This prospective registry study of 7,040 primary THA patients challenged the validity of the Lewinnek safe zone as a reliable predictor of hip dislocation. It asked whether any radiographic acetabular position reliably confers low dislocation risk, and whether patient and implant factors matter more than cup angles.
Every THA surgeon aims for the Lewinnek safe zone — but this study shows that landing within it does not make a hip stable, and landing outside it does not guarantee dislocation.
The core problem is that a single AP radiograph captures a static, 2D projection of cup position. It cannot reflect pelvic tilt during sitting or rising from a chair, pelvic rotation, or femoral stem anteversion — all of which alter the functional arc of motion and the true risk of impingement.
The practical takeaway is that dislocation workup and prevention must go beyond cup angles. Age extremes (under 50 and 70 or older) independently increase risk. Low preoperative activity in younger patients adds further risk. Soft tissue repair of the posterior capsule and external rotators, restoration of combined anteversion, and functional pelvic alignment are the factors that a simple AP radiograph cannot capture but that likely drive instability.
For boards: know that the Lewinnek safe zone (inclination 40° ± 10°, anteversion 15° ± 10°) was derived from only 9 dislocations in 300 patients. This study — the largest prospective cohort to examine the question — found no radiographic zone that meaningfully reduced dislocation risk.
This prospective registry study of 7,040 primary THA patients challenged the validity of the Lewinnek safe zone as a reliable predictor of hip dislocation. It asked whether any radiographic acetabular position reliably confers low dislocation risk, and whether patient and implant factors matter more than cup angles.
Every THA surgeon aims for the Lewinnek safe zone — but this study shows that landing within it does not make a hip stable, and landing outside it does not guarantee dislocation.
The core problem is that a single AP radiograph captures a static, 2D projection of cup position. It cannot reflect pelvic tilt during sitting or rising from a chair, pelvic rotation, or femoral stem anteversion — all of which alter the functional arc of motion and the true risk of impingement.
The practical takeaway is that dislocation workup and prevention must go beyond cup angles. Age extremes (under 50 and 70 or older) independently increase risk. Low preoperative activity in younger patients adds further risk. Soft tissue repair of the posterior capsule and external rotators, restoration of combined anteversion, and functional pelvic alignment are the factors that a simple AP radiograph cannot capture but that likely drive instability.
For boards: know that the Lewinnek safe zone (inclination 40° ± 10°, anteversion 15° ± 10°) was derived from only 9 dislocations in 300 patients. This study — the largest prospective cohort to examine the question — found no radiographic zone that meaningfully reduced dislocation risk.