Soft layered prostheses for arthritic hip joints: a study of materials degradation

A great deal of interest is being shown in complaint bearing surfaces for artificial joints. These produce very low friction because of the fluid-film lubrication that they exhibit, and therefore should produce lower wear than current prosthetic materials as the two surfaces of the joint are completely separated by a film of synovial fluid. However, one problem with soft elastic materials in vivo is that the elastomers may degrade with time. Specimens of four polyurethanes were kept in Ringer's solution at 37 degrees C for about 4 months to investigate changes in mechanical properties and the mode of lubrication with time of immersion. The materials tested were a polyether-urethane (E57), a polyester-urethane (E58), an aromatic polyether-urethane (P1) and an aliphatic polyether-urethane (A1). Samples of E57 showed a decrease in hardness and elastic modulus of 21% and 28% respectively. This was much better than E58 which showed decreases of 32% and 42% respectively. Better results were achieved with A1 (hardness change 2%) and P1 (decrease in hardness of 14% and modulus 17%). When tested in a hip-function simulator, A1 showed virtually no change in its very low coefficient of friction (0.004), P1 showed an increase of 60%, while E57 and and E58 both showed a doubling in friction over the course of the study. The modes of in vitro degradation were considered and surface effects determined to be most damaging. Currently the best candidate for a compliant material in hip replacement is the aliphatic polyether-urethane which maintains fluid-film lubrication.