Overview
- Cup position drives dislocation, wear, impingement and squeaking
- Lewinnek safe zone is 40° ± 10° inclination and 15° ± 10° anteversion
- Many dislocating hips lie within it
- Spinopelvic mobility changes functional cup position
- Positioning, transverse acetabular ligament, imaging, navigation and robotics reduce malposition
- Plan combined anteversion and assess spinal stiffness for each patient
Western Health Orthopaedic Registrar presentation – Avoiding Malpositioning of the Acetabulum in THR
Complications
- Excess inclination, edge loading, wear, squeaking, metal ion release and lateral instability
- Insufficient inclination, limited abduction and impingement
- Excess anteversion, anterior dislocation and iliopsoas irritation
- Retroversion, posterior dislocation
Pathology
- Posterior pelvic tilt on sitting increases functional anteversion and protects against anterior impingement
- Stiff spines cause anterior impingement and posterior dislocation when seated
- Sacral slope change under 10 degrees from standing to sitting suggests stiffness
Investigations
- Standing and sitting lateral spinopelvic radiographs identify high risk patients
- Template on a calibrated AP pelvis for size, centre and depth relative to the teardrop
Management
- Lewinnek safe zone, 40 ± 10 degrees inclination and 15 ± 10 degrees anteversion
- Many dislocations occur inside the Lewinnek zone
- Murray showed anatomical, operative and radiographic definitions differ
- Combined anteversion target about 25 to 50 degrees
- High risk spines may need more anteversion, larger heads or dual mobility
- Align the cup parallel to and just inside the transverse acetabular ligament
- Check the pelvis after draping, as it often rolls or tilts in the lateral position
- Mechanical guides assume a neutral pelvis and are unreliable alone
- Robotic surgery improves accuracy and reduces outliers
- Evidence for fewer dislocations and revisions with technology is inconsistent
Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 10 October 2026.