Abstract
Background
Various postoperative evaluations using fluoroscopy have reported in vivo knee flexion kinematics under weight bearing conditions. This method has been used to investigate which design features are more important for restoring normal knee function. The objective of this study is to evaluate the kinematics of a Low Contact Stress total knee arthroplasty (LCS TKA) in weight bearing deep knee flexion using 2D/3D registration technique.
Patients and methods
We investigated the in vivo knee kinematics of 6 knees (4 patients) implanted with the LCS meniscal bearing TKA (LCS Mobile-Bearing Knee System, Depuy, Warsaw, IN). Mean period between operation and surveillance was 170.7±14.2 months. Under fluoroscopic surveillance, each patient did a deep knee flexion under weight-bearing condition. Femorotibial motion was analyzed using 2D/3D registration technique, which uses computer-assisted design (CAD) models to reproduce the spatial position of the femoral, tibial components from single-view fluoroscopic images. We evaluated the knee flexion angle, femoral axial rotation, and antero-posterior translation of contact positions.
Results
The mean maximum knee flexion angle was 109.3±9.1°. The mean axial rotation of the femoral component exhibited gradual external rotation from full extension to maximum flexion reaching 9.4±5.9°. At full extension, the medial contact position was −3.7±2.9 mm, and the lateral contact position was −4.4±4.7 mm. The medial contact position moved 2.1 mm anteriorly from full extension to 80° of knee flexion, and then moved 0.4 mm posteriorly until maximum flexion. On the other hand, the lateral contact position stayed constant from full extension to 80° of knee flexion, and then moved 2.3 mm posteriorly until maximum flexion. At maximum flexion, the medial contact position moved anteriorly to a final position of 1.3±4.0 mm and the lateral contact position moved posteriorly to a final position of −6.8±3.8 mm. From the results of bilateral contact positions at each flexion angle, patterns of kinematic pathways were determined. From full extension to 80° of knee flexion, the kinematic pattern was a lateral pivot pattern, where the medial contact position kept moving forward while the lateral contact position remained constant. With more than 80° of knee flexion, kinematics changed into a medial pivot pattern.
Discussion
This study has investigated the kinematics of a LCS meniscal bearing TKA. The typical subjects exhibited a lateral pivot pattern from full extension to 80° of knee flexion, that are not usually observed in normal knees. It might be caused by the geometry of replaced articular surfaces and the mobility of the meniscal-bearing insert. Further investigation should be necessary in more number of cases not only in this implant but also in other types of LCS.
