Ring avulsion injuries: a biomechanical study.

The biomechanics of ring avulsion injuries was studied in a cadaveric simulation model. Custom-fitted metal rings attached to a rigid frame were placed over the proximal phalanx of fresh or thawed fresh-frozen specimens. Ring avulsion injuries in 44 fingers were produced with a standardized force applied to the proximal ulna. The progress of injury was evaluated with simultaneous high-speed cinematography and continuous force measurements. The injured digits were x-rayed and categorized according to Urbaniak's classification. Continuous force measurements produced similar curves for all classes of injuries. The average maximum force resulting in class I injuries was 80 N. The average maximum force producing amputation in class III injuries was 154 N, a force much lower than expected. Force measurements for class II injuries were nearly identical to those of class III. This surprisingly minimal force resulting in digit amputation was explained by high-speed cinematography, which showed that the rings tilt on the digits concentrating disruption forces as a result of ring angulation on the finger. Incomplete amputations were due to loss of ring purchase by skin flap eversion. Finally, comparison of high-speed cinematography with force curves suggests that skin rupture rather than skeletal or tendon disruption accounted for the maximum force during ring avulsion injury.