Why do some titanium-alloy total hip arthroplasty modular necks fail?

BACKGROUND

Increased modularity in total hip arthroplasty (THA) through extra junction between the neck and the femoral stem is gaining popularity among orthopaedic community. However, the advantage of the additional junction is shadowed by an increased risk of mechanical failure. The aim of this study was to describe the exact mechanism of fracture of the modular femoral neck in an uncemented stem.

METHODS

Clinical, metallurgical, and mechanical analysis including finite-element modelling and elemental-sensitive tissue analysis with the micro-PIXE method was performed on two patients treated with fully modular primary THA made from Titanium alloy of the same oval taper-cone design. In patient A revision was performed 7.8 years after the unilateral primary procedure because of modular femoral neck fracture, while patient B was left-side revised 15 years after the bilateral primary procedure because of aseptic loosening of the femoral stem.

RESULTS

Body weight was 30% higher and the arm of implanted modular femoral neck was 51% longer in patient A compared to patient B. Therefore, the stress ratio on the modular femoral neck of patient A was calculated to be 2.45 times higher than in patient B, preventing cold welding and producing taper damage and degradation at the neck-stem junction. Large clusters of metallic debris containing Titanium and Vanadium from the alloy were present in the periprosthetic soft tissues of patient A.

CONCLUSIONS

Patients with higher body mass index treated with fully modular Ti-alloy THA may be at increased risk to experience catastrophic failure of the device. Orthopaedic surgeons should avoid using long necks whenever possible, as these are especially prone to develop a vicious circle starting with the fretting process and crevice corrosion at the taper-cone connection, leading to crack initiation and crack propagation, accelerated by the increased vulnerability of the Ti-alloy in biologic media, ultimately ending as fracture at the typical site. Serum Ti concentration may represent a rough estimation of taper degradation and patients with elevated levels should be warned and followed accordingly.