ORIGINAL ARTICLE

Effect of Calcar Screw Use in Surgical Neck Fractures of the Proximal Humerus With Unstable Medial Support: A Biomechanical Study Lu Bai, MD,* Zhongguo Fu, MD,† Shuai An, MD,† Peixun Zhang, MD,† Dianying Zhang, MD,† and Baoguo Jiang, MD†

Objectives: To evaluate the effect of calcar screw use in proximal humeral fractures with unstable medial support treated with locked plates.

Methods: Standard osteotomies were performed in 36 cadaveric humeri to create a surgical neck fracture proximal humeral model. For static testing, 12 pairs of humeri were divided into 4 groups: normal alignment and varus deformity groups with and without 5-mm medial deficiencies. Calcar screw function was measured in each group by axial, shear, and torsion stiffness tests. Another 6 pairs of humeri with 5-mm medial deficiencies were subjected to cyclic loading tests in the normal alignment model with and without calcar screw application. Results: Calcar screws improved rotational stability in the normal alignment (P = 0.007) and varus (P = 0.002) groups. Calcar screws improved static and cyclic axial (P = 0.004) and shear (P = 0.017) stability in the normal alignment group with medial deficiency. In specimens with normal alignment and intact medial cortex, calcar screws provided no advantage in axial (P = 0.535) or shear (P = 0.537) stiffness. Calcar screws did not provide sufficient axial (P = 0.782) or shear (P = 0.772) stability to avoid reduction loss in humeri with varus malreduction. Conclusions: In humeri with normal alignment, calcar screws can provide additional stability even when a medial deficiency exists. The use of calcar screws in humeri with varus deformity showed no biomechanical superiority. Key Words: proximal humeral fracture, calcar screw, biomechanical test, alignment

INTRODUCTION Proximal humeral fracture is a common shoulder injury, particularly in elderly patients; it accounts for about 15% of all fracture cases in patients aged older than 50 years.1,2 Internal fixation with locking plates is used widely in clinical practice to treat this injury.3 The locking structure and fixation pattern of these plates provide a biomechanical advantage, especially for osteoporotic bone.4 However, despite the biomechanical superiority and clinical availability of this treatment approach, several multicenter clinical trials have documented unacceptable rates of reduction loss and varus deformity in proximal humeral fractures treated with locking plates.3,5–7 A common pattern of failure is the increased varus of the humeral head, leading to secondary screw penetration or plate breakage.8 The absence of medial support is a potential biomechanical risk factor for the reduction loss and varus displacement. The use of calcar screws in a locked plate configuration was introduced to resolve the issues associated with medial calcar deficiency.9 Although existing clinical data have demonstrated that calcar screws maintain alignment in proximal humeral fractures with unstable medial support, reduction loss and varus malunion continue to occur.9–11 The aim of this biomechanical study was to explore the effectiveness of calcar screws in avoiding reduction loss and varus deformity in proximal humeral fractures with unstable medial support.

(J Orthop Trauma 2014;28:452–457) Accepted for publication December 17, 2013. From the *Department of Sports Medicine, Shenzhen Hospital, Peking University, Shenzhen, China; and †Department of Orthopedics and Traumatology, Peoples’ Hospital, Traffic Medicine Center, Peking University, Beijing, China. Supported by a grant from the Key Projects in the National Science and Technology Pillar Program in the Eleventh Five-Year Plan Period for Treatment of Difficult Fractures (No. 2007BAI04B06) and by the New Century Excellent Talents Support Project of the Chinese Ministry of Education (BMU20110270). The authors report no conflict of interest. Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions this article on the journal’s Web site (www.jorthotrauma.com). Reprints: Baoguo Jiang, MD, Department of Orthopedics and Traumatology, Peoples’ Hospital, Traffic Medicine Center, Peking University, 11 South XiZhiMen Avenue, Beijing 100044, China (e-mail: jiangbaoguo@vip. sina.com). Copyright © 2013 by Lippincott Williams & Wilkins

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MATERIALS AND METHODS Specimen Preparation and Experimental Groups Eighteen embalmed cadavers (10 women, 8 men) were obtained from a cadaver bank (Department of Anatomy, Peking University, Beijing, China). The average age at the time of death was 66.2 (range, 37–81) years. All humeri were examined with anteroposterior and lateral radiographs. Only specimens with normal macroscopic and radiographic appearance were used. The soft tissues were stripped off, and the specimens were kept frozen at 2208C until the day before preparation, at which time they were thawed overnight. Although the embalming process has a minimal impact on bone density and mechanical properties,12 we measured bone mineral density (BMD) in every specimen for quality J Orthop Trauma  Volume 28, Number 8, August 2014

