The concept of a universal safe zone is history: a retrospective cohort study of 1520 total hip arthroplasties

Abstract

Introduction:

Lewinnek safe zone has guided optimal acetabular cup positioning for decades. This study aimed to provide new data on dislocation rates after total hip arthroplasty (THA), assess the effects of cup positioning, and evaluate how well the event of dislocation can be predicted.

Methods:

A total of 1520 THAs performed at our hospital were included. The primary outcome was dislocation. Hips were divided into 20° × 20° areas with 5° intervals based on positioning, and crude dislocation rates were compared. Logistic regression models were used and the results reported using odds ratios with 95% confidence intervals. Predictive performances were assessed with R² and C-indexes

Results:

59 dislocations (48 posterior, 11 anterior) occurred, giving a dislocation rate of 3.9%. Neither a universal nor component-specific safe zone could be created. Cup brand was the most significant factor. Overall predicative values were 0.041–0.134 with R² values and 0.665–0.775 with C-index values.

Conclusions:

Dislocation after THA cannot be explained by cup positioning alone. Risk profiles differ between acetabular cup systems, and even though cup design emerged as a key factor, its explanatory power remained modest. These findings emphasize the multifactorial nature of dislocation and the limitations of universal safe zones.