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The influence of miniscrew insertion torque

The influence of miniscrew insertion torque Summary Objective The aim of this in vitro study was to evaluate the progressive development of surface microdamage produced following the insertion of orthodontic miniscrews (OMs) into 1.5 mm thick porcine tibia bone using maximum insertion torque values of 12 Ncm, 18 Ncm, and 24 Ncm. Methods Aarhus OMs (diameter 1.5 mm; length 6 mm) were inserted into 1.5 mm porcine bone using a torque limiting hand screwdriver set at 12 Ncm, 18 Ncm, and 24 Ncm. A custom rig equipped with a compression load cell was used to record the compression force exerted during manual insertion. A sequential staining technique was used to identify microdamage viewed under laser confocal microscopy. Virtual slices were created and stitched together to form a compressed two-dimensional composition of the microdamage. Histomorphometric parameters, including total damage area, diffuse damage area, maximum crack length, maximum damage radius, and maximum diffuse damage radius, were measured. Kruskal–Wallis Tests and Wilcoxon Rank-Sum Tests were used to analyse the generated data. Results All OMs inserted using 12 Ncm failed to insert completely, while partial insertion was observed for two OMs inserted at 18 Ncm. Complete insertion was achieved for all OMs inserted at 24 Ncm. Histomorphometrically, OMs inserted using 24 Ncm produced a significantly larger diffuse damage area (P < 0.05; P < 0.05) and maximum diffuse damage radius (P < 0.05; P < 0.05), for both the entry and exit surfaces, respectively, compared with the 12 Ncm and 18 Ncm groups. Conclusions Insertion torque can influence the degree of OM insertion and, subsequently, the amount of microdamage formed following insertion into 1.5 mm thick porcine tibia bone. An increase in insertion torque corresponds with greater insertion depth and larger amounts of microdamage. Introduction The use of orthodontic miniscrews (OMs) has gained immense popularity within the orthodontic discipline due to their perceived ability to enhance anchorage in difficult clinical situations. While OMs provide minimal biomechanical requirements on patient compliance or on the pre-existing dental arch (1), their stability is not consistent, with a reported success ranging from 59.2–100.0% (2). OM stability initially relies on the mechanical interlocking obtained between the screw thread and bone immediately following placement. The bone–OM contact formed represents primary stability and plays an important role in OM success, particularly during the initial stages prior to bone remodelling and healing (7, 8). The healing process is targeted (9) and acts to maintain the integrity and strength of the bony matrix (10) as a result of the extensive damage produced following insertion (3–5). Although this characteristic bone remodelling process is vital to the integrity of the adjacent bone and the long-term stability of the OM (11), the formation of local osteoclastic resorptive cavities adjacent to the OM can be detrimental to success (5). The delicate balance between obtaining high primary stability, while minimising microdamage, appears to be the key to long-term OM success. Insertion torque has been reported to exhibit a strong influence on primary stability due to its direct characteristic association with the bone–OM friction encountered during insertion (12, 13). Although high insertion torque values might be associated with high primary stability, excessive torque has been linked to increased degeneration and damage along the interfacial bone (14, 15). The increased strain associated with necrosis and osteocyte apoptosis (15), corresponds with the activation of osteoclasts to maintain the strength of the bone (16). Therefore, extensive microdamage caused by high insertion torque values might negatively impact OM success (7). Avoiding the use of high insertion torque could be beneficial to maintain the integrity of the surrounding bone (15). A number of studies have investigated the relationship between insertion torque values and OM success. Motoyoshi et al. (13) reported a significant difference in OM success rates for different insertion torque values. Based on these results, it was advocated that an insertion torque range of 5–10 Ncm increased the success rate of 1.6 mm diameter OMs and is now widely accepted. However, Lim et al. (17) reported that higher insertion torque values, in the order of 19.5 Ncm, are required to completely insert a 1.5 mm diameter OM. These studies suggest the importance of understanding the consequences on the formation of bone damage following OM insertion with differing torque levels. Insertion torque values are highly dependent on the local bone properties, as well as cortical bone thickness and screw geometry, particularly OM diameter (18). Minimal histological research has been conducted to validate the conflicting insertion torque recommendations related to damage created in the surrounding tissue. The ability to achieve good primary stability from an insertion torque value, without excessive deformation of the surrounding bony matrix, would be favourable for OM success. Therefore, the aim of the present study was to evaluate the progressive development of surface microdamage produced following insertion of an OM in 1.5 mm thick porcine bone, using maximum insertion torque values of 12 Ncm, 18 Ncm, and 24 Ncm. The null hypothesis was that insertion torque value has no influence on microdamage formed. Materials and methods Preparation of bone specimens Three porcine tibia bones were freshly obtained from a local abattoir and used with approval of the University of Adelaide Animal Ethics Committee, number M-2015–032. The bone specimens were sectioned into quarters and the proximal and distal ends of the bones were removed with a bandsaw. Fifteen plate-like bone specimens were wet machined using a low speed diamond saw. The nominal measurements were 1.5 mm thick, 15 mm wide, and 20 mm long. The nominal value of 1.5 mm thick cortical bone was chosen based on the average cortical bone thickness found in the maxilla and mandible (19). The bone specimens were then polished with increasing grades of silicon carbide micromesh paper, to remove any iatrogenic cracks created during the preparation of the bone specimens. The thickness of each specimen was measured using digital callipers. A single scratch was then placed in the lower left corner of each specimen to differentiate the entry surface from the exit surface of the cortical bone. The prepared specimens were wrapped in phosphate buffered saline solution (PBS) soaked gauze and stored at −20°C to maintain adequate hydration and the physical properties of the bone until testing (20). Testing was conducted within one month of preparation. The bone specimens created from the three porcine tibia bones were randomly allocated into three groups of five based on intended insertion torque values: Group A (12 Ncm), Group B (18 Ncm), and Group C (24 Ncm). A total of fifteen 1.5 × 6.0 mm self-drilling, untreated, cylindrical Aarhus Anchor Screws (MEDICON eG, Tuttlingen, Germany) were used. Sequential staining and OM insertion The prepared bone specimens were immersed into xylenol orange [5 × 10−4 M] solution for thirty minutes and rinsed thoroughly under distilled water for eight minutes to identify residual iatrogenic damage formed during the preparation of the specimen. The xylenol stained bone specimens were then placed in a customised apparatus, which was equipped with an Omega miniature industrial compression load cell with a range of 0–100 N (R4-F6-76535, N2Surplus Inc., Roanoke, VA, USA) and personal computer with LabVIEW software (National instruments Australia, Macquarie Park, NSW, AUS) to digitise the data obtained from the compression load cell (Figure 1). The OMs were inserted manually into the bone specimens, by one clinician (MN), using a torque limiting hand driver (G00234, Rocky Mountain Orthodontics, Denver, CO, USA) set to 12 Ncm, 18 Ncm, and 24 Ncm. Data obtained from LabVIEW was then transferred to Microsoft Excel 2003, to determine the number of peaks, maximum peak value, mean peak value, and duration of insertion above the mean peak value, as shown in Figure 2. Figure 1 View largeDownload slide Customised rig. OM is being inserted into the cortical bone, which is secured in the customised Perspex rig fitted with compression load cell below. Figure 1 View largeDownload slide Customised rig. OM is being inserted into the cortical bone, which is secured in the customised Perspex rig