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Biomechanics and anterior cruciate ligament reconstruction

Biomechanics and anterior cruciate ligament reconstruction For years, bioengineers and orthopaedic surgeons have applied the principles of mechanics to gain valuable information about the complex function of the anterior cruciate ligament (ACL). The results of these investigations have provided scientific data for surgeons to improve methods of ACL reconstruction and postoperative rehabilitation. This review paper will present specific examples of how the field of biomechanics has impacted the evolution of ACL research. The anatomy and biomechanics of the ACL as well as the discovery of new tools in ACL-related biomechanical study are first introduced. Some important factors affecting the surgical outcome of ACL reconstruction, including graft selection, tunnel placement, initial graft tension, graft fixation, graft tunnel motion and healing, are then discussed. The scientific basis for the new surgical procedure, i.e., anatomic double bundle ACL reconstruction, designed to regain rotatory stability of the knee, is presented. To conclude, the future role of biomechanics in gaining valuable in-vivo data that can further advance the understanding of the ACL and ACL graft function in order to improve the patient outcome following ACL reconstruction is suggested. Background geons on patients with a ruptured ACL. It is estimated that An anterior cruciate ligament (ACL) rupture is one of the approximately 100,000 primary ACL reconstruction sur- most common knee injuries in sports. It is estimated that geries are performed annually in the United States [1,3]. the annual incidence is about 1 in 3,000 within the gen- The direct cost for these operations is estimated to be over eral population in the United States, which translates into $2 billion [4]. more than 150,000 new ACL tears every year [1,2]. Unlike many tendons and ligaments, a mid-substance ACL tear The goal of an ACL reconstruction is to reproduce the cannot heal and the manifestation is moderate to severe functions of the native ACL. Over the past three decades, disability with "giving way" episodes in activities of daily clinically relevant biomechanical studies have provided living, especially during sporting activities with demand- us with important data on the ACL, particularly on its ing cutting and pivoting maneuvers. Further, it can cause complex anatomy and functions in stabilizing the knee injuries to other soft tissues in and around the knee, par- joint in multiple degrees of freedom (DOF). As such, sur- ticularly the menisci, and lead to early onset osteoarthritis gical reconstruction of the ACL has not been able to repro- of the knee. Therefore, surgical treatment using tissue duce its complex function. Both short and long term autografts or allografts is frequently performed by sur- clinical outcome studies reveal an 11–32% less than satis- Page 1 of 9 (page number not for citation purposes) Journal of Orthopaedic Surgery and Research 2006, 1:2 http://www.josr-online.com/content/1/1/2 factory outcome for patients [5-8], among whom up to Discovery of tools for biomechanical studies of 10% may require revision ACL reconstruction [9]. Indeed, the ACL and ACL grafts There have been many tools, including buckle transduc- ACL reconstruction remains a significant clinical problem to date as there have been over 3,000 papers published in ers, load cells, strain gauges, and so on, designed to meas- the last 10 years, with over half focusing on techniques, a ure the forces within the ACL when a load is applied to the large number on complications and related issues, and knee [14-19]. All have contributed significantly to the only a small percentage on clinical outcome. knowledge of the function of the ACL. However, they all make contact with the ACL. This review paper will provide a perspective on how bio- mechanics has helped in understanding the complex Other investigators prefer to measure the force in the ACL function of the normal ACL as well as in advancing ACL without contact. These include the use of radiographic or reconstruction. Firstly, the anatomy and function of the kinematic linkage systems attached to the bones and ACL as well as available tools in ACL-related biomechan- determine the forces in the ACL by combining kinematic ical study are briefly introduced. Secondly, the contribu- data from the intact knee and the load-deformation curves tions of biomechanics in determining some key factors of the ACL [12,20]. More recently, computer modeling that affect the surgical outcomes of ACL reconstruction are and simulations have also been used to estimate the forces discussed. Thirdly, the role of biomechanics in developing in the ACL during gait [21]. a new ACL reconstruction procedure, i.e., anatomic dou- ble bundle ACL reconstruction, is presented. Finally, the In our research center, we have pioneered the use of a future role of biomechanics in gaining the needed in-vivo robotic manipulator together with a 6-DOF universal data to further improve the results of ACL reconstruction force-moment sensor (UFS), as illustrated in Figure 1[22]. for better patient outcome is suggested. Anatomy and biomechanics of the ACL The ACL extends from the lateral femoral condyle within the intercondylar notch, to its insertion at the anterior part of the central tibial plateau. The cross-sectional areas of the ACL at the two insertion sites are larger than those at the mid substance. The cross-sectional shape of the ACL is also irregular[10]. Functionally, the ACL consists of the anteromedial (AM) bundle and the posterolateral (PL) bundle [11]. It has been shown that the AM bundle lengthens and tightens in flexion, while the PL bundle does the same in extension [12]. These complex anato- mies make the ACL particularly well suited for limiting excessive anterior tibial translation as well as axial tibial and valgus knee rotations. Laboratory studies have determined load-elongation curve of a bone-ligament-bone complex by a uniaxial ten- sile test. The stiffness and ultimate load are obtained to represent its structural properties. In the same test, a stress-strain relationship can also be obtained, from which the modulus, tensile strength, ultimate strain, and strain energy density can be measured to represent the mechanical properties [13]. In addition, forces in the ACL can be measured by studying the knee kinematics in 6 DOF in response to externally applied loads. For instance, when a knee is subjected to an anterior tibial load, it undergoes anterior tibial translation, as well as internal (a) The s fo Figure 1 yrce stem de s in robotic/universal force- 6 DO signed F to measure knmo ee kin mee nt se matics and in situ nsor (UFS) testing tibial rotation. Thus, biomechanics is useful to determine (a) The robotic/universal force-moment sensor (UFS) testing the inter-relationships between the ACL and knee kine- system designed to measure knee kinematics and in situ matics as the data serve as the basis for the goal of a forces in 6 DOF. (b) A human cadaveric knee specimen mounted on the robotic/UFS testing system. replacement graft. Page 2 of 9 (page number not for citation purposes) Journal of Orthopaedic Surgery and Research 2006, 1:2 http://www.josr-online.com/content/1/1/2 This robotic/UFS testing system can be used to measure Biomechanics for ACL reconstruction the in situ force vectors of the ACL and the ACL graft in The ultimate aim of an ACL reconstruction