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Introduction: The literature regarding undisplaced femoral neck fractures (FNF) is sparse. The aim of this prospective feasibility study is to compare the clinical outcome after undisplaced FNF treated with internal fixation (IF) and displaced FNF treated with hip arthroplasty. We hypothesized that hip arthroplasty would give a lower incidence of reoperations.

Methods: A total of 235 patients were included with a median age of 84 years (range 65-99). A consecutive series of 65 patients with undisplaced FNF were treated with IF, and 170 patients with displaced FNF were treated with either a total hip arthroplasty or a hemiarthroplasty. Follow-up interviews were conducted at 1 year using the Harris Hip Score (HHS), WOMAC, and pain numeric rating scale (PNRS). The minimum follow-up time was 22 months. There was no difference in baseline data between the groups.

Results: Nineteen (8%) hips required reoperation at least once at a mean of 6 months (range 0-35). The rate of reoperation was higher in the IF group compared to the hip arthroplasty group (13.8% vs. 5.9%, 95% CI 0.9-6.4). The overall 1-year and 2-year mortality was 28% and 40%, respectively, with no difference between the groups. The most common reasons for reoperations in the IF group were non-union and avascular necrosis, and 6 patients were treated with hip or excision arthroplasty. In the arthroplasty group, the most common indications were deep infection and dislocation. We did not find any differences between the groups in terms of HHS, WOMAC, and PNRS.

Conclusions: In this feasibility study, we found no differences in patient-reported outcomes between the groups although IF required a higher rate of reoperations. Further randomized trials are needed to establish the optimal treatment of undisplaced FNF in the elderly.

Trial registration: ClinicalTrial.org, NCT03392285. Retrospectively registered on 5 February 2018.