Causes of failure in flexible metacarpophalangeal prostheses

Rheumatoid arthritis is a common and debilitating disease which often affliets the small joints of the hands, especially the metacarpophalangeal joints. Although there have been recent improvements in drug treatments, for a number of patients the final clinical intervention entails the replacement of the natural joint with a prosthesis. The vast majority of such implants are single-piece, flexible, silicone designs. This group includes the Swanson, Sutter and NeuFlex metacarpophalangeal prostheses. The most common mode of failure of these implants is fracture, often at the junction of the distal stem and the hinge. Although such fracture does not always mean that a revision operation is required, it is self-evident that if the fracture rate could be reduced, such implants should provide enhanced results in the hands of patients. A clear way to reduce the fracture rate is to understand how fracture has come about. A cohort of 12 failed and explanted Sutter metacarpophalangeal prostheses were obtained from the hands of three rheumatoid patients. After a macroscopic analysis, the explanted prostheses were sliced so that the fracture faces could be examined under a scanning electron microscope. From the results obtained, a number of features consistent with fatigue failure were identified. These features showed that fracture was initiated on the dorsal aspect of the distal stem and travelled in a palmar-ulnar direction.