['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.', 'Ti-6Al-4V trunnions (n = 60) and two different sizes of cobalt-chromium (Co-Cr) heads (28\u2008mm and 36\u2008mm; 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.', '36\u2008mm diameter heads had significantly lower pull-off forces than 28\u2008mm heads when impacted at 4\u2008kN and 5\u2008kN (p\u2008<\u20080.001; p\u2008<\u20080.001), but not at 6\u2008kN (p\u2008=\u20080.21). Mean pull-off forces at 4 kN and 5\u2008kN impaction forces were approximately 20% larger for 28\u2008mm heads compared with 36\u2008mm 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.'] Do large-diameter total hip arthroplasty modular heads require greater assembly forces for initial stability?