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Retrograde nailing versus locking plate osteosynthesis of proximal humeral fractures: a biomechanical study
J Shoulder Elbow Surg (2012) 21, 618-624
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Retrograde nailing versus locking plate osteosynthesis of proximal humeral fractures: a biomechanical study Sven-Oliver Dietz, MD*, Frank Hartmann, MD, Thomas Schwarz, Tobias E. Nowak, MD, Annalisa Enders, MD, Sebastian Kuhn, MD, Alexander Hofmann, MD, Pol Maria Rommens, MD Center for Musculoskeletal Surgery, Department of Trauma Surgery, University Medical Center, Johannes Gutenberg University, Mainz, Germany Background: In locking plate osteosynthesis of proximal humeral fractures, secondary varus malalignment is a specific complication. Retron nails (Tantum AG, Neumunster, Germany), among others, have been designed to improve medial support of the calcar humeri. The aim of our biomechanical study was to examine whether Retron nails provide increased stiffness for axial loads and adequate stiffness for torsional loads when compared with Philos plates (Synthes AG, Umkirch, Germany). Materials and methods: Twenty-two fresh-frozen paired humeri were collected. After potting the specimens, intact bones were exposed to sinusoidal axial (10-120 N) and torsional (2.5 Nm) loading for 8 cycles to calculate the initial stiffness and exclude pairs with differences. Afterward, an unstable proximal humeral fracture (AO 11-A3) was created by means of an oscillating saw, and the respective osteosynthesis devices were implanted. After another 4 cycles, initial changes in stiffness were measured. Subsequently, all specimens were tested for 1,000 cycles of loading before final stiffness was assessed. Results: We found no statistically significant differences between Retron and Philos specimens after 4 or 1,000 cycles of loading. Conclusion: Our study suggests that retrograde nailing provides sufficient stability for axial and torsional loading in 2-part fractures of proximal humeri. Level of evidence: Basic Science Study, Biomechanical Study. Ó 2012 Journal of Shoulder and Elbow Surgery Board of Trustees. Keywords: Proximal humeral fracture; locking plates; intramedullary implants; biomechanics of proximal humerus; Retron; retrograde nailing
Fractures of the proximal humerus are, along with hip, spine, and distal forearm fractures, referred to as major osteoporotic fractures. A recent trend analysis from *Reprint requests: Sven-Oliver Dietz, MD, Center for Musculoskeletal Surgery, Department of Trauma Surgery, University Medical Center, Johannes Gutenberg University, Langenbeckstrasse 1, 55101 Mainz, Germany. E-mail address: [email protected] (S.-O. Dietz).
Switzerland showed that, between 2000 and 2007, the absolute number of proximal humeral fractures increased by 22.8% in women and 35.5% in men.14 Because locking plates were shown to have superior properties to conventional plates in biomechanical investigations,1,5,7,11,22 these implants became widely used in clinical practice. Although numerous articles have provided excellent accounts of good clinical results,4,16-18,28 multiple implant-specific obstacles
1058-2746/$ - see front matter Ó 2012 Journal of Shoulder and Elbow Surgery Board of Trustees. doi:10.1016/j.jse.2011.04.013
Retrograde proximal humerus osteosynthesis have become apparent.2,3,8,9,13,21,23,25 In addition to primary and secondary screw perforation, secondary loss of reposition and varus deformity were the most common complications.8,23 Furthermore, surgical approaches for applying a locking plate compromise the soft tissue and blood supply to the humeral head. Different surgical approaches, including deltopectoral and anterior delta split, cannot eliminate these effects.10,24,26 Minimally invasive devices have been constructed to address these problems. Initial clinical studies showed good results, but implantrelated complications were found in 17% of cases and avascular head necrosis in 5.5%.18 Furthermore, only a limited number of screws can be applied percutaneously because of the risks of axillary nerve injury.21 Because inadequate medial support of the calcar humeri has a significant influence on secondary varus deformities, we investigated the biomechanical properties of a new locking device. Retron nails (Tantum AG, Neumunster, Germany) are implanted through small incisions and have to be inserted near the calcar humeri. To our knowledge, the clinical and biomechanical performances of this new implant have never been investigated. The purpose of our study was to assess the biomechanical properties of this new nail in unstable surgical neck fractures. As a reference implant, the Philos plate (Synthes AG, Umkirch, Germany) was used. The hypothesis was that the nails would provide more stable fixation for axial loads than locking plates and that they would adequately maintain reductions for torsional loads. Ultimate load to failure and fracture alignment with the devices were not subjects of our study.
Materials and methods Specimens Eleven pairs of fresh-frozen human proximal humeri (22 humeri) were dissected from all soft tissues and shortened to a length of 20 cm from the proximal aspect. Specimens with clinical or radiologic abnormalities were eliminated. Two groups were formed based on paired comparison. In group 1, osteosynthesis was performed with the angular-stable plate, whereas in group 2, the retrograde nail was used. Each pair was randomized for the specific date of the experiment. In each pair, the specimens were randomly assigned to the relevant group. An unstable 2-part fracture of the proximal humerus (AO 11-A3 injury) was simulated by means of an oscillating pneumatic saw. A wedge osteotomy with a 10-mm defect at the humeral neck was created. The base of the resected triangle was at the medial side (Fig. 1). To ease osteosynthesis, the medial osteotomy was completed after application of the implant. Each specimen was potted proximally in polymethyl methacrylate (PMMA) cement. Contact between the PMMA and implant was avoided. To prevent loosening between the articular surface of the proximal humerus and the PMMA, 2 additional screws were inserted into the humeral head. Distally, all specimens were fixed by means of a metal rod (Fig. 1).
