Corrosion-Induced Fracture of a Double-Modular Hip Prosthesis

Abstract

Modularity is an important aspect of total hip replacement design. A modular Morse taper is commonly used to attach the femoral head to the femoral stem. Newer double-modular designs incorporate a second interface at the neck-stem junction. Increased modularity purportedly allows the surgeon to more closely restore patient anatomy, such as limb length, lateral offset, and femoral anteversion, and to better balance the soft tissue to achieve optimal biomechanics. However, modularity also increases the number of mechanical junctions that may lead to fretting (micromotion)1, corrosion2,3, and ultimately fracture. We have evaluated a double-modular PROFEMUR Z stem (Wright Medical Technology, Arlington, Tennessee) after catastrophic failure due to a fractured neck component at the neck-stem junction. This retrieval allowed us to analyze the mechanical failure mechanisms associated with a double-modular design. We hypothesized that modularity at the neck-stem junction resulted in fretting and crevice corrosion, which led to crack initiation and, on the application of an overload event, to fracture. The patient was informed that data concerning the case would be submitted for publication, and he consented. In June 2006, a thirty-year-old man with rheumatoid arthritis who was 6 ft and 6 in (2 m) tall, weighed 242 lb (109.8 kg), and had a body mass index of 29 kg/m2 underwent an uncomplicated right total hip arthroplasty with a cementless PROFEMUR Z hip stem with a long, straight neck (no varus/valgus offset) and a ceramic-on-ceramic articulation. The ceramic-on-ceramic modular implant was considered to be indicated in this particular patient because of his young age, high body mass index, active lifestyle, and profession. A modular stem was used in an effort to reproduce the anatomic femoral anteversion, offset, and lower-limb length to optimize hip abductor and biomechanical function. The hip functioned well until April 2008, when …