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Effect of Calcar Screw

FIGURE 1. Experimental protocol for static testing. Dotted lines means “calcar screw added”.

control and to avoid systematic deviation.13 Before testing, a quantitative computed tomographic (TSX-101A; Toshiba, Tokyo, Japan) image of each specimen was obtained, and the BMD was assessed using specialized software (Mindways, QCTPro 2.0; GE, Waukesha, WI). The mean BMD of all specimens was 0.141 6 0.013 (range, 0.128–0.164) g/cm3. BMD did not differ significantly between the left and right sides of each donor (paired t test, P = 0.882). Figure 1 illustrates the experimental protocol of this study. Six specimen pairs, each were assigned to the normal alignment (anatomic reduction of humeral head–shaft angle) and varus deformity (20-degree varus of humeral head) groups. Within each group, specimens were randomized into

2 subgroups: medial cortex contact (CC) and medial cortex removal (CR). A paired t test was used to ensure that BMD did not differ between groups (P = 0.763). In the CR subgroups, the inferomedial region of each proximal humerus was removed by a T-saw to create a 5-mm deficiency. The specimens belonging to all 4 subgroups underwent baseline static axial compression, shear, and torsion tests to obtain stiffness values. During static testing, each specimen was kept within the linear elastic region to prevent permanent damage. No visual evidence of damage was noted, which was confirmed by an average linearity coefficient (R2) . 0.99. Calcar screws were then applied in each static testing group to evaluate their effects on the mechanical properties of the construct. Next, cyclic axial compression testing was performed to simulate the reduction loss in clinical practice.8,10,11,14 For these tests, 6 additional pairs of humeri with normal alignment and medial cortex deficiency were divided into 2 subgroups according to the presence or absence of calcar screw fixation (Fig. 2).

Fracture Model

FIGURE 2. Experimental protocol for dynamic cycling tests. Ó 2013 Lippincott Williams & Wilkins

The distal ends of all humeri were cut, so that the specimens extended 200 mm distally from the most proximal portion of the humeral head. Osteotomies were performed on an X-Y table. A T-saw was used to cut around the cortical bone at the site of the surgical neck. A medial deficiency model was prepared according to the criteria of Sanders et al15 and Voigt et al16; a 5-mm transverse closed wedge osteotomy was performed with a T-saw to create a bone deficiency in the medial calcar region while preserving the dorsal third of the greater tuberosity cortex, which served to brace the locking plate. www.jorthotrauma.com |

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A varus malreduction model was created by inducing a 20-degree collapse of the humeral head. Each specimen was fixed with a Philos proximal humeral internal locking system (Synthes, Oberdorf, Switzerland). According to the recommendation of the AO Foundation, the plates were positioned at least 10 mm distal to the apex of the greater tuberosity and 5 mm lateral of the bicipital groove. For calcar screw fixation, 2 locking screws were applied to provide medial support. Screws were as long as possible to obtain purchase in the subchondral zone. Operations were performed under an imaging intensifier.

Mechanical Testing All biomechanical tests were performed using an Instron 8874 machine (Canton, MA) with the following load cell characteristics: 2-kN capacity, 0.1-N resolution, and 61% accuracy in axial mode; and 10-N$m capacity, 0.0001-N$m resolution, and 61% accuracy in torsional mode. After testing without calcar screws, the screws were applied in subgroups, and axial and shear tests were performed again. Torsion testing was performed only in the CR group because few reports have documented rotational reduction loss in proximal humeral fractures, and rotational displacement complications are rare in stable medial fractures of this type. Each intact humerus was oriented vertically in the coronal and sagittal planes. The distal end of each specimen was mounted rigidly in an industrial vise and reinforced with bone cement. For the axial and shear tests, bone cement was used to shape the load cell to match the shape of the proximal humerus. Before the shear test, a base with a 20degree angle was added to enable the application of a shear force. For static axial and shear stiffness tests, a vertical load was applied at the apex of the humeral head using displacement control (maximum deflection, 5 mm; load rate, 5 mm/min; preload, 50 N; maximum load, 500 N). Not all specimens achieved the 5-mm limit of deflection, which was selected because the Instron machine measures not only the fracture line displacement but also the bending deformation of the long bone. Thus, the 5-mm limit includes both of these variables. Static torsion tests (maximum deflection, 5 degrees; load rate, 5 degrees per minute) were performed in the clockwise and counterclockwise directions. Cyclic tests (minimum force, 50 N; maximum force, 300 N; maximum deflection, 5 mm) consisted of 5000 cycles. Maximum and minimum displacements were recorded during every load cycle. Displacements in 1, 10, 200, 400, 600, 800, 1000, 2000, 3000, 4000, and 5000 cycles were compared. In static tests, the slope of the load deflection curve was used to compute the baseline axial stiffness. Each test was repeated 3 times, and average stiffness was calculated. Although the loading values used in this study were below the physiological loads produced by many activities of daily life, they were chosen to prevent permanent damage to the humeri during testing and to enable comparison with tests of specimens with calcar screws.