is to restore the response to applied loads to the knee. This system is capa- function of the intact ACL. Laboratory study on human ble of accurately recording and repeating translations and cadaveric knee designed to evaluate the effectiveness of rotation of less than 0.2 mm and 0.2°, respectively [23]. ACL reconstruction under clinical maneuvers, i.e anterior Interested readers may refer to Woo, et al. for the princi- drawer and Lachman test, reveal that most of the current ples and detailed operation of this testing system [22,24]. reconstruction procedures are satisfactory during anterior tibial loads [29]. However, they fail to restore both the Through the use of the robotic/UFS testing system, a thor- kinematics and the in situ forces in the ACL under rotatory ough understanding of the function of the ACL, and more loads (Figures 3 and 4) and muscle loads [30,31]. importantly its AM and PL bundles, was possible. For instance, it has been found that under an anterior tibial Factors affecting the outcome of an ACL reconstruction load, the PL bundle actually carried a higher load than the Factors that could determine the fate of an ACL recon- AM bundle with the knee near extension, and the AM struction include graft selection, tunnel placement, initial bundle carried a higher load with the knee flexion angle graft tension, graft fixation, graft tunnel motion, and rate larger than 30° (Figure 2) [25]. It was also found that of graft healing. We believe that there is a logical sequence when the knee was under combined rotatory loads of val- to examine these factors in order to achieve the ideal gus and internal tibial torques, the AM and PL bundles results (Figure 5). almost evenly shared the load at 15° of knee flexion [25]. Thus, it is clear that the smaller PL bundle does play a sig- Graft selection nificant role in controlling rotatory stability due to its Over the years, a variety of autografts and allografts have more lateral femoral position. been used for ACL reconstruction. Synthetic grafts had also been tried and are seldom used because of poor results. For autografts, the bone-patellar tendon-bone ACL reconstruction The first intra-articular ACL reconstruction began with Hey-Groves in 1917; however, it was made popular by O'Donoghue in 1950. The introduction of arthroscopic equipment has further led to revolutionary changes in ACL surgery [26-28]. There has since been a significant increase in the frequency of ACL reconstruction as well as research on this procedure. Coup 5-Nm internal tibia the intact, Figure 3 led anterior 2) ACL-deficient, tibial translat l torque an and 3) ACL-recon d 1 ion in response to combin 0-Nm valgus tor struc que for 1) ted knee ed Coupled anterior tibial translation in response to combined 5-Nm internal tibial torque and 10-Nm valgus torque for 1) the intact, 2) ACL-deficient, and 3) ACL-reconstructed knee. Magnitu b a Figure 2 n undle in d n = 10) de of the in situ response to 134 N a force in the nterior tibia intact AM b l loadundle an (mean ± SD d PL * indicates significant difference when compared with the Magnitude of the in situ force in the intact AM bundle and PL intact knee, † indicates significant difference when compared bundle in response to 134 N anterior tibial load (mean ± SD with the anatomic reconstruction (mean ± SD and n = 10). and n = 10). (Reproduced with permission from Gabriel MT, (Reproduced with permission from Yagi M, Wong EK, Kan- Wong EK, Woo SL, Yagi M, Debski RE: Distribution of in situ amori A, Debski RE, Fu FH, Woo SL: Biomechanical analysis forces in the anterior cruciate ligament in response to rota- of an anatomic anterior cruciate ligament reconstruction. Am tory loads. J Orthop Res 2004, 22:85–89). J Sports Med 2002, 30:660–666.) Page 3 of 9 (page number not for citation purposes) Journal of Orthopaedic Surgery and Research 2006, 1:2 http://www.josr-online.com/content/1/1/2 hamstring function (to reduce anterior tibial translation) are of concern [37,38]. Tunnel placement Femoral tunnel placement will have a profound effect on knee kinematics. In recent years, most surgeons choose to move the femoral tunnel to the footprint of the AM bun- dle of the ACL, i.e., near the 11 o'clock position on the frontal view of a right knee. Biomechanical studies have suggested that this femoral tunnel placement could not satisfactorily achieve the needed rotatory knee stability, whereas a more lateral placement towards the footprint of the PL bundle, i.e., the 10 o'clock position yielded better results [39]. Further, in addition to the frontal plane (i.e., the clock position), the tunnel position in the sagittal plane must also be considered [40]. In revision ACL sur- gery, it was discovered that there were a large percentage of wrong graft tunnel placement in this plane [41]. Still, it has been shown that there is no single position that could produce the rotatory knee stability close to that of the intact knee [39]. As a result, biomechanical studies have In situ force in response to a combin t ions (mea Figure 4 orque ann ± SD and n = 10 d 10-Nm the v ACL and aed rotatory lgus to)r the quereplac a load of 5- t 15ement grafts in ° and Nm 30°int kn eee rn fl al ex tib-ial In situ force in the ACL and the replacement grafts in been conducted to evaluate an anatomic double bundle response to a combined rotatory load of 5-Nm internal tibial ACL reconstruction. The details will be discussed in a later torque and 10-Nm valgus torque at 15° and 30° knee flex- section. ions (mean ± SD and n = 10). (Reproduced with permission from Yagi M, Wong EK, Kanamori A, Debski RE, Fu FH, Initial graft tension Woo SL: Biomechanical analysis of an anatomic anterior cru- Laboratory studies have found that an initial graft tension ciate ligament reconstruction. Am J Sports Med 2002, 30:660– of 88 N resulted in an overly constrained knee; while a 666.) lower initial graft tension of 44 N would be more suitable [42]. On the contrary, an in vivo study on goats found no significant differences in knee kinematics and in situ forces, between high (35 N) and low (5 N) initial tension (BPTB) and hamstrings tendons are the most common, groups at 6 weeks after surgery [43]. Viscoelastic studies albeit some surgeons also use the quadriceps tendon and revealed that the tension in the graft can decrease by as the iliotibial band. BPTB autografts have been proclaimed much as 50% within a short time after fixation because of as the "gold standard" in ACL reconstruction. Recently, its stress relaxation behavior [44,45]. More recently, a 2- issues relating to donor site morbidity, such as arthrofi- year follow up study evaluating a range of graft tensions of brosis, kneeling/patello-femoral pain, and quadriceps 20 N, 40 N, and 80 N found that the highest graft tension weakness, have caused a paradigm shift from 86.9% to of 80 N produced a significantly more stable knee (p < 21.2% between 2000 to 2004 to quadrupled semitendi- 0.05) [46]. Thus, the literature is confusing and definitive nosus and gracilis tendon (QSTG) autografts [32,33]. answers on initial graft tension remain unknown [47]. Graft fixation Biomechanically, a 10-mm wide BPTB graft has stiffness and ultimate load values of 210 ± 65 N/mm and 1784 ± There are advocates of early and aggressive postoperative 580 N, respectively [34], which compare well with those rehabilitation as well as neuromuscular training to help of the young human femur-ACL-tibia complex (FATC) athletes return to sports as early as possible [26]. To meet (242 ± 