Large-diameter total hip arthroplasty modular heads require greater assembly forces for initial stability.
Bone Joint Res
; 5(8): 338-46, 2016 Aug.
Article
em En
| MEDLINE
| ID: mdl-27496914
ABSTRACT
OBJECTIVES:
Modular junctions are ubiquitous in contemporary hip arthroplasty. The head-trunnion junction is implicated in the failure of large diameter metal-on-metal (MoM) hips which are the currently the topic of one the largest legal actions in the history of orthopaedics (estimated costs are stated to exceed $4 billion). Several factors are known to influence the strength of these press-fit modular connections. 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. MATERIALS ANDMETHODS:
Ti-6Al-4V trunnions (n = 60) and two different sizes of cobalt-chromium (Co-Cr) heads (28âmm and 36âmm; 30 of each size) were used in the study. Three different levels of assembly force were considered 4 kN; 5 kN; 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. The statistical differences in pull-off force were examined using a Kruskal-Wallis test and two-sample Mann-Whitney U test. Finite element and analytical models were developed to understand the reasons for the experimentally observed differences.RESULTS:
36âmm diameter heads had significantly lower pull-off forces than 28âmm heads when impacted at 4âkN and 5âkN (pâ<â0.001; pâ<â0.001), but not at 6âkN (pâ=â0.21). Mean pull-off forces at 4 kN and 5âkN impaction forces were approximately 20% larger for 28âmm heads compared with 36âmm 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.CONCLUSION:
This is the first study to show that 36âmm Co-Cr heads have up to 20% lower pull-off connection strength compared with 28âmm heads for equivalent assembly forces. This effect is likely to play a role in the high failure rates of large diameter MoM hips.Cite this article A. R. MacLeod, N. P. T. Sullivan, M. R. Whitehouse, H. S. Gill. Large-diameter total hip arthroplasty modular heads require greater assembly forces for initial stability. Bone Joint Res 2016;5338-346. DOI 10.1302/2046-3758.58.BJR-2016-0044.R1.
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MEDLINE
Idioma:
En
Ano de publicação:
2016
Tipo de documento:
Article