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Statistical Analysis Statistical analyses were performed with SPSS software (version 12.0; SPSS Inc, Chicago, IL). The data are summarized as medians (ranges). The nonparametric Mann– Whitney–Wilcoxon test with no correction for multiple comparisons was applied to detect differences between subgroups with and without calcar screw fixation. Correlations of BMD with maximum load and age were analyzed with bivariate Pearson tests. The level of statistical significance was defined as P , 0.05.

RESULTS Static Testing The results of axial compression and shear tests are shown in Table 1. Axial and shear stiffness with and without calcar screws differed significantly in the CR subgroup of the normal alignment group (P = 0.004 and 0.017, respectively) but not in the CC subgroup. Thus, calcar screw use had no significant effect in specimens with a normal head–shaft angle and intact medial cortex, but played a significant role in fixation stability in specimens with normal humeral head alignment and medial deficiency. In the varus group, calcar screw use had no significant effect on axial or shear stiffness in the CC or CR subgroup. Thus, the medial supporting effect of the calcar screws was weakened in specimens with humeral heads in a nonanatomic position. In humeri with normal alignment, stiffness values were 0.788 6 0.23 N$m per degree in specimens with calcar screws and 0.48 6 0.19 N$m per degree in specimens without calcar screws (P = 0.007). In humeri with 20degree varus deformities, stiffness values were 0.730 6 0.23 N$m per degree in specimens with calcar screws and 0.50 6 0.13 N$m per degree in specimens without calcar screws (P = 0.002).

Dynamic Cycling Test Humeri with normal alignment and medial CR were subjected to dynamic cycling tests with or without calcar screws. No failure, including subsidence, cut out, or plate breakage, was observed. No significant displacement occurred in either group during the first 2000 cycles. At 3000, 4000, and 5000 cycles, the calcar screws provided a significantly stiffer bone implant structure in comparison with specimens lacking screws (Table 2). Figure 3 (Supplemental Digital Content 1, http://links.lww.com/BOT/A164) shows average maximum and minimum displacement values according to load cycle. In the 6 pairs of humeri subjected to cyclic testing, BMD was correlated significantly with mean displacement at 5000 cycles (r = 20.594, P = 0.042; see Figure 4, Supplemental Digital Content 2, http://links.lww.com/BOT/A165).

DISCUSSION In the past 20 years, proximal humeral fractures have been fixed with blade plates, standard plates, and locking plates. New implant designs have increased the success of clinical management. The advantages of locking plates over nonlocking plate types in the treatment of proximal humeral fractures have been proven clinically17 and biomechanically,18–21 but recent clinical Ó 2013 Lippincott Williams & Wilkins

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TABLE 1. Results of Axial Compression and Shear (Load Displacement) Tests Group Normal alignment CC CR Varus deformity CC CR

Calcar Screw

N

Axial Stiffness (N/mm); Median (Range)

No Yes No Yes

6 6 6 6

745.5 792.0 278.5 183.5

(696–889) (715–912) (212–346) (129–231)

No Yes No Yes

6 6 6 6

160.5 200.5 143.5 149.0

(143–177) (132–215) (119–158) (126–168)

P* 0.635 0.004†

0.535 0.782

P*

Shear (N/mm); Median (Range) 555.5 601.0 275.0 191.0

(487–688) (503–705) (189–304) (145–251)

