A Novel Failure Interface in Constrained Total Hip Arthroplasty: Dissociation at the Constrained Liner Insert-Bipolar Head Junction Following Periprosthetic Joint Infection.

Introduction: Constrained acetabular liners are used as a salvage option for recurrent instability in revision total hip arthroplasty (THA). While failure mechanisms, including pelvic fixation loss, liner-shell dissociation, biomaterial fracture, and femoral head dislocation, are described in the literature, failure at the interface between the constrained liner insert and the bipolar head remains underreported. Case Report: A 45-year-old male factory worker presented 7 weeks after right hip hemiarthroplasty with signs of periprosthetic joint infection and dislocation. He underwent staged treatment including wound debridement, antibiotic cement spacer placement, and revision THA with constrained liner. The patient subsequently experienced recurrent instability necessitating a second revision. Intraoperatively, the failure was identified at the factory-assembled constrained liner insert-bipolar head junction without any locking ring fracture. This represents a previously undescribed failure interface in constrained THA. Conclusion: This case highlights a novel anatomical failure interface in constrained THA systems. Surgeons must be aware that dissociation at the constrained liner insert-bipolar head junction can occur independently of locking ring failure, particularly in multiply-revised hips with impingement. Careful implant selection, optimized component positioning, and rigorous rehabilitation, compliance are critical to long-term success.