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Effect of Coracoid Drilling for Acromioclavicular Joint Reconstruction Techniques on Coracoid Fracture Risk: A Biomechanical Study
Effect of Coracoid Drilling for Acromioclavicular Joint Reconstruction Techniques on Coracoid Fracture Risk: A Biomechanical Study Frank Martetschläger, M.D., Ph.D., Tim Saier, M.D., Annabelle Weigert, Elmar Herbst, M.D., Martin Winkler, Dipl.-Ing., Julia Henschel, Dipl.-Ing., Peter Augat, M.D., Ph.D., Andreas B. Imhoff, M.D., Ph.D., and Sepp Braun, M.D., Ph.D.
Purpose: To biomechanically compare the stability of the coracoid process after an anatomic double-tunnel technique using two 4-mm drill holes or a single-tunnel technique using one 4-mm or one 2.4-mm drill hole. Methods: For biomechanical testing, 18 fresh-frozen cadaveric scapulae were used and randomly assigned to one of the following groups: two 4-mm drill holes (group 1), one 4-mm drill hole (group 2), or one 2.4-mm drill hole (group 3). After standardized coracoid drilling, load was applied to the conjoined tendons at a rate of 120 mm/min and ultimate failure load, along with the failure mode, was recorded. Results: There was no significant difference between groups regarding load to failure. Mean load to failure in group 1 was 392 N; group 2, 459 N; and group 3, 506 N. The corresponding P values were .55, .74, and .20 for group 1 versus group 2, group 2 versus group 3, and group 1 versus group 3, respectively. However, the failure mode for the group with one 4-mm drill hole and the group with two 4-mm drill holes was coracoid fracture, whereas the group with one 2.4-mm drill hole showed 5 tears of the conjoined tendons and only 1 coracoid fracture (P ¼ .015). Conclusions: Although there was no significant difference regarding load-to-failure testing between groups, the failure mechanism analysis showed that one 2.4-mm drill hole led to less destabilization of the coracoid than one or two 4-mm drill holes. Clinical Relevance: Techniques with small, 2.4-mm drill holes might decrease the risk of severe iatrogenic fracture complications.
lthough surgical treatment of high-grade acromioclavicular (AC) joint disruptions of Rockwood type IV through VI is still lacking a gold-standard procedure, arthroscopically assisted reconstruction techniques using cortical fixation buttons (CFBs) have gained increased popularity.1-6 For these techniques, different coracoclavicular (CC) drilling diameters and configurations are described. Most commonly, 4-mm tunnel sizes have been used for inserting the CFBs.2,4,5,7 For anatomic reconstruction of the 2 CC ligaments, a technique with two separate 4-mm drill
A
holes has been proposed.5,8 However, the modern techniques have introduced new complications such as fractures of the lateral clavicle or the coracoid process.2,3 The biomechanical consequences of CC drilling on clavicle strength have been reported recently.9 Little is known about the influence of different coracoid drilling configurations on coracoid fracture risk. Therefore the purpose of this study was to biomechanically compare the stability of the coracoid process after an anatomic double-tunnel technique using two 4-mm drill holes or a single-tunnel technique using one
From the Department of Orthopaedic Sports Medicine, Klinikum rechts der Isar, TU Munich (F.M., A.W., E.H., A.B.I., S.B.), Munich, Germany; Center for Shoulder and Elbow Surgery, ATOS Clinic Munich (F.M.), Munich, Germany; Trauma Center Murnau, Berufsgenossenschaftliche Klinik Murnau (T.S.), Murnau, Germany; Institute of Biomechanics, Berufsgenossenschaftliche Unfallklinik Murnau (T.S., M.W., J.H., P.A.), Murnau, Germany; and Institute of Biomechanics, Paracelsus Medical University (P.A.), Salzburg, Austria. The authors report the following potential conflict of interest or source of funding: F.M. receives support from Arthrex. T.S. receives support from Arthrex. A.W. receives support from Arthrex. E.H. receives support from Arthrex. M.W. receives support from Arthrex, Stryker, and Citieffe. J.H.
receives support from Arthrex, Stryker, and Citieffe. P.A. receives support from Stryker, Arthrex, and Citieffe. A.B.I. receives support from Arthrex. S.B. receives support from Arthrex. This research was supported by a research grant from Arthrex and performed at the Institute of Biomechanics, Trauma Center Murnau, Murnau, Germany. Received June 28, 2015; accepted November 23, 2015. Address correspondence to Andreas B. Imhoff, M.D., Ph.D., Ismaninger Strasse 22, 81675 Munich, Germany. E-mail: [email protected] Ó 2016 by the Arthroscopy Association of North America 0749-8063/15576/$36.00 http://dx.doi.org/10.1016/j.arthro.2015.11.049
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol
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No
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(Month), 2016: pp 1-6
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F. MARTETSCHLÄGER ET AL. Table 2. One-Way Analysis of Variance With Tukey Post Hoc Test for Age and BMD
P value for age P value for BMD
Two 4-mm Tunnels v One 4-mm Tunnel .78 .87
One 4-mm Tunnel v One 2.4-mm Tunnel .78 .73
Two 4-mm Tunnels v One 2.4-mm Tunnel .78 .96
BMD, bone mineral density.
