An 82-year-old woman sustains a displaced intracapsular fracture of the femoral neck and undergoes hip hemiarthroplasty with a bipolar prosthesis. Compared with a unipolar prosthesis such as the Austin Moore, the principal theoretical advantage of the bipolar design is:

• A Lower risk of postoperative dislocation because the head diameter is larger
• B Better fixation in the femoral canal without cement
• C Reduced acetabular cartilage erosion because motion can occur at both articulations ✓
• D Elimination of stress shielding in the proximal femur

Explanation

In a bipolar prosthesis the inner metal head articulates with a polyethylene liner within an outer metal shell that also articulates with acetabular cartilage. Motion shared between both interfaces theoretically reduces shear on acetabular cartilage and delays erosion and central protrusion compared with a unipolar Thompson or Austin Moore implant. Dislocation rates between unipolar and bipolar designs are comparable in most series, so option A is the tempting but incorrect distractor.

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

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Written and medically reviewed by the StethoPrep medical team.