fitted with compression load cell below. Figure 2 View largeDownload slide Compression load cell data with variables. Figure 2 View largeDownload slide Compression load cell data with variables. Immediately following insertion, the specimens were immersed into calcein [1 × 10−3 M] solution for 30 minutes, followed by an 8-minute rinse under distilled water, to identify damage caused during miniscrew insertion. The specimens were then placed back into the rig and the OMs were removed for analysis in another study. Any gross bony debris protruding above the surface of the cortical bone was removed using a scalpel for imaging purposes. The specimens were then immersed in calcein blue solution [1 × 10−4 M] for 30 minutes to identify damage caused during removal of the OM and bony debris, and then thoroughly rinsed in distilled water for 30 minutes. Microdamage detection and analysis The entry and exit surfaces of the specimens were imaged at 63 times magnification, using a Leica SP5 spectral scanning confocal microscope, with a 561DPSS laser (569–700 nm red spectrum), 488 Argon laser (496–593 nm green spectrum), and 405 Diode laser (413–480 nm blue spectrum), for the excitation and detection of xylenol orange, calcein, and calcein blue, respectively. A series of 10 μm thick tissue sections were optically created to a maximum depth of 500 μm from the entry and exit surface. The images were stitched together using ImageJ (National Institutes of Health, Bethesda, Maryland, USA), to form a compressed two-dimensional composition of the bony damage as shown in Figure 3. Figure 3 View largeDownload slide Imaging process. (a) Schematic diagram of the cortical bone specimen in profile view (not to scale). (b) Optical virtual slices created by the laser confocal microscopy program. (c) Compression of all the virtual slices. (d) Compressed virtual slice stitched with other compressed slices to form the entire microdamage area. Figure 3 View largeDownload slide Imaging process. (a) Schematic diagram of the cortical bone specimen in profile view (not to scale). (b) Optical virtual slices created by the laser confocal microscopy program. (c) Compression of all the virtual slices. (d) Compressed virtual slice stitched with other compressed slices to form the entire microdamage area. The microdamage formed on the entry and exit surfaces was quantified into regions of diffuse damage and linear microcracks. Diffuse damage was defined as regions of intense halo staining of small-sized cracks, measuring 1 μm (21), in a crosshatched appearance (22). Microcracks, conversely, were identified as intermediate stained sharp linear defects in the order of 100 μm (23). The following histomorphometric measurements were made on the stitched images using the ImageJ program: total damage, diffuse damage, maximum crack length, maximum damage radius, and maximum diffuse damage radius (Figure 4). Figure 4 View largeDownload slide Histomorphometric parameters. (a) Total damaged area. (b) Diffuse damage area. (c) Maximum crack length. (d) Maximum damage radius. (e) Maximum diffuse damage radius. Figure 4 View largeDownload slide Histomorphometric parameters. (a) Total damaged area. (b) Diffuse damage area. (c) Maximum crack length. (d) Maximum damage radius. (e) Maximum diffuse damage radius. A quantification analysis was performed on five randomly selected images four weeks after the initial histomorphometric measurements to determine intra-rater reliability. In addition, a second clinician (WL) analysed five randomly selected images to determine the inter-rater reliability of the histomorphometric analysis. Statistical analysis Descriptive analyses were calculated for the specimens using SAS 9.3 (SAS Institute Inc., Cary, NC, USA). Intraclass correlation coefficient analysis was used to determine intra- and inter-rater reliability. A linear regression model was used to indicate discrepancies within compression load cell variable between the three groups. Kruskal–Wallis Tests and Wilcoxon Rank-Sum Tests were then performed to investigate the differences in histomorphometric effects of torque value on the microdamage. Results The cortical bone thickness in Group A (12 Ncm), B (18 Ncm), and C (24 Ncm) did not differ significantly from the nominal thickness of 1.5 mm, with mean values of 1.51 mm (SD 0.07), 1.57 mm (SD 0.05), and 1.5 mm (SD 0.04), respectively. The compression load data showed some variability between the three groups (Tables 1 and 2), however, overall the mean values were similar. Table 1 Compression load data.   Number of peaks  Max. peak value (N)  Mean peak value (N)  Duration above the mean peak value (s)  Mean  SD  Mean  SD  Mean  SD  Mean  SD  Group A  37.6  7.44  21.6  1.35  14.5  11.18  7.9  2.00  Group B  46.4  6.23  29.9  10.73  20.2  6.04  9.6  1.95  Group C  50.8  10.18  26.8  2.55  16.1  1.53  10.2  1.56    Number of peaks  Max. peak value (N)  Mean peak value (N)  Duration above the mean peak value (s)  Mean  SD  Mean  SD  Mean  SD  Mean  SD  Group A  37.6  7.44  21.6  1.35  14.5  11.18  7.9  2.00  Group B  46.4  6.23  29.9  10.73  20.2  6.04  9.6  1.95  Group C  50.8  10.18  26.8  2.55  16.1  1.53  10.2  1.56  View Large Table 2 Significance of compression load cell data.   Number of peaks  Max. peak value  Mean peak value  Duration above the mean peak value  P value  95% CI  P value  95% CI  P value  95% CI  P value  95% CI  Group A vs Group B  0.02*  [−16.26 to −1.34]  0.04*  [0.34 to 16.28]  0.01*  [1.80 to 9.500]  0.05*  [−3.41 to −0.02]  Group A vs Group C  0.00*  [−20.66 to −5.74]  0.85  [−7.28 to 8.66]  0.10  [−0.68 to 7.02]  0.01*  [−4.03 to −0.64]  Group B vs Group C  0.22  [−11.86 to 3.06]  0.06  [−15.59 to 0.35]  0.19  [−6.33 to 1.37]  0.44  [−2.31 to 1.08]    Number of peaks  Max. peak value  Mean peak value  Duration above the mean peak value  P value  95% CI  P value  95% CI  P value  95% CI  P value  95% CI  Group A vs Group B  0.02*  [−16.26 to −1.34]  0.04*  [0.34 to 16.28]  0.01*  [1.80 to 9.500]  0.05*  [−3.41 to −0.02]  Group A vs Group C  0.00*  [−20.66 to −5.74]  0.85  [−7.28 to 8.66]  0.10  [−0.68 to 7.02]  0.01*  [−4.03 to −0.64]  Group B vs Group C  0.22  [−11.86 to 3.06]  0.06  [−15.59 to 0.35]  0.19  [−6.33 to 1.37]  0.44  [−2.31 to 1.08]  *Statistically significant (P < 0.05). View Large Intra- and inter-rater reliability regarding all histomorphometric measurements was excellent, with an intraclass correlation coefficient value ranging from 0.785–0.986 and 0.786–0.982, respectively. Partial insertion was observed for all five OMs in Group A (12 Ncm) and two OMs from Group B (18 Ncm). Incomplete insertion was defined as the inability of the OM collar to achieve contact with the entry surface of the bone specimen. Although the tips of the partially inserted OMs penetrated the exit surface of the cortical bone, complete insertion was not achieved using the assigned torque value. The maximum torque limit was reached and further insertion could not be achieved. All OMs with incomplete insertion displayed exposed threads above the entry surface of the bone specimen. Figure 5 shows an incomplete and a completely inserted OM. Conversely, all five OMs in Group C (24 Ncm) inserted completely. Figure 5 View largeDownload slide OM insertion. (a) Complete insertion of an OM at 18 Ncm. (b) Incomplete insertion of an OM at 18 Ncm. Figure 5 View largeDownload slide OM insertion. (a) Complete insertion of an OM at 18 Ncm. (b) Incomplete insertion of an OM at 18 Ncm. For the entry surface, Group C (24 Ncm) showed a statistically higher amount of diffuse damage (P < 0.05) and maximum diffuse damage radius (P < 0.05) compared to Group A (12 Ncm) and Group B (18 Ncm) (Figure 6a and 6b). No other statistically significant measurements were observed between the three groups for the entry surface. Figure 6 View largeDownload slide (a) Histomorphometric area measurements for the entry surface. (b) Histomorphometric length measurements for the entry surface. DDA, diffuse damage area; MaxCL, maximum crack length; MaxDR, maximum damage radius; MaxDDR, maximum diffuse damage radius; TDA, Total damaged area. *Statistically significant (P < 0.05). Figure 6 View largeDownload slide (a) Histomorphometric area measurements for the entry surface. (b) Histomorphometric length measurements for the entry surface. DDA, diffuse damage area; MaxCL, maximum crack length; MaxDR, maximum damage radius; MaxDDR, maximum diffuse damage radius; TDA, Total damaged area. *Statistically significant (P < 0.05). Regarding the exit surface, a significantly larger amount of total damage area (P < 0.05) was observed for Group B (18 Ncm) compared to