Problem Wrong positioning of the acetabular cup leads to impingement, luxations and limited ROM. To improve the cup position computer assistant navigation systems had been developed. They are expensive, time consuming and sometimes no easy to handle.

Solution A new concept for the implantation of THP will be introduced: The cup position is not orientated as usual according to the body axis, but according to the stem position. Stem and cup are a closed biomechanical system, cup anteversion and inclination will be positioned according CCD angle and torsion of the stem.

With a new mechanical navigation system for the cup with a special cup probe and guiding system the best position is found intraoperatively according to ROM, impingement, luxation and tension of the muscles.

Results 150 patients had been operated. Optime is extended only 4–6 minutes, dysplasia inlays, impingement and luxations had disappered. The system is easy to handle and to sterilize routinely with the other op.-instruments.

Aims: To evaluate the clinical signiþcance of isolated femoral malrotation after otherwise well performed painful TKAñs. Methods: 11 painful TKAñs (5 female, 6 male, mean 61 years, range 41 to 73) with normal mechanical axis, patella tracking, stability in extension and no signs of infection or loosening were included in this prospective study. The knees were evaluated by routine clinical and radiographic examinations in combination with a standardized computer tomography (CT) to measure the rotation of the components, using the transepicondylar axis as a reference for the femur. Results: All patients had progressive persistent pain from the time of surgery which was resistant to conservative therapy. There were two groups according to the clinical symptoms: limited ßexion and medial pain at the proximal tibia (4) or ßexion > 90û with anterior knee pain during stairs descending or raising from a chair (7). Standard x-rays were normal but in the special CT all patients showed isolated internal malrotation of the femoral component mean 7û (2 to 10). 9 patients required revision surgery with correction of the malrotated femoral component. The two patients who did not want revision surgery had only minor malrotation (< 4û).Conclusions: In painful TKAñs with unknown persistent pain but limited ßexion and/or lateral instability in the ßexion gap evaluation of the femoral component rotation with a special CT should be performed.