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Background: Dislocation is the most common early surgical complication in patients with hip fractures treated with total hip replacement (THR). Dual mobility total hip replacement (DM-THR) was developed to increase joint stability, but no randomised trial has investigated its efficacy and safety. We aimed to compare the risk of dislocation in patients receiving DM-THR versus conventional THR.

Methods: We included people aged 65 years or older with a displaced femoral neck fracture attending 20 Swedish and 24 UK hospitals in this pragmatic, registry-based, randomised superiority trial. Participants eligible for THR were randomly allocated in a 1:1 ratio to either DM-THR or standard THR with the use of remote, web-based, country-specific sequences. Neither participants nor members of the direct clinical care teams were masked, but research investigators and statisticians were masked until the trial analysis was completed. The primary outcome was dislocation of the index joint, treated with closed reduction or open surgery within 1 year, analysed in the modified intention-to-treat population including all consenting participants receiving any type of hip replacement and excluding those with major protocol deviations. This trial is registered with ClinicalTrials.gov (NCT03909815) and ISRCTN11895196.

Findings: Between Jan 9, 2020, and April 2, 2024, 2934 eligible patients were screened, of whom 1334 were not enrolled, mainly due to surgeon's preference (n=560), because they had already been treated at the time of screening (n=257), or because informed consent was not obtained (n=213). 1600 participants (64% female, 36% male) were thus randomly assigned to either DM-THR (n=798) or THR (n=802). After exclusion of 34 participants due to prespecified protocol deviations, 1566 participants (97·9%) contributed to the modified intention-to-treat analysis. A dislocation, the primary outcome, occurred in ten (1·3%) of 779 participants in the DM-THR group and in 33 (4·2%) of 787 in the THR group (adjusted hazard ratio 0·27, 95% CI 0·13−0·56; p<0·0001).

Interpretation: The use of DM-THR substantially reduced the risk of dislocation and of any surgical complication among people treated with THR after a displaced femoral neck fracture. DM-THR can thus be recommended for this group of patients.