MEASUREMENT OF THE SPATIAL MICROMOVEMENT OF HIP ENDOPROSTHESES AS A FUNCTION OF THE SPATIAL FORCE SYSTEM - A PILOT-STUDY

A precision measurement method (resolution: 0.1 mu m) is described tha t simultaneously tracks spatial movement of the prosthesis and its ben ding as a function of the spatial force system (three force and three torque components). With this technique, we examined the effects of co mplete separation between shaft and cement on bending and micromovemen t. With regard to mechanical stability a completely tied and a loosene d prosthesis differ only in respect to torsional load: the loosened pr osthesis bears a 2.5-fold larger torsion flexibility. In both cases th e shaft does not move like a rigid body: it bends up or down depending on the way the torque acts. It is the torques (that are produced by t he forces) rather than the forces themselves that are responsible for the extent of micromovement and bending. In the completely loosened sh aft, the shaft's apex only moves either intrusively or extrusively acc ording to the directions of the torque vector: the horizontal movement cannot be resolved. The data suggest that shafts that are freely moun ted into its cement quiver load the cement layer in a less damaging fa shion than tied shafts.