Friction in hip hemiendoprostheses. Review of literature and own model using cadaveric acetabula.

The science of tribology concerning hip arthroplasty has mainly dealt with total endoprosthesis, whereas measurement values of hemiendoprosthetic implants are rare. The small amount of experimental tribologic data concerning hemiendoprosthetic implants in the form of pendulum trials, animal experiments, in vivo measurements on human hip joints and pin on disc studies will be reviewed in the following work. The reported frictional coefficients in these studies were between 0.014-0.57. In order to test the friction coefficients of different femur head hemiendoprostheses (unipolar ceramic- and metal heads) against fresh cadaveric acetabula, the HEPFlEx-hip simulator (Hemi-EndoProsthesis Flexion Extension) was developed. In the simulator, the various hemiendoprosthetic heads are placed on a special cone and tested against a cadaver acetabulum cast in MCP 47 woodmetal. The plane of movement of the apparatus is uniaxial with a flexion-extension movement of 35 degrees. The force is produced pneumatically with amounts of up to 5 kN. Newborn calf serum serves as a lubricant. A PC collects the data from torque-, force-, and angle-sensors on-line and allows the simultaneous processing and visualization of the data. The frictional coefficients produced by the different head materials and the relevance of the play between the hemiendoprothesis head size and acetabulum can be determined. Preliminary results showed that the mean friction coefficient at 1 kN loading was =0.024-0.063 for ceramic against cartilage and =0.033-0.075 for metal against cartilage. (Hip International 2002; 2: 126-34).