In total hip arthroplasty (THA), the 'safe zone' (Lewinnek zone) for acetabular cup placement describes an abduction angle of ___° and anteversion of ___°, with ±10° tolerance:

• A Abduction 30°, anteversion 10°
• B Abduction 40°, anteversion 15° ✓
• C Abduction 50°, anteversion 20°
• D Abduction 35°, anteversion 5°

Explanation

Lewinnek's original 1978 study defined the safe zone for acetabular cup placement as radiographic abduction (inclination) of 40° ± 10° and radiographic anteversion of 15° ± 10°. Cups placed outside this zone have a significantly higher dislocation rate. Insufficient anteversion predisposes to posterior dislocation (most common direction in THA); excessive anteversion causes anterior dislocation and edge loading. Cup abduction >55° increases edge loading and accelerates polyethylene wear. Modern THA planning uses CT-based measurements given that radiographic and anatomical anteversion differ.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.