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Implants A Philos plate was used as an example of a locking plate, and a Retron nail was used as an example of a retrograde nail. Both implants are made of titanium. The retrograde nail was placed in the medial third of a humeral head through the lateral aspect of the humeral shaft. The nail is cannulated, has a diameter of 8 mm and a length of 76 or 86 mm, and can be laterally lengthened by a diaphysis anchor. A selftapping thread at the proximal end of the nail fixes the nail in the subchondral bone of the humeral head. All nails were locked by three 4-mm self-tapping angular-stable locking bolts in the humeral head and two self-tapping angular-stable locking bolts in the proximal humeral shaft. An aiming device was used to insert the locking bolts. All nails were connected to a diaphysis anchor of a particular length. The latter was fixed by a 4-mm self-tapping screw in the humeral shaft. A locking plate of 11.4 mm in length was placed at the lateral aspect of the proximal humerus by use of an aiming device to ensure correct orientation. The plate was fixed in the humeral head by six 3.5-mm self-tapping angular-stable screws. To provide maximum support of the humeral head, these screws are converting in the anteroposterior plane and diverting in the sagittal plane. An additional three 3.5-mm locking screws secured the shaft.
Biomechanical testing All specimens were mounted on a pneumatic testing machine (SyncoTec, Clausthal-Zellerfeld, Germany) (Fig. 2), loaded in the axial direction, and torqued in 3 subsequent steps. All specimens received the same loading profile. All intact humeri were pretested (step 1) to rule out intraindividual differences. These measures were used as initial values. Afterward, the specimens were tested after osteotomy and insertion of the respective implant for 4 cycles (step 2). Finally, all specimens were tested for 1,000 cycles (step 3).
Pretesting/step 1 After potting, all specimens were subjected to 8 cycles of axial and torque loading. First, the specimen was subjected to 10 N of axial loading for 15 seconds while torsion was applied in a sinusoidal manner (0.5-2 Nm). Afterward, the torsion load was constant at 0.5 Nm whereas the axial load was raised in a sinusoidal manner to 120 N. During loading, translation and angles along the testing axes were recorded. The initial stiffness of the specimens was calculated from these values. Humeral pairs with differences in initial stiffness of greater than 20% were excluded.
Step 2 After implantation of the respective osteosynthesis devices, all specimens were tested in an identical manner to step 1 for 4 cycles. The loss of stiffness after osteosynthesis was calculated from these values.
Step 3 All specimens were then subjected to 1,000 cycles of axial and torque loading. The loading parameters remained unchanged.
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Figure 1 (A) Radiographs of paired cadaveric humeri after osteotomy and implantation of Retron nail (right) and Philos plate (left) in anteroposterior and lateral projections. (B) Corresponding specimens (Philos plate [bottom] and Retron nail [top]).
no statistically significant intraindividual differences in torsion and so were used for further testing. The mean value of torsional stiffness was 0.73 Nm/ (SD, 0.10 Nm/ ; minimum, 0.56 Nm/ ; maximum, 0.91 Nm/ ) for group 1 and 0.72 Nm/ (SD, 0.12 Nm/ ; minimum, 0.48 Nm/ ; maximum, 0.83 Nm/ ) for group 2 (P ¼ .767).
Figure 2 Retron specimen after embedding, loaded into pneumatic testing machine.
Failure was defined as an irreversible closure of the osteotomy gap, peri-implant fracture, or interruption of the linear loading curve in association with a sudden deviation in the forcedeformation diagram.
Statistics Interindividual comparisons were based on 2-sample Wilcoxon tests. Torsion and axial stiffness were investigated. The values of the implants were computed, and the distribution was described in medians and percentiles. P < .05 was regarded as statistically significant. SPSS (release 15.0 for Windows; SPSS, Chicago, IL, USA) was used for numeric and graphical analyses.
Results Torsional stiffness Pretesting/step 1 Before osteosynthesis, all specimens were potted and tested for axial and torsional stiffness. Of the 11 pairs, 9 showed
Step 2 After pretesting, the specimens in group 1 gained 68.17% of their initial stiffness (SD, 8.61%; minimum, 57.90%; maximum, 84.67%). Humeral specimens with retrograde humeral nails reached 62.94% of their initial stiffness (SD, 11.65%; minimum, 45.45%; maximum, 82.56%). These values were not statistically significant (P ¼ .260). Step 3 After 1,000 cycles, we found no statistically significant differences in torsional stiffness between the 2 groups. Whereas group 1 specimens reached 98.68% of their initial stiffness (SD, 7.62%; minimum, 86.36%; maximum, 112.67%), the specimens in group 2 gained a mean value of 104.48% of their initial stiffness (SD, 8.10%; minimum, 92.28%; maximum, 120.68%). These findings were not statistically relevant (P ¼ .110) (Fig. 3).