28 N/mm and 2160 ± 157 N, respectively) [35]. It these requirements, increased rigidity of mechanical fixa- also has the advantage of having bone blocks available for tion of the grafts has been promoted and a wide variety of graft fixation in the osseous tunnels that leads to better fixation devices are now available. knee stability for earlier return to sports. The QSTG autograft, evolved from a single-strand semitendinosus Biomechanically speaking, for a tendon graft with a bone tendon graft, has very high stiffness and ultimate load val- block on one or both ends (e.g., quadriceps tendon, Achil- ues of (776 ± 204 N/mm, 4090 ± 295 N, respectively) les tendon, and BPTB), interference screws have been suc- [36]. Issues relating to graft tunnel motion and a slower cessfully used [48,49]. An interference screw fixation has rate of tendon to bone healing, as well as the reduction of an initial stiffness of 51 ± 17 N/mm [50], only about 25% Page 4 of 9 (page number not for citation purposes) Journal of Orthopaedic Surgery and Research 2006, 1:2 http://www.josr-online.com/content/1/1/2 A Figure 5 logical sequence of factors to be considered in ACL reconstruction in order to improve the results A logical sequence of factors to be considered in ACL reconstruction in order to improve the results. of that of the intact ACL. Such fixation can be at the native Another type of fixation is the so-called "suspensory fixa- ligament footprint (at the articular surface) and thus can tion", such as the use of EndoButton (Smith & Nephew, limit graft-tunnel motion and increase knee stability. New Inc., Andover, MA) to fix the graft at the lateral femoral interference screws with blunt threads have also been used cortex. The reported stiffness and ultimate load were 61 ± for soft tissue grafts in the bony tunnel with minimal graft 11 N/mm and 572 ± 105 N, respectively [58]. Cross-pin laceration. Recently, bioabsorbable screws have become fixation, such as TransFix (Arthrex, Inc., Naples, FL), is available. They have stiffness and ultimate load values of another method, and has a stiffness and ultimate load of 60 ± 11 N/mm and 830 ± 168 N, respectively, which are 240 ± 74 N/mm and 934 ± 296 N, respectively [59]. It comparable to those for metal screw fixation [51-54]. The should be noted that as the graft is fixed further from the advantages of these screws are that they do not need to be joint surface, the graft tunnel motion will increase. For the removed in cases of revision or arthroplasty, or for MRI. tibial side, cortical screws and washers are used. The ulti- The disadvantages include possible screw breakage during mate load of the fixation is around 800–900 N [60,61]. the insertion, inflammatory response, and inadequate fix- ation due to early degradation of the implant before graft In addition to the devices, the selection of knee flexion incorporation in the bone tunnel [55-57]. angle for graft fixation is also an important biomechanical Page 5 of 9 (page number not for citation purposes) Journal of Orthopaedic Surgery and Research 2006, 1:2 http://www.josr-online.com/content/1/1/2 consideration. It has been shown fixing the graft at full time of application, and dosage levels so that clinical knee extension helps with the range of knee motion, application can be a reality. while fixing at 30° of knee flexion increases the knee sta- bility [62]. A developing trend for ACL reconstruction As traditional single bundle ACL reconstruction could not Tunnel motion fully restore rotatory knee stability, investigators have A goat model study showed that a soft tissue graft secured explored anatomic double bundle ACL reconstruction for by an EndoButton and polyester tape can yield up to 0.8 ACL replacement [70-73]. An anatomic double bundle ± 0.4 mm longitudinal graft tunnel motion and 0.5 ± 0.2 ACL reconstruction utilizes two separate grafts to replace mm transverse motion [38]. In contrast, using a biode- the AM and PL bundles of the ACL. Biomechanical studies gradable interference screw could reduce these motions to have revealed that an anatomic double bundle ACL recon- 0.2 ± 0.1 mm and 0.1 ± 0.1 mm, respectively. In addition, struction has clear advantages in terms of achieving kine- the anterior tibial translation in response to an anterior matics at the level of the intact knee with concomitant tibial load for the EndoButton fixation was significantly improvement of the in situ forces in the ACL graft closer to larger than those fixed with a biointerference screw (5.3 ± those of the intact ACL, even when the knee is subjected 1.2 mm and 4.2 ± 0.9 mm, respectively. p < 0.05) [38]. to rotatory loads [30]. Shown in Figures 3 and 4 are the Our research center has further demonstrated that with coupled anterior tibial translation and the in situ force in EndoButton and polyester tape fixation, the elongation the ACL and ACL grafts in response to combined rotatory of the hamstring graft under cyclic tensile load (50 N), loads of 5 N-m internal tibial torque and 10 N-m valgus was between 14–50% of the total graft tunnel motion, torque. It is worth noting that the coupled anterior tibial suggesting that the majority of motion came from the tape translation after anatomic double bundle ACL reconstruc- [63]. tion was 24% less than that after traditional single bundle ACL reconstruction. In addition, the in situ force in the Graft-tunnel healing ACL graft was 93% of the intact ACL as compared to only Early and improved graft-tunnel healing is obviously 68% for single bundle ACL reconstruction. desirable. Grafts that allow for bone-to-bone healing gen- erally heal faster, i.e., 6 weeks. In contrast, soft tissue grafts Of course, anatomic double bundle ACL reconstruction require tendon-to-bone healing and take 10–12 weeks involves more surgical variables which could affect the [64,65]. Animal model studies showed that the stiffness final outcome. One of the major concerns is the force dis- and ultimate load of the bone patellar tendon-bone tribution between the AM and PL grafts and the potential autograft healing in rabbits at 8 weeks were 84 ± 18 N/mm of overloading either one of the two grafts [25]. Shorter in and 142 ± 34 N, respectively, which were significantly length and smaller in diameter, the PL graft would have a higher compared to 45 ± 9 N/mm and 99 ± 26 N, respec- higher risk of graft failure. To find a range of knee flexion tively, for the tendon autograft healing (p < 0.05) [66]. angles for graft fixation that would be safe for both of the grafts, our research center has performed a series of exper- Various biologically active substances have been used to iments and has discovered that when both the AM and PL accelerate graft healing. Bone morphogenetic protein-2 grafts were fixed at 30°, the in situ force in the PL graft was was delivered to the bone-tendon interface using adenovi- 34% and 67% higher than that in the intact PL bundle in ral gene transfer techniques (AdBMP-2) in rabbits. The response to an anterior tibial load and combined rotatory results showed that at 8 weeks, the stiffness and ultimate loads, respectively. Meanwhile, when the AM graft was load (29 ± 7 N/mm and 109 ± 51 N, respectively) fixed at 60° and the PL graft was fixed at full extension, the increased significantly, as compared to only 17 ± 8 N/mm force in the AM graft was 46% higher than that in the and 45 ± 18 N, respectively, for untreated controls (p < intact AM bundle under an anterior tibial load [74]. A fol- 0.05) [67]. Exogenous transforming growth