Lewinnek safe zone

Dislocations after total hip-replacement arthroplasties.

Lewinnek GE, Lewis JL, Tarr R, et al. The Journal of bone and joint surgery. American volume 1978 · PMID 641088 · doi:10.1007/978-1-4471-5451-8_27

Question

Does acetabular cup orientation influence the risk of dislocation after total hip replacement?

Design

Case series of total hip replacements with precise radiographic measurement of cup orientation.

Patients

Patients who underwent total hip replacement.

Intervention

Cup within a defined orientation range

Comparison

Cup outside that range

Primary outcome

Dislocation

Key result

Cups within about 15 degrees (plus or minus 10) of anteversion and 40 degrees (plus or minus 10) of inclination dislocated less often than those outside this 'safe zone'; increased anteversion was linked to anterior dislocation, and risk was highest early after surgery and in previously operated hips.

Limitations

Small, old cohort; later registry work has shown many dislocations occur with cups inside the safe zone, reflecting spinopelvic and soft-tissue factors.

Impact on practice

Remains the classic target for cup positioning that every trainee is expected to know, although modern thinking adds spinopelvic alignment.