Review Question - QID 216233

A 73-year-old male underwent left total hip arthroplasty 15 years ago. He had an uneventful postoperative course and was doing well up until 8 months ago when he began having insidious onset left hip pain that worsens with activity. Figure A shows his most recent radiographs obtained 2 week ago. ESR and CRP at that time were normal. The patient presents to the ED today with the radiograph shown in Figure B. He reports this this is the first issue he's had with his THA. Which of the following is the MOST LIKELY reason for this patient’s late instability?