Abstract
Introduction
The majority of primary total hip arthroplasty (THA) procedures performed throughout the world use modular junctions, such as the trunnion-head interface; however, the failure of these press-fit junctions is currently a key issue that may be exacerbated by the use of large diameter heads. Several factors are known to influence the strength of the initial connection, 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.
Methods
Ti-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. Finite element and analytical models were also developed to better understand the mechanics of the problem.
Results
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.
Discussion
This is the first study to demonstrate that the head-trunnion connection strength of larger diameter heads is compromised at the point of assembly. Our finite element and analytical models show that the differences in pull-off strength can be explained by differences in structural rigidity and the resulting interface pressures. It should be noted that this effect is likely to be even more pronounced for head diameters larger than 36 mm.
Conclusion
We recommend that surgeons use 20% larger impaction forces for 36 mm heads than they would ordinarily use for 28 mm heads.