factor-β and low-up study found that when the PL graft was fixed at epidermal growth factor have also been applied in dog sti- 15° and the AM graft was fixed at either 45° or 15° of fle joints to enhance BPTB autograft healing after ACL knee flexion, the in situ forces in the AM and PL grafts were reconstruction. At 12 weeks, the stiffness and ultimate below those of the AM and PL bundles, i.e., neither graft load of the femur-graft-tibia complex reached 94 ± 20 N/ was overloaded. Thus, these flexion angles are safe for mm and 303 ± 108 N, respectively, almost doubling those graft fixation [75]. of the control group (54 ± 18 N/mm and 176 ± 74 N, respectively) [68]. Recently, periosteum has been sutured Future roles of biomechanics in ACL onto the tendon and inserted into the bone tunnel, result- reconstruction ing in superior and stronger healing [69]. These positive In this review paper, we have summarized how in vitro results have led to more studies on specific growth factors, biomechanical studies have made many significant con- tributions to the understanding of the ACL and ACL Page 6 of 9 (page number not for citation purposes) Journal of Orthopaedic Surgery and Research 2006, 1:2 http://www.josr-online.com/content/1/1/2 replacement grafts and how these data have helped the kinematic data, the in situ forces in the ACL and ACL grafts surgeons. In the future, biomechanical studies must can be calculated. When the calculated in situ forces are involve more realistic in vivo loading conditions. We matched by those obtained experimentally, the computa- envisage an approach that involves both experimental tional model is then validated and can be used to com- and computational methods (see Figure 6). Continuous pute the stress and strain distributions in the ACL and ACL advancements in the development of ways to measure in grafts, as well as to predict in situ forces in the ACL and vivo kinematics of the knee during daily activities are ACL grafts during more complex in vivo motions that being made. Recently, a dual orthogonal fluoroscopic sys- could not be done in laboratory experiments. In the end, tem has been used to measure in vivo knee kinematics, it will be possible to develop a large database on the func- with an accuracy of 0.1 mm and 0.1° for objects with tions of ACL and ACL grafts that are based on subject-spe- known shapes, positions and orientations [76]. Once col- cific data (such as age, gender, and geometry), to elucidate lected, the in vivo kinematic data can be replayed on specific mechanisms of ACL injury, to customize patient cadaveric specimens using the robotics/UFS testing system specific surgical management (including surgical pre- in order to determine the in situ forces in the ACL and ACL planning), as well as to design appropriate rehabilitation grafts. In parallel, subject-specific computational models protocols. We believe such a biomechanics based of the knee can be constructed. Based on the same in vivo approach will provide clinicians with valuable scientific A Figure 6 flow chart detailing a combined approach of experiment and computational modeling based on in vivo kinematics A flow chart detailing a combined approach of experiment and computational modeling based on in vivo kinematics. 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Arthroscopy 2003, under failure tensile loading and cyclic submaximal tensile 19:297-304. loading. Am J Sports Med 2002, 30:549-557. 40. Bernard M, Hertel P, Hornung H, Cierpinski T: Femoral insertion 59. Brown CH Jr, Wilson DR, Hecker AT, Ferragamo M: Graft-bone of the ACL. Radiographic quadrant method. Am J Knee Surg motion and tensile properties of hamstring and patellar ten- 1997, 10:14-22. don anterior cruciate ligament femoral graft fixation under 41. Sommer C, Friederich NF, Muller W: Improperly placed anterior cyclic loading. Arthroscopy 2004, 20:922-935. cruciate ligament grafts: correlation between radiological 60. Steiner ME, Hecker AT, Brown CH Jr, Hayes WC: Anterior cruci- parameters and clinical results. Knee Surg Sports Traumatol ate ligament graft fixation: comparison of hamstring and Arthrosc 2000, 8:207-213. patellar tendon grafts. Am J Sports Med 1994, 22:240-247. 42. Mae T, Shino K, Miyama T, Shinjo H, Ochi T, Yoshikawa H, Fujie H: 61. Magen HE, Howell SM, Hull ML: Structural properties of six tibial Single- versus two-femoral socket anterior cruciate liga- fixation methods for anterior cruciate ligament soft tissue ment reconstruction technique: Biomechanical analysis grafts. Am J Sports Med 1999, 27:35-43. using a robotic simulator. Arthroscopy 2001, 17:708-716. 62. Asahina S, Muneta T, Ishibashi T, Yamamoto H: Effects of knee flex- 43. Abramowitch SD, Papageorgiou CD, Withrow JD, Gilbert TW, Woo ion angle at graft fixation on the outcome of anterior cruci- SL: The effect of initial graft tension on the biomechanical ate ligament reconstruction. Arthroscopy 1996, 12:70-75. properties of a healing ACL replacement graft: a study in 63. Hoher J, Livesay GA, Ma CB, Withrow JD, Fu FH, Woo SL: Ham- goats. J Orthop Res 2003, 21:708-715. string graft motion in the femoral bone tunnel when using 44. Boylan D, Greis PE, West JR, Bachus KN, Burks RT: Effects of initial titanium button/polyester tape fixation. Knee Surg Sports Trau- graft tension on knee stability after anterior cruciate liga- matol Arthrosc 1999, 7:215-219. ment reconstruction using hamstring tendons: a cadaver 64. Papageorgiou CD, Ma CB, Abramowitch SD, Clineff TD, Woo SL: A study. Arthroscopy 2003, 19:700-705. multidisciplinary study of the healing of an intraarticular 45. Johnson GA, Tramaglini DM, Levine RE, Ohno K, Choi NY, Woo SL: anterior cruciate ligament graft in a goat model. Am J Sports Tensile and viscoelastic properties of human patellar ten- Med 2001, 29:620-626. don. J Orthop Res 1994, 12:796-803. 65. Rodeo SA, Arnoczky SP, Torzilli PA: Tendon-healing in a bone 46. Yasuda K, Tsujino J, Tanabe Y, Kaneda K: Effects of initial graft Tunnel. A biomechanical and histological study in the dog. J tension on clinical outcome after anterior cruciate ligament Bone Joint Surg Am 1993, 75:1795-803. reconstruction. Autogenous doubled hamstring tendons 66. Park MJ, Lee MC, Seong SC: A comparative study of the healing connected in series with polyester tapes. Am J Sports Med 1997, of tendon autograft and tendon-bone autograft using patel- 25:99-106. lar tendon in rabbits. Int Orthop 2001, 25:35-39. 47. Nicholas SJ, D'Amato MJ, Mullaney MJ, Tyler TF, Kolstad K, McHugh 67. Martinek V, Latterman C, Usas A, Abramowitch S, Woo SL, Fu FH, MP: A prospectively randomized double-blind study on the Huard J: Enhancement of tendon-bone integration of anterior effect of initial graft tension on knee stability after anterior cruciate ligament grafts with bone morphogenetic protein-2 cruciate ligament reconstruction. Am J Sports Med 2004, gene transfer: a histological and biomechanical study. J Bone 32:1881-1886. Joint Surg Am 2002, 84:1123-1131. 