Fig 1. Prepared specimen before potting process. The tweezers point to the coracoid process. (CT, conjoined tendons.)
4-mm or one 2.4-mm drill hole. We hypothesized that 2 drill holes and a larger drill diameter would critically destabilize the coracoid process and make it more prone to fracturing.
Methods This biomechanical study was approved by the institutional review board of the Technical University of Munich. Specimen Preparation Testing was performed using 18 fresh-frozen human cadaveric specimens, obtained from the Institution for Forensic Medicine, Technical University Munich (4 female and 5 male cadavers; mean age, 58.4 years; age range, 45 to 68 years). Specimens were thawed at room temperature 24 hours before testing. The humeri and clavicles were removed, along with the soft tissues from the scapula. The conjoined tendons were left intact and attached to the tip of the coracoid process (Fig 1). The microstructure of the specimens was captured by microecomputed tomography (CT) analysis (mCT 80;
Scanco Medical, Brüttisellen, Switzerland). In the region of the planned drill holes, 500 slices with a voxel size of 36 mm were obtained. Bone mineral density (BMD) measurement (dual-energy x-ray absorptiometry [in grams per square centimeter]) of the coracoid process was performed for each specimen to guarantee consistent BMD values between groups (mean BMD, 0.65 g/cm2; range, 0.58 to 0.67 g/cm2; Table 1). Coracoid dimensions were measured using a digital caliper (Mitutoyo, Kanagawa, Japan). Height and width have been measured at the insertion site of the conoid ligament close to the base.10 For inclusion, several criteria were mandatory: (1) standardized coracoid morphology, where the standard measures were defined according to prior studies as 45 2 mm for length, 25 1 mm for width, and 12 1 mm for height10,11; (2) intact scapula and conjoined tendons; and (3) cadaveric age younger than 70 years. Specimens with pre-existing injuries to the scapula and/or the conjoined tendons, reduced bone quality (<0.50 g/cm2), or divergent coracoid morphology were excluded from the study. Biomechanical Testing Scapulae were randomly assigned to the following groups by lottery: (1) two 4-mm tunnels, (2) one 4-mm tunnel, or (3) one 2.4-mm tunnel. There were no significant differences between groups regarding BMD, morphology, and age (Table 2). All tests were performed at room temperature, and the conjoined tendon was constantly kept moist using 0.9% saline solution. For the group with two 4-mm tunnels, the tunnels were drilled in a standard manner, entering at the anatomic insertion points of the CC ligaments according to Salzmann et al.11 and exiting the central section of the coracoid process. For the group with one 4-mm drill
Table 1. Demographic Data Specimens, n BMD, g/cm2 Mean SD (range) 95% CI Age, mean SD (range), yr
Two 4-mm Tunnels 6
One 4-mm Tunnel 6
One 2.4-mm Tunnel 6
All Specimens 18
0.65 0.03 (0.61-0.67) 0.62-0.67 54.8 9.7 (45-66)
0.64 0.03 (0.59-0.67) 0.61-0.67 54.8 9.7 (45-66)
0.65 0.02 (0.63-0.67) 0.63-0.67 65.7 1.9 (64-68)
0.65 0.03 (0.58-0.67) 0.63-0.66 58.4 9.1 (45-68)
BMD, bone mineral density; CI, confidence interval.
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FRACTURE RISK AFTER CORACOID DRILLING
Table 4. One-Way Analysis of Variance With Tukey Post Hoc Test for Load to Failure
P value
Two 4-mm Tunnels v One 4-mm Tunnel .55
One 4-mm Tunnel v One 2.4-mm Tunnel .74
Two 4-mm Tunnels v One 2.4-mm Tunnel .20
load was applied at a rate of 120 mm/min until failure.12,13 Ultimate failure load, along with the mode of failure, was recorded for all specimens (Tables 3 and 4). Micro-CT was used to analyze coracoid fracture patterns.
Fig 2. Testing setup. The specimen is mounted on the testing machine. The conjoined tendons (CT) are connected to the mechanical clamp. (CP, coracoid process.)