Group A (12 Ncm) and Group C (24 Ncm). Group C (24 Ncm), however, displayed a statistically higher diffuse damage area (P < 0.05) and maximum diffuse damage radius (P < 0.05) compared with the other two groups (12 Ncm and 18 Ncm) (Figure 7a and 7b). Figure 7 View largeDownload slide (a) Histomorphometric area measurements for the exit surface. (b) Histomorphometric length measurements for the entry surface. DDA, diffuse damage area; MaxCL, maximum crack length; MaxDR, maximum damage radius; MaxDDR, maximum diffuse damage radius; TDA, total damaged area. *Statistically significant (P < 0.05). Figure 7 View largeDownload slide (a) Histomorphometric area measurements for the exit surface. (b) Histomorphometric length measurements for the entry surface. DDA, diffuse damage area; MaxCL, maximum crack length; MaxDR, maximum damage radius; MaxDDR, maximum diffuse damage radius; TDA, total damaged area. *Statistically significant (P < 0.05). The histomorphometric variables measured between the entry and exit surfaces were then compared per group (Table 3). For Group A (12 Ncm), the entry surface only displayed a statistically larger amount of diffuse damage (P < 0.05) compared with the exit side. For Group B (18 Ncm), the exit surface showed a statistically significant maximum crack length (P < 0.05) compared with the entry side. No statistically significant histomorphometric measurements were observed for Group C (24 Ncm) when the insertion and exit surfaces were compared. Table 3 Comparison of the histomorphometric measurements for the entry and exit surfaces for Group A, B and C. Group A (12 Ncm)  Total damage area (mm2)  Diffuse damage area (mm2)  Maximum crack length (mm)  Maximum damage radius (mm)  Maximum diffuse damage radius (mm)    Mean  SD  Mean  SD  Mean  SD  Mean  SD  Mean  SD  Side   Entry  8.26  1.71  5.04  1.67  1.68  0.37  2.50  0.40  1.37  0.32   Exit  6.68  1.36  2.51  0.66  1.88  0.28  2.44  0.28  1.11  0.19   P value  0.14  0.02*  0.40  0.75  0.14  Group B (18 Ncm)  Total damage area (mm2)  Diffuse damage area (mm2)  Maximum crack length (mm)  Maximum damage radius (mm)  Maximum diffuse damage radius (mm)    Mean  SD  Mean  SD  Mean  SD  Mean  SD  Mean  SD  Side   Entry  9.51  1.29  7.13  0.38  1.32  0.51  2.51  0.35  1.89  0.11   Exit  10.54  1.54  7.01  1.63  2.28  0.40  2.84  0.25  1.97  0.35   P value  0.09  0.83  0.06  0.04*  0.83  Group C (24 Ncm)  Total damage area (mm2)  Diffuse damage area (mm2)  Maximum crack length (mm)  Maximum damage radius (mm)  Maximum diffuse damage radius (mm)    Mean  SD  Mean  SD  Mean  SD  Mean  SD  Mean  SD  Side   Entry  9.27  1.06  7.42  0.74  1.54  0.30  2.72  0.34  1.90  0.15   Exit  8.73  3.39  8.16  0.82  1.64  0.96  2.39  0.84  2.00  0.20   P value  0.83  0.53  0.68  1.00  1.00  Group A (12 Ncm)  Total damage area (mm2)  Diffuse damage area (mm2)  Maximum crack length (mm)  Maximum damage radius (mm)  Maximum diffuse damage radius (mm)    Mean  SD  Mean  SD  Mean  SD  Mean  SD  Mean  SD  Side   Entry  8.26  1.71  5.04  1.67  1.68  0.37  2.50  0.40  1.37  0.32   Exit  6.68  1.36  2.51  0.66  1.88  0.28  2.44  0.28  1.11  0.19   P value  0.14  0.02*  0.40  0.75  0.14  Group B (18 Ncm)  Total damage area (mm2)  Diffuse damage area (mm2)  Maximum crack length (mm)  Maximum damage radius (mm)  Maximum diffuse damage radius (mm)    Mean  SD  Mean  SD  Mean  SD  Mean  SD  Mean  SD  Side   Entry  9.51  1.29  7.13  0.38  1.32  0.51  2.51  0.35  1.89  0.11   Exit  10.54  1.54  7.01  1.63  2.28  0.40  2.84  0.25  1.97  0.35   P value  0.09  0.83  0.06  0.04*  0.83  Group C (24 Ncm)  Total damage area (mm2)  Diffuse damage area (mm2)  Maximum crack length (mm)  Maximum damage radius (mm)  Maximum diffuse damage radius (mm)    Mean  SD  Mean  SD  Mean  SD  Mean  SD  Mean  SD  Side   Entry  9.27  1.06  7.42  0.74  1.54  0.30  2.72  0.34  1.90  0.15   Exit  8.73  3.39  8.16  0.82  1.64  0.96  2.39  0.84  2.00  0.20   P value  0.83  0.53  0.68  1.00  1.00  *Statistically significant (P < 0.05). View Large Discussion The aim of the present study was to evaluate the progressive development of surface microdamage following the insertion of an OM into 1.5 mm thick porcine bone, using maximum insertion torque values of 12 Ncm, 18 Ncm, and 24 Ncm. The amount of microdamage observed for the entry surface showed a trend towards an increase in microdamage following an increase in torque value. Some statistical differences between the insertion torque values were observed and, therefore, the null hypothesis was rejected. Group C (24Ncm) displayed larger mean values for all histomorphometric measurements compared with Group A and B, with the exception of the maximum crack length variable (Table 3). The increase in microdamage can be attributed to the complete insertion of all OMs in Group C (24 Ncm) compared with the other groups, in which partial insertion was observed for all OMs in Group A (12 Ncm) and two OMs in Group B (18 Ncm). Although maximum torque was never reached, all OMs in Group C (24 Ncm) were inserted completely and, subsequently, the number of threads that passed through the cortical bone specimen was higher compared with Group A (12 Ncm) and Group B (18 Ncm). It appears as though a higher insertion torque value is required to overcome the increase in friction to achieve complete insertion in 1.5mm thick porcine bone. These results reflect observations made by Lim et al. (17) and Cho and Baek (24). Lim et al. (17) reported an exponential increase in torque as the insertion of 1.5 × 7.0 mm OMs into 1.5 mm thick artificial bone progressed, with a maximum insertion torque of 19.5 Ncm. Cho and Baek (24) reported a similar exponential torque pattern for both tapered and cylindrical OMs inserted into artificial bone, with a maximum insertion torque value of 16.31 Ncm and 17.82 Ncm for cylindrical and tapered OMs, respectively. The exponential torque pattern was thought to reflect the increase in frictional resistance encountered as bony debris accumulated in the OM threads during insertion. The ascending torque value recorded by Lim et al. and Cho and Baek may explain why, despite the OM tips piercing through the exit surface of the cortical bone, the OM insertion failure occurred at 12 Ncm and even at 18 Ncm. Further torque would be required to overcome the increase in friction from the accumulation of bony debris during the progressive insertion. An increase in torque can be associated with increase insertion depth and, based on the mean histomorphometric values reported in this study, an increase in the amount of microdamage formed. In regards to the maximum crack length, Group A (12 Ncm) recorded the largest mean measurement compared with Group B (18 Ncm) and C (24 Ncm) (Table 3). This unexpected result might be attributed to the propagation characteristics of microcracks. According to Vashishth et al. (25) and Reilly and Currey (26), microcracks tend to travel along the path associated with the weakest resistance and are, therefore, influenced by bony structures including osteocytes and vasculature. Although the bony specimens were randomly distributed amongst the three groups, the individual anatomical characteristic of the bone might have influenced the results. The mean histomorphometric values recorded for the exit surface also exhibited a trend towards an increase in microdamage following an increase in torque value (Figure 7). Statistically, however, the results are not as consistent compared to the entry surface. This can be attributed to the partial insertion observed for all OM in Group A (12 Ncm) and two OM in Group B (18 Ncm). The number of threads that passed through the cortical bone would be reduced compared with Group C (24 Ncm), thereby decreasing adjacent bone deformation and reducing the amount of microdamage formed. Based on these results, it was expected that the entry surface would exhibit higher histomorphometric values compared to the exit surface. The entry surface for Group A (12 Ncm) recorded a statistically larger diffuse damage area compared with the exit surface; however, Group B (18 Ncm) and Group C (24 Ncm) did not exhibited this trend. The exit surface for Group B (18 Ncm) displayed a significantly higher maximum damage radius compared with the entry surface, while no statistically significant results were reported for Group C (24 Ncm) when the entry surface damage was compared to the exit side (Table 3). The unexpected results might be due to the biomechanical strength of bone under compressive and tensile force. During insertion, the entry and exit surfaces of the cortical bone undergo compression and tensile forces, respectively. The