Abstract
Purpose
Our aim was to compare the passive kinematics and coronal plane stability throughout flexion in the native and the replaced knee, using three different TKA designs: posterior stabilized (PS), bi-cruciate substituting (BCS), and ultracongruent (UC). Our hypotheses were: 1.) a guided motion knee replacement (BCS) offers the closest replication of native knee kinematics in terms of femoral rollback 2.) the replaced knee will be significantly more stable in the coronal plane than the native knee; 3.) No difference exists in coronal plane stability between the 3 implants/designs throughout flexion.
Methods
After IRB approval, two cadaveric specimens were used for a pilot study to determine sample size. Five fresh-frozen hip-to-toe cadaveric specimens then underwent TKA using an anatomic measured resection technique with a computer-navigated robotic femoral cutting-guide. The PS, BCS, and UC TKA designs were implanted in each knee using the same distal and posterior femoral cuts to standardize the position of the implants. Computer navigation was then utilized to record the varus/valgus laxity of each implant at 0°, 30°, 60° and 90° of flexion while applying a standardized 9.8Nm moment.
Passive tibiofemoral kinematics were measured in a continuous passive motion machine from 10° to 110°. Femoral rollback on the tibia was calculated for the native and replaced knees by measuring the closest point (CP) on the femoral condyle to a transverse plane perpendicular to the mechanical axis of the tibia at each flexion angle.
Results
Average coronal plane laxity increased with flexion from 0°-90° in the native and replaced knees. All three knee implant designs had comparable varus/valgus laxity throughout flexion with maximum differences between designs of <1°-2°, but were all more stable on average than the native knee at all flexion angles. The BCS design matched the native knee kinematics most closely, but exhibited more posterior translation and “sliding” of the femoral condyles on the tibial insert than observed in the native knee.
Conclusions
BCS TKA exhibited passive knee kinematic patterns that most closely resembled the native knee but with more femoral rollback. All three knee implant designs had comparable varus/valgus laxity throughout flexion, but were all more stable on average than the native knee throughout flexion.