Axial stiffness Pretesting/step 1 Before osteosynthesis, all specimens were potted and tested for axial and torsional stiffness. Of the 11 pairs, 9 showed no statistically significant intraindividual differences in axial loading. These specimens were used for further testing. Before osteotomy and osteosynthesis, the mean axial stiffness was 214.94 N/mm for group 1 (SD, 72.50 N/mm; minimum, 126.54 N/mm; maximum, 321.70 N/mm) and 209.17 N/mm for group 2 (SD, 85.97 N/mm; minimum, 129.00 N/mm; maximum, 368.50 N/mm). These findings were not statistically significant (P ¼ .674).
Retrograde proximal humerus osteosynthesis
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Figure 3 Box plots for distribution of differences in torsional loading for both groups after pretesting/step 1, step 2, and step 3 (horizontal lines display median and first and third quartiles and vertical lines indicate maximum and minimum values).
Step 2 After 4 cycles of loading, locking plateetreated specimens reached 97.51% (SD, 46.23%; minimum, 45.50%; maximum, 178.20%) of their initial stiffness. Humeri treated with retrograde nails gained 108.79% (SD, 45.21%; minimum, 44.08%; maximum, 167.84%) of their initial stiffness on axial loading. These values were not statistically significant (P ¼ .263).
Step 3 After 1,000 cycles, specimens stabilized with locking plates gained 116.33% (SD, 16.60%; minimum, 92.67%; maximum, 140.27%). In group 2 humeri reached 125.16% of their initial stiffness for axial loading (SD, 17.57%; minimum, 108.83%; maximum, 154.20%). These results were not statistically significant (P ¼ .401) (Fig. 4).
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Figure 4 Box plots for distribution of differences in axial loading for both groups after pretesting/step 1, step 2, and step 3 (horizontal lines display median and first and third quartiles and vertical lines indicate maximum and minimum values).
Discussion Because conventional plates showed insufficient biomechanical properties in the treatment of proximal humeral fractures,1,11 locking plates were introduced and have become popular during the last decade. A number of biomechanical evaluations confirmed that these locking plates are superior to conventional plates. However, the
biomechanical evaluation findings for locked internal-load carriers are not consistent. Edwards et al5 found superior biomechanical characteristics for locking plates in bending and torsion, and Sanders et al20 found higher stiffness on valgus loading with locking plates when compared with internal-load carriers. Other groups found no significant differences between locking plates and angular-stable nails for axial load and torsion11 or superior biomechanical
Retrograde proximal humerus osteosynthesis properties for bending and torsional loading using an internal-load carrier.7 In clinical situations, locking plates have been shown to be excellent devices with which to manage proximal humeral fractures.4,23,28 The findings of our study did not show differences in axial or torsional loading stiffness in our specimens between Philos plates and Retron nails. Both implants fulfilled the requirements of stable biomechanical applications. Because our institution uses locking plates as the standard implant to treat proximal 2-part fractures of the humerus with metaphyseal comminution, and because their biomechanical characteristics have been evaluated in numerous studies,5,7,11,22,27 angular-stable plates were chosen as the reference implant for our study. Even though clinical results of locked plating are often excellent or good, some implant-related complications have been shown over the years. In a systematic review of the literature, Sproul et al23 showed that varus malunion was the most common complication. One reason for this phenomenon is insufficient support of the calcar humeri. Gardner et al8 found that the presence of medial support had a significant influence on secondary loss of reduction. These findings were validated by Kuchle et al.12 The Retron nail is a new device that has been developed to solve the problem of insufficient medial support. Similar to dynamic hip screws, the nail is implanted in an ascending way close to the calcar humeri, with anchoring in the subchondral bone of the humeral head. It is implanted from the proximal part of the diaphysis. This technique is thought to spare the vascularization of the humeral head and avoid damage to the subacromial space. To date, there are no clinical or biomechanical data about this new nail. With our biomechanical testing, Retron nail specimens gained 108% of the initial stiffness of the unbroken humeri for axial loading after implantation and gap osteotomy. This shows that the design of the new implant fulfills the requirements of biomechanically stable osteosynthesis on axial loading. Foruria et al6 and Roderer et al19 proved in their biomechanical studies on cadaveric proximal humeri that locking plates possess superior biomechanical properties under rotational load compared with locked intramedullary nails. Comparing their results with those from a former biomechanical study from our research laboratory, we expected the locking plate to be superior for torsional loading.11 However, we could not prove this assumption in this study because no significant differences could be detected. Both implants were biomechanically stable. Our results propose that the configuration of the angular-stable locking bolts in the retrograde nail provides adequate mechanical strength for torsional loading. We chose a gap model for simulating a medial comminuted 2-part fracture. Gap models have been widely used for this purpose.5,7,11 We chose a 2-part fracture for our model because it is easier to reproduce than 3- or 4-part fractures. Furthermore, 2-part fractures are seen most frequently in patients with proximal humeral fractures. We
623 loaded our implants with up to 120 N of force for axial loading and up to 2.0 Nm for torsional loading. These measures have been previously established in our laboratory,11 and comparable loads have been applied by other groups.20 Although it has been stated that maximum load of the proximal humerus is up to 0.9 times body weight, we find our loadings to be adequate because we simulated the situation after osteosynthesis. The amount of torque expected in daily activities is approximately 2 Nm.27 Assessing load to failure was not an objective of this study. The specimens were homogeneous. This was confirmed by evaluation of intact humeri. We are aware that intraindividual differences exist. Marti et al15 showed a high correlation in bone quality between pairs. To minimize these effects, stiffness was measured as a percentage of the initial stiffness. Our analysis aimed to assess the primary stability of 2 implants on axial and torsional loading. Primary seating effects could be excluded by subsequent measures. The fracture model and experimental setup were comparable to those used in other studies.5,7,11,27 We are aware that our study has limitations. The study focuses purely on a specific set of biomechanical outcome measurements. Our simple loading patterns do not match the complex multidirectional forces that are present in reality. A weakness of our study is that we cannot comment on biological aspects. We cannot say whether the Retron nail is superior to the Philos plate for soft-tissue management, avascular necrosis of the humeral head, or irritations of the axillary nerve. Furthermore, fracture alignment with the devices was not tested in our study. These questions should be addressed by future clinical studies.