hole and the group with one 2.4-mm drill hole, one central tunnel was created in the center between the anatomic insertion points of the CC ligaments. To perform biomechanical testing, the scapular body of each specimen was potted in casting resin (RENCAST FC53; Huntsman Advanced Materials, Bergkamen, Germany) using a custom-made jig (Fig 1). The specimens were then fixed to a servo-electric testing machine (Zwick 010; Zwick, Ulm, Germany). The conjoined tendons were gripped by a mechanical softtissue clamp 30 mm from the coracoid insertion (Fig 2). The distance was measured with a digital caliper (Mitutoyo) to ensure consistent conditions across groups. The specimens were aligned with the vector of force being directed inferiorly, mimicking the force created by the biceps and coracobrachialis muscle. The
Statistical Analysis A post hoc power analysis using G*Power (version 3.1.9.2; Franz Paul, Kiel, Germany) was performed to determine the power of the study. On the basis of the results of the Fisher exact test, an effect size of 0.85 was calculated. With this effect size, an a of .05, and the sample size of 18, a power of 0.81 was calculated. For statistical analysis, SPSS software (version 22.0; IBM, New York, NY) was used. Normal distribution was tested and confirmed with the KolmogorovSmirnov test. Quantitative parameters are given as means, standard deviations, and 95% confidence intervals. To evaluate the differences regarding BMD, specimen age, and load to failure between groups, a 1-way analysis of variance with a Tukey post hoc test was used. The Fisher exact test was used to analyze the failure modes between the testing groups. A significance level of P < .05 was accepted as a statistically significant difference.
Results Demographic data for the 18 tested specimens and BMD data are displayed in Table 1. After randomization into the different groups, there was no statistically significant difference between groups regarding BMD and age (Table 2). Load-to-Failure Testing No failure at the clamp-tendon interface was recorded during load-to-failure testing of the 18 specimens. Mean load to failure increased from 392 N (95% CI, 288.7-494.9 N) for the group with two 4-mm tunnels to
Table 3. Load to Failure (Load Application of 120 mm/min) Specimens (N ¼ 18), n Load to failure, N Mean SD (range) 95% CI CI, confidence interval.
Two 4-mm Tunnels 6
One 4-mm Tunnel 6
One 2.4-mm Tunnel 6
392 98.2 (249-517) 288.7-494.9
459 143.8 (241-605) 308.2-610.1
506 73.3 (396-613) 429.4-583.2
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Table 5. Mode of Failure With Fisher Exact Test for Significance Between Groups
Coracoid fracture, n P value for fracture risk
Two 4-mm Tunnels v One 4-mm Tunnel 6v6
One 4-mm Tunnel v One 2.4-mm Tunnel 6v1
Two 4-mm Tunnels v One 2.4-mm Tunnel 6v1
> .99
.015
.015
459 N (95% CI, 308.2-610.1 N) for the group with one 4-mm tunnel and 506 N (95% CI, 429.4-583.2 N) for the group with one 2.4-mm tunnel (Table 3). However, statistical analysis showed no significant differences between groups (P ¼ .55, P ¼ .74, and P ¼ .20; Table 4). Failure Mode Analysis of the failure mode showed that all specimens in the group with two 4-mm tunnels and the group with one 4-mm tunnel failed by fracture of the coracoid process (6 of 6 specimens in each group). In contrast, 5 of 6 specimens in the group with one 2.4mm tunnel failed by tearing of the conjoined tendons and only 1 failed by coracoid fracture. When we compared the group with two 4-mm tunnels and the group with one 4-mm tunnel versus the group with one 2.4-mm tunnel, a statistically significant difference regarding coracoid fracture was detected (P ¼ .015 and P ¼ .015, respectively; Table 5, Fig 3). Micro-CT Analysis For all fractured coracoids, micro-CT data showed a fracture running through the drill holes (Fig 4). The remaining coracoids were confirmed to be intact without any fracture signs.
Fig 3. Failure by coracoid fracture. The fracture is running through the distal drill hole.
Fig 4. Microecomputed tomography analysis showing fracture line running through drill hole.
Discussion The failure mechanism analysis of this study showed that one 2.4-mm drill hole led to a reduced risk of coracoid process fracture compared with one or two 4mm drill holes, whereas there was no significant difference regarding load-to-failure testing between groups. The purpose of this laboratory study was to analyze the impact of 3 different coracoid drilling configurations widely used for modern AC joint reconstruction on the risk of coracoid process fracture. We hypothesized that the number of drill holes and/or the drill hole diameter would have a crucial influence on the biomechanical stability of the coracoid process. Although load-to-failure testing did not show any significant differences between groups, the results of the failure mode analysis clearly supported the hypothesis that 2 drill holes and a larger drill diameter would critically destabilize the coracoid process and make it more prone to fracturing. Modern arthroscopically assisted techniques for reconstruction of the unstable AC joint have been shown to provide reliable and good clinical results.2,5,8,14 However, the newer techniques using drill holes through the clavicle and coracoid process have introduced new complication patterns. Several authors
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FRACTURE RISK AFTER CORACOID DRILLING
