Trunnion fracture of femoral prosthesis following a large metal-on-metal uncemented total hip arthroplasty: a case report.

backgroundTrunnion fracture of the femoral prosthesis is an extremely rare complication following Total Hip Arthroplasty (THA). There are very few reports in literature on trunnion fracture. All previously reported cases are of prostheses with smaller heads, unlike the large metal-on-metal articulation in our case, which is unique. CASE PRESENTATION: A 29-year-old male patient with juvenile idiopathic arthropathy presented to us with a fracture of the trunnion of his left THA, with no history of trauma. He had undergone staged bilateral THA for avascular necrosis of the hips 10 years ago (with AML-A stem and large metal-on-metal articulation bilaterally). We revised the THA and exchanged both femoral and acetabular components. He had a good functional outcome at 3 years' follow up.

conclusionThe primary reason for the trunnion fracture in our case was the faulty stem design of the AML [with a small (9/10) taper, and a large head], causing excessive bending moment at the trunnion, worsening the cantilever effect and leading to subsequent cyclic fatigue failure. Whilst gross trunnion failure (GTF) with dissociation of the head from the taper is commonly reported, trunnion fracture per se is a rare and devastating complication. The AML-A stem has since been recalled by the company and there is a need to constantly monitor these patients for potential trunnion-related complications.