Biomechanical Strength of Scaphoid Partial Unions

Abstract

Grant received from: VICTR Consulting Fee: Biomet (Lee) It is not known how much force a partially united scaphoid can sustain without refracturing. This is critically important in determining when to discontinue immobilization in laborers or active individuals and when considering return to play in athletes. The purpose of this study was to test the biomechanical strength of simulated partially united scaphoids using a validated testing model for scaphoid fractures. We hypothesized that there would be no difference in the strength of scaphoids with 50% or greater bone at the waist compared to intact scaphoids. Forty-one lightly embalmed cadaver scaphoids were divided into 4 groups, 3 experimental osteotomy groups (25%, 50% and 75% of the scaphoid waist) and 1 control group (intact). Each specimen was potted and tested using a validated protocol. Using a materials testing machine (MTS), each scaphoid was subjected to a dorsal to volar cantilever force of 80 N to 120 N for 4,000 cycles, representing a sub-failure physiological load, followed by load-to-failure. Permanent deformation was measured during the physiologic load phase and stiffness, max force, work to failure and mechanism of failure were recorded during load-to-failure testing. Analysis of variance between groups was determined using the Kruskal-Wallis test. All scaphoids survived the sub-failure conditioning with no significant difference in permanent deformation (P = 0.34) (Table 57-1). Intact scaphoids endured an average maximum load of 333N prior to failure, compared to 321N for the 25% group, 297N for the 50% group and 324N for the 75% group, with no statistically significant variance (P = 0.86) (Fig. 57-1). There were also no statistically significant differences in stiffness (P = 0.15) or work to failure (P = 0.93) between the intact, 25%, 50% and 75% groups. One specimen from each osteotomy group failed by fracturing through the osteotomy site; all others failed by fracturing at the distal pole near the loading site. This is consistent with previous reports in the literature. •Scaphoids with at least 25% of intact cortical bone at the waist can withstand normal physiologic loads.•The data provide valuable information regarding partial scaphoid union and supports return to play or mobilization once the scaphoid has reached 25% union across the fracture site.Table 57-1Average Values During Physiologic Sub-failure Conditioning and Load-to-failure TestingNPermanent Deformation (mm)Stiffness (N/mm)Max Force to Failure (N)Work to Failure (N ∙ mm)Intact101.4226333478125%112.1327132172050%102.4219429765575%101.82168343703 Open table in a new tab