48. Lambert KL: Vascularized patellar tendon graft with rigid 68. Yasuda K, Tomita F, Yamazaki S, Minami A, Tohyama H: The effect internal fixation for anterior cruciate ligament insufficiency. of growth factors on biomechanical properties of the bone- Clin Orthop 1983, 172:85-89. patellar tendon-bone graft after anterior cruciate ligament 49. Kurosaka M, Yoshiya S, Andrish JT: A biomechanical comparison reconstruction: a canine model study. Am J Sports Med 2004, of different surgical techniques of graft fixation in anterior 32:870-880. cruciate ligament reconstruction. Am J Sports Med 1987, 69. Chen CH, Chen WJ, Shih CH, Chou SW: Arthroscopic anterior 15:225-229. cruciate ligament reconstruction with periosteum-envelop- 50. Rowden NJ, Sher D, Rogers GJ, Schindhelm K: Anterior cruciate ing hamstring tendon graft. Knee Surg Sports Traumatol Arthrosc ligament graft fixation. Initial comparison of patellar tendon 2004, 12:398-405. and semitendinosus autografts in young fresh cadavers. Am J 70. Adachi N, Ochi M, Uchio Y, Iwasa J, Kuriwaka M, Ito Y: Reconstruc- Sports Med 1997, 25:472-478. tion of the anterior cruciate ligament. Single- versus double- 51. Walton M: Absorbable and metal interference screws: com- bundle multistranded hamstring tendons. J Bone Joint Surg Br parison of graft security during healing. Arthroscopy 1999, 2004, 86:515-520. 15:818-826. 71. Bellier G, Christel P, Colombet P, Djian P, Franceschi JP, Sbihi A: 52. Caborn DN, Coen M, Neef R, Hamilton D, Nyland J, Johnson DL: Double-stranded hamstring graft for anterior cruciate liga- Quadrupled semitendinosus – gracilis autograft fixation in ment reconstruction7. Arthroscopy 2004, 20:890-894. the femoral tunnel: a comparison between a metal and a bio- 72. Yasuda K, Kondo E, Ichiyama H, Kitamura N, Tanabe Y, Tohyama H, absorbable interference screw. Arthroscopy 1998, 14:241-245. Minami A: Anatomic reconstruction of the anteromedial and 53. Pena F, Grontvedt T, Brown GA, Aune AK, Engebretsen L: Compar- posterolateral bundles of the anterior cruciate ligament ison of failure strength between metallic and absorbable using hamstring tendon grafts. Arthroscopy 2004, 20:1015-1025. interference screws. Influence of insertional torque, tunnel – 73. Zelle BA, Brucker PU, Feng MT, Fu FH: Anatomical double-bun- bone block gap, bone mineral density, and interference. Am dle anterior cruciate ligament reconstruction. Sports Med J Sports Med 1996, 24:329-334. 2006, 36:99-108. 54. Weiler A, Windhagen HG, Raschke MJ, Laumeyer A, Hoffmann RF: 74. Miura K, Woo SL, Brinkley R, Fu YC, Noorani S: Effects of knee Biodegradable interference screw fixation exhibits pull-out flexion angles for graft fixation on its force distribution in force and stiffness similar to titanium screws. Am J Sports Med double bundle anterior cruciate ligament reconstruction. 1998, 26:119-128. Am J Sports Med 2006, 34:577-585. 55. Lajtai G, Humer K, Aitzetmuller G, Unger F, Noszian I, Orthner E: 75. Vercillo F, Noorani S, Dede O, Miura K, Woo SL: Basic science on Serial magnetic resonance imaging evaluation of a bioab- double bundle anterior cruciate ligament reconstruction sorbable interference screw and the adjacent bone. Arthros- and safe knee flexion angles for graft fixation [abstract]. 6th copy 1999, 15:481-488. International Symposium on Ligaments and Tendons, Chicago, IL 2006. 56. Kousa P, Jarvinen TL, Kannus P, Jarvinen M: Initial fixation 76. Li G, Wuerz TH, DeFrate LE: Feasibility of using orthogonal strength of bioabsorbable and titanium interference screws fluoroscopic images to measure in vivo joint kinematics. J Bio- in anterior cruciate ligament reconstruction. Am J Sports Med mech Eng 2004, 126:314-318. 2001, 29:420-425. 57. Martinek V, Friederich NF: Tibial and pretibial cyst formation after anterior cruciate ligament reconstruction with bioab- sorbable interference screw fixation. Arthroscopy 1999, 15:317-320. 58. Honl M, Carrero V, Hille E, Schneider E, Morlock MM: Bone-patel- lar tendon-bone grafts for anterior cruciate ligament recon- struction: an in vitro comparison of mechanical behavior Page 9 of 9 (page number not for citation purposes) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Orthopaedic Surgery and Research Springer Journals

Biomechanics and anterior cruciate ligament reconstruction

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Springer Journals
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Copyright © 2006 by Woo et al; licensee BioMed Central Ltd.
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Medicine & Public Health; Orthopedics; Surgical Orthopedics
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1749-799X
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10.1186/1749-799X-1-2
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17150122
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Abstract

For years, bioengineers and orthopaedic surgeons have applied the principles of mechanics to gain valuable information about the complex function of the anterior cruciate ligament (ACL). The results of these investigations have provided scientific data for surgeons to improve methods of ACL reconstruction and postoperative rehabilitation. This review paper will present specific examples of how the field of biomechanics has impacted the evolution of ACL research. The anatomy and biomechanics of the ACL as well as the discovery of new tools in ACL-related biomechanical study are first introduced. Some important factors affecting the surgical outcome of ACL reconstruction, including graft selection, tunnel placement, initial graft tension, graft fixation, graft tunnel motion and healing, are then discussed. The scientific basis for the new surgical procedure, i.e., anatomic double bundle ACL reconstruction, designed to regain rotatory stability of the knee, is presented. To conclude, the future role of biomechanics in gaining valuable in-vivo data that can further advance the understanding of the ACL and ACL graft function in order to improve the patient outcome following ACL reconstruction is suggested. Background geons on patients with a ruptured ACL. It is estimated that An anterior cruciate ligament (ACL) rupture is one of the approximately 100,000 primary ACL reconstruction sur- most common knee injuries in sports. It is estimated that geries are performed annually in the United States [1,3]. the annual incidence is about 1 in 3,000 within the gen- The direct cost for these operations is estimated to be over eral population in the United States, which translates into $2 billion [4]. more than 150,000 new ACL tears every year [1,2]. Unlike many tendons and ligaments, a mid-substance ACL tear The goal of an ACL reconstruction is to reproduce the cannot heal and the manifestation is moderate to severe functions of the native ACL. Over the past three decades, disability with "giving way" episodes in activities of daily clinically relevant biomechanical studies have provided living, especially during sporting activities with demand- us with important data on the ACL, particularly on its ing cutting and pivoting maneuvers. Further, it can cause complex anatomy and functions in stabilizing the knee injuries to other soft tissues in and around the knee, par- joint in multiple degrees of freedom (DOF). As such, sur- ticularly the menisci, and lead to early onset osteoarthritis gical reconstruction of the ACL has not been able to repro- of the knee. Therefore, surgical treatment using tissue duce its complex function. Both short and long term autografts or allografts is frequently performed by sur- clinical outcome studies reveal an 11–32% less than satis- Page 1 of 9 (page number not for citation purposes) Journal of Orthopaedic Surgery and Research 2006, 1:2 http://www.josr-online.com/content/1/1/2 factory outcome for patients [5-8], among whom up to Discovery of tools for biomechanical studies of 10% may require revision ACL reconstruction [9]. Indeed, the ACL and ACL grafts There have been many tools, including buckle transduc- ACL reconstruction remains a significant clinical problem