Abstract Tibial insert locking mechanisms are intended to limit interface motion and “backside” wear in modular total knee arthroplasty (TKA). Nevertheless, anterior polyethylene subluxation is occasionally apparent on lateral radiographs after TKA, suggesting locking mechanism failure. We retrospectively identified 10 modular posterior-stabilized implants of a single design that were found to have failure of the locking mechanism at the time of revision surgery for osteolysis. Operative reports were reviewed for the presence of backside wear, and preoperative radiographs were inspected for polyethylene subluxation. All 10 implants demonstrated significant backside wear. Nine had anterior polyethylene subluxation evident on preoperative radiographs. Anterior polyethylene subluxation on the lateral radiograph is a subtle sign of failure of the locking mechanism in this modular posterior-stabilized TKA.