Conclusion Our study suggests that retrograde nailing of unstable 2part fractures of the proximal humerus using a Retron nail provides sufficient stability for axial and torsional loading. There are no significant differences with the Philos plate.
Disclaimer The hardware for the Retron nail was donated by Tantum AG, and the hardware for the Philos plate was donated by Synthes GmbH. The authors, their immediate families, and any research foundations with which they are affiliated have not received any financial payments or other benefits from any commercial entity related to the subject of this article.
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References 1. Chudik S. Fixed-angle plate fixation in simulated fractures of the proximal humerus: a biomechanical study of a new device. J Shoulder Elbow Surg 2003;12:578-88. doi:10.1016/s1058-2746(03)00217-9 2. Clavert P, Adam P, Bevort A, Bonnomet F, Kempf JF. Pitfalls and complications with locking plate for proximal humerus fracture. J Shoulder Elbow Surg 2010;19:489-94. doi:10.1016/j.jse.2009. 09.005 3. Dietrich M, Meier C, Lattmann T, Zingg U, Gruninger P, Platz A. Complex fracture of the proximal humerus in the elderly. Locking plate osteosynthesis vs hemiarthroplasty [in German]. Chirurg 2008; 79:231-40. doi:10.1007/s00104-007-1436-z 4. Duralde XA, Leddy LR. The results of ORIF of displaced unstable proximal humeral fractures using a locking plate. J Shoulder Elbow Surg 2010;19:480-8. doi:10.1016/j.jse.2009.08.008 5. Edwards SL, Wilson NA, Zhang LQ, Flores S, Merk BR. Two-part surgical neck fractures of the proximal part of the humerus. A biomechanical evaluation of two fixation techniques. J Bone Joint Surg Am 2006;88:2258-64. doi:10.2106/jbjs.e.00757 6. Foruria AM, Carrascal MT, Revilla C, Munuera L, Sanchez-Sotelo J. Proximal humerus fracture rotational stability after fixation using a locking plate or a fixed-angle locked nail: the role of implant stiffness. Clin Biomech (Bristol, Avon) 2010;25:307-11. doi:10.1016/j. clinbiomech.2010.01.009 7. F€ uchtmeier B, May R, Hente R, Maghsudi M, V€olk M, Hammer J, et al. Proximal humerus fractures: a comparative biomechanical analysis of intra and extramedullary implants. Arch Orthop Trauma Surg 2007;127:441-7. doi:10.1007/s00402-007-0319-6 8. Gardner MJ, Weil Y, Barker JU, Kelly BT, Helfet DL, Lorich DG. The importance of medial support in locked plating of proximal humerus fractures. J Orthop Trauma 2007;21:185-91. doi:10.1097/BOT. 0b013e3180333094 9. Helwig P, Bahrs C, Epple B, Oehm J, Eingartner C, Weise K. Does fixed-angle plate osteosynthesis solve the problems of a fractured proximal humerus? A prospective series of 87 patients. Acta Orthop 2009;80:92-6. 10. Hepp P, Theopold J, Voigt C, Engel T, Josten C, Lill H. The surgical approach for locking plate osteosynthesis of displaced proximal humeral fractures influences the functional outcome. J Shoulder Elbow Surg 2008;17:21-8. doi:10.1016/j.jse.2007.03.029 11. Hessmann MH, Hansen WSM, Krummenauer F, Pol TF, Rommens M. Locked plate fixation and intramedullary nailing for proximal humerus fractures: a biomechanical evaluation. J Trauma 2005;58:1194-201. doi:10.1097/01.ta.0000170400.68994.ab 12. Kuchle R, Hofmann A, Hessmann M, Rommens PM. The cloverleaf plate for osteosynthesis of humeral head fractures. Definition of the current position [in German]. Unfallchirurg 2006;109:1017-24. doi:10. 1007/s00113-006-1159-5 13. Lill H, Hepp P, Rose T, Konig K, Josten C. The angle stable lockingproximal-humerus-plate (LPHP) for proximal humeral fractures using a small anterior-lateral-deltoid-splitting-approachdtechnique and first results [in German]. Zentralbl Chir 2004;129:43-8. doi:10.1055/s2004-44870
S.-O. Dietz et al. 14. Lippuner K, Popp AW, Schwab P, Gitlin M, Schaufler T, Senn C, et al. Fracture hospitalizations between years 2000 and 2007 in Switzerland: a trend analysis. Osteoporosis Int. Epub ahead of press 2011. doi:10. 1007/s00198-010-1487-8 15. Marti A, Fankhauser C, Frenk A, Cordey J, Gasser B. Biomechanical evaluation of the less invasive stabilization system for the internal fixation of distal femur fractures. J Orthop Trauma 2001;15:482-7. 