MORPHOLOGY OF ROD STOCK AND COMPRESSION MOLDED SHEETS OF ULTRA-HIGH-MOLECULAR-WEIGHT POLYETHYLENE USED IN ORTHOPEDIC IMPLANTS

Compression-moulded sheets and extruded rods of ultra-high-molecular-w eight polyethylene (UHMWPE) are currently used in the production of jo int replacement prostheses. Crystallographic texture present in rods a nd sheets of UHMWPE was measured using a combination of small-angle X- ray scattering and wide-angle X-ray diffraction. Crystallographic text ure 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 c hains within crystallites. There was a spatial variation in crystallog raphic orientation in extruded rods. The direction of chain alignment within crystallites located near the outer surface of rods was orthogo nal to the radial direction, whereas the chain direction was orthogona l to the axial or extrusion direction in crystallites located near the centreline of extruded rods. Crystallographic texture was spatially u niform in compression-moulded sheets with the chain direction within c rystallites aligned orthogonal to the moulding direction. In both case s the induced crystallographic texture can be explained in terms of cr ystallization from an oriented melt. (C) 1996 Elsevier Science Limited .