In navigated knee arthroplasty the hip centre is determined by rotary motion of the femur (pivoting). The accuracy of this functional hip centre determination in vivo is unclear. In the following paper the accuracy of pivoting in the determination of the hip centre was examined.
Navigated (TC-PLUS, Solution, PLUS Orthopedics) total knee arthroplasty (PI Galileo, PLUS Orthopedics) was performed on 25 patients with primary arthritis of the knee joint. The position of the femoral component and the hip centre were postoperatively determined by computer tomography. Through comparison with the intraoperatively documented data, the deviation of the pivoted from the true hip centre in the frontal and sagittal planes was calculated. The degree of arthritis of the hip was determined on plain radiographs according to Kellgren.
The mean deviation was determined to 1.0 +/- 0.7 degrees in the frontal plane and 2.5 +/- 1.6 degrees in the sagittal plane (p = 0.002). This corresponds to a mean overall deviation of 20 +/- 10 mm. The data were continuously, non-parametrically distributed without any outliers. A great range of motion (ROM) in the frontal as well as sagittal planes during pivoting resulted in a less accurate determination of the hip centre. There was no correlation to the degree of arthritis of the hip.
The results indicate a recommendable ROM during pivoting for maximal accuracy of hip centre determination of 20 to 30 degrees in the sagittal plane and 30 to 40 degrees in the frontal plane. Arthritis of the hip is not a contraindication for functional determination of the hip centre.