16. Matziolis D, Kaeaeb M, Zandi SS, Perka C, Greiner S. Surgical treatment of two-part fractures of the proximal humerus: comparison of fixed-angle plate osteosynthesis and Zifko nails. Injury 2010;41: 1041-6. doi:10.1016/j.injury.2010.04.017 17. Olerud P, Ahrengart L, S€oderqvist A, Saving J, Tidermark J. Quality of life and functional outcome after a 2-part proximal humeral fracture: a prospective cohort study on 50 patients treated with a locking plate. J Shoulder Elbow Surg 2010;19:814-22. doi:10.1016/j.jse.2009.11.046 18. Roderer G, Erhardt J, Graf M, Kinzl L, Gebhard F. Clinical results for minimally invasive locked plating of proximal humerus fractures. J Orthop Trauma 2010;24:400-6. doi:10.1097/BOT.0b013e3181ccafb3 19. Roderer G, Gebhard F, Krischak G, Wilke HJ, Claes L. Biomechanical in vitro assessment of fixed angle plating using a new concept of locking for the treatment of osteoporotic proximal humerus fractures. Int Orthop 2010;35:535-41. doi:10.1007/s00264-010-1021-9 20. Sanders BS, Bullington AB, McGillivary GR, Hutton WC. Biomechanical evaluation of locked plating in proximal humeral fractures. J Shoulder Elbow Surg 2007;16:229-34. doi:10.1016/j.jse.2006.03.013 21. Saran N, Bergeron SG, Benoit B, Reindl R, Harvey EJ, Berry GK. Risk of axillary nerve injury during percutaneous proximal humerus locking plate insertion using an external aiming guide. Injury 2010;41: 1037-40. doi:10.1016/j.injury.2010.04.014 22. Siffri PC, Peindl RD, Coley ER, Norton J, Connor PM, Kellam JF. Biomechanical analysis of blade plate versus locking plate fixation for a proximal humerus fracture: comparison using cadaveric and synthetic humeri. J Orthop Trauma 2006;20:547-54. doi:10.1097/01. bot.0000244997.52751.58 23. Sproul RC, Iyengar JJ, Devcic Z, Feeley BT. A systematic review of locking plate fixation of proximal humerus fractures. Injury 2011;42: 408-13. doi:10.1016/j.injury.2010.11.058 24. Sturzenegger M, Fornaro E, Jakob RP. Results of surgical treatment of multifragmented fractures of the humeral head. Arch Orthop Trauma Surg 1982;100:249-59. 25. Sudkamp N, Bayer J, Hepp P, Voigt C, Oestern H, Kaab M, et al. Open reduction and internal fixation of proximal humeral fractures with use of the locking proximal humerus plate. results of a prospective, multicenter, observational study. J Bone Joint Surg Am 2009;91:13208. doi:10.2106/jbjs.h.00006 26. Tingart M, Bathis H, Bouillon B, Tiling T. The displaced proximal humeral fracture: is there evidence for therapeutic concepts? [in German]. Chirurg 2001;72:1284-91. 27. Wheeler DL, Colville MR. Biomechanical comparison of intramedullary and percutaneous pin fixation for proximal humeral fracture fixation. J Orthop Trauma 1997;11:363-7. 28. Yang H, Li Z, Zhou F, Wang D, Zhong B. A prospective clinical study of proximal humerus fractures treated with a locking proximal humerus plate. J Orthop Trauma 2011;25:11-7. doi:10.1097/BOT. 0b013e3181d2d04c
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Retrograde nailing versus locking plate osteosynthesis of proximal humeral fractures: a biomechanical study Sven-Oliver Dietz, MD*, Frank Hartmann, MD, Thomas Schwarz, Tobias E. Nowak, MD, Annalisa Enders, MD, Sebastian Kuhn, MD, Alexander Hofmann, MD, Pol Maria Rommens, MD Center for Musculoskeletal Surgery, Department of Trauma Surgery, University Medical Center, Johannes Gutenberg University, Mainz, Germany Background: In locking plate osteosynthesis of proximal humeral fractures, secondary varus malalignment is a specific complication. Retron nails (Tantum AG, Neumunster, Germany), among others, have been designed to improve medial support of the calcar humeri. The aim of our biomechanical study was to examine whether Retron nails provide increased stiffness for axial loads and adequate stiffness for torsional loads when compared with Philos plates (Synthes AG, Umkirch, Germany). Materials and methods: Twenty-two fresh-frozen paired humeri were collected. After potting the specimens, intact bones were exposed to sinusoidal axial (10-120 N) and torsional (2.5 Nm) loading for 8 cycles to calculate the initial stiffness and exclude pairs with differences. Afterward, an unstable proximal humeral fracture (AO 11-A3) was created by means of an oscillating saw, and the respective osteosynthesis devices were implanted. After another 4 cycles, initial changes in stiffness were measured. Subsequently, all specimens were tested for 1,000 cycles of loading before final stiffness was assessed. Results: We found no statistically significant differences between Retron and Philos specimens after 4 or 1,000 cycles of loading. Conclusion: Our study suggests that retrograde nailing provides sufficient stability for axial and torsional loading in 2-part fractures of proximal humeri. Level of evidence: Basic Science Study, Biomechanical Study. Ó 2012 Journal of Shoulder and Elbow Surgery Board of Trustees. Keywords: Proximal humeral fracture; locking plates; intramedullary implants; biomechanics of proximal humerus; Retron; retrograde nailing
Fractures of the proximal humerus are, along with hip, spine, and distal forearm fractures, referred to as major osteoporotic fractures. A recent trend analysis from *Reprint requests: Sven-Oliver Dietz, MD, Center for Musculoskeletal Surgery, Department of Trauma Surgery, University Medical Center, Johannes Gutenberg University, Langenbeckstrasse 1, 55101 Mainz, Germany. E-mail address: [email protected] (S.-O. Dietz).