to date as there have been over 3,000 papers published in ers, load cells, strain gauges, and so on, designed to meas- the last 10 years, with over half focusing on techniques, a ure the forces within the ACL when a load is applied to the large number on complications and related issues, and knee [14-19]. All have contributed significantly to the only a small percentage on clinical outcome. knowledge of the function of the ACL. However, they all make contact with the ACL. This review paper will provide a perspective on how bio- mechanics has helped in understanding the complex Other investigators prefer to measure the force in the ACL function of the normal ACL as well as in advancing ACL without contact. These include the use of radiographic or reconstruction. Firstly, the anatomy and function of the kinematic linkage systems attached to the bones and ACL as well as available tools in ACL-related biomechan- determine the forces in the ACL by combining kinematic ical study are briefly introduced. Secondly, the contribu- data from the intact knee and the load-deformation curves tions of biomechanics in determining some key factors of the ACL [12,20]. More recently, computer modeling that affect the surgical outcomes of ACL reconstruction are and simulations have also been used to estimate the forces discussed. Thirdly, the role of biomechanics in developing in the ACL during gait [21]. a new ACL reconstruction procedure, i.e., anatomic dou- ble bundle ACL reconstruction, is presented. Finally, the In our research center, we have pioneered the use of a future role of biomechanics in gaining the needed in-vivo robotic manipulator together with a 6-DOF universal data to further improve the results of ACL reconstruction force-moment sensor (UFS), as illustrated in Figure 1[22]. for better patient outcome is suggested. Anatomy and biomechanics of the ACL The ACL extends from the lateral femoral condyle within the intercondylar notch, to its insertion at the anterior part of the central tibial plateau. The cross-sectional areas of the ACL at the two insertion sites are larger than those at the mid substance. The cross-sectional shape of the ACL is also irregular[10]. Functionally, the ACL consists of the anteromedial (AM) bundle and the posterolateral (PL) bundle [11]. It has been shown that the AM bundle lengthens and tightens in flexion, while the PL bundle does the same in extension [12]. These complex anato- mies make the ACL particularly well suited for limiting excessive anterior tibial translation as well as axial tibial and valgus knee rotations. Laboratory studies have determined load-elongation curve of a bone-ligament-bone complex by a uniaxial ten- sile test. The stiffness and ultimate load are obtained to represent its structural properties. In the same test, a stress-strain relationship can also be obtained, from which the modulus, tensile strength, ultimate strain, and strain energy density can be measured to represent the mechanical properties [13]. In addition, forces in the ACL can be measured by studying the knee kinematics in 6 DOF in response to externally applied loads. For instance, when a knee is subjected to an anterior tibial load, it undergoes anterior tibial translation, as well as internal (a) The s fo Figure 1 yrce stem de s in robotic/universal force- 6 DO signed F to measure knmo ee kin mee nt se matics and in situ nsor (UFS) testing tibial rotation. Thus, biomechanics is useful to determine (a) The robotic/universal force-moment sensor (UFS) testing the inter-relationships between the ACL and knee kine- system designed to measure knee kinematics and in situ matics as the data serve as the basis for the goal of a forces in 6 DOF. (b) A human cadaveric knee specimen mounted on the robotic/UFS testing system. replacement graft. Page 2 of 9 (page number not for citation purposes) Journal of Orthopaedic Surgery and Research 2006, 1:2 http://www.josr-online.com/content/1/1/2 This robotic/UFS testing system can be used to measure Biomechanics for ACL reconstruction the in situ force vectors of the ACL and the ACL graft in The ultimate aim of an ACL reconstruction is to restore the response to applied loads to the knee. This system is capa- function of the intact ACL. Laboratory study on human ble of accurately recording and repeating translations and cadaveric knee designed to evaluate the effectiveness of rotation of less than 0.2 mm and 0.2°, respectively [23]. ACL reconstruction under clinical maneuvers, i.e anterior Interested readers may refer to Woo, et al. for the princi- drawer and Lachman test, reveal that most of the current ples and detailed operation of this testing system [22,24]. reconstruction procedures are satisfactory during anterior tibial loads [29]. However, they fail to restore both the Through the use of the robotic/UFS testing system, a thor- kinematics and the in situ forces in the ACL under rotatory ough understanding of the function of the ACL, and more loads (Figures 3 and 4) and muscle loads [30,31]. importantly its AM and PL bundles, was possible. For instance, it has been found that under an anterior tibial Factors affecting the outcome of an ACL reconstruction load, the PL bundle actually carried a higher load than the Factors that could determine the fate of an ACL recon- AM bundle with the knee near extension, and the AM struction include graft selection, tunnel placement, initial bundle carried a higher load with the knee flexion angle graft tension, graft fixation, graft tunnel motion, and rate larger than 30° (Figure 2) [25]. It was also found that of graft healing. We believe that there is a logical sequence when the knee was under combined rotatory loads of val- to examine these factors in order to achieve the ideal gus and internal tibial torques, the AM and PL bundles results (Figure 5). almost evenly shared the load at 15° of knee flexion [25]. Thus, it is clear that the smaller PL bundle does play a sig- Graft selection nificant role in controlling rotatory stability due to its Over the years, a variety of autografts and allografts have more lateral femoral position. been used for ACL reconstruction. Synthetic grafts had also been tried and are seldom used because of poor results. For autografts, the bone-patellar tendon-bone ACL reconstruction The first intra-articular ACL reconstruction began with Hey-Groves in 1917; however, it was made popular by O'Donoghue in 1950. The introduction of arthroscopic equipment has further led to revolutionary changes in ACL surgery [26-28]. There has since been a significant increase in the frequency of ACL reconstruction as well as research on this procedure. Coup 5-Nm internal tibia the intact, Figure 3 led anterior 2) ACL-deficient, tibial translat l torque an and 3) ACL-recon d 1 ion in response to combin 0-Nm valgus tor struc que for 1) ted knee ed Coupled anterior tibial translation in response to combined 5-Nm internal tibial torque and 10-Nm valgus torque for 1) the intact, 2) ACL-deficient, and 3) ACL-reconstructed knee. Magnitu b a Figure 2 n undle in d n = 10) de of the in situ response to 134 N a force in the nterior tibia intact AM b l loadundle an (mean ± SD d PL * indicates significant difference when compared with the Magnitude of the in situ force in the intact AM bundle and PL intact knee, † indicates significant difference when compared bundle in response to 134 N anterior tibial load (mean ± SD with the anatomic reconstruction (mean ± SD and n = 10). and n = 10). (Reproduced with permission from Gabriel