1 Discussion

Failure is due to fatigue; however, fatigue is always initiated at point of stress concentration or discontinuity. The manufacturer had changed the Corail stem's neck geometry from matt to polished finish some years ago to extend the fatigue life. Additionally, the neck had flattened in the antero-posterior planes to increase range of motion. The antero-posterior flats are common to a number of manufacturer's necks. Consequently it is believed that something else may have initiated the fatigue failure in this Corail stem. Similar failures have been reported in the past on titanium stems, authors have attributed failure to overload, geometry, and laser markings.⁴ Other causes for catastrophic failure of a femoral stem are material design, implant positioning (varus alignment) and patient characteristics.^5–7

It has been documented in the literature that the anterior and lateral aspect of femoral prostheses are subjected to the greatest tensile stresses during walking, initiating sit to stand, and climbing stairs.⁸ Cracks initiate under tensile stresses. In our case, the manufacturer laser-etched their product codes onto the tensile aspect of the femoral stem. These laser-etchings caused a stress-riser, which initiated the fatigue fracture whilst the patient was gardening in the squatting position. It has been previously described by Lee et al that laser etching at the neck–shoulder junction of the femoral stem causes heat-induced changes in the microstructure as well as an area of local stress.⁹

Woolson et al observed fatigue fractures through etched characters on the anterolateral aspect of femoral stems of all the prostheses that were analysed by the manufacturer using scanning electron microscopes.¹⁰

The affected Corail AMT femoral stems were recalled in February 2004 once DePuy France was aware of multiple incidents of post-operative fatigue fractures through Post Marketing Surveillance.³ Changes in prosthesis design need to be thoroughly tested in a controlled laboratory environment before being offered to surgeons and the community. Catastrophic failures of implants due to poor design and inadequate quality control can have a devastating financial and health impact on the general population.

Conflicts of interest

All authors have none to declare.