Switzerland showed that, between 2000 and 2007, the absolute number of proximal humeral fractures increased by 22.8% in women and 35.5% in men.14 Because locking plates were shown to have superior properties to conventional plates in biomechanical investigations,1,5,7,11,22 these implants became widely used in clinical practice. Although numerous articles have provided excellent accounts of good clinical results,4,16-18,28 multiple implant-specific obstacles
1058-2746/$ - see front matter Ó 2012 Journal of Shoulder and Elbow Surgery Board of Trustees. doi:10.1016/j.jse.2011.04.013
Retrograde proximal humerus osteosynthesis have become apparent.2,3,8,9,13,21,23,25 In addition to primary and secondary screw perforation, secondary loss of reposition and varus deformity were the most common complications.8,23 Furthermore, surgical approaches for applying a locking plate compromise the soft tissue and blood supply to the humeral head. Different surgical approaches, including deltopectoral and anterior delta split, cannot eliminate these effects.10,24,26 Minimally invasive devices have been constructed to address these problems. Initial clinical studies showed good results, but implantrelated complications were found in 17% of cases and avascular head necrosis in 5.5%.18 Furthermore, only a limited number of screws can be applied percutaneously because of the risks of axillary nerve injury.21 Because inadequate medial support of the calcar humeri has a significant influence on secondary varus deformities, we investigated the biomechanical properties of a new locking device. Retron nails (Tantum AG, Neumunster, Germany) are implanted through small incisions and have to be inserted near the calcar humeri. To our knowledge, the clinical and biomechanical performances of this new implant have never been investigated. The purpose of our study was to assess the biomechanical properties of this new nail in unstable surgical neck fractures. As a reference implant, the Philos plate (Synthes AG, Umkirch, Germany) was used. The hypothesis was that the nails would provide more stable fixation for axial loads than locking plates and that they would adequately maintain reductions for torsional loads. Ultimate load to failure and fracture alignment with the devices were not subjects of our study.
Materials and methods Specimens Eleven pairs of fresh-frozen human proximal humeri (22 humeri) were dissected from all soft tissues and shortened to a length of 20 cm from the proximal aspect. Specimens with clinical or radiologic abnormalities were eliminated. Two groups were formed based on paired comparison. In group 1, osteosynthesis was performed with the angular-stable plate, whereas in group 2, the retrograde nail was used. Each pair was randomized for the specific date of the experiment. In each pair, the specimens were randomly assigned to the relevant group. An unstable 2-part fracture of the proximal humerus (AO 11-A3 injury) was simulated by means of an oscillating pneumatic saw. A wedge osteotomy with a 10-mm defect at the humeral neck was created. The base of the resected triangle was at the medial side (Fig. 1). To ease osteosynthesis, the medial osteotomy was completed after application of the implant. Each specimen was potted proximally in polymethyl methacrylate (PMMA) cement. Contact between the PMMA and implant was avoided. To prevent loosening between the articular surface of the proximal humerus and the PMMA, 2 additional screws were inserted into the humeral head. Distally, all specimens were fixed by means of a metal rod (Fig. 1).
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Implants A Philos plate was used as an example of a locking plate, and a Retron nail was used as an example of a retrograde nail. Both implants are made of titanium. The retrograde nail was placed in the medial third of a humeral head through the lateral aspect of the humeral shaft. The nail is cannulated, has a diameter of 8 mm and a length of 76 or 86 mm, and can be laterally lengthened by a diaphysis anchor. A selftapping thread at the proximal end of the nail fixes the nail in the subchondral bone of the humeral head. All nails were locked by three 4-mm self-tapping angular-stable locking bolts in the humeral head and two self-tapping angular-stable locking bolts in the proximal humeral shaft. An aiming device was used to insert the locking bolts. All nails were connected to a diaphysis anchor of a particular length. The latter was fixed by a 4-mm self-tapping screw in the humeral shaft. A locking plate of 11.4 mm in length was placed at the lateral aspect of the proximal humerus by use of an aiming device to ensure correct orientation. The plate was fixed in the humeral head by six 3.5-mm self-tapping angular-stable screws. To provide maximum support of the humeral head, these screws are converting in the anteroposterior plane and diverting in the sagittal plane. An additional three 3.5-mm locking screws secured the shaft.
