Does a 36-mm Head Increase Cumulative Revision Rate in Total Hip Arthroplasty When Compared to a 32-mm Head? A Study From the Australian Orthopaedic Association National Joint Replacement Registry

BACKGROUND: Registry and industry data show increasing utilization of large (36 mm) heads in primary total hip arthroplasty (THA). Recent analysis of the Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) has reported reduced dislocation rates with 36 mm heads compared to 32- and 28-mm heads; however, the effect of age, fixation, approach, cup size, and bearing surface upon revision risk and head size has not been examined, with little data on all-cause revision. METHODS: The AOANJRR data were examined for all ceramic and metal head THA with highly crosslinked polyethylene between September 1999 and December 2022. There were 272,258 THAs identified. Cumulative percent revision (CPR) following THA was examined with further subanalysis of age, fixation, approach, cup size, and bearing surface for 32- and 36-mm heads. RESULTS: The CPR was higher for 36 mm heads from 1 month (HR hazard ratio 1.14 (1.08 to 1.20), P < 0.001). Subgroup analysis showed these differences varied depending on age, sex, approach, cup size, and bearing surface. There were differences in reasons for revision between head sizes, with significantly more revisions with 36 mm heads for fracture (HR 1.30 (1.18 to 1.42), P < 0.001), particularly for ≥ 65-year-olds, and loosening (HR 1.21 (1.09 to 1.34, P < 0.001), and significantly more revisions for dislocation (to a lesser degree) with 32 mm heads (HR 1.18 (1.07 to 1.30), P < 0.001). CONCLUSIONS: Our study shows an association between larger head size and increased CPR. This difference is most clearly seen in metal-on-highly crosslinked polyethylene articulations, anterior approach, and 54- to 55-mm cup size in men < 65 years, although it still exists to a lesser extent in men ≥ 65 years. For women, the pattern was similar, although not as apparent. Comparing 36- to 32-mm heads, 36 mm showed reduced early dislocation; however, revision for fracture and loosening was increased. With an increasing trend toward larger head sizes both in the AOANJRR and elsewhere, consideration must be given in these particular subgroups as to whether larger head size confers the intended survival advantages.