Predictors for Long-Term Hip Survivorship Following Acetabular Fracture Surgery: Importance of Gap Compared with Step Displacement.

BACKGROUND: Historically, the greatest residual (gap or step) displacement is used to predict clinical outcome following acetabular fracture surgery. Gap and step displacement may, however, impact the outcome to different degrees. We assessed the individual relationship between gap or step displacement and hip survivorship and determined their independent association with conversion to total hip arthroplasty.

METHODS: Patients who had acetabular fracture fixation (from 1992 through 2014), follow-up of ≥2 years (or early conversion to total hip arthroplasty), and postoperative computed tomography (CT) scans were included. Of 227 patients, 55 (24.2%) had conversion to total hip arthroplasty at a mean follow-up (and standard deviation) of 8.7 ± 5.6 years. Residual gap and step displacement were measured using a standardized CT-based method, and assessors were blinded to the outcome. Kaplan-Meier survivorship curves for the hips were plotted and compared (log-rank test) using critical cutoff values for gap and step displacement. These values were identified using receiver operating characteristic curves. Multivariate analysis was performed to identify independent variables associated with conversion to total hip arthroplasty. Subgroup analysis was performed in younger patients (<50 years old).

RESULTS: The critical CT cutoff value for total hip arthroplasty conversion was 5 mm for gap and 1 mm for step displacement. Hip survivorship at 10 years was 82.0% for patients with a gap of <5 mm compared with 56.5% for a gap of ≥5 mm (p < 0.001) and 80.0% for a step of <1.0 mm versus 65.5% for a step of ≥1.0 mm (p = 0.012). A gap of ≥5 mm (hazard ratio [HR], 2.3; p = 0.012) and an age of ≥50 years (HR, 4.2; p < 0.001) were independently associated with conversion to total hip arthroplasty in all patients. In the subgroup of younger patients, only a step of ≥1 mm (HR, 6.4; p = 0.017) was an independent factor for conversion to total hip arthroplasty.

CONCLUSIONS: Residual gap and step displacement as measured on CT scans are both related to long-term hip survivorship, but step displacement (1 mm) is tolerated less than gap displacement (5 mm). Of the 2 types of displacement, only a large gap displacement (≥5 mm) was independently associated with conversion to total hip arthroplasty. In younger patients who had less articular impaction with smaller residual gaps, only step displacement (≥1 mm) appeared to be associated with this outcome.

LEVEL OF EVIDENCE: Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.