Morphology of rod stock and compression-moulded sheets of ultra-high-molecular-weight polyethylene used in orthopaedic implants.

Compression-moulded sheets and extruded rods of ultra-high-molecular-weight polyethylene (UHMWPE) are currently used in the production of joint replacement prostheses. Crystallographic texture present in rods and sheets of UHMWPE was measured using a combination of small-angle X-ray scattering and wide-angle X-ray diffraction. Crystallographic texture can induce anisotropy in macroscopic properties of polymers, such as modulus and yield stress. Both rods and sheets of UHMWPE revealed a low but discernible degree of preferred orientation of polyethylene chains within crystallites. There was a spatial variation in crystallographic orientation in extruded rods. The direction of chain alignment within crystallites located near the outer surface of rods was orthogonal to the radial direction, whereas the chain direction was orthogonal to the axial or extrusion direction in crystallites located near the centreline of extruded rods. Crystallographic texture was spatially uniform in compression-moulded sheets with the chain direction within crystallites aligned orthogonal to the moulding direction. In both cases the induced crystallographic texture can be explained in terms of crystallization from an oriented melt.