Large-diameter total hip arthroplasty modular heads require greater assembly forces for initial stability.

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 AND METHODS: Ti-6Al-4V trunnions (n = 60) and two different sizes of cobalt-chromium (Co-Cr) heads (28 mm and 36 mm; 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: 36 mm diameter heads had significantly lower pull-off forces than 28 mm heads when impacted at 4 kN and 5 kN (p < 0.001; p < 0.001), but not at 6 kN (p = 0.21). Mean pull-off forces at 4 kN and 5 kN impaction forces were approximately 20% larger for 28 mm heads compared with 36 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. CONCLUSION: This is the first study to show that 36 mm Co-Cr heads have up to 20% lower pull-off connection strength compared with 28 mm 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;5:338-346. DOI: 10.1302/2046-3758.58.BJR-2016-0044.R1.

Bristol index of patellar width to thickness (BIPWiT): a reproducible measure of patellar thickness from adult MRI.

BACKGROUND: The restoration of an adequate patellar thickness is a key to the successful outcome of knee arthroplasty. This study investigated the relationship between the thickness of the native patellar and medial-lateral patellar width using magnetic resonance imaging (MRI). METHODS: 75 MRI scans of young adults, with an average age of 27 (range 16-40) were studied. Exclusion criteria included a diagnosis of degenerative joint disease, patello-femoral pathology or age under 16/over 40 (170 patients). The bony thickness of the patellar, the chondral thickness and patellar width were measured, as was the location of maximal patellar thickness. Inter/intraobserver variability was calculated and correlation analysis was performed. RESULTS: We found a strong correlation between patellar width and thickness (bone plus cartilage) (Pearson 0.75, P<0.001). The mean width to thickness ratio was 1.8:1 (standard deviation 0.1, 95% confidence interval 1.78-1.83). Without cartilage the ratio was 2.16:1 (SD 0.15, 95% CI 2.11-2.21), correlation was moderate (Pearson 0.59, P<0.001). The average maximal patellar cartilage thickness was 4.1mm (SD 1.3). CONCLUSION: The strong correlation and narrow confidence intervals for the ratio of patellar width to thickness, suggest that patellar width might be used as a guide for accurate restoration of patellar thickness during total knee or patello-femoral replacement. After removing osteophytes we would recommend a ratio of 1.8:1. Further work is required to establish whether there is a relationship between anterior knee pain post total knee arthroplasty and an abnormal patellar width:thickness ratio. LEVEL OF EVIDENCE: Level III.