Background Studies of elderly patients with Garden-I and Garden-II femoral neck fractures (FNFs) suggest that a preoperative posterior tilt of the femoral head of at least 20° increases the risk of fixation failure. A recently published treatment algorithm recommended hemiarthroplasty over internal fixation for elderly patients with Garden-I and Garden-II FNFs and a preoperative posterior tilt of at least 20°. However, the reliability of the method used to measure the posterior tilt has not been assessed according to recommended standards for reliability trials. Methods Four orthopedic registrars and four consultants measured the posterior tilt angle in 50 preoperative lateral radiographs at two occasions six weeks apart. We estimated inter- and intrarater reliability by intraclass correlation coefficient (ICC). We also assessed repeatability by the repeatability coefficient (RC) and agreement by the minimal detectable change (MDC). Based on the suggested cutoff value of 20°, we reported the overall percentage and specific agreement for the choice of implant. Results Inter- and intrarater reliability for all raters was excellent with an ICC (95% CI) of 0.77 (0.69–0.85) and 0.77 (0.67–0.86), respectively. The RC was 13.9 and the MDC 14.1. Specific agreement for choosing arthroplasty was 61.3 and 54.6% for the first and second test occasion, respectively. Conclusions Eight orthopedic surgeons measured the posterior tilt in 50 Garden-I and Garden-II FNFs and achieved excellent inter- and intrarater reliability. However, variations in repeated measurements and variations in measurements made by different raters, as assessed by the RC and the MDC respectively, ranged from 13.9° to 14.1°. The variations in posterior tilt measurements should be taken into account when choosing the type of implant for elderly patients with Garden-I and Garden-II femoral neck fractures.
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