have described fractures of the lateral clavicle or the coracoid process after arthroscopically assisted reconstruction of the AC joint.2,3,8,15 Recently, Spiegl et al.9 investigated the biomechanical consequences of CC reconstruction techniques on clavicle strength. They showed that tendon graft reconstruction with two 6mm drill holes and tenodesis screws caused significantly more reduction of clavicle strength than a CFB technique with two 2.4-mm drill holes. In contrast, possible destabilization of the coracoid process by different coracoid drilling techniques (number and width of tunnels used) has not been investigated yet. Ferreira et al.16 investigated the influence of the drilling location on coracoid stability while drilling one or two 6-mm holes into the coracoid process for AC joint repair. They were able to show that the coracoid process resisted a higher peak load when the drill hole entry point was located in the central or medial sector and the exit point was in the central sector. For our study, the standard entry points were located at the anatomic insertion site of the CC ligaments for the double-tunnel reconstruction or in the center between these for the single-tunnel reconstruction. The exit point was in the central section of the coracoid process. Therefore this study examines a best-case scenario, given that the entry points of the drill cannot be visualized during arthroscopically assisted AC joint reconstruction from the subcoracoid space and the course of the tunnel might influence stability.16 This possible drawback of anatomic CC ligament reconstruction has recently been investigated by Coale et al.17 using threedimensional computed tomography models. They concluded that transclavicular-transcoracoid reconstruction techniques using a linear 6-mm tunnel could not restore the anatomic footprint of the CC ligaments without significant risk of cortical breach and fracture. In a similar study using 3-dimensional computed tomography models, Xue et al.18 found cortical breach of the clavicle and/or the coracoid process in 95 of 105 virtual models (91%) when simulating a “truly anatomic CC reconstruction” with two 4-mm drill holes. Despite the lack of medial or lateral cortical breach during the standardized drilling procedure in this simplified model, failure mode analysis showed a higher risk of coracoid fracture for the groups with one 4-mm and two 4-mm tunnels when compared with one smaller, 2.4-mm tunnel. Therefore, to minimize the coracoid fracture risk, we have changed our technique to a 2.4-mm single-tunnel technique using 2 cortical button devices and 2 high-strength suture tapes. To date, little is known about the influence of the number of drill holes and/or the drill hole diameter on the risk of coracoid process fracture. Therefore this study provides potentially important information for
surgeons dealing with this injury in daily clinical practice and might help to avoid this difficult-to-handle complication. Limitations This study has several limitations. As with all biomechanical studies, it has the inherent problem that only time-zero stability was evaluated. However, assumption of quick bony consolidation of the bone tunnels is wrong, given that implant material is in place and several studies have shown tunnel widening rather than bony consolidation.19,20 Therefore fracture risk might even increase over time. Nevertheless, the changes the drill holes undergo over time cannot be answered by this study. A further limitation of our study is that cadaveric scapulae were used for testing, not necessarily showing the same properties as healthy young persons, in whom AC dislocations mostly occur. To minimize this potential limitation, only specimens from cadavers younger than 70 years have been included, and a BMD measurement has been performed to reduce bias. Furthermore, for this simplified cadaveric study, we standardized the drilling in the laboratory, not accounting for any technical and visual problems that might occur during transclaviculartranscoracoid drilling in the operating room. Finally, minimal differences in quality and/or measures or anatomy of the coracoid process or the conjoined tendons might be a potential bias. However, only specimens that met the standard coracoid dimensions, as previously described, and that showed completely intact conjoined tendons were used for the study.
Conclusions Although there was no significant difference regarding load-to-failure testing between groups, the failure mechanism analysis showed that one 2.4-mm drill hole led to less destabilization of the coracoid than one or two 4-mm drill holes.
References 1. Braun S, Martetschläger F, Imhoff AB. Arthroscopically assisted reconstruction of acute and chronic AC joint separations. Oper Orthop Traumatol 2014;26:228-236 [in German]. 2. Martetschläger F, Horan MP, Warth RJ, Millett PJ. Complications after anatomic fixation and reconstruction of the coracoclavicular ligaments. Am J Sports Med 2013;41: 2896-2903. 3. Milewski MD, Tompkins M, Giugale JM, Carson EW, Miller MD, Diduch DR. Complications related to anatomic reconstruction of the coracoclavicular ligaments. Am J Sports Med 2012;40:1628-1634. 4. Salzmann GM, Walz L, Schoettle PB, Imhoff AB. Arthroscopic anatomical reconstruction of the acromioclavicular joint. Acta Orthop Belg 2008;74:397-400.
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5. Venjakob AJ, Salzmann GM, Gabel F, et al. Arthroscopically assisted 2-bundle anatomic reduction of acute acromioclavicular joint separations: 58-month findings. Am J Sports Med 2013;41:615-621. 6. Warth RJ, Martetschläger F, Gaskill TR, Millett PJ. Acromioclavicular joint separations. Curr Rev Musculoskelet Med 2013;6:71-78. 7. Hosseini H, Friedmann S, Troger M, Lobenhoffer P, Agneskirchner JD. Arthroscopic reconstruction of chronic AC joint dislocations by transposition of the coracoacromial ligament augmented by the Tight Rope device: A technical note. Knee Surg Sports Traumatol Arthrosc 2009;17:92-97. 8. Salzmann GM, Walz L, Buchmann S, Glabgly P, Venjakob A, Imhoff AB. Arthroscopically assisted 2bundle anatomical reduction of acute acromioclavicular joint separations. Am J Sports Med 2010;38:1179-1187. 9. Spiegl UJ, Smith SD, Euler SA, Dornan GJ, Millett PJ, Wijdicks CA. Biomechanical consequences of coracoclavicular reconstruction techniques on clavicle strength. Am J Sports Med 2014;42:1724-1730. 10. Rios CG, Arciero RA, Mazzocca AD. Anatomy of the clavicle and coracoid process for reconstruction of the coracoclavicular ligaments. Am J Sports Med 2007;35:811-817. 11. Salzmann GM, Paul J, Sandmann GH, Imhoff AB, Schottle PB. The coracoidal insertion of the coracoclavicular ligaments: An anatomic study. Am J Sports Med 2008;36:2392-2397. 12. Mazzocca AD, Santangelo SA, Johnson ST, Rios CG, Dumonski ML, Arciero RA. A biomechanical evaluation of an anatomical coracoclavicular ligament reconstruction. Am J Sports Med 2006;34:236-246.