128.5 154.0 109.0 113.0

(111–141) (100–187) (88–133) (97–141)

0.821 0.017†

0.537 0.937

*Mann–Whitney–Wilcoxon test, with versus without calcar screws. †P , 0.05.

secondary varus.3 An in vivo biomechanical study24 found that the force applied to the humeral head by the glenoid at rest was about 30% of body weight immediately after shoulder arthroplasty and increased to the approximate level of body weight after 7 weeks of rehabilitation. This force was about 130% of body weight during daily activity. Also, similar to our study, a recent biomechanical study showed significantly less axial and shear force resistance in varus humeral heads than in those with normal alignment.25 These findings indicate that, although the shoulder is not a weightbearing joint, the axial and shear forces applied to the proximal humerus by deltoid and rotator cuff activity may lead to reduction loss and varus deformity. Lack of medial support and varus malreduction of the humeral head were found to be risk factors for increased reduction loss.3 Clinically, Osterhoff et al10 found that reduction loss occurred despite the consistent use of calcar screws in all cases of medial deficiency. Increasing varus deformity of the humeral head was especially common in cases of initial varus

trials have shown that reduction loss and increased varus deformity are common complications in fractures with medial cortex deficiency or varus malreduction treated with locking plates.3,22 Südkamp et al11 reported a 40% incidence of such complications. Agudelo et al5 found that humeral heads fixed in a varus position were prone to reduction loss or varus collapse. Gardner et al9 reported that the placement of a superiorly directed oblique locked screw in the inferomedial region of the proximal humeral fragment may achieve a more stable medial column support and enable better maintenance of reduction. Nho et al23 also emphasized the importance of such screws, calling them “calcar screws.” In this way, 2 surgical techniques have been used in attempts to avoid reduction loss and varus deformity: (1) recovering the continuity of the medial cortex to provide cortical support for the humeral head, or (2) using calcar screws to provide angular stability, facilitating the maintenance of humeral head reduction. However, these techniques have not satisfactorily resolved the issues of reduction loss and

TABLE 2. Minimum and Maximum Displacement in Cycling Tests No. of Cycles 1 10 200 400 600 800 1000 2000 3000 4000 5000

Minimum Displacement (mm)* No Screw (n = 6) 0.00 0.13 0.24 0.30 0.34 0.24 0.41 0.50 0.64 0.78 0.69

Calcar Screw (n = 6)

(0.00–0.01) (0.01–0.38) (0.04–0.61) (0.05–0.64) (0.07–0.68) (0.08–0.70) (0.09–0.72) (0.07–0.82) (0.32–0.96) (0.20–1.08) (0.22–1.16)

0.00 0.08 0.03 0.06 0.09 0.10 0.12 0.17 0.19 0.22 0.24

(0.00–0.00) (0.01–0.15) (0.02–0.25) (0.01–0.31) (0.03–0.36) (0.04–0.39) (0.03–0.41) (0.07–0.50) (0.08–0.34) (0.09–0.52) (0.08–0.50)

Maximum Displacement (mm)* P† 0.818 0.485 0.067 0.038‡ 0.042‡ 0.180 0.106 0.063 0.032‡ 0.035‡ 0.036‡

No Screw (n = 6) 0.27 0.44 1.12 1.58 1.70 1.75 1.82 1.94 2.09 2.11 2.12

(0.02–0.34) (0.14–0.64) (0.37–2.07) (0.46–2.13) (0.48–2.16) (0.50–2.18) (0.50–2.18) (0.89–2.31) (1.08–2.50) (0.92–2.66) (1.38–2.79)

P†

Calcar Screw (n = 6) 0.37 0.30 0.73 0.84 0.88 0.91 0.93 1.01 1.41 1.13 1.16

(0.13–0.31) (0.06–0.40) (0.12–1.02) (0.13–1.13) (0.15–1.16) (0.18–1.17) (0.24–1.19) (0.55–1.20) (0.59–1.28) (0.58–1.31) (0.58–1.38)

0.310 0.394 0.394 0.132 0.167 0.093 0.077 0.02‡ 0.022‡ 0.025‡ 0.024‡

*Data are presented as median (range). †Mann–Whitney–Wilcoxon test, with versus without calcar screws. ‡P , 0.05.

