Stiffness and strength design of composite bone plates

Although the potential of composites used for bone plate implantation has been realized for a long period of time, not much literature has addressed optimal design of composite bone plates upon variation in reinforcing fiber structures, plate thicknesses, and so on. In this paper, we present a preliminary but critical design procedure for composite bone plate with target on both its stiffness and its ultimate strength. While the material system used is the same, i.e. carbon fibers and PEEK matrix, three kinds of fibrous preforms namely unidirectional (UD) prepregs, braided fabrics, and knitted fabrics are considered as reinforcements, constituting six different candidate laminated plates. They are UD [0°]ns, angle plied [±20°]ns, multidirectional [0°/±20°/0°]ns, two braiding angle diamond braid fabrics [5°]ns and [15°]ns, and plain weft knitted fabric laminates. By varying the number of layers (n) in the lamination, different plate thicknesses can be attained. The structure-property relationship of the composite plates is realized through the use of a micromechanics bridging model. A clinically used stainless steel plate is taken as a design benchmark. It has been found that the composite bone plates with all the considered lamination patterns except for the knitted fabric reinforcement and having a thickness near to that of the stainless steel plate can exhibit required properties of bending rigidity and maximum bending moment. The plate thickness is more controlled by strength rather than by stiffness requirement. However, the 5° braided fabric composite bone plate shows an overall superiority as compared to the other lamination forms, and is thus recommended as the first choice.