Biomechanical testing All specimens were mounted on a pneumatic testing machine (SyncoTec, Clausthal-Zellerfeld, Germany) (Fig. 2), loaded in the axial direction, and torqued in 3 subsequent steps. All specimens received the same loading profile. All intact humeri were pretested (step 1) to rule out intraindividual differences. These measures were used as initial values. Afterward, the specimens were tested after osteotomy and insertion of the respective implant for 4 cycles (step 2). Finally, all specimens were tested for 1,000 cycles (step 3).
Pretesting/step 1 After potting, all specimens were subjected to 8 cycles of axial and torque loading. First, the specimen was subjected to 10 N of axial loading for 15 seconds while torsion was applied in a sinusoidal manner (0.5-2 Nm). Afterward, the torsion load was constant at 0.5 Nm whereas the axial load was raised in a sinusoidal manner to 120 N. During loading, translation and angles along the testing axes were recorded. The initial stiffness of the specimens was calculated from these values. Humeral pairs with differences in initial stiffness of greater than 20% were excluded.
Step 2 After implantation of the respective osteosynthesis devices, all specimens were tested in an identical manner to step 1 for 4 cycles. The loss of stiffness after osteosynthesis was calculated from these values.
Step 3 All specimens were then subjected to 1,000 cycles of axial and torque loading. The loading parameters remained unchanged.
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Figure 1 (A) Radiographs of paired cadaveric humeri after osteotomy and implantation of Retron nail (right) and Philos plate (left) in anteroposterior and lateral projections. (B) Corresponding specimens (Philos plate [bottom] and Retron nail [top]).
no statistically significant intraindividual differences in torsion and so were used for further testing. The mean value of torsional stiffness was 0.73 Nm/ (SD, 0.10 Nm/ ; minimum, 0.56 Nm/ ; maximum, 0.91 Nm/ ) for group 1 and 0.72 Nm/ (SD, 0.12 Nm/ ; minimum, 0.48 Nm/ ; maximum, 0.83 Nm/ ) for group 2 (P ¼ .767).
Figure 2 Retron specimen after embedding, loaded into pneumatic testing machine.
Failure was defined as an irreversible closure of the osteotomy gap, peri-implant fracture, or interruption of the linear loading curve in association with a sudden deviation in the forcedeformation diagram.
Statistics Interindividual comparisons were based on 2-sample Wilcoxon tests. Torsion and axial stiffness were investigated. The values of the implants were computed, and the distribution was described in medians and percentiles. P < .05 was regarded as statistically significant. SPSS (release 15.0 for Windows; SPSS, Chicago, IL, USA) was used for numeric and graphical analyses.
Results Torsional stiffness Pretesting/step 1 Before osteosynthesis, all specimens were potted and tested for axial and torsional stiffness. Of the 11 pairs, 9 showed
Step 2 After pretesting, the specimens in group 1 gained 68.17% of their initial stiffness (SD, 8.61%; minimum, 57.90%; maximum, 84.67%). Humeral specimens with retrograde humeral nails reached 62.94% of their initial stiffness (SD, 11.65%; minimum, 45.45%; maximum, 82.56%). These values were not statistically significant (P ¼ .260). Step 3 After 1,000 cycles, we found no statistically significant differences in torsional stiffness between the 2 groups. Whereas group 1 specimens reached 98.68% of their initial stiffness (SD, 7.62%; minimum, 86.36%; maximum, 112.67%), the specimens in group 2 gained a mean value of 104.48% of their initial stiffness (SD, 8.10%; minimum, 92.28%; maximum, 120.68%). These findings were not statistically relevant (P ¼ .110) (Fig. 3).
Axial stiffness Pretesting/step 1 Before osteosynthesis, all specimens were potted and tested for axial and torsional stiffness. Of the 11 pairs, 9 showed no statistically significant intraindividual differences in axial loading. These specimens were used for further testing. Before osteotomy and osteosynthesis, the mean axial stiffness was 214.94 N/mm for group 1 (SD, 72.50 N/mm; minimum, 126.54 N/mm; maximum, 321.70 N/mm) and 209.17 N/mm for group 2 (SD, 85.97 N/mm; minimum, 129.00 N/mm; maximum, 368.50 N/mm). These findings were not statistically significant (P ¼ .674).
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Figure 3 Box plots for distribution of differences in torsional loading for both groups after pretesting/step 1, step 2, and step 3 (horizontal lines display median and first and third quartiles and vertical lines indicate maximum and minimum values).
Step 2 After 4 cycles of loading, locking plateetreated specimens reached 97.51% (SD, 46.23%; minimum, 45.50%; maximum, 178.20%) of their initial stiffness. Humeri treated with retrograde nails gained 108.79% (SD, 45.21%; minimum, 44.08%; maximum, 167.84%) of their initial stiffness on axial loading. These values were not statistically significant (P ¼ .263).
Step 3 After 1,000 cycles, specimens stabilized with locking plates gained 116.33% (SD, 16.60%; minimum, 92.67%; maximum, 140.27%). In group 2 humeri reached 125.16% of their initial stiffness for axial loading (SD, 17.57%; minimum, 108.83%; maximum, 154.20%). These results were not statistically significant (P ¼ .401) (Fig. 4).
