The effect of cast design on the stabilization of underlying long bones

Two composite model systems, each representing a bone overlying soft tissues and a surrounding cast, were tested for strain along the length of the bone while torsional stress was applied through the cast. The two models were identical in every dimension except that one was square in cross section, and one was circular. Three triaxial rosette strain gages, spaced evenly along the bone, generated output to a strain gage conditioner and amplifier. Data from the two models are compared. In two of the nine gages, there was a significant difference between models, with the square model displaying greater strains. Calculations of principal stress followed a similar trend with the square model displaying consistently larger values of stress. The data suggest that a quadrilateral cast design may provide added stabilization to a bone contained within, compared to the traditional form-fitting cast