Modern total knee replacements aim to reconstruct a physiological kinematic behaviour, and specifically femoral roll-back and automatic tibial rotation. A specific software derived from a clinically used navigation system was developed to allow in vivo registration of the knee kinematics before and after total knee replacement. The study was designed to test for the feasibility of the intra-operative registration of the knee kinematics during standard, navigated total knee replacement.

The software measures the respective movement of the femur and the tibia, and specially antero-posterior translation and tibial rotation during passive knee flexion. Kinematic registration was performed twice during an usual procedure of navigated total knee replacement: 1) Before any bone resection or ligamentous balancing; 2) After fixation of the final implants. 200 cases of total knee replacement have been analysed. Post-operative kinematic was classified as following: 1) Occurrence of a normal femoral roll-back during knee flexion, no roll-back or paradoxical femoral roll-forward. 2) Occurrence of a normal tibial internal rotation during knee flexion, no tibial rotation or paradoxical tibial external rotation. All patients were followed up for a minimal period of 12 months, and reevaluated at the latest follow-up visit for clinical and functional results with completion of the Knee Society Scores.

Recording the kinematic was possible in all cases. The results of both pre-operative and post-operative registrations were analysed on a qualitative manner. The results were close to those already published in both experimental and clinical studies. About femoral roll-back, 54% had a normal femoral roll-back during knee flexion after total knee replacement, 13% had no significant roll-back and 33% had a paradoxical femoral roll-forward. About tibia rotation, 65% had a normal tibia internal rotation during knee flexion, 16% had no significant tibia rotation and 19 had a paradoxical tibia external rotation. The mean Knee Score was 92/100 ± 10 points. There was a significant correlation between the post-operative kinematic behaviour and the Function Score, with better score for the patients having a physiological femoral roll-back and a physiological tibial internal rotation during knee flexion (p<0.01).

Intra-operative analysis of the kinematic of the knee during total knee replacement may offer the chance to modify the kinematic behaviour of the implant and to choose the best fitted constraint to the patient's native knee in order to impact positively the functional result.

Navigation systems have proved to improve the accuracy of the bone resection during total knee replacement (TKR). They might also be helpful to assess intra-operatively the knee kinematics before and after prosthesis implantation.

We are using the OrthoPilot^® system (Aesculap, Tuttlingen, FRG) on a routine basis for TKR. The current standard version of the software helps the surgeon orienting the bone resections and allows measuring the ligamentous balancing. This version was modified to allow a continuous tracking of the 3D tibio-femoral movement during passive knee flexion and extension. The kinematics was assessed by measuring the tibial movement in these three planes with the femur as reference.

For the purpose of the study, following data were registered before and after implanting the prosthesis: flexion-extension angle, varus-valgus angle, rotational angle, antero-posterior translation. Additionally, the gap between the contact point of the femoral component and the corresponding point of the tibial resection was measured after prosthesis implantation. Two successive registrations were performed by each of the 100 patients of the study before and after prosthesis implantation. The pre-and post-implantation kinematic curves were respectively compared by each patient to assess reproducibility. The pre-and postimplantation kinematic curves were compared by each patient to assess the modification due to prosthesis implantation. The results were compared to the current available literature.

The kinematic curves were plotted from maximal extension to maximal flexion. The observed 3D kinematics seem to be in agreement with the current literature in both in-vitro and in-vivo studies. We could observe the tibial internal rotation and the femoral roll-back during flexion. Some patients experienced paradoxical movement, both before and after implantation. However the post-implantation kinematics was generally closer to the expected one than the pre-implantation kinematics.

The software has definitely the potential to assess the intra-operative knee kinematics during various surgical procedures. It might help to try several solutions (orientation of the resections, implant combination or design, ligamentous balancing… ) before final implantation, in order to choose the best individual compromise. The actual relevance of such a study remains to be defined. It might be interesting to compare these data with in-vivo kinematic studies by the same patients.