MT, (Reproduced with permission from Yagi M, Wong EK, Kan- Wong EK, Woo SL, Yagi M, Debski RE: Distribution of in situ amori A, Debski RE, Fu FH, Woo SL: Biomechanical analysis forces in the anterior cruciate ligament in response to rota- of an anatomic anterior cruciate ligament reconstruction. Am tory loads. J Orthop Res 2004, 22:85–89). J Sports Med 2002, 30:660–666.) Page 3 of 9 (page number not for citation purposes) Journal of Orthopaedic Surgery and Research 2006, 1:2 http://www.josr-online.com/content/1/1/2 hamstring function (to reduce anterior tibial translation) are of concern [37,38]. Tunnel placement Femoral tunnel placement will have a profound effect on knee kinematics. In recent years, most surgeons choose to move the femoral tunnel to the footprint of the AM bun- dle of the ACL, i.e., near the 11 o'clock position on the frontal view of a right knee. Biomechanical studies have suggested that this femoral tunnel placement could not satisfactorily achieve the needed rotatory knee stability, whereas a more lateral placement towards the footprint of the PL bundle, i.e., the 10 o'clock position yielded better results [39]. Further, in addition to the frontal plane (i.e., the clock position), the tunnel position in the sagittal plane must also be considered [40]. In revision ACL sur- gery, it was discovered that there were a large percentage of wrong graft tunnel placement in this plane [41]. Still, it has been shown that there is no single position that could produce the rotatory knee stability close to that of the intact knee [39]. As a result, biomechanical studies have In situ force in response to a combin t ions (mea Figure 4 orque ann ± SD and n = 10 d 10-Nm the v ACL and aed rotatory lgus to)r the quereplac a load of 5- t 15ement grafts in ° and Nm 30°int kn eee rn fl al ex tib-ial In situ force in the ACL and the replacement grafts in been conducted to evaluate an anatomic double bundle response to a combined rotatory load of 5-Nm internal tibial ACL reconstruction. The details will be discussed in a later torque and 10-Nm valgus torque at 15° and 30° knee flex- section. ions (mean ± SD and n = 10). (Reproduced with permission from Yagi M, Wong EK, Kanamori A, Debski RE, Fu FH, Initial graft tension Woo SL: Biomechanical analysis of an anatomic anterior cru- Laboratory studies have found that an initial graft tension ciate ligament reconstruction. Am J Sports Med 2002, 30:660– of 88 N resulted in an overly constrained knee; while a 666.) lower initial graft tension of 44 N would be more suitable [42]. On the contrary, an in vivo study on goats found no significant differences in knee kinematics and in situ forces, between high (35 N) and low (5 N) initial tension (BPTB) and hamstrings tendons are the most common, groups at 6 weeks after surgery [43]. Viscoelastic studies albeit some surgeons also use the quadriceps tendon and revealed that the tension in the graft can decrease by as the iliotibial band. BPTB autografts have been proclaimed much as 50% within a short time after fixation because of as the "gold standard" in ACL reconstruction. Recently, its stress relaxation behavior [44,45]. More recently, a 2- issues relating to donor site morbidity, such as arthrofi- year follow up study evaluating a range of graft tensions of brosis, kneeling/patello-femoral pain, and quadriceps 20 N, 40 N, and 80 N found that the highest graft tension weakness, have caused a paradigm shift from 86.9% to of 80 N produced a significantly more stable knee (p < 21.2% between 2000 to 2004 to quadrupled semitendi- 0.05) [46]. Thus, the literature is confusing and definitive nosus and gracilis tendon (QSTG) autografts [32,33]. answers on initial graft tension remain unknown [47]. Graft fixation Biomechanically, a 10-mm wide BPTB graft has stiffness and ultimate load values of 210 ± 65 N/mm and 1784 ± There are advocates of early and aggressive postoperative 580 N, respectively [34], which compare well with those rehabilitation as well as neuromuscular training to help of the young human femur-ACL-tibia complex (FATC) athletes return to sports as early as possible [26]. To meet (242 ± 28 N/mm and 2160 ± 157 N, respectively) [35]. It these requirements, increased rigidity of mechanical fixa- also has the advantage of having bone blocks available for tion of the grafts has been promoted and a wide variety of graft fixation in the osseous tunnels that leads to better fixation devices are now available. knee stability for earlier return to sports. The QSTG autograft, evolved from a single-strand semitendinosus Biomechanically speaking, for a tendon graft with a bone tendon graft, has very high stiffness and ultimate load val- block on one or both ends (e.g., quadriceps tendon, Achil- ues of (776 ± 204 N/mm, 4090 ± 295 N, respectively) les tendon, and BPTB), interference screws have been suc- [36]. Issues relating to graft tunnel motion and a slower cessfully used [48,49]. An interference screw fixation has rate of tendon to bone healing, as well as the reduction of an initial stiffness of 51 ± 17 N/mm [50], only about 25% Page 4 of 9 (page number not for citation purposes) Journal of Orthopaedic Surgery and Research 2006, 1:2 http://www.josr-online.com/content/1/1/2 A Figure 5 logical sequence of factors to be considered in ACL reconstruction in order to improve the results A logical sequence of factors to be considered in ACL reconstruction in order to improve the results. of that of the intact ACL. Such fixation can be at the native Another type of fixation is the so-called "suspensory fixa- ligament footprint (at the articular surface) and thus can tion", such as the use of EndoButton (Smith & Nephew, limit graft-tunnel motion and increase knee stability. New Inc., Andover, MA) to fix the graft at the lateral femoral interference screws with blunt threads have also been used cortex. The reported stiffness and ultimate load were 61 ± for soft tissue grafts in the bony tunnel with minimal graft 11 N/mm and 572 ± 105 N, respectively [58]. Cross-pin laceration. Recently, bioabsorbable screws have become fixation, such as TransFix (Arthrex, Inc., Naples, FL), is available. They have stiffness and ultimate load values of another method, and has a stiffness and ultimate load of 60 ± 11 N/mm and 830 ± 168 N, respectively, which are 240 ± 74 N/mm and 934 ± 296 N, respectively [59]. It comparable to those for metal screw fixation [51-54]. The should be noted that as the graft is fixed further from the advantages of these screws are that they do not need to be joint surface, the graft tunnel motion will increase. For the removed in cases of revision or arthroplasty, or for MRI. tibial side, cortical screws and washers are used. The ulti- The disadvantages include possible screw breakage during mate load of the fixation is around 800–900 N [60,61]. the insertion, inflammatory response, and inadequate fix- ation due to early degradation of the implant before graft In addition to the devices, the selection of knee flexion incorporation in the bone tunnel [55-57]. angle for graft fixation is also an important biomechanical Page 5 of 9 (page number not for citation purposes) Journal of Orthopaedic Surgery and Research 2006, 1:2 http://www.josr-online.com/content/1/1/2 consideration. It has been shown fixing the graft at full time of application, and dosage levels so that clinical knee extension helps with the range of knee motion, application can be a reality. while fixing at 30° of knee flexion increases the knee sta- bility [62]. A developing