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malreduction. To explore the biomechanical features of calcar screws in the contexts of varus and medial cortex deficiency, we thus designed the experiments reported here. Our study found that the role of calcar screws was not important in the CC subgroup. In clinical practice, good results may also be achieved with stable medial support in proximal humeral fractures (eg, AO classification 11 A2.1).6,7,11 Anatomic reduction and medial cortex support can provide fixation stability. This study showed that calcar screws increased axial and shear stiffness in humeri with normal alignment and no medial support. Calcar screws pass over the inferolateral wall of the great tuberosity, oblique to the medial cortex, providing additional support to the humeral head. The advantage of this fixation configuration has been proven in a biomechanical study of blade plates.26 We also found that displacement increased with the number of dynamic test cycles in humeri with no medial cortex support and calcar screw fixation. This finding indicates that insufficient bony support may lead to reduction loss or even implant failure. In contrast, the supporting effect of calcar screws was not notable in varus (.20 degrees) humeral heads fixed with locking plates. Reasons for this may be: (1) changes in calcar screw position and alignment may reduce axial and shear stiffness without superior fixation; (2) calcar screws may increase stability but not perform well in varus humeri, which show lower axial and shear stiffness23,27; or (3) varus may reduce the amount of cancelous bone at the screw–bone interface, leading to unstable fixation. Thus, surgeons should seek to prevent reduction loss and varus deformity first by restoring normal alignment of the humeral head. We also found that calcar screws increased torque stability, which may be related to the distribution of locking screws in the Philos system; the screws are oriented in converging and diverging directions to maximize purchase in cancelous bone of the humeral head. Another explanation is that the screws increase the contact area at the screw–bone interface, thereby increasing stability.3 We did not evaluate rotational deformity of the humeral head in this study because no standard clinical criterion is available, and because varus malreduction is seen more commonly in clinical practice. This study was conducted with a small number of specimens, and studies with much larger samples are needed to confirm the findings. Despite this limitation, the results of our biomechanical study suggest that the following measures can be taken in clinical practice to avoid reduction loss and varus collapse in unstable proximal humeral fractures. First, humeral head reduction should create proper alignment by recovering the normal head–shaft angle. After reduction, the medial cortex should be evaluated. If the shaft was medialized and impacted into the head fragment or if medial cortex is deficient, calcar screws may provide ideal fixation of the anatomically reduced humeral head.28 In cases of severe varus combined with medial cortex deficiency,29 more aggressive treatments such as the use of fibular grafts or allograft should be considered.30 REFERENCES 1. Bell JE, Leung BC, Spratt KF, et al. Trends and variation in incidence, surgical treatment, and repeat surgery of proximal humeral fractures in the elderly. J Bone Joint Surg Am. 2011;93:121–131.