stress of the different loading conditions could influence the damage observed (27). The amount of compressive force applied during insertion could have also influenced the histomorphometric results. O’Sullivan et al (28) reported an association between increased compression and the likelihood of localised cortical bone damage and, therefore, to minimise variation in compressive force during manual insertion, one clinician inserted all experimental OMs. Results from the linear regression analysis appears to indicate an overall consistent application of compressive force during insertion, with no statistically significant difference observed between the three groups in regards to the maximum peak value and the duration of insertion above the mean peak value (Tables 1 and 2). Discrepancies, however, were observed with the other variables examined. The significant differences can be attributed to the complete and partial insertion observed. Overall, the mean histomorphometric measurements observed for Group B (18 Ncm) are comparable with Group C (24 Ncm) despite inclusion of the incomplete insertion data for Group B (18 Ncm) (Figure 6 and 7). These results suggest that an association between the degree of OM insertion and torque value exists and that the presence of a torque threshold for 1.5 mm thick porcine bone could be between 18–24 Ncm. Following attainment of insertion, application of additional torque above the threshold might not increase the amount of microdamage given the histomorphometric variables observed for Group C (24 Ncm) was similar to damage shown by Group B (18 Ncm). The incomplete insertion at 12 Ncm and even at 18 Ncm does not support the widely cited 5-10Ncm insertion torque recommendations (13, 29, 30). All of these studies, however, utilized a predrill protocol. Predrilling is an effective adjunct procedure prescribed to reduce the resistance encountered during OM insertion (1). Although the amount of microdamage caused during insertion is significantly reduced following predrilling (3, 4, 31), the reduction of the insertion torque value can negatively impact primary stability (1, 32). According to Wilmes et al. (8), the 5–10 Ncm ideal insertion torque range advocated in the literature, appears to be achievable only when the cortical bone thickness is between 0.5 mm and 1.0 mm. This range is markedly thin in comparison to cortical bone thicknesses encountered in most placement sites within the maxilla or mandible (19, 33, 34). Furthermore, cortical bone thickness of less than 1.0 mm has been associated with high stress adjacent to OM threads (35) and failure clinically (29). Therefore, to achieve the advocated 5–10 Ncm-insertion torque range, the required cortical bone thickness may not provide sufficient primary stability for the long-term OM success as first thought. Thicker cortical bone and higher insertion torque values may be required to achieve OM stability. The outcome of the current study indicates that an insertion torque range of 18–24 Ncm is sufficient, under the conditions of the project, to obtain complete insertion of a 1.5 mm diameter OM. Although two OMs in Group B (18 Ncm) failed to insert completely, this could potentially be attributed to the manual perpendicular insertion of the OMs. Inaccuracies could have occurred in the maintenance of a perpendicular insertion path. Recommendations for angled insertions have been proposed to increase OM stability (36); however, some controversy still exists. Angled insertions require higher insertion torque values (36) due to the increase in apparent cortical bone thickness the OM must penetrate (34). A number of authors, however, assert perpendicular insertion to avoid increase bone stress adjacent to the OM (37, 38) and to increase anchorage stability (39). Clinically, however, perpendicular insertion of an OM is challenging to achieve given access difficulties associated with the variable anatomical contour of the bony surfaces. To provide the most clinically relevant information using an ex-vivo study model, manual insertion of OMs was adopted. Although care was taken to insert OMs perpendicular to the cortical bone, the insertion angle may have contributed to the insertion failure of two of the OMs inserted at 18 Ncm. Partial insertion of OM is not clinically acceptable due to the high risk of the leverage force, increase bone stress adjacent to the OM (37) and subsequent inability to provide sufficient anchorage (40). OMs should be inserted such that the head contacts the cortical bone (41) and, therefore, clinicians tend to achieve complete insertion irrespective of the torque value required. This inaccurate insertion protocol is reflected by reports that 78.3% of orthodontists do not measure insertion torque (42). Although imprecise in comparison with the meticulous dental implant insertion protocol, the insertion of OMs until complete insertion is achieved may be beneficial given the amount of microdamage created between Group B (18 Ncm) and Group C (24 Ncm) torque values were similar in this experimental model. Insertion torque is dependent on cortical bone thickness. The outcomes obtained exhibit biological plausibility; however, the results should be interpreted in reference to the methodology. To simplify the experimental model, cortical bone was utilized to investigate insertion torque values. Cortical bone was investigated due to the strong association with OM stability (7, 8). Cancellous bone has been reported to exhibit minimal stress during insertion (37, 38) and, therefore, was not included in this experimental model. The combination of cortical and cancellous bone, however, may produce different histomorphometric observations and, therefore, further research is required to examine the interaction and stress distribution histologically. In previous histomorphometric studies (3–6), iatrogenic microcracks may have been produced during the dehydration and preservation procedure prior to en bloc staining or serial sectioning of the bone specimen for imaging purposes. The risk of over- or under-estimation of quantitative histomorphometric parameters is highly possible with these previous techniques. Subsequently, a new staining protocol and imaging technique was used in the present study in an attempt to accurately measure the microdamage caused during insertion. The characteristic calcium affinities of xylenol orange, calcein, and calcein blue were used in a sequential manner to distinguish iatrogenic damage caused during bone preparation or OM removal from true damage caused during OM insertion. The amount of iatrogenic damage caused by bone specimen preparation and the removal of the OM was minimal and, subsequently, was included in the overall damage observed for insertion. The quantification method for microdamage was consistent for all groups and, therefore, the measurements can still be considered valid. Conclusion The findings of the present in vitro study indicated that insertion torque values could impact on the level of OM insertion and, subsequently, the amount of microdamage formed following insertion into 1.5 mm thick porcine tibia bone. An insertion torque range of 18–24 Ncm appears to be adequate for this experimental model. Application of additional torque above the insertion torque threshold does not appear to produce additional microdamage; however, further histomorphometric investigations are required to verify this finding. Conflict of interest None to declare. Acknowledgements The authors thank the Australian Society of Orthodontists, Foundation for Research and Education, for support of this research project. We would also like to thank Suzanne Edwards, statistician at the University of Adelaide, for assistance with the statistical analysis and interpretation of the data. The authors acknowledge the facilities, and the scientific and technical assistance, of the Australian Microscopy and Microanalysis Research Facility at the University of Adelaide. References 1. Baumgaertel S. Razavi M.R. and Hans M.G. ( 2008) Mini-implant anchorage for the orthodontic practitioner. American Journal of Orthodontics and Dentofacial Orthopedics , 133, 621– 627. Google Scholar CrossRef Search ADS PubMed  2. Papageorgiou S.N. Zogakis I.P. and Papadopoulos M.A. ( 2012) Failure rates and associated risk factors of orthodontic miniscrew implants: a meta-analysis. American Journal of Orthodontics and Dentofacial Orthopedics , 142, 577– 595.e7. 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( 2009) Impact of insertion depth and predrilling diameter on primary stability of orthodontic mini-implants. Angle Orthodontist , 79, 609– 614. Google Scholar CrossRef Search ADS PubMed  42. Buschang P.H. Carrillo R. Ozenbaugh B. and Rossouw P.E. ( 2008) 2008 survey of AAO members on miniscrew usage. Journal of Clinical Orthodontics , 42, 513– 518. Google Scholar PubMed  © The Author(s) 2017. Published by Oxford University Press on behalf of the European Orthodontic Society. All rights reserved. For permissions, please email: [email protected] http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png The European Journal of Orthodontics Oxford University Press