13. Sethi P, Obopilwe E, Rincon L, Miller S, Mazzocca A. Biomechanical evaluation of distal biceps reconstruction with cortical button and interference screw fixation. J Shoulder Elbow Surg 2010;19:53-57. 14. Scheibel M, Droschel S, Gerhardt C, Kraus N. Arthroscopically assisted stabilization of acute high-grade acromioclavicular joint separations. Am J Sports Med 2011;39: 1507-1516. 15. Gerhardt DC, VanDerWerf JD, Rylander LS, McCarty EC. Postoperative coracoid fracture after transcoracoid acromioclavicular joint reconstruction. J Shoulder Elbow Surg 2011;20:e6-e10. 16. Ferreira JV, Chowaniec D, Obopilwe E, Nowak MD, Arciero RA, Mazzocca AD. Biomechanical evaluation of effect of coracoid tunnel placement on load to failure of fixation during repair of acromioclavicular joint dislocations. Arthroscopy 2012;28:1230-1236. 17. Coale RM, Hollister SJ, Dines JS, Allen AA, Bedi A. Anatomic considerations of transclavicular-transcoracoid drilling for coracoclavicular ligament reconstruction. J Shoulder Elbow Surg 2013;22:137-144. 18. Xue C, Zhang M, Zheng TS, et al. Clavicle and coracoid process drilling technique for truly anatomic coracoclavicular ligament reconstruction. Injury 2013;44: 1314-1320. 19. Epstein D, Day M, Rokito A. Current concepts in the surgical management of acromioclavicular joint injuries. Bull NYU Hosp Jt Dis 2012;70:11-24. 20. Yoo JC, Choi NH, Kim SY, Lim TK. Distal clavicle tunnel widening after coracoclavicular ligament reconstruction with semitendinous tendon: A case report. J Shoulder Elbow Surg 2006;15:256-259.
Purpose: To biomechanically compare the stability of the coracoid process after an anatomic double-tunnel technique using two 4-mm drill holes or a single-tunnel technique using one 4-mm or one 2.4-mm drill hole. Methods: For biomechanical testing, 18 fresh-frozen cadaveric scapulae were used and randomly assigned to one of the following groups: two 4-mm drill holes (group 1), one 4-mm drill hole (group 2), or one 2.4-mm drill hole (group 3). After standardized coracoid drilling, load was applied to the conjoined tendons at a rate of 120 mm/min and ultimate failure load, along with the failure mode, was recorded. Results: There was no significant difference between groups regarding load to failure. Mean load to failure in group 1 was 392 N; group 2, 459 N; and group 3, 506 N. The corresponding P values were .55, .74, and .20 for group 1 versus group 2, group 2 versus group 3, and group 1 versus group 3, respectively. However, the failure mode for the group with one 4-mm drill hole and the group with two 4-mm drill holes was coracoid fracture, whereas the group with one 2.4-mm drill hole showed 5 tears of the conjoined tendons and only 1 coracoid fracture (P ¼ .015). Conclusions: Although there was no significant difference regarding load-to-failure testing between groups, the failure mechanism analysis showed that one 2.4-mm drill hole led to less destabilization of the coracoid than one or two 4-mm drill holes. Clinical Relevance: Techniques with small, 2.4-mm drill holes might decrease the risk of severe iatrogenic fracture complications.
