To minimise the risk of dislocation and edge loading after total hip arthroplasty, the classic Lewinnek safe zone defines acceptable radiographic acetabular cup positioning as:

• A Inclination of 40 degrees plus or minus 10 degrees and anteversion of 15 degrees plus or minus 10 degrees ✓
• B Inclination of 20 degrees and anteversion of 40 degrees
• C Inclination of 15 degrees plus or minus 10 degrees and anteversion of 45 degrees plus or minus 5 degrees
• D Inclination of 55 degrees and neutral anteversion

Explanation

Lewinnek described a safe zone of 40 degrees of abduction (inclination) with a range of about 30 to 50 degrees, combined with 15 degrees of anteversion ranging roughly 5 to 25 degrees. Cups outside this zone carry higher dislocation rates. Option C simply swaps the two angles, a common examination trap, and cups with inclination near 55 degrees are prone to edge loading and dislocation.

Reference: Campbell's Operative Orthopaedics, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.