The influence of miniscrew insertion torque

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Oxford University Press
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© The Author(s) 2017. Published by Oxford University Press on behalf of the European Orthodontic Society. All rights reserved. For permissions, please email: [email protected]
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0141-5387
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10.1093/ejo/cjx026
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Abstract

Summary Objective The aim of this in vitro study was to evaluate the progressive development of surface microdamage produced following the insertion of orthodontic miniscrews (OMs) into 1.5 mm thick porcine tibia bone using maximum insertion torque values of 12 Ncm, 18 Ncm, and 24 Ncm. Methods Aarhus OMs (diameter 1.5 mm; length 6 mm) were inserted into 1.5 mm porcine bone using a torque limiting hand screwdriver set at 12 Ncm, 18 Ncm, and 24 Ncm. A custom rig equipped with a compression load cell was used to record the compression force exerted during manual insertion. A sequential staining technique was used to identify microdamage viewed under laser confocal microscopy. Virtual slices were created and stitched together to form a compressed two-dimensional composition of the microdamage. Histomorphometric parameters, including total damage area, diffuse damage area, maximum crack length, maximum damage radius, and maximum diffuse damage radius, were measured. Kruskal–Wallis Tests and Wilcoxon Rank-Sum Tests were used to analyse the generated data. Results All OMs inserted using 12 Ncm failed to insert completely, while partial insertion was observed for two OMs inserted at 18 Ncm. Complete insertion was achieved for all OMs inserted at 24 Ncm. Histomorphometrically, OMs inserted using 24 Ncm produced a significantly larger diffuse damage area (P < 0.05; P < 0.05) and maximum diffuse damage radius (P < 0.05; P < 0.05), for both the entry and exit surfaces, respectively, compared with the 12 Ncm and 18 Ncm groups. Conclusions Insertion torque can influence the degree of OM insertion and, subsequently, the amount of microdamage formed following insertion into 1.5 mm thick porcine tibia bone. An increase in insertion torque corresponds with greater insertion depth and larger amounts of microdamage. Introduction The use of orthodontic miniscrews (OMs) has gained immense popularity within the orthodontic discipline due to their perceived ability to enhance anchorage in difficult clinical situations. While OMs provide minimal biomechanical requirements on patient compliance or on the pre-existing dental arch (1), their stability is not consistent, with a reported success ranging from 59.2–100.0% (2). OM stability initially relies on the mechanical interlocking obtained between the screw thread and bone immediately following placement. The bone–OM contact formed represents primary stability and plays an important role in OM success, particularly during the initial stages prior to bone remodelling and healing (7, 8). The healing process is targeted (9) and acts to maintain the integrity and strength of the bony matrix (10) as a result of the extensive damage produced following insertion (3–5). Although this characteristic bone remodelling process is vital to the integrity of the adjacent bone and the long-term stability of the OM (11), the formation of local osteoclastic resorptive cavities adjacent to the OM can be detrimental to success (5). The delicate balance between obtaining high primary stability, while minimising microdamage, appears to be the key to long-term OM success. Insertion torque has been reported to exhibit a strong influence on primary stability due to its direct characteristic association with the bone–OM friction encountered during insertion (12, 13). Although high insertion torque values might be associated with high primary stability, excessive torque has been linked to increased degeneration and damage along the interfacial bone (14, 15). The increased strain associated with necrosis and osteocyte apoptosis (15), corresponds with the activation of osteoclasts to maintain the strength of the bone (16). Therefore, extensive microdamage caused by high insertion torque values might negatively impact OM success (7). Avoiding the use of high insertion torque could be beneficial to maintain the integrity of the surrounding bone (15). A number of studies have investigated the relationship between insertion torque values and OM success. Motoyoshi et al. (13) reported a significant difference in OM success rates for different insertion torque values. Based on these results, it was advocated that an insertion torque range of 5–10 Ncm increased the success rate of 1.6 mm diameter OMs and is now widely accepted. However, Lim et al. (17) reported that higher insertion torque values, in the order of 19.5 Ncm, are required to completely insert a 1.5 mm diameter OM. These studies suggest the importance of understanding the consequences on the formation of bone damage following OM insertion with differing torque levels. Insertion torque values are highly dependent on the local bone properties, as well as cortical bone thickness and screw geometry, particularly OM diameter (18). Minimal histological research has been conducted to validate the conflicting insertion torque recommendations related to damage created in the surrounding tissue. The ability to achieve good primary stability from an insertion torque value, without excessive deformation of the surrounding bony matrix, would be favourable for OM success. Therefore, the aim of the present study was to evaluate the progressive development of surface microdamage produced following insertion of an OM in 1.5 mm thick porcine bone, using maximum insertion torque values of 12 Ncm, 18 Ncm, and 24 Ncm. The null hypothesis was that insertion torque value has no influence on microdamage formed. Materials and methods Preparation of bone specimens Three porcine tibia bones were freshly obtained from a local abattoir and used with approval of the University of Adelaide Animal Ethics Committee, number M-2015–032. The bone specimens were sectioned into quarters and the proximal and distal ends of the bones were removed with a bandsaw. Fifteen plate-like bone specimens were wet machined using a low speed diamond saw. The nominal measurements were 1.5 mm thick, 15 mm wide, and 20 mm long. The nominal value of 1.5 mm thick cortical bone was chosen based on the average cortical bone thickness found in the maxilla and mandible (19). The bone specimens were then polished with increasing grades of silicon carbide micromesh paper, to remove any iatrogenic cracks created during the preparation of the bone specimens. The thickness of each specimen was measured using digital callipers. A single scratch was then placed in the lower left corner of each specimen to differentiate the entry surface from the exit surface of the cortical bone. The prepared specimens were wrapped in phosphate buffered saline solution (PBS) soaked gauze and stored at −20°C to maintain adequate hydration and the physical properties of the bone until testing (20). Testing was conducted within one month of preparation. The bone specimens created from the three porcine tibia bones were randomly allocated into three groups of five based on intended insertion torque values: Group A (12 Ncm), Group B (18 Ncm), and Group C (24 Ncm). A total of fifteen 1.5 × 6.0 mm self-drilling, untreated, cylindrical Aarhus Anchor Screws (MEDICON eG, Tuttlingen, Germany) were used. Sequential staining and OM insertion The prepared bone specimens were immersed into xylenol orange [5 × 10−4 M] solution for thirty minutes and rinsed thoroughly under distilled water for eight minutes to identify residual iatrogenic damage formed during the preparation of the specimen. The xylenol stained bone specimens were then placed in a customised apparatus, which was equipped with an Omega miniature industrial compression load cell with a range of 0–100 N (R4-F6-76535, N2Surplus