Le but de notre étude était d'étudier l'influence du positionnement de l'interligne articulaire tibiofémoral distal du genou prothésé sur la cinématique tridimensionnelle du genou et sur le comportement des ligaments collatéraux au cours du mouvement in vitro .

Nous avons mis en place une prothèse totale du genou postérostabilisée sur sept pièces cadavériques fraîchement congelées. Un appareil expérimental original mettant l'articulation du genou en mouvement a permis d'enregistrer en trois dimensions par un système optoélectronique Vicon^® la cinématique fémorotibiale et fémoropatellaire du genou. Deux goniomètres électroniques ont mesuré l'écartement des ligaments au cours du mouvement.

Cinq configurations ont été enregistrées pour un même genou : genou sain, genou prothésé, abaissement de l'interligne articulaire tibiofémoral distal du genou prothésé de 2 mm puis 4 mm et rehaussement de l'interligne de 2 mm sans que soit modifié l'équilibrage ligamentaire en extension. Les courbes obtenues ont été comparées entre elles en calculant un coefficient de multi-corrélation.

Ces variations du positionnement de l'interligne du genou modifient significativement la cinématique de la patella (rotation et abduction-rotation) et ne modifient pas significativement la cinématique fémorotibiale du genou prothésé. Tout en conservant un bon équilibrage ligamentaire en extension, si l'interligne est abaissé il apparaît une laxité en flexion, si l'interligne est rehaussé il apparaît une tension des ligaments au cours de la flexion.

The level of the joint space can be modified after implantation of a total knee prosthesis. Likewise, ligament balance is a cardinal point of the surgical technique. The purpose of this in vitro work was to study the influence of the position of the distal tibiofemoral joint space after implantation of a total knee prosthesis on the three-dimensional kinetics of the knee joint and on the behavior of the lateral ligaments.

Total knee arthroplasty (TKA) with a posterior stabilized prosthesis was performed on seven fresh-frozen cadaver specimens. A specially-designed experimental device was used to achieve continuous knee motion simulating hip flexion from a vertical position. The Vicon^® optoelectronic system was used to record the femorotibial and femoropatellar kinematics in three dimensions. Two electronic goniometers were positioned on the insertions of the lateral ligaments to measure ligament displacements during knee movements. Five configurations were recorded on each knee: healthy knee, same knee after TKA, and 2-mm and 4-mm upward displacement of the prosthetic distal tibiofemoral joint space. Ligament balance at extension was preserved in all configurations. The kinematic curves obtained were compared with the coefficient of multiple correlation.

Changing the position of the joint space had a significant effect on the kinematics of the patella (rotation and abduction-rotation) but did not have a significant effect on the femorotibial kinematics. Variations in the length of the lateral ligaments were of small amplitude. Lowering the joint space led to laxity at flexion. Raising the joint space tightened the ligaments at flexion.

These results confirm our clinical impression when the level of the distal femur cut is set to achieve tension on the ligaments at knee extension. If the joint space is lowered, i.e. with a more sparing distal femur cut, the prosthesis takes up less space during flexion, leading to laxity at flexion. If the joint space is raised, i.e. with an excessive distal femur cut, the prosthesis takes up more space during flexion, tightening the lateral ligaments.

The position of the joint space must be rigorously reproduced during TKA not only to maintain correct femorotibial kinematics, but most importantly to preserve patellar kinematics and proper behavior of the lateral ligaments. Ideally, the height of the joint space should be restored first, followed by control of the ligament balance. An over- or undercut of the femur can lead to defective femoropatellar kinematics and ligament tension at flexion despite good ligament balance at extension. In addition, ligament balance should not be achieved by displacing the tibial cut or by modifying the thickness of the tibial component, which would have an effect not only at extension but also at flexion.