Navigation enables to precisely reproduce pre-operative planning. Most systems however do not tell us the target values of the planning parameters. As we operate more and more young patients with full or only slightly reduced mobility, kinematic properties of artificial hip joints become more important. The range of motion of artificial hip joints is limited, finaly resulting in impingement and thereby in wear and loosening.

This limited range of motion has to be devided under the different planes of movement and at the same time dislocation stability has to be considered. We have to search for the ideal compromise.

For this purpose a three-dimensional computer model has been developed. Input are the implant geometries and the implantation angles. As result the range of motion of the different planes of movement is calculated. Thus the surgeon knows the consequences of his implant choice and his pre-operative planning.

As a result of this investigation flexion proved to be the critical plane of movement, above all in full profile cups and if anteversion of the cup and the stem are not considered. Another critical plane of movement is external rotation.

Concerning implant design extralong heads as well as antidislocation rims proved to be problematic. The same problem we face in antidislocation cups with a closing angle of more than 180° of course.

Only by considering the kinematic results of an individual THR planning full benefit can be taken out of navigation.