Inc., Roanoke, VA, USA) and personal computer with LabVIEW software (National instruments Australia, Macquarie Park, NSW, AUS) to digitise the data obtained from the compression load cell (Figure 1). The OMs were inserted manually into the bone specimens, by one clinician (MN), using a torque limiting hand driver (G00234, Rocky Mountain Orthodontics, Denver, CO, USA) set to 12 Ncm, 18 Ncm, and 24 Ncm. Data obtained from LabVIEW was then transferred to Microsoft Excel 2003, to determine the number of peaks, maximum peak value, mean peak value, and duration of insertion above the mean peak value, as shown in Figure 2. Figure 1 View largeDownload slide Customised rig. OM is being inserted into the cortical bone, which is secured in the customised Perspex rig fitted with compression load cell below. Figure 1 View largeDownload slide Customised rig. OM is being inserted into the cortical bone, which is secured in the customised Perspex rig fitted with compression load cell below. Figure 2 View largeDownload slide Compression load cell data with variables. Figure 2 View largeDownload slide Compression load cell data with variables. Immediately following insertion, the specimens were immersed into calcein [1 × 10−3 M] solution for 30 minutes, followed by an 8-minute rinse under distilled water, to identify damage caused during miniscrew insertion. The specimens were then placed back into the rig and the OMs were removed for analysis in another study. Any gross bony debris protruding above the surface of the cortical bone was removed using a scalpel for imaging purposes. The specimens were then immersed in calcein blue solution [1 × 10−4 M] for 30 minutes to identify damage caused during removal of the OM and bony debris, and then thoroughly rinsed in distilled water for 30 minutes. Microdamage detection and analysis The entry and exit surfaces of the specimens were imaged at 63 times magnification, using a Leica SP5 spectral scanning confocal microscope, with a 561DPSS laser (569–700 nm red spectrum), 488 Argon laser (496–593 nm green spectrum), and 405 Diode laser (413–480 nm blue spectrum), for the excitation and detection of xylenol orange, calcein, and calcein blue, respectively. A series of 10 μm thick tissue sections were optically created to a maximum depth of 500 μm from the entry and exit surface. The images were stitched together using ImageJ (National Institutes of Health, Bethesda, Maryland, USA), to form a compressed two-dimensional composition of the bony damage as shown in Figure 3. Figure 3 View largeDownload slide Imaging process. (a) Schematic diagram of the cortical bone specimen in profile view (not to scale). (b) Optical virtual slices created by the laser confocal microscopy program. (c) Compression of all the virtual slices. (d) Compressed virtual slice stitched with other compressed slices to form the entire microdamage area. Figure 3 View largeDownload slide Imaging process. (a) Schematic diagram of the cortical bone specimen in profile view (not to scale). (b) Optical virtual slices created by the laser confocal microscopy program. (c) Compression of all the virtual slices. (d) Compressed virtual slice stitched with other compressed slices to form the entire microdamage area. The microdamage formed on the entry and exit surfaces was quantified into regions of diffuse damage and linear microcracks. Diffuse damage was defined as regions of intense halo staining of small-sized cracks, measuring 1 μm (21), in a crosshatched appearance (22). Microcracks, conversely, were identified as intermediate stained sharp linear defects in the order of 100 μm (23). The following histomorphometric measurements were made on the stitched images using the ImageJ program: total damage, diffuse damage, maximum crack length, maximum damage radius, and maximum diffuse damage radius (Figure 4). Figure 4 View largeDownload slide Histomorphometric parameters. (a) Total damaged area. (b) Diffuse damage area. (c) Maximum crack length. (d) Maximum damage radius. (e) Maximum diffuse damage radius. Figure 4 View largeDownload slide Histomorphometric parameters. (a) Total damaged area. (b) Diffuse damage area. (c) Maximum crack length. (d) Maximum damage radius. (e) Maximum diffuse damage radius. A quantification analysis was performed on five randomly selected images four weeks after the initial histomorphometric measurements to determine intra-rater reliability. In addition, a second clinician (WL) analysed five randomly selected images to determine the inter-rater reliability of the histomorphometric analysis. Statistical analysis Descriptive analyses were calculated for the specimens using SAS 9.3 (SAS Institute Inc., Cary, NC, USA). Intraclass correlation coefficient analysis was used to determine intra- and inter-rater reliability. A linear regression model was used to indicate discrepancies within compression load cell variable between the three groups. Kruskal–Wallis Tests and Wilcoxon Rank-Sum Tests were then performed to investigate the differences in histomorphometric effects of torque value on the microdamage. Results The cortical bone thickness in Group A (12 Ncm), B (18 Ncm), and C (24 Ncm) did not differ significantly from the nominal thickness of 1.5 mm, with mean values of 1.51 mm (SD 0.07), 1.57 mm (SD 0.05), and 1.5 mm (SD 0.04), respectively. The compression load data showed some variability between the three groups (Tables 1 and 2), however, overall the mean values were similar. Table 1 Compression load data.   Number of peaks  Max. peak value (N)  Mean peak value (N)  Duration above the mean peak value (s)  Mean  SD  Mean  SD  Mean  SD  Mean  SD  Group A  37.6  7.44  21.6  1.35  14.5  11.18  7.9  2.00  Group B  46.4  6.23  29.9  10.73  20.2  6.04  9.6  1.95  Group C  50.8  10.18  26.8  2.55  16.1  1.53  10.2  1.56    Number of peaks  Max. peak value (N)  Mean peak value (N)  Duration above the mean peak value (s)  Mean  SD  Mean  SD  Mean  SD  Mean  SD  Group A  37.6  7.44  21.6  1.35  14.5  11.18  7.9  2.00  Group B  46.4  6.23  29.9  10.73  20.2  6.04  9.6  1.95  Group C  50.8  10.18  26.8  2.55  16.1  1.53  10.2  1.56  View Large Table 2 Significance of compression load cell data.   Number of peaks  Max. peak value  Mean peak value  Duration above the mean peak value  P value  95% CI  P value  95% CI  P value  95% CI  P value  95% CI  Group A vs Group B  0.02*  [−16.26 to −1.34]  0.04*  [0.34 to 16.28]  0.01*  [1.80 to 9.500]  0.05*  [−3.41 to −0.02]  Group A vs Group C  0.00*  [−20.66 to −5.74]  0.85  [−7.28 to 8.66]  0.10  [−0.68 to 7.02]  0.01*  [−4.03 to −0.64]  Group B vs Group C  0.22  [−11.86 to 3.06]  0.06  [−15.59 to 0.35]  0.19  [−6.33 to 1.37]  0.44  [−2.31 to 1.08]    Number of peaks  Max. peak value  Mean peak value  Duration above the mean peak value  P value  95% CI  P value  95% CI  P value  95% CI  P value  95% CI  Group A vs Group B  0.02*  [−16.26 to −1.34]  0.04*  [0.34 to 16.28]  0.01*  [1.80 to 9.500]  0.05*  [−3.41 to −0.02]  Group A vs Group C  0.00*  [−20.66 to −5.74]  0.85  [−7.28 to 8.66]  0.10  [−0.68 to 7.02]  0.01*  [−4.03 to −0.64]  Group B vs Group C  0.22  [−11.86 to 3.06]  0.06  [−15.59 to 0.35]  0.19  [−6.33 to 1.37]  0.44  [−2.31 to 1.08]  *Statistically significant (P < 0.05). View Large Intra- and inter-rater reliability regarding all histomorphometric measurements was excellent, with an intraclass correlation coefficient value ranging from 0.785–0.986 and 0.786–0.982, respectively. Partial insertion was observed for all five OMs in Group A (12 Ncm) and two OMs from Group B (18 Ncm). Incomplete insertion was defined as the inability of the OM collar to achieve contact with the entry surface of the bone specimen. Although the tips of the partially inserted OMs penetrated the exit surface of the cortical bone, complete insertion was not achieved using the assigned torque value. The maximum torque limit was reached and further insertion could not be achieved. All OMs with incomplete insertion displayed exposed threads above the entry surface of the bone specimen. Figure 5 shows an incomplete and a completely inserted OM. Conversely, all five OMs in Group C (24 Ncm) inserted completely. Figure 5 View largeDownload slide OM insertion. (a) Complete insertion of an OM at 18 Ncm. (b) Incomplete insertion of an OM at 18 Ncm. Figure 5 View largeDownload slide OM insertion. (a) Complete insertion of an OM at 18 Ncm. (b) Incomplete insertion of an OM at 18 Ncm. For the entry surface, Group C (24 Ncm) showed a statistically higher amount of diffuse damage (P < 0.05) and maximum diffuse damage radius (P < 0.05) compared to Group A (12 Ncm) and Group B (18 Ncm) (Figure 6a