Large Diameter Total Hip Arthroplasty Modular Heads Require Greater Assembly Forces for Initial Stability

摘要

ObjectivesModular junctions are a potential site of failure in contemporary hip arthroplasty. Failure of the trunnion-head interface is currently a key issue that may be exacerbated by the use of large diameter heads. These components are assembled using a press-fit connection comprising a ‘male’ taper (trunnion) and a ‘female’ ball (head). Several factors are known to influence the strength of the initial fit, however, the influence of different head sizes has not previously been investigated. The aim of the study was to establish whether the choice of head size influences the initial strength of the trunnion-head connection.MethodsTi‐6Al-4V trunnions (n = 60) and two different sizes of Co-Cr heads (28 mm and 36 mm) were used in the study. Three different levels of assembly force were considered; 4, 5 and 6 kN (n = 10 each). The strength of the press-fit connection was subsequently evaluated by measuring the pull-off force required to break the connection.ResultsFor the first time, we report that 36 mm diameter heads had significantly lower pull-off forces when impacted at 4 and 5 kN (p  0.001; p  0.001) but not at 6 kN (p = 0.21) compared to 28 mm heads. Mean pull-off forces at 4 and 5 kN impaction forces were 20% lower for 36 mm heads compared to 28 mm heads. Finite element and analytical models demonstrate that the differences in pull-off strength can be explained by differences in structural rigidity and the resulting interface pressures.ConclusionWe have demonstrated that 36 mm heads require 20% larger impaction forces to obtain equivalent initial connections strengths compared to 28 mm heads.