Mechanical properties and material characteristics of orthopaedic casting material

In this study, two types of orthopaedic casting materials were evaluated: The Johnson & Johnson Specialist plaster bandage and the 3M Scotch-cast Plus fiber glass bandage. The materials were evaluated using tensile tests to determine the elastic modulus, yield strength, and ultimate tensile strength. To determine the structural characteristics and stiffness of a cylindrical cast, each material was formed around a foam cylinder core and tested in a four-point bend jig. A computer-based model using the finite element method (FEM) was developed for a cylindrical cast of both types and compared with the experimental findings. A second FEM model with loads applied at the periphery was performed to simulate the clinical observations of plaster bandage breakdown at the ends of a cast. It is with these tests that the two bandage materials were compared and evaluated. It was concluded that the plaster bandage, while initially stiffer than the fiber glass bandage, had a much lower yield stress. This implies that the plaster cast may break down under loads that would leave the fiber glass cast intact. It was also determined that the plaster bandage load displacement curve is bilinear. The bilinear characteristic of the plaster bandage explains its breakdown at the ends of a cast.