Mechanical failure of femoral stems at the modular junction of revision hip arthroplasty systems has been reported only infrequently. In the current study, the cause of six stem fractures, which occurred in vivo, was analyzed with use of clinical data and failure analysis.
Six patients with a fracture at the mid-stem junction of a modular revision hip implant were identified in our database of patients who had undergone revision arthroplasty. The characteristics of the patients with a fractured stem were compared with those of 165 patients from the same prospective database who had a modular stem implanted, had at least two years of follow-up, and had not had a fracture of the stem. Failure analysis of three implants (six fracture surfaces) was carried out, with use of microscopic, chemical, and microhardness characterization techniques.
Patients with a fractured stem had significantly higher body mass indices than patients without a stem fracture. Radiographs demonstrated that these femoral implants lacked adequate osseous support of the junction area of the stem. All stems failed approximately 1 to 2 mm proximal to the body-stem junction, thus indicating the presence of a bending moment. The chemical composition and microhardness matched those of Ti-6Al-4V. Evidence of wear and fatigue were found on the fracture surface. A wear strip was also observed along the circumference of the stem near the junction.
We concluded that the stem failure was initiated by a fretting fatigue mechanism and was propagated by a pure bending fatigue mechanism. Risk factors for fractures of the modular junction include excessive body weight and inadequate proximal osseous support because of trochanteric osteotomy, reduced preoperative bone stock, osteolysis, loosening, and/or implant undersizing. Surgeons should consider the use of implants with strengthened junctions when using modular stems in such patients.