Review Question - QID 219492

A 75-year-old male comes into the clinic with a painful metal-on-polyethylene total hip arthroplasty that was performed nearly seven years ago. His erythrocyte sedimentation rate (ESR) and c-reactive protein (CRP) levels are within normal limits, his cobalt ion levels are 2 ng/mL, and his chromium ion levels are 1 ng/mL. He undergoes a revision total hip arthroplasty, and his recovered femoral stem is demonstrated in Figure A. Which of the following implant design features has been shown to contribute most to the type of corrosion that necessitated this patient's revision surgery?