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2. Kannus P, Palvanen M, Niemi S, et al. Rate of proximal humeral fractures in older Finnish women between 1970 and 2007. Bone. 2009;44:656–659. 3. Helmy N, Hintermann B. New trends in the treatment of proximal humerus fractures. Clin Orthop Relat Res. 2006;442:100–108. 4. Ring D. Current concepts in plate and screw fixation of osteoporotic proximal humerus fractures. Injury. 2007;38:59–68. 5. Agudelo J, Schürmann M, Stahel P, et al. Analysis of efficacy and failure in proximal humerus fractures treated with locking plates. J Orthop Trauma. 2007;21:676–681. 6. Solberg BD, Moon CN, Franco DP, et al. Surgical treatment of three and four-part proximal humeral fractures. J Bone Joint Surg Am. 2009;91: 1689–1697. 7. Sproul RC, Iyengar JJ, Devcic Z, et al. A systematic review of locking plate fixation of proximal humerus fractures. Injury. 2011;42:408–413. 8. Egol KA, Ong CC, Walsh M, et al. Early complications in proximal humerus fractures (OTA Types 11) treated with locked plates. J Orthop Trauma. 2008;22:159–164. 9. Gardner MJ, Weil Y, Barker JU, et al. The importance of medial support in locked plating of proximal humerus fractures. J Orthop Trauma. 2007; 21:185–191. 10. Osterhoff G, Ossendorf C, Wanner GA, et al. The calcar screw in angular stable plate fixation of proximal humeral fractures—a case study. J Orthop Surg Res. 2011;6:50. 11. Südkamp NP, Audigé L, Lambert S, et al. Path analysis of factors for functional outcome at one year in 463 proximal humeral fractures. J Shoulder Elbow Surg. 2011;20:1207–1216. 12. McElhaney J, Fogle J, Byars E, et al. Effect of embalming on the mechanical properties of beef bone. J Appl Physiol. 1964;19:1234–1236. 13. Hepp P, Lill H, Bail H, et al. Where should implants be anchored in the humeral head? Clin Orthop Relat Res. 2003;415:139–147. 14. Clavert P, Adam P, Bevort A, et al. Pitfalls and complications with locking plate for proximal humerus fracture. J Shoulder Elbow Surg. 2010;19:489–494. 15. Sanders BS, Bullington AB, McGillivary GR, et al. Biomechanical evaluation of locked plating in proximal humeral fractures. J Shoulder Elbow Surg. 2007;16:229–234. 16. Voigt C, Kreienborg S, Megatli O, et al. How does a varus deformity of the humeral head affect elevation forces and shoulder function? A biomechanical study with human shoulder specimens. J Orthop Trauma. 2011;25:399–405. 17. Lanting B, MacDermid J, Drosdowech D, et al. Proximal humeral fractures: a systematic review of treatment modalities. J Shoulder Elbow Surg. 2008;17:42–54. 18. Siffri PC, Peindl RD, Coley ER, et al. Biomechanical analysis of blade plate versus locking plate fixation for a proximal humerus fracture: comparison using cadaveric and synthetic humeri. J Orthop Trauma. 2006;20:547–554. 19. Ruch DS, Glisson RR, Marr AW, et al. Fixation of three-part proximal humeral fractures: a biomechanical evaluation. J Orthop Trauma. 2000; 14:36–40. 20. Walsh S, Reindl R, Harvey E, et al. Biomechanical comparison of a unique locking plate versus a standard plate for internal fixation of proximal humerus fractures in a cadaveric model. Clin Biomech (Bristol, Avon). 2006;21:1027–1031. 21. Brunner F, Sommer C, Bahrs C, et al. Open reduction and internal fixation of proximal humerus fractures using a proximal humeral locked plate: a prospective multicenter analysis. J Orthop Trauma. 2009;23:163–172. 22. Kralinger F, Voigt C, Platz A, et al. Proximal humerus fractures-the influence of local bone status on complications after surgical treatment. An international multicenter study. J Bone Joint Surg Am. 2012;94: 162. 23. Nho SJ, Brophy RH, Barker JU, et al. Management of proximal humeral fractures based on current literature. J Bone Joint Surg Am. 2007;89:44–58. 24. Bergmann G, Graichen F, Bender A, et al. In vivo glenohumeral contact forces—measurements in the first patient 7 months postoperatively. J Biomech. 2007;40:2139–2149. 25. Lescheid J, Zdero R, Shah S, et al. The biomechanics of locked plating for repairing proximal humerus fractures with or without medial cortical support. J Trauma. 2010;69:1235–1242. 26. Lever JP, Aksenov SA, Zdero R, et al. Biomechanical analysis of plate osteosynthesis systems for proximal humerus fractures. J Orthop Trauma. 2008;22:23–29.

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J Orthop Trauma  Volume 28, Number 8, August 2014 27. Kralinger F, Unger S, Wambacher M, et al. The medial periosteal hinge, a key structure in fractures of the proximal humerus: a biomechanical cadaver study of its mechanical properties. J Bone Joint Surg Br. 2009; 91:973–976. 28. Zhang L, Zheng J, Wang W, et al. The clinical benefit of medial support screws in locking plating of proximal humerus fractures: a prospective randomized study. Int Orthop. 2011;35:1655–1661.

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Effect of Calcar Screw 29. Robinson CM, Wylie JR, Ray AG, et al. Proximal humeral fractures with a severe varus deformity treated by fixation with a locking plate. J Bone Joint Surg Br. 2010;92:672–678. 30. Mathison C, Chaudhary R, Beaupre L, et al. Biomechanical analysis of proximal humeral fixation using locking plate fixation with an intramedullary fibular allograft. Clin Biomech (Bristol, Avon). 2010;25: 642–646.

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Effect of Calcar Screw Use in Surgical Neck Fractures of the Proximal Humerus With Unstable Medial Support: A Biomechanical Study.

To evaluate the effect of calcar screw use in proximal humeral fractures with unstable medial support treated with locked plates...
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