trend for ACL reconstruction As traditional single bundle ACL reconstruction could not Tunnel motion fully restore rotatory knee stability, investigators have A goat model study showed that a soft tissue graft secured explored anatomic double bundle ACL reconstruction for by an EndoButton and polyester tape can yield up to 0.8 ACL replacement [70-73]. An anatomic double bundle ± 0.4 mm longitudinal graft tunnel motion and 0.5 ± 0.2 ACL reconstruction utilizes two separate grafts to replace mm transverse motion [38]. In contrast, using a biode- the AM and PL bundles of the ACL. Biomechanical studies gradable interference screw could reduce these motions to have revealed that an anatomic double bundle ACL recon- 0.2 ± 0.1 mm and 0.1 ± 0.1 mm, respectively. In addition, struction has clear advantages in terms of achieving kine- the anterior tibial translation in response to an anterior matics at the level of the intact knee with concomitant tibial load for the EndoButton fixation was significantly improvement of the in situ forces in the ACL graft closer to larger than those fixed with a biointerference screw (5.3 ± those of the intact ACL, even when the knee is subjected 1.2 mm and 4.2 ± 0.9 mm, respectively. p < 0.05) [38]. to rotatory loads [30]. Shown in Figures 3 and 4 are the Our research center has further demonstrated that with coupled anterior tibial translation and the in situ force in EndoButton and polyester tape fixation, the elongation the ACL and ACL grafts in response to combined rotatory of the hamstring graft under cyclic tensile load (50 N), loads of 5 N-m internal tibial torque and 10 N-m valgus was between 14–50% of the total graft tunnel motion, torque. It is worth noting that the coupled anterior tibial suggesting that the majority of motion came from the tape translation after anatomic double bundle ACL reconstruc- [63]. tion was 24% less than that after traditional single bundle ACL reconstruction. In addition, the in situ force in the Graft-tunnel healing ACL graft was 93% of the intact ACL as compared to only Early and improved graft-tunnel healing is obviously 68% for single bundle ACL reconstruction. desirable. Grafts that allow for bone-to-bone healing gen- erally heal faster, i.e., 6 weeks. In contrast, soft tissue grafts Of course, anatomic double bundle ACL reconstruction require tendon-to-bone healing and take 10–12 weeks involves more surgical variables which could affect the [64,65]. Animal model studies showed that the stiffness final outcome. One of the major concerns is the force dis- and ultimate load of the bone patellar tendon-bone tribution between the AM and PL grafts and the potential autograft healing in rabbits at 8 weeks were 84 ± 18 N/mm of overloading either one of the two grafts [25]. Shorter in and 142 ± 34 N, respectively, which were significantly length and smaller in diameter, the PL graft would have a higher compared to 45 ± 9 N/mm and 99 ± 26 N, respec- higher risk of graft failure. To find a range of knee flexion tively, for the tendon autograft healing (p < 0.05) [66]. angles for graft fixation that would be safe for both of the grafts, our research center has performed a series of exper- Various biologically active substances have been used to iments and has discovered that when both the AM and PL accelerate graft healing. Bone morphogenetic protein-2 grafts were fixed at 30°, the in situ force in the PL graft was was delivered to the bone-tendon interface using adenovi- 34% and 67% higher than that in the intact PL bundle in ral gene transfer techniques (AdBMP-2) in rabbits. The response to an anterior tibial load and combined rotatory results showed that at 8 weeks, the stiffness and ultimate loads, respectively. Meanwhile, when the AM graft was load (29 ± 7 N/mm and 109 ± 51 N, respectively) fixed at 60° and the PL graft was fixed at full extension, the increased significantly, as compared to only 17 ± 8 N/mm force in the AM graft was 46% higher than that in the and 45 ± 18 N, respectively, for untreated controls (p < intact AM bundle under an anterior tibial load [74]. A fol- 0.05) [67]. Exogenous transforming growth factor-β and low-up study found that when the PL graft was fixed at epidermal growth factor have also been applied in dog sti- 15° and the AM graft was fixed at either 45° or 15° of fle joints to enhance BPTB autograft healing after ACL knee flexion, the in situ forces in the AM and PL grafts were reconstruction. At 12 weeks, the stiffness and ultimate below those of the AM and PL bundles, i.e., neither graft load of the femur-graft-tibia complex reached 94 ± 20 N/ was overloaded. Thus, these flexion angles are safe for mm and 303 ± 108 N, respectively, almost doubling those graft fixation [75]. of the control group (54 ± 18 N/mm and 176 ± 74 N, respectively) [68]. Recently, periosteum has been sutured Future roles of biomechanics in ACL onto the tendon and inserted into the bone tunnel, result- reconstruction ing in superior and stronger healing [69]. These positive In this review paper, we have summarized how in vitro results have led to more studies on specific growth factors, biomechanical studies have made many significant con- tributions to the understanding of the ACL and ACL Page 6 of 9 (page number not for citation purposes) Journal of Orthopaedic Surgery and Research 2006, 1:2 http://www.josr-online.com/content/1/1/2 replacement grafts and how these data have helped the kinematic data, the in situ forces in the ACL and ACL grafts surgeons. In the future, biomechanical studies must can be calculated. When the calculated in situ forces are involve more realistic in vivo loading conditions. We matched by those obtained experimentally, the computa- envisage an approach that involves both experimental tional model is then validated and can be used to com- and computational methods (see Figure 6). Continuous pute the stress and strain distributions in the ACL and ACL advancements in the development of ways to measure in grafts, as well as to predict in situ forces in the ACL and vivo kinematics of the knee during daily activities are ACL grafts during more complex in vivo motions that being made. Recently, a dual orthogonal fluoroscopic sys- could not be done in laboratory experiments. In the end, tem has been used to measure in vivo knee kinematics, it will be possible to develop a large database on the func- with an accuracy of 0.1 mm and 0.1° for objects with tions of ACL and ACL grafts that are based on subject-spe- known shapes, positions and orientations [76]. Once col- cific data (such as age, gender, and geometry), to elucidate lected, the in vivo kinematic data can be replayed on specific mechanisms of ACL injury, to customize patient cadaveric specimens using the robotics/UFS testing system specific surgical management (including surgical pre- in order to determine the in situ forces in the ACL and ACL planning), as well as to design appropriate rehabilitation grafts. In parallel, subject-specific computational models protocols. We believe such a biomechanics based of the knee can be constructed. Based on the same in vivo approach will provide clinicians with valuable scientific A Figure 6 flow chart detailing a combined approach of experiment and computational modeling based on in vivo kinematics A flow chart detailing a combined approach of experiment and computational modeling based on in vivo kinematics. 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Published: Sep 25, 2006

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