THE EFFECTS OF HARDNESS AND SURFACE QUALITY OF METAL TAPERS ON THE FRACTURE LOAD OF CERAMIC BALL HEADS IN HIP ENDOPROSTHESES

The safety of ceramic ball heads attached to hip endoprostheses by means of conical fitting requires that the connection be designed with the ceramic material in mind. The ISO 7206-5 standard describes a static testing protocol to measure the maximal loading limit of the conical connection. In order to achieve high fracture loads in this test, it is necessary to optimize the load-bearing parts of the connection to prevent stress concentrations, to which the ceramic ball is sensitive. For this reason, manufacturers of ceramic balls give their customers specifications for the design of the metal taper, specifying tolerances for angle, roughness, roundness and straightness. In order to investigate which parameters of the metal taper have an influence on the fracture load achievable according to ISO 7206-5, fracture load tests were conducted with zircon oxide (ZIO-LOX(R)) heads in combination with test tapers from various manufacturers. The test tapers were made of Ti- or CoCr-based alloys. Before the tests the tapers were completely measured and characterized according to geometric parameters of roughness, roundness, straightness, profile and angle, and physical parameters such as hardness and elastic modulus. Subsequent measurements after the fracture tests characterized changes that occured in the metal tapers. The fracture loads achieved depended on the potential for deformation of the taper surface. This potential can be described by a factor computed as hardness/roughness. A dependence of the fracture load on elastic modulus, form deviations or taper angle could not be established within the Cerasiv specifications.