LT, Rouben DP, Foley KT. Minimally invasive transforaminal lumbar interbody fusion (TLIF): technical feasibility and initial results. J Spinal Disord Tech . 2005; 18( suppl): S1- S6. Google Scholar CrossRef Search ADS PubMed  2. Fan S, Hu Z, Zhao F, Zhao X, Huang Y, Fang X. Multifidus muscle changes and clinical effects of one-level posterior lumbar interbody fusion: minimally invasive procedure versus conventional open approach. Eur Spine J . 2010; 19( 2): 316- 324. 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Bakhsheshian J, Khanna R, Choy W et al.   Incidence of graft extrusion following minimally invasive transforaminal lumbar interbody fusion. J Clin Neurosci . 2016; 24: 88- 93. Google Scholar CrossRef Search ADS PubMed  34. Lindley TE, Viljoen SV, Dahdaleh NS. Effect of steerable cage placement during minimally invasive transforaminal lumbar interbody fusion on lumbar lordosis. J Clin Neurosci . 2014; 21( 3): 441- 444. Google Scholar CrossRef Search ADS PubMed  35. Wu RH, Fraser JF, Hartl R. Minimal access versus open transforaminal lumbar interbody fusion: meta-analysis of fusion rates. Spine . 2010; 35( 26): 2273- 2281. Google Scholar CrossRef Search ADS PubMed  36. Closkey RF, Parsons JR, Lee CK, Blacksin MF, Zimmerman MC. Mechanics of interbody spinal fusion. Analysis of critical bone graft area. Spine . 1993; 18( 8): 1011- 1015. Google Scholar CrossRef Search ADS PubMed  37. Abe K, Orita S, Mannoji C et al.   Perioperative complications in 155 patients who underwent oblique lateral interbody fusion surgery: perspectives and indications from a retrospective, multicenter survey. Spine . 2017; 42( 1): 55- 62. Google Scholar CrossRef Search ADS PubMed  38. Grant JP, Oxland TR, Dvorak MF. Mapping the structural properties of the lumbosacral vertebral endplates. Spine . 2001; 26( 8): 889- 896. Google Scholar CrossRef Search ADS PubMed  39. Lowe TG, Hashim S, Wilson LA et al.   A biomechanical study of regional endplate strength and cage morphology as it relates to structural interbody support. Spine . 2004; 29( 21): 2389- 2394. Google Scholar CrossRef Search ADS PubMed  40. Labrom RD, Tan JS, Reilly CW, Tredwell SJ, Fisher CG, Oxland TR. The effect of interbody cage positioning on lumbosacral vertebral endplate failure in compression. Spine . 2005; 30( 19): E556- E561. Google Scholar CrossRef Search ADS PubMed  41. Wang SJ, Han YC, Pan FM, Ma B, Tan J. Single transverse-orientation cage via MIS-TLIF approach for the treatment of degenerative lumbar disease: a technical note. Int J Clin Exp Med . 2015; 8( 8): 14154- 14160. Google Scholar PubMed  42. Kuslich SD, Ulstrom CL, Griffith SL, Ahern JW, Dowdle JD. The Bagby and Kuslich method of lumbar interbody fusion. History, techniques, and 2-year follow-up results of a United States prospective, multicenter trial. Spine . 1998; 23( 11): 1267- 1278; discussion 1279. Google Scholar CrossRef Search ADS PubMed  43. Lim TH, Kwon H, Jeon CH et al.   Effect of endplate conditions and bone mineral density on the compressive strength of the graft-endplate interface in anterior cervical spine fusion. Spine . 2001; 26( 8): 951- 956. Google Scholar CrossRef Search ADS PubMed  COMMENTS In their manuscript, the authors present the results of a randomized prospective trial comparing the clinical and radiographic outcomes of 2 different cage types (banana-shaped vs straight-cages) used in MIS-TLIF. In total, 84 patients were included in this study. The authors found a significantly higher increase in DH and segmental lordotic angle immediately postoperatively. However, upon follow-up these differences were lost. Clinical outcome and fusion rates were similar for both groups. However, banana-shaped cages were associated with a significantly higher rate of cage subsidence which may be caused by the more medial position of the banana-shaped cages. The study is interesting in several regards. Firstly, it sets out to investigate unanswered questions regarding the operative technique (MIS-TLIF) that have not been thoroughly answered yet. In daily practice, it can be very helpful for a surgeon to know which benefits different cage types offer. Additionally, the study is designed as a prospective randomized trial, which adds impact to the results of this study. However, the study has some limitations that are discussed by the authors as well. The patient numbers are rather low and the follow-up period is short. As there appears to be a significantly higher subsidence rate with banana-shaped cages, it would be interesting to see whether this affects the long-term outcome. Based on the results presented in the manuscript, surgeons are free to choose either cage based on their preference. But even if its effect on the clinical and radiographic outcome remains unclear, it seems reasonable to try to place banana-shaped cages as anteriorly as possible in order to lower the risk of cage subsidence. Roger Hartl New York, New York The authors have presented a timely and thoughtful randomized clinical trial asking the question whether or not there is a difference (both clinical and radiographic) between minimally invasive transforaminal lumbar interbody fusions performed with a straight cage or banana-shaped cage. A rigorous radiographic evaluation was completed preoperatively and postoperatively in patients undergoing an MIS-TLIF, to include position of the interbody, fusion status, disc height, segmental lordotic angle, subsidence, and spinopelvic parameters, along with validated clinical outcome scales. Reasonably sized cohorts were randomized into 2 groups and then followed for 12 months. Fusion rates were nearly equivalent in the 2 groups (95.2% vs 96.6%), indicating that to achieve an arthrodesis, the type of interbody that is selected is meaningless. Instead it is meticulous preparation of the endplates that will reliably achieve an arthrodesis. After all, Cloward reliably achieved interbody fusions without instrumentation.1 The authors made several interesting observations: disc height and segmental lordotic angle was greater in the banana-shaped cage group in the immediate postoperative period than in the straight cage group. However, the incidence of subsidence during follow-up was significantly higher in the banana-shaped group resulting in the loss of lordosis and disc height achieved through the operation. Such an observation also reinforces the old adage that nothing ruins a perfectly good