and 6b). No other statistically significant measurements were observed between the three groups for the entry surface. Figure 6 View largeDownload slide (a) Histomorphometric area measurements for the entry surface. (b) Histomorphometric length measurements for the entry surface. DDA, diffuse damage area; MaxCL, maximum crack length; MaxDR, maximum damage radius; MaxDDR, maximum diffuse damage radius; TDA, Total damaged area. *Statistically significant (P < 0.05). Figure 6 View largeDownload slide (a) Histomorphometric area measurements for the entry surface. (b) Histomorphometric length measurements for the entry surface. DDA, diffuse damage area; MaxCL, maximum crack length; MaxDR, maximum damage radius; MaxDDR, maximum diffuse damage radius; TDA, Total damaged area. *Statistically significant (P < 0.05). Regarding the exit surface, a significantly larger amount of total damage area (P < 0.05) was observed for Group B (18 Ncm) compared to Group A (12 Ncm) and Group C (24 Ncm). Group C (24 Ncm), however, displayed a statistically higher diffuse damage area (P < 0.05) and maximum diffuse damage radius (P < 0.05) compared with the other two groups (12 Ncm and 18 Ncm) (Figure 7a and 7b). Figure 7 View largeDownload slide (a) Histomorphometric area measurements for the exit surface. (b) Histomorphometric length measurements for the entry surface. DDA, diffuse damage area; MaxCL, maximum crack length; MaxDR, maximum damage radius; MaxDDR, maximum diffuse damage radius; TDA, total damaged area. *Statistically significant (P < 0.05). Figure 7 View largeDownload slide (a) Histomorphometric area measurements for the exit surface. (b) Histomorphometric length measurements for the entry surface. DDA, diffuse damage area; MaxCL, maximum crack length; MaxDR, maximum damage radius; MaxDDR, maximum diffuse damage radius; TDA, total damaged area. *Statistically significant (P < 0.05). The histomorphometric variables measured between the entry and exit surfaces were then compared per group (Table 3). For Group A (12 Ncm), the entry surface only displayed a statistically larger amount of diffuse damage (P < 0.05) compared with the exit side. For Group B (18 Ncm), the exit surface showed a statistically significant maximum crack length (P < 0.05) compared with the entry side. No statistically significant histomorphometric measurements were observed for Group C (24 Ncm) when the insertion and exit surfaces were compared. Table 3 Comparison of the histomorphometric measurements for the entry and exit surfaces for Group A, B and C. Group A (12 Ncm)  Total damage area (mm2)  Diffuse damage area (mm2)  Maximum crack length (mm)  Maximum damage radius (mm)  Maximum diffuse damage radius (mm)    Mean  SD  Mean  SD  Mean  SD  Mean  SD  Mean  SD  Side   Entry  8.26  1.71  5.04  1.67  1.68  0.37  2.50  0.40  1.37  0.32   Exit  6.68  1.36  2.51  0.66  1.88  0.28  2.44  0.28  1.11  0.19   P value  0.14  0.02*  0.40  0.75  0.14  Group B (18 Ncm)  Total damage area (mm2)  Diffuse damage area (mm2)  Maximum crack length (mm)  Maximum damage radius (mm)  Maximum diffuse damage radius (mm)    Mean  SD  Mean  SD  Mean  SD  Mean  SD  Mean  SD  Side   Entry  9.51  1.29  7.13  0.38  1.32  0.51  2.51  0.35  1.89  0.11   Exit  10.54  1.54  7.01  1.63  2.28  0.40  2.84  0.25  1.97  0.35   P value  0.09  0.83  0.06  0.04*  0.83  Group C (24 Ncm)  Total damage area (mm2)  Diffuse damage area (mm2)  Maximum crack length (mm)  Maximum damage radius (mm)  Maximum diffuse damage radius (mm)    Mean  SD  Mean  SD  Mean  SD  Mean  SD  Mean  SD  Side   Entry  9.27  1.06  7.42  0.74  1.54  0.30  2.72  0.34  1.90  0.15   Exit  8.73  3.39  8.16  0.82  1.64  0.96  2.39  0.84  2.00  0.20   P value  0.83  0.53  0.68  1.00  1.00  Group A (12 Ncm)  Total damage area (mm2)  Diffuse damage area (mm2)  Maximum crack length (mm)  Maximum damage radius (mm)  Maximum diffuse damage radius (mm)    Mean  SD  Mean  SD  Mean  SD  Mean  SD  Mean  SD  Side   Entry  8.26  1.71  5.04  1.67  1.68  0.37  2.50  0.40  1.37  0.32   Exit  6.68  1.36  2.51  0.66  1.88  0.28  2.44  0.28  1.11  0.19   P value  0.14  0.02*  0.40  0.75  0.14  Group B (18 Ncm)  Total damage area (mm2)  Diffuse damage area (mm2)  Maximum crack length (mm)  Maximum damage radius (mm)  Maximum diffuse damage radius (mm)    Mean  SD  Mean  SD  Mean  SD  Mean  SD  Mean  SD  Side   Entry  9.51  1.29  7.13  0.38  1.32  0.51  2.51  0.35  1.89  0.11   Exit  10.54  1.54  7.01  1.63  2.28  0.40  2.84  0.25  1.97  0.35   P value  0.09  0.83  0.06  0.04*  0.83  Group C (24 Ncm)  Total damage area (mm2)  Diffuse damage area (mm2)  Maximum crack length (mm)  Maximum damage radius (mm)  Maximum diffuse damage radius (mm)    Mean  SD  Mean  SD  Mean  SD  Mean  SD  Mean  SD  Side   Entry  9.27  1.06  7.42  0.74  1.54  0.30  2.72  0.34  1.90  0.15   Exit  8.73  3.39  8.16  0.82  1.64  0.96  2.39  0.84  2.00  0.20   P value  0.83  0.53  0.68  1.00  1.00  *Statistically significant (P < 0.05). View Large Discussion The aim of the present study was to evaluate the progressive development of surface microdamage following the insertion of an OM into 1.5 mm thick porcine bone, using maximum insertion torque values of 12 Ncm, 18 Ncm, and 24 Ncm. The amount of microdamage observed for the entry surface showed a trend towards an increase in microdamage following an increase in torque value. Some statistical differences between the insertion torque values were observed and, therefore, the null hypothesis was rejected. Group C (24Ncm) displayed larger mean values for all histomorphometric measurements compared with Group A and B, with the exception of the maximum crack length variable (Table 3). The increase in microdamage can be attributed to the complete insertion of all OMs in Group C (24 Ncm) compared with the other groups, in which partial insertion was observed for all OMs in Group A (12 Ncm) and two OMs in Group B (18 Ncm). Although maximum torque was never reached, all OMs in Group C (24 Ncm) were inserted completely and, subsequently, the number of threads that passed through the cortical bone specimen was higher compared with Group A (12 Ncm) and Group B (18 Ncm). It appears as though a higher insertion torque value is required to overcome the increase in friction to achieve complete insertion in 1.5mm thick porcine bone. These results reflect observations made by Lim et al. (17) and Cho and Baek (24). Lim et al. (17) reported an exponential increase in torque as the insertion of 1.5 × 7.0 mm OMs into 1.5 mm thick artificial bone progressed, with a maximum insertion torque of 19.5 Ncm. Cho and Baek (24) reported a similar exponential torque pattern for both tapered and cylindrical OMs inserted into artificial bone, with a maximum insertion torque value of 16.31 Ncm and 17.82 Ncm for cylindrical and tapered OMs, respectively. The exponential torque pattern was thought to reflect the increase in frictional resistance encountered as bony debris accumulated in the OM threads during insertion. The ascending torque value recorded by Lim et al. and Cho and Baek may explain why, despite the OM tips piercing through the exit surface of the cortical bone, the OM insertion failure occurred at 12 Ncm and even at 18 Ncm. Further torque would be required to overcome the increase in friction from the accumulation of bony debris during the progressive insertion. An increase in torque can be associated with increase insertion depth and, based on the mean histomorphometric values reported in this study, an increase in the amount of microdamage formed. In regards to the maximum crack length, Group A (12 Ncm) recorded the largest mean measurement compared with Group B (18 Ncm) and C (24 Ncm) (Table 3). This unexpected result might be attributed to the propagation characteristics of microcracks. According to Vashishth et al. (25) and Reilly and Currey (26), microcracks tend to travel along the path associated with the weakest resistance and are, therefore, influenced by bony structures including osteocytes and vasculature. Although the bony specimens were randomly distributed amongst the three groups, the individual anatomical characteristic of the bone might have influenced the results. The mean histomorphometric values recorded for the exit surface also exhibited a trend towards an increase in microdamage following an increase in torque value (Figure 7). Statistically, however, the results are not as consistent compared to the entry surface. This can be attributed to the partial insertion observed for all OM in Group A (12 Ncm) and two OM in Group B (18 Ncm). The number of threads that passed through the cortical bone would be reduced compared with