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Figure 4 Box plots for distribution of differences in axial loading for both groups after pretesting/step 1, step 2, and step 3 (horizontal lines display median and first and third quartiles and vertical lines indicate maximum and minimum values).
Discussion Because conventional plates showed insufficient biomechanical properties in the treatment of proximal humeral fractures,1,11 locking plates were introduced and have become popular during the last decade. A number of biomechanical evaluations confirmed that these locking plates are superior to conventional plates. However, the
biomechanical evaluation findings for locked internal-load carriers are not consistent. Edwards et al5 found superior biomechanical characteristics for locking plates in bending and torsion, and Sanders et al20 found higher stiffness on valgus loading with locking plates when compared with internal-load carriers. Other groups found no significant differences between locking plates and angular-stable nails for axial load and torsion11 or superior biomechanical
Retrograde proximal humerus osteosynthesis properties for bending and torsional loading using an internal-load carrier.7 In clinical situations, locking plates have been shown to be excellent devices with which to manage proximal humeral fractures.4,23,28 The findings of our study did not show differences in axial or torsional loading stiffness in our specimens between Philos plates and Retron nails. Both implants fulfilled the requirements of stable biomechanical applications. Because our institution uses locking plates as the standard implant to treat proximal 2-part fractures of the humerus with metaphyseal comminution, and because their biomechanical characteristics have been evaluated in numerous studies,5,7,11,22,27 angular-stable plates were chosen as the reference implant for our study. Even though clinical results of locked plating are often excellent or good, some implant-related complications have been shown over the years. In a systematic review of the literature, Sproul et al23 showed that varus malunion was the most common complication. One reason for this phenomenon is insufficient support of the calcar humeri. Gardner et al8 found that the presence of medial support had a significant influence on secondary loss of reduction. These findings were validated by Kuchle et al.12 The Retron nail is a new device that has been developed to solve the problem of insufficient medial support. Similar to dynamic hip screws, the nail is implanted in an ascending way close to the calcar humeri, with anchoring in the subchondral bone of the humeral head. It is implanted from the proximal part of the diaphysis. This technique is thought to spare the vascularization of the humeral head and avoid damage to the subacromial space. To date, there are no clinical or biomechanical data about this new nail. With our biomechanical testing, Retron nail specimens gained 108% of the initial stiffness of the unbroken humeri for axial loading after implantation and gap osteotomy. This shows that the design of the new implant fulfills the requirements of biomechanically stable osteosynthesis on axial loading. Foruria et al6 and Roderer et al19 proved in their biomechanical studies on cadaveric proximal humeri that locking plates possess superior biomechanical properties under rotational load compared with locked intramedullary nails. Comparing their results with those from a former biomechanical study from our research laboratory, we expected the locking plate to be superior for torsional loading.11 However, we could not prove this assumption in this study because no significant differences could be detected. Both implants were biomechanically stable. Our results propose that the configuration of the angular-stable locking bolts in the retrograde nail provides adequate mechanical strength for torsional loading. We chose a gap model for simulating a medial comminuted 2-part fracture. Gap models have been widely used for this purpose.5,7,11 We chose a 2-part fracture for our model because it is easier to reproduce than 3- or 4-part fractures. Furthermore, 2-part fractures are seen most frequently in patients with proximal humeral fractures. We
623 loaded our implants with up to 120 N of force for axial loading and up to 2.0 Nm for torsional loading. These measures have been previously established in our laboratory,11 and comparable loads have been applied by other groups.20 Although it has been stated that maximum load of the proximal humerus is up to 0.9 times body weight, we find our loadings to be adequate because we simulated the situation after osteosynthesis. The amount of torque expected in daily activities is approximately 2 Nm.27 Assessing load to failure was not an objective of this study. The specimens were homogeneous. This was confirmed by evaluation of intact humeri. We are aware that intraindividual differences exist. Marti et al15 showed a high correlation in bone quality between pairs. To minimize these effects, stiffness was measured as a percentage of the initial stiffness. Our analysis aimed to assess the primary stability of 2 implants on axial and torsional loading. Primary seating effects could be excluded by subsequent measures. The fracture model and experimental setup were comparable to those used in other studies.5,7,11,27 We are aware that our study has limitations. The study focuses purely on a specific set of biomechanical outcome measurements. Our simple loading patterns do not match the complex multidirectional forces that are present in reality. A weakness of our study is that we cannot comment on biological aspects. We cannot say whether the Retron nail is superior to the Philos plate for soft-tissue management, avascular necrosis of the humeral head, or irritations of the axillary nerve. Furthermore, fracture alignment with the devices was not tested in our study. These questions should be addressed by future clinical studies.
Conclusion Our study suggests that retrograde nailing of unstable 2part fractures of the proximal humerus using a Retron nail provides sufficient stability for axial and torsional loading. There are no significant differences with the Philos plate.
Disclaimer The hardware for the Retron nail was donated by Tantum AG, and the hardware for the Philos plate was donated by Synthes GmbH. The authors, their immediate families, and any research foundations with which they are affiliated have not received any financial payments or other benefits from any commercial entity related to the subject of this article.
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