Abstract
Introduction
Total-knee-arthroplasty (TKA) is used to restore knee function and is a well-established treatment of osteoarthritis. Along with the widely used fixed bearing TKA design, some surgeons opt to use mobile bearing designs. The mobile-bearing TKA is believed to allow for more freedom in placement of the tibial plate, greater range of motion in internal-external (IE) rotation and greater constraint through the articular surface. This current study evaluates 1) the kinematics of a high constraint three condyle mobile bearing TKA, 2) the insert rotation relative to the tibia, and 3) compares them with the intact knee joint kinematics during laxity tests and activities-of-daily-living (lunge, level walking, stairs down). We hypothesize that 1) in contrast to the intact state the anterior-posterior (AP) stability of the implanted joint increases when increasing compression level while 2) maintaining the IE mobility, and that 3) the high constraint does not prevent differential femorotibial rollback during lunge.
Methods
Six fresh-frozen human cadaveric knee joints with a mean donor age of 64.5 (±2.4) years and BMI of 23.3 (±7.3) were tested on a robot (KR140, KUKA) in two different states: 1) intact, 2) after implantation of a three condyle mobile bearing TKA. The tibia plateau and the insert of each tested specimen were equipped with a sensor to measure the insert rotation during testing. Laxity tests were done at extension and under flexion (15°, 30°, 45°, 60° 90°, 120°) by applying subsequent forces in AP and medial-lateral (ML) of ±100N and moments in IE and varus-valgus (VV) rotation (6Nm/4Nm, 12 Nm/-). Testing was performed under low (44N) and weight bearing compression (500N). Loading during the lunge, level walking and stairs descent activity was based on in-vivo data. Resulting data was averaged and compared with the kinematics of the intact knee.
Results
Increasing the joint compression resulted in a 90% reduced AP laxity (increased stability) for the implanted case while the intact knee laxity stayed similar. In high compression the implanted IE mobility was reduced by 45% for low and mid flexion angles and by 20% for high flexion angles, while the intact knee IE mobility was reduced by 30% at low and mid flexion and 20% at high flexion. The trend of the rollback behaviour was similar for the implanted and intact joints and showed higher lateral than medial rollback (Figure 3 A). The average insert-rotation was highest during level walking (+ 5° to −2.5°) and lowest during lunge (−3.5° to 2.5° over flexion).
Conclusion
The established hypotheses were supported by the above listed results. Increasing the joint compression in the mobile bearing design stabilized the knee in the AP direction and maintained the IE mobility similar to the intact knee. This can be directly related to the design of the TKA articular surface, which has a high impact on constraint as soon as the joint is loaded. However, the high constraint of the TKA did not prevent differential rollback.