Group C (24 Ncm), thereby decreasing adjacent bone deformation and reducing the amount of microdamage formed. Based on these results, it was expected that the entry surface would exhibit higher histomorphometric values compared to the exit surface. The entry surface for Group A (12 Ncm) recorded a statistically larger diffuse damage area compared with the exit surface; however, Group B (18 Ncm) and Group C (24 Ncm) did not exhibited this trend. The exit surface for Group B (18 Ncm) displayed a significantly higher maximum damage radius compared with the entry surface, while no statistically significant results were reported for Group C (24 Ncm) when the entry surface damage was compared to the exit side (Table 3). The unexpected results might be due to the biomechanical strength of bone under compressive and tensile force. During insertion, the entry and exit surfaces of the cortical bone undergo compression and tensile forces, respectively. The stress of the different loading conditions could influence the damage observed (27). The amount of compressive force applied during insertion could have also influenced the histomorphometric results. O’Sullivan et al (28) reported an association between increased compression and the likelihood of localised cortical bone damage and, therefore, to minimise variation in compressive force during manual insertion, one clinician inserted all experimental OMs. Results from the linear regression analysis appears to indicate an overall consistent application of compressive force during insertion, with no statistically significant difference observed between the three groups in regards to the maximum peak value and the duration of insertion above the mean peak value (Tables 1 and 2). Discrepancies, however, were observed with the other variables examined. The significant differences can be attributed to the complete and partial insertion observed. Overall, the mean histomorphometric measurements observed for Group B (18 Ncm) are comparable with Group C (24 Ncm) despite inclusion of the incomplete insertion data for Group B (18 Ncm) (Figure 6 and 7). These results suggest that an association between the degree of OM insertion and torque value exists and that the presence of a torque threshold for 1.5 mm thick porcine bone could be between 18–24 Ncm. Following attainment of insertion, application of additional torque above the threshold might not increase the amount of microdamage given the histomorphometric variables observed for Group C (24 Ncm) was similar to damage shown by Group B (18 Ncm). The incomplete insertion at 12 Ncm and even at 18 Ncm does not support the widely cited 5-10Ncm insertion torque recommendations (13, 29, 30). All of these studies, however, utilized a predrill protocol. Predrilling is an effective adjunct procedure prescribed to reduce the resistance encountered during OM insertion (1). Although the amount of microdamage caused during insertion is significantly reduced following predrilling (3, 4, 31), the reduction of the insertion torque value can negatively impact primary stability (1, 32). According to Wilmes et al. (8), the 5–10 Ncm ideal insertion torque range advocated in the literature, appears to be achievable only when the cortical bone thickness is between 0.5 mm and 1.0 mm. This range is markedly thin in comparison to cortical bone thicknesses encountered in most placement sites within the maxilla or mandible (19, 33, 34). Furthermore, cortical bone thickness of less than 1.0 mm has been associated with high stress adjacent to OM threads (35) and failure clinically (29). Therefore, to achieve the advocated 5–10 Ncm-insertion torque range, the required cortical bone thickness may not provide sufficient primary stability for the long-term OM success as first thought. Thicker cortical bone and higher insertion torque values may be required to achieve OM stability. The outcome of the current study indicates that an insertion torque range of 18–24 Ncm is sufficient, under the conditions of the project, to obtain complete insertion of a 1.5 mm diameter OM. Although two OMs in Group B (18 Ncm) failed to insert completely, this could potentially be attributed to the manual perpendicular insertion of the OMs. Inaccuracies could have occurred in the maintenance of a perpendicular insertion path. Recommendations for angled insertions have been proposed to increase OM stability (36); however, some controversy still exists. Angled insertions require higher insertion torque values (36) due to the increase in apparent cortical bone thickness the OM must penetrate (34). A number of authors, however, assert perpendicular insertion to avoid increase bone stress adjacent to the OM (37, 38) and to increase anchorage stability (39). Clinically, however, perpendicular insertion of an OM is challenging to achieve given access difficulties associated with the variable anatomical contour of the bony surfaces. To provide the most clinically relevant information using an ex-vivo study model, manual insertion of OMs was adopted. Although care was taken to insert OMs perpendicular to the cortical bone, the insertion angle may have contributed to the insertion failure of two of the OMs inserted at 18 Ncm. Partial insertion of OM is not clinically acceptable due to the high risk of the leverage force, increase bone stress adjacent to the OM (37) and subsequent inability to provide sufficient anchorage (40). OMs should be inserted such that the head contacts the cortical bone (41) and, therefore, clinicians tend to achieve complete insertion irrespective of the torque value required. This inaccurate insertion protocol is reflected by reports that 78.3% of orthodontists do not measure insertion torque (42). Although imprecise in comparison with the meticulous dental implant insertion protocol, the insertion of OMs until complete insertion is achieved may be beneficial given the amount of microdamage created between Group B (18 Ncm) and Group C (24 Ncm) torque values were similar in this experimental model. Insertion torque is dependent on cortical bone thickness. The outcomes obtained exhibit biological plausibility; however, the results should be interpreted in reference to the methodology. To simplify the experimental model, cortical bone was utilized to investigate insertion torque values. Cortical bone was investigated due to the strong association with OM stability (7, 8). Cancellous bone has been reported to exhibit minimal stress during insertion (37, 38) and, therefore, was not included in this experimental model. The combination of cortical and cancellous bone, however, may produce different histomorphometric observations and, therefore, further research is required to examine the interaction and stress distribution histologically. In previous histomorphometric studies (3–6), iatrogenic microcracks may have been produced during the dehydration and preservation procedure prior to en bloc staining or serial sectioning of the bone specimen for imaging purposes. The risk of over- or under-estimation of quantitative histomorphometric parameters is highly possible with these previous techniques. Subsequently, a new staining protocol and imaging technique was used in the present study in an attempt to accurately measure the microdamage caused during insertion. The characteristic calcium affinities of xylenol orange, calcein, and calcein blue were used in a sequential manner to distinguish iatrogenic damage caused during bone preparation or OM removal from true damage caused during OM insertion. The amount of iatrogenic damage caused by bone specimen preparation and the removal of the OM was minimal and, subsequently, was included in the overall damage observed for insertion. The quantification method for microdamage was consistent for all groups and, therefore, the measurements can still be considered valid. Conclusion The findings of the present in vitro study indicated that insertion torque values could impact on the level of OM insertion and, subsequently, the amount of microdamage formed following insertion into 1.5 mm thick porcine tibia bone. An insertion torque range of 18–24 Ncm appears to be adequate for this experimental model. Application of additional torque above the insertion torque threshold does not appear to produce additional microdamage; however, further histomorphometric investigations are required to verify this finding. Conflict of interest None to declare. 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Journal

The European Journal of Orthodontics – Oxford University Press

Published: Feb 1, 2018

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