What is the Lewinnek safe zone for acetabular cup positioning and why is it clinically relevant?

• A. Anteversion about 5 degrees (±10) and abduction about 20 degrees (±10); within this range reduces dislocation risk.
• B. Anteversion about 25 degrees and abduction about 60 degrees; does not affect dislocation.
• C. Anteversion 45 degrees and abduction 20 degrees; misaligned zone.
• D. Anteversion about 15 degrees (±10) and abduction about 40 degrees (±10); positioning within this zone reduces dislocation risk.

Answer: (D)

The key idea here is that there is a target range for placing the acetabular cup that tends to make the hip more stable and less prone to dislocation. Lewinnek’s safe zone defines this target by specifying about 15 degrees of anteversion and about 40 degrees of abduction, each with a tolerance of roughly ±10 degrees. Positioning the cup within these angles helps keep the ball of the femur aligned with the socket across common hip motions, reducing the chances that the head will edge out of the cup or that impingement will occur.

This is why the option with anteversion around 15 degrees (±10) and abduction around 40 degrees (±10) is the best choice: it corresponds to the classic Lewinnek safe zone and has been associated with lower dislocation risk in the original analysis.

Keep in mind that while this zone guides implant positioning, real-world factors like pelvic tilt, spinopelvic mobility, patient activity, and newer concepts of functional orientation can influence dislocation risk, so some surgeons tailor targets to individual patients.