lthough surgical treatment of high-grade acromioclavicular (AC) joint disruptions of Rockwood type IV through VI is still lacking a gold-standard procedure, arthroscopically assisted reconstruction techniques using cortical fixation buttons (CFBs) have gained increased popularity.1-6 For these techniques, different coracoclavicular (CC) drilling diameters and configurations are described. Most commonly, 4-mm tunnel sizes have been used for inserting the CFBs.2,4,5,7 For anatomic reconstruction of the 2 CC ligaments, a technique with two separate 4-mm drill
A
holes has been proposed.5,8 However, the modern techniques have introduced new complications such as fractures of the lateral clavicle or the coracoid process.2,3 The biomechanical consequences of CC drilling on clavicle strength have been reported recently.9 Little is known about the influence of different coracoid drilling configurations on coracoid fracture risk. Therefore the purpose of this study was to biomechanically compare the stability of the coracoid process after an anatomic double-tunnel technique using two 4-mm drill holes or a single-tunnel technique using one
From the Department of Orthopaedic Sports Medicine, Klinikum rechts der Isar, TU Munich (F.M., A.W., E.H., A.B.I., S.B.), Munich, Germany; Center for Shoulder and Elbow Surgery, ATOS Clinic Munich (F.M.), Munich, Germany; Trauma Center Murnau, Berufsgenossenschaftliche Klinik Murnau (T.S.), Murnau, Germany; Institute of Biomechanics, Berufsgenossenschaftliche Unfallklinik Murnau (T.S., M.W., J.H., P.A.), Murnau, Germany; and Institute of Biomechanics, Paracelsus Medical University (P.A.), Salzburg, Austria. The authors report the following potential conflict of interest or source of funding: F.M. receives support from Arthrex. T.S. receives support from Arthrex. A.W. receives support from Arthrex. E.H. receives support from Arthrex. M.W. receives support from Arthrex, Stryker, and Citieffe. J.H.
receives support from Arthrex, Stryker, and Citieffe. P.A. receives support from Stryker, Arthrex, and Citieffe. A.B.I. receives support from Arthrex. S.B. receives support from Arthrex. This research was supported by a research grant from Arthrex and performed at the Institute of Biomechanics, Trauma Center Murnau, Murnau, Germany. Received June 28, 2015; accepted November 23, 2015. Address correspondence to Andreas B. Imhoff, M.D., Ph.D., Ismaninger Strasse 22, 81675 Munich, Germany. E-mail: [email protected] Ó 2016 by the Arthroscopy Association of North America 0749-8063/15576/$36.00 http://dx.doi.org/10.1016/j.arthro.2015.11.049
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol
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No
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(Month), 2016: pp 1-6
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F. MARTETSCHLÄGER ET AL. Table 2. One-Way Analysis of Variance With Tukey Post Hoc Test for Age and BMD
P value for age P value for BMD
Two 4-mm Tunnels v One 4-mm Tunnel .78 .87
One 4-mm Tunnel v One 2.4-mm Tunnel .78 .73
Two 4-mm Tunnels v One 2.4-mm Tunnel .78 .96
BMD, bone mineral density.
Fig 1. Prepared specimen before potting process. The tweezers point to the coracoid process. (CT, conjoined tendons.)
4-mm or one 2.4-mm drill hole. We hypothesized that 2 drill holes and a larger drill diameter would critically destabilize the coracoid process and make it more prone to fracturing.
Methods This biomechanical study was approved by the institutional review board of the Technical University of Munich. Specimen Preparation Testing was performed using 18 fresh-frozen human cadaveric specimens, obtained from the Institution for Forensic Medicine, Technical University Munich (4 female and 5 male cadavers; mean age, 58.4 years; age range, 45 to 68 years). Specimens were thawed at room temperature 24 hours before testing. The humeri and clavicles were removed, along with the soft tissues from the scapula. The conjoined tendons were left intact and attached to the tip of the coracoid process (Fig 1). The microstructure of the specimens was captured by microecomputed tomography (CT) analysis (mCT 80;
Scanco Medical, Brüttisellen, Switzerland). In the region of the planned drill holes, 500 slices with a voxel size of 36 mm were obtained. Bone mineral density (BMD) measurement (dual-energy x-ray absorptiometry [in grams per square centimeter]) of the coracoid process was performed for each specimen to guarantee consistent BMD values between groups (mean BMD, 0.65 g/cm2; range, 0.58 to 0.67 g/cm2; Table 1). Coracoid dimensions were measured using a digital caliper (Mitutoyo, Kanagawa, Japan). Height and width have been measured at the insertion site of the conoid ligament close to the base.10 For inclusion, several criteria were mandatory: (1) standardized coracoid morphology, where the standard measures were defined according to prior studies as 45 2 mm for length, 25 1 mm for width, and 12 1 mm for height10,11; (2) intact scapula and conjoined tendons; and (3) cadaveric age younger than 70 years. Specimens with pre-existing injuries to the scapula and/or the conjoined tendons, reduced bone quality (<0.50 g/cm2), or divergent coracoid morphology were excluded from the study. Biomechanical Testing Scapulae were randomly assigned to the following groups by lottery: (1) two 4-mm tunnels, (2) one 4-mm tunnel, or (3) one 2.4-mm tunnel. There were no significant differences between groups regarding BMD, morphology, and age (Table 2). All tests were performed at room temperature, and the conjoined tendon was constantly kept moist using 0.9% saline solution. For the group with two 4-mm tunnels, the tunnels were drilled in a standard manner, entering at the anatomic insertion points of the CC ligaments according to Salzmann et al.11 and exiting the central section of the coracoid process. For the group with one 4-mm drill
Table 1. Demographic Data Specimens, n BMD, g/cm2 Mean SD (range) 95% CI Age, mean SD (range), yr
Two 4-mm Tunnels 6
One 4-mm Tunnel 6
One 2.4-mm Tunnel 6
All Specimens 18
0.65 0.03 (0.61-0.67) 0.62-0.67 54.8 9.7 (45-66)
0.64 0.03 (0.59-0.67) 0.61-0.67 54.8 9.7 (45-66)
0.65 0.02 (0.63-0.67) 0.63-0.67 65.7 1.9 (64-68)
0.65 0.03 (0.58-0.67) 0.63-0.66 58.4 9.1 (45-68)
BMD, bone mineral density; CI, confidence interval.
