Biomechanical analysis of the role of wrist guards in "splint-top" forearm fractures [skating application]

The use of wrist guards has substantially reduced the incidence of wrist and forearm injuries. There is concern, however, that current wrist guard designs do not sufficiently protect the forearm. This study tests the hypothesis that, during a fall while wearing wrist guards, mid-shaft forearm stress can exceed the ultimate strength of cortical bone. Further, it investigates possible mechanisms by examining impacts at (1) the thenar eminence, vs. (2) the metacarpal heads, as well as examining stresses along the length of the forearm. A model of forearm, palm, and wrist guard biomechanics implemented parameter values from the literature and direct measurement to compute link forces and moments. Normal, shear and principal stresses were computed in cortical bone as a function of position along the forearm. The model supported the hypothesis: under a 2000N impact to the thenar eminence, the principal stress exceeds cortical bone ultimate strength, at 256 MPa. At the metacarpal heads, the maximum principal stress was 225 MPa, suggesting this unusual load pattern is not a mechanism for mid-shaft fracture. Forearm stresses did show a sharp increase at the proximal wrist guard edge, reflecting an abrupt end to load-sharing and suggesting large local stresses. In conclusion, the results suggest a new wrist guard design incorporating a gradual tapering of bending moment of inertia will diminish the load transfer rate at the wrist guard edge, possibly reducing the occurrence of "splint-top" fractures.