Stability of the Proximal Humeral Fracture Treated with Compression, Locking Plating and Inlay Allograft Struts

INTRODUCTION: Stability of the proximal humeral fracture can be difficult to achieved if osteoporosis or severe loss of bone stock is presented. Recently, locking plate technology has included these fractures, but appears that may not be predictable as hoped. Recent studies have demonstrated the importance of both reduction and mechanical support of the medial column in proximal humerus fracture fixation. Although the open reduction and internal fixation with a compression plate in conjunction with autogenous bone grafting is the most successful management, however, it was found that fractures without medial column support was obtained, reduction loss and articular screw penetration occurred in 29% of cases. Therefore, the medial column mechanical support of these fractures could play a significant role. This study experimentally compares the strength and fixation stability for proximal humeral fractures treated with cloverleaf conventional, locking plating and inlay strut allografts. The results are intended to lead to a better understanding of the load transfer and inlay graft technique in proximal humeral fracture stabilization. MATERIALS AND METHODS: Twenty humeral sawbone specimens (Synbone 5011, Swiss), conventional and locking cloverleaf plating system (Synthes 3.5 mm cloverleaf plate), with and without augmented inlay strut allograft, were experimentally compared in this study. All specimens were divided into four groups: (I) specimens were fixed with a conventional (compression) plate; (II) with a locking plate; (III) a conventional plate and augmented with an inlay allograft strut; and (IV) a locking plate with an inlay allograft strut. All plates were seated over the greater tuberosity then positioned and fixed with two holes (middle stem) and four in the shaft (two cortex). A 10-mm gap transverse osteotomy was performed at the level of the surgical neck to simulate the nonunion. The mid-shaft of the specimen was cut and inserted into a circle trough and then embedding this end in cement. The humeral head was smeared butter oil then semiinserted in the cement to mold a partial cup. Each construct was installed and statically tested in a material testing machine (Universal testing machine, HT-2402, Hungta, Taiwan). Increasing axial load was applied through a ball bearing with a displacement rate of 5 mm/min until the load displacement curve showed a clear decay (screw loosening/construct failure) or the osteotomy gap of the specimens closed (the fracture site contacted). The axial stiffness was then defined as the load divided by displacement and to be evaluated according to the slope of linear portion of the loaddisplacement curve. SigmaStat 17 (SPSS, Chicago, IL) statistics package was used for analysis. One way ANOVA was performed with stiffness and ultimate load as the outcome variable, post-hoc test compared the statistical significance between groups as p≦0.05. RESULTS: Table 1 listed the experimental results of stiffness and ultimate strength for four groups. We can find that the locking plate with inlay allograft provided the highest axial stiffness and the axial ultimate strength. The strength of the specimens that fixed with inlay allograft struts obtained to be about 3 times of those of conventional and locking specimens without allograft struts augmented. In the conventional plate system, there is no significant difference in stiffness between the conventional plate and conventional plate with inlay allograft. However, the locking plate conjunction with inlay strut showed about two times of the locking plate system in stiffness and ultimate strength. Table 1 Experimental results of averaged stiffness and ultimate strength No Conv. Conv. + inlay strut Locking Locking + inlay strut Stiffness (N/mm) 194.91 (±52.05) 204.48 (±37.90) 149.16 (±21.27) 336.47 (±50.38) Ultimate Strength (N) 496.58 (±118.92) 955.81 (±97.57) 582.40 (±57.64) 1051.71 (±91.12) Fig. 1 compared the averaged stiffness between groups. The graphs revealed that the locking plate with inlay allograft strut group was significantly greater than other groups in stiffness (356.5 N/mm verses 186.7, 227.5 and 169.4). Fig. 2 also showed the locking plate with inlay allograft strut which provided higher loading resistance when compared to other groups (1130.4 verses 496.6, 955.8 and 582.4). 0 50 100 150 200 250 300 350 400 450 Conventional Conventional + Inlay strut Locking Locking + Inlay strut St iff ne ss (N /m m ) Fig. 1: Graph of averaged stiffness with standard deviations. Significant differences were found at the locking plate with inlay allograft strut group (p<0.05). 0 100 200 300 400 500 600 700 800 90