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FRACTURE RISK AFTER CORACOID DRILLING
Table 4. One-Way Analysis of Variance With Tukey Post Hoc Test for Load to Failure
P value
Two 4-mm Tunnels v One 4-mm Tunnel .55
One 4-mm Tunnel v One 2.4-mm Tunnel .74
Two 4-mm Tunnels v One 2.4-mm Tunnel .20
load was applied at a rate of 120 mm/min until failure.12,13 Ultimate failure load, along with the mode of failure, was recorded for all specimens (Tables 3 and 4). Micro-CT was used to analyze coracoid fracture patterns.
Fig 2. Testing setup. The specimen is mounted on the testing machine. The conjoined tendons (CT) are connected to the mechanical clamp. (CP, coracoid process.)
hole and the group with one 2.4-mm drill hole, one central tunnel was created in the center between the anatomic insertion points of the CC ligaments. To perform biomechanical testing, the scapular body of each specimen was potted in casting resin (RENCAST FC53; Huntsman Advanced Materials, Bergkamen, Germany) using a custom-made jig (Fig 1). The specimens were then fixed to a servo-electric testing machine (Zwick 010; Zwick, Ulm, Germany). The conjoined tendons were gripped by a mechanical softtissue clamp 30 mm from the coracoid insertion (Fig 2). The distance was measured with a digital caliper (Mitutoyo) to ensure consistent conditions across groups. The specimens were aligned with the vector of force being directed inferiorly, mimicking the force created by the biceps and coracobrachialis muscle. The
Statistical Analysis A post hoc power analysis using G*Power (version 3.1.9.2; Franz Paul, Kiel, Germany) was performed to determine the power of the study. On the basis of the results of the Fisher exact test, an effect size of 0.85 was calculated. With this effect size, an a of .05, and the sample size of 18, a power of 0.81 was calculated. For statistical analysis, SPSS software (version 22.0; IBM, New York, NY) was used. Normal distribution was tested and confirmed with the KolmogorovSmirnov test. Quantitative parameters are given as means, standard deviations, and 95% confidence intervals. To evaluate the differences regarding BMD, specimen age, and load to failure between groups, a 1-way analysis of variance with a Tukey post hoc test was used. The Fisher exact test was used to analyze the failure modes between the testing groups. A significance level of P < .05 was accepted as a statistically significant difference.
Results Demographic data for the 18 tested specimens and BMD data are displayed in Table 1. After randomization into the different groups, there was no statistically significant difference between groups regarding BMD and age (Table 2). Load-to-Failure Testing No failure at the clamp-tendon interface was recorded during load-to-failure testing of the 18 specimens. Mean load to failure increased from 392 N (95% CI, 288.7-494.9 N) for the group with two 4-mm tunnels to
Table 3. Load to Failure (Load Application of 120 mm/min) Specimens (N ¼ 18), n Load to failure, N Mean SD (range) 95% CI CI, confidence interval.
Two 4-mm Tunnels 6
One 4-mm Tunnel 6
One 2.4-mm Tunnel 6
392 98.2 (249-517) 288.7-494.9
459 143.8 (241-605) 308.2-610.1
506 73.3 (396-613) 429.4-583.2
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F. MARTETSCHLÄGER ET AL.
Table 5. Mode of Failure With Fisher Exact Test for Significance Between Groups
Coracoid fracture, n P value for fracture risk
Two 4-mm Tunnels v One 4-mm Tunnel 6v6
One 4-mm Tunnel v One 2.4-mm Tunnel 6v1
Two 4-mm Tunnels v One 2.4-mm Tunnel 6v1
> .99
.015
.015
459 N (95% CI, 308.2-610.1 N) for the group with one 4-mm tunnel and 506 N (95% CI, 429.4-583.2 N) for the group with one 2.4-mm tunnel (Table 3). However, statistical analysis showed no significant differences between groups (P ¼ .55, P ¼ .74, and P ¼ .20; Table 4). Failure Mode Analysis of the failure mode showed that all specimens in the group with two 4-mm tunnels and the group with one 4-mm tunnel failed by fracture of the coracoid process (6 of 6 specimens in each group). In contrast, 5 of 6 specimens in the group with one 2.4mm tunnel failed by tearing of the conjoined tendons and only 1 failed by coracoid fracture. When we compared the group with two 4-mm tunnels and the group with one 4-mm tunnel versus the group with one 2.4-mm tunnel, a statistically significant difference regarding coracoid fracture was detected (P ¼ .015 and P ¼ .015, respectively; Table 5, Fig 3). Micro-CT Analysis For all fractured coracoids, micro-CT data showed a fracture running through the drill holes (Fig 4). The remaining coracoids were confirmed to be intact without any fracture signs.
