Functional Safe Zone Is Superior to the Lewinnek Safe Zone for Total Hip Arthroplasty: Why the Lewinnek Safe Zone Is Not Always Predictive of Stability

Background: The Lewinnek "safe zone" is not always predictive of stability after total hip arthroplasty (THA). Recent studies have focused on functional hip motion as observed on lateral spine-pelvis-hip x-rays. The purpose of this study was to assess the correlation between the Lewinnek safe zone and the functional safe zone based on hip and pelvic motion in the sagittal plane.

Methods: Three hundred twenty hips (291 patients) underwent primary THA using computer navigation. Two hundred ninety-six of these hips (92.5%) were within the Lewinnek safe zone as determined by inclination of 40° ± 10° and anteversion of 15° ± 10°. All patients had preoperative and postoperative standing and sitting lateral spinopelvic x-rays. The combined sagittal index (CSI), a combination of sagittal acetabular and femoral position, was measured for each patient and used to assess the functional safe zone. Data analysis was performed to identify hips in the Lewinnek safe zone inside and outside the sagittal functional safe zone. Predictive factors for hips outside the functional safe zone were identified.

Results: Of the 296 hips within the Lewinnek safe zone, 254 (85.8%) were also in the functional safe zone. Forty-two patients were outside the functional safe zone based on CSI; 19 had an increased standing CSI and 23 had a decreased sitting CSI, all were considered at risk for dislocation. Predictive factors for falling outside the functional safe zone were increased femoral mobility (P < .001, r = 0.632), decreased spinopelvic mobility (P < .001, r = 0.455), and pelvic incidence (P < .001, r = 0.400).

Conclusion: In this study, 14.2% of hips within the Lewinnek safe zone were outside the functional safe zone, identifying a potential reason hips dislocate despite having "normal" cup angles. The best predictor for falling outside the functional safe zone, both preoperatively and postoperatively, was femoral mobility, not the sagittal cup position (ie, cup anteinclination).

Level of evidence: Level III, retrospective review.