This 1978 retrospective study of 300 total hip replacements asked whether acetabular cup orientation predicts dislocation risk. It introduced the concept of a quantitative 'safe zone' for cup placement based on measured anteversion and lateral opening angles. Nine hips (3%) dislocated, providing the dataset for the field's most cited positioning reference.
The Lewinnek safe zone is one of the most cited concepts in hip reconstruction, yet it was derived from just nine dislocation events with an unvalidated radiographic measurement technique — a limitation worth understanding before applying it dogmatically.
In practice, these numbers remain your starting point: target anteversion 15 ± 10° and abduction 40 ± 10° on every primary THA. When reviewing post-operative films on a dislocated hip, check anteversion first. If the cup is ≥ 25°, excessive anteversion is the likely driver of anterior instability.
For revision cases, flag prior ipsilateral surgery as an independent risk factor before you even open the chart. The dislocation rate in that population dwarfs primary THA, and soft-tissue deficiency compounds whatever cup position you achieve.
The broader lesson from the senior surgeon's 0.5% rate: hitting the safe zone is necessary but not sufficient. Intraoperative stability testing and post-operative adduction restriction are part of the same risk-reduction strategy. Modern functional alignment concepts. Adjusting cup position for spinopelvic mobility. Build directly on the limitations Lewinnek's own data revealed.
This 1978 retrospective study of 300 total hip replacements asked whether acetabular cup orientation predicts dislocation risk. It introduced the concept of a quantitative 'safe zone' for cup placement based on measured anteversion and lateral opening angles. Nine hips (3%) dislocated, providing the dataset for the field's most cited positioning reference.
The Lewinnek safe zone is one of the most cited concepts in hip reconstruction, yet it was derived from just nine dislocation events with an unvalidated radiographic measurement technique — a limitation worth understanding before applying it dogmatically.
In practice, these numbers remain your starting point: target anteversion 15 ± 10° and abduction 40 ± 10° on every primary THA. When reviewing post-operative films on a dislocated hip, check anteversion first. If the cup is ≥ 25°, excessive anteversion is the likely driver of anterior instability.
For revision cases, flag prior ipsilateral surgery as an independent risk factor before you even open the chart. The dislocation rate in that population dwarfs primary THA, and soft-tissue deficiency compounds whatever cup position you achieve.
The broader lesson from the senior surgeon's 0.5% rate: hitting the safe zone is necessary but not sufficient. Intraoperative stability testing and post-operative adduction restriction are part of the same risk-reduction strategy. Modern functional alignment concepts. Adjusting cup position for spinopelvic mobility. Build directly on the limitations Lewinnek's own data revealed.