Fig 3. Failure by coracoid fracture. The fracture is running through the distal drill hole.
Fig 4. Microecomputed tomography analysis showing fracture line running through drill hole.
Discussion The failure mechanism analysis of this study showed that one 2.4-mm drill hole led to a reduced risk of coracoid process fracture compared with one or two 4mm drill holes, whereas there was no significant difference regarding load-to-failure testing between groups. The purpose of this laboratory study was to analyze the impact of 3 different coracoid drilling configurations widely used for modern AC joint reconstruction on the risk of coracoid process fracture. We hypothesized that the number of drill holes and/or the drill hole diameter would have a crucial influence on the biomechanical stability of the coracoid process. Although load-to-failure testing did not show any significant differences between groups, the results of the failure mode analysis clearly supported the hypothesis that 2 drill holes and a larger drill diameter would critically destabilize the coracoid process and make it more prone to fracturing. Modern arthroscopically assisted techniques for reconstruction of the unstable AC joint have been shown to provide reliable and good clinical results.2,5,8,14 However, the newer techniques using drill holes through the clavicle and coracoid process have introduced new complication patterns. Several authors
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FRACTURE RISK AFTER CORACOID DRILLING
have described fractures of the lateral clavicle or the coracoid process after arthroscopically assisted reconstruction of the AC joint.2,3,8,15 Recently, Spiegl et al.9 investigated the biomechanical consequences of CC reconstruction techniques on clavicle strength. They showed that tendon graft reconstruction with two 6mm drill holes and tenodesis screws caused significantly more reduction of clavicle strength than a CFB technique with two 2.4-mm drill holes. In contrast, possible destabilization of the coracoid process by different coracoid drilling techniques (number and width of tunnels used) has not been investigated yet. Ferreira et al.16 investigated the influence of the drilling location on coracoid stability while drilling one or two 6-mm holes into the coracoid process for AC joint repair. They were able to show that the coracoid process resisted a higher peak load when the drill hole entry point was located in the central or medial sector and the exit point was in the central sector. For our study, the standard entry points were located at the anatomic insertion site of the CC ligaments for the double-tunnel reconstruction or in the center between these for the single-tunnel reconstruction. The exit point was in the central section of the coracoid process. Therefore this study examines a best-case scenario, given that the entry points of the drill cannot be visualized during arthroscopically assisted AC joint reconstruction from the subcoracoid space and the course of the tunnel might influence stability.16 This possible drawback of anatomic CC ligament reconstruction has recently been investigated by Coale et al.17 using threedimensional computed tomography models. They concluded that transclavicular-transcoracoid reconstruction techniques using a linear 6-mm tunnel could not restore the anatomic footprint of the CC ligaments without significant risk of cortical breach and fracture. In a similar study using 3-dimensional computed tomography models, Xue et al.18 found cortical breach of the clavicle and/or the coracoid process in 95 of 105 virtual models (91%) when simulating a “truly anatomic CC reconstruction” with two 4-mm drill holes. Despite the lack of medial or lateral cortical breach during the standardized drilling procedure in this simplified model, failure mode analysis showed a higher risk of coracoid fracture for the groups with one 4-mm and two 4-mm tunnels when compared with one smaller, 2.4-mm tunnel. Therefore, to minimize the coracoid fracture risk, we have changed our technique to a 2.4-mm single-tunnel technique using 2 cortical button devices and 2 high-strength suture tapes. To date, little is known about the influence of the number of drill holes and/or the drill hole diameter on the risk of coracoid process fracture. Therefore this study provides potentially important information for
surgeons dealing with this injury in daily clinical practice and might help to avoid this difficult-to-handle complication. Limitations This study has several limitations. As with all biomechanical studies, it has the inherent problem that only time-zero stability was evaluated. However, assumption of quick bony consolidation of the bone tunnels is wrong, given that implant material is in place and several studies have shown tunnel widening rather than bony consolidation.19,20 Therefore fracture risk might even increase over time. Nevertheless, the changes the drill holes undergo over time cannot be answered by this study. A further limitation of our study is that cadaveric scapulae were used for testing, not necessarily showing the same properties as healthy young persons, in whom AC dislocations mostly occur. To minimize this potential limitation, only specimens from cadavers younger than 70 years have been included, and a BMD measurement has been performed to reduce bias. Furthermore, for this simplified cadaveric study, we standardized the drilling in the laboratory, not accounting for any technical and visual problems that might occur during transclaviculartranscoracoid drilling in the operating room. Finally, minimal differences in quality and/or measures or anatomy of the coracoid process or the conjoined tendons might be a potential bias. However, only specimens that met the standard coracoid dimensions, as previously described, and that showed completely intact conjoined tendons were used for the study.
Conclusions Although there was no significant difference regarding load-to-failure testing between groups, the failure mechanism analysis showed that one 2.4-mm drill hole led to less destabilization of the coracoid than one or two 4-mm drill holes.
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