operation like follow-up. There are 2 especially important findings in this prospective randomized study. The first is the issue of segmental lordosis. The authors found that the mean segmental lordosis angle increased in both groups, but the increase was greater in the banana-shaped cage than the straight cage. Such an observation is important because other authors have reported the exact opposite with a loss of lordosis in transforaminal approaches, leading some to suggest that the TLIF is a kyphosing operation.2 The data presented by these authors clearly refutes this and would suggest the capacity to achieve lordosis in a TLIF is once again technique related. The second observation is the location of the interbody spacer. I share the authors' surprise to learn that contrary to the intuitive assumption, the banana-shaped interbody would have better coverage of the anterior portion of the endplates, this study demonstrated the exact opposite. While I do not refute the rigorous radiographic analysis performed by the authors, I would suggest that a conclusion that straight cages have better anterior coverage than banana cages applies only for this study and should not be broadly applied. In this reviewer's experience, broader coverage of the anterior aspect of the endplate is actually better achieved with banana-shaped interbody than straight cages. The evolution of the interbodies and their use is especially germane in this circumstance. Straight cages were designed based on Cloward's PLIF operation, where straight cages were place bilaterally with bilateral access to the disc space. Unilateral access to the disc space introduced by Harms spawned the development of a geometry that could achieve coverage across the disc space from that unilateral approach.3 The straight cages used in transforaminal approaches were never intended to be placed obliquely through a transforaminal corridor. However, the familiarity with that configuration fit naturally in the hands of those surgeons who migrated from PLIFs to TLIFs and the use of straight interbodies in TLIF has become commonplace. As the authors mention, rotating an interbody into the anterior half and center of the disc space is more technically demanding than securing a straight interbody obliquely across the disc space. It is likely more the consequence of technique than geometry that explains the authors' radiographic observation that straight cages had better coverage of the anterior and posterior portions of the endplates, while the coverage of the lateral aspects of the endplates was slightly higher with the banana-shaped geometry. The authors found banana-shaped interbodies are placed mostly central and lateral, while straight interbodies cover the anterior and posterior portion more evenly. It is the position of the interbody within the disc space that led directly to the authors next observation of subsidence, which they found more likely in the banana-shaped interbody than the straight caged interbody. While the authors do not specify the dimensions of the banana-shaped interbodies used, they do report a range: 25-36 mm. It would stand to reason that the smaller the interbody the less likely it would be that the interbody would reach the apophyseal ring. Without the support of the apophyseal ring the result was subsidence. The authors specifically discuss this in their discussion, citing studies that the posterior and lateral aspects of the vertebral body endplate are the most resistant to a compressive load. They obviously recognize the technique component that contributes to subsidence. Specifically, the authors mention that over filling the anterior aspect of the disc space with graft material, will prevent the interbody from reaching the anterior aspect of the disc space. The authors acknowledge that the technique guide along with various operative technique papers have emphasized the importance of placing the interbody in the anterior aspect of this disc space. Another option, not currently available, is a graft with dimensions that when rotated through a transforaminal corridor reaches the apophyseal ring. Perhaps with the long-term radiographic results of the banana-cage demonstrated with this study, the anatomical basis for longer banana-shaped interbodies that can span from apophyseal ring to apophyseal ring may be further explored. Regardless, the authors are to be commended on orchestrating a prospective, randomized clinical trial attempting to answer the question that undoubtedly countless of surgeons have asked who feel strongly about the geometry of interbody that they use. It is increasingly difficult to accomplish these types of studies in the current climate of medicine (A CT scan of the lumbar spine in an asymptomatic patient is no longer available to most clinicians in the United States.) While the results of this study actually demonstrate a remarkable parity in the 2 geometries (straight and banana) in the long-term, the question that needs to be asked after reviewing the long-term radiographic analysis is how to capture the short-term radiographic benefits of banana-shaped cages for the long-term. We can only speculate as to the answer to that question, but based on the data provided by this manuscript, one can safely venture that it would be a combination of geometry, dimensions, and surgical technique. Perhaps it will be these modifications that will lead to long-term restoration of disc height, preservation of segmental lordosis and reliably achieve an arthrodesis in the MIS TLIF patients of tomorrow. Luis Manuel Tumialan Phoenix, Arizona 1. Cloward RB. The treatment of ruptured lumbar intervertebral discs by vertebral body fusion. I. Indications, operative technique, after care. Journal of neurosurgery. Mar 1953;10(2):154-168. 2. Hsieh PC, Koski TR, O'Shaughnessy BA, et al. Anterior lumbar interbody fusion in comparison with transforaminal lumbar interbody fusion: implications for the restoration of foraminal height, local disc angle, lumbar lordosis, and sagittal balance. Journal of neurosurgery. Spine. Oct 2007;7(4):379-386. 3. Harms J, Jeszenszky D. The Unilateral, Transforaminal Approach for Posterior Lumbar Interbody Fusion. Orthopaedics and Traumatology. 1998;6(2):88-99. Copyright © 2017 by the Congress of Neurological Surgeons

Journal

NeurosurgeryOxford University Press

Published: Mar 1, 2018

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