Effect of femoral head size on risk of revision for dislocation after total hip arthroplasty: a population-based analysis of 42,379 primary procedures from the Finnish Arthroplasty Register.

BACKGROUND AND PURPOSE: Previous population-based registry studies have shown that larger femoral head size is associated with reduced risk of revision for dislocation. However, the previous data have not included large numbers of hip resurfacing arthroplasties or large metal-on-metal (> 36-mm) femoral head arthroplasties. We evaluated the association between femoral component head size and the risk of revision for dislocation after THA by using Finnish Arthroplasty Register data.

PATIENTS AND METHODS: 42,379 patients who were operated during 1996-2010 fulfilled our criteria. 18 different cup/stem combinations were included. The head-size groups studied (numbers of cases) were 28 mm (23,800), 32 mm (4,815), 36 mm (3,320), and > 36 mm (10,444). Other risk factors studied were sex, age group (18-49 years, 50-59 years, 60-69 years, 70-79 years, and > 80 years), and time period of operation (1996-2000, 2001-2005, 2006-2010).

RESULTS: The adjusted risk ratio in the Cox model for a revision operation due to dislocation was 0.40 (95% CI: 0.26-0.62) for 32-mm head size, 0.41 (0.24-0.70) for 36-mm head size, and 0.09 (0.05-0.17) for > 36-mm head size compared to implants with a head size of 28 mm.

INTERPRETATION: Larger femoral heads clearly reduce the risk of dislocation. The difference in using heads of > 36 mm as opposed to 28-mm heads for the overall revision rate at 10 years follow-up is about 2%. Thus, although attractive from a mechanical point of view, based on recent less favorable clinical outcome data on these large heads, consisting mainly of metal-on-metal prostheses, one should be cautious using these implants.