Long-term cancer risk after metal-on-metal hip arthroplasty: a Finnish population-based study with a mean follow-up of 12.8 years

OBJECTIVE: Metal-on-metal (MoM) total hip arthroplasty (THA) and hip-resurfacing arthroplasty were widely used worldwide, including in Finland, at the beginning of the century. This study aimed to assess the long-term cancer risk among all Finnish patients with any kind of MoM hip device and to compare it with that of the general Finnish population and patients with conventional THA. DESIGN: Population-based study. SETTING: Nationwide retrospective comparative register-based study. PARTICIPANTS: A total of 18 922 patients who underwent MoM hip arthroplasty (HA) and 29 247 patients who underwent conventional THA (reference cohort) were recorded in the Finnish Arthroplasty Register (FAR) between 2001 and 2013. Data on cases of cancer up to 2021 for these cohorts were extracted from the Finnish Cancer Registry (FCR). Data from the FAR and FCR, as well as information on deaths from the Population Register Centre, were linked using personal identification codes. MAIN OUTCOME MEASURES: The relative risk of cancer was expressed as the ratio of the observed to expected number of cases in the Finnish population using the standardised incidence ratio (SIR). Poisson regression was used to estimate the relative risk of cancer in the MoM cohort compared with the non-MoM cohort as a ratio of SIRs. RESULTS: The overall risk of cancer in the MoM cohort was decreased compared with that of the general Finnish population (1381 observed versus 1521 expected; SIR 0.91; 95% CI 0.86 to 0.96; p