Abstract
Introduction
The Bi-Cruciate Stabilized (BCS) total knee arthroplasty (TKA) incorporates two cam-post mechanisms to reproduce the functionality and stability provided by the anterior cruciate ligament and posterior cruciate ligament in the native knee. The anterior cam-post mechanism provides stability in full extension and early flexion (≤20°) while the posterior cam-post mechanism prevents anterior sliding of the femur during deeper flexion (≥60°). Recently (2012), a second generation BCS design introduced more normal shapes to the femur and tibial bearing geometries that provides delayed lateral femoral condyle rollback and encourages more stable positioning of the medial femoral condyle. The purpose of this study was to compare the in vivo kinematics exhibited by the two generations during weight bearing flexion.
Methods
In vivo kinematics were derived for 126 patients. Eighty-six subjects were implanted with a first generation BCS (BCS 1) TKA and 40 with the second generation BCS (BCS 2) TKA. Fluoroscopic videos were captured for patients while they performed a deep knee bend (DKB) from full extension to maximum flexion. Anterior-posterior motion of the lateral femoral condyle (LAP) and the medial femoral condyle (MAP), as well as tibio-femoral axial rotation, were analyzed at 30° increments from full extension to maximum flexion using a 3D-to-2D image registration technique. Statistical analysis was conducted at the 95% confidence level.
Results
From full extension to 120° of flexion the amount of posterior femoral rollback (PFR) for the lateral condyle was −25.8±5.87 mm and −14.4±4.75 mm for the BCS1 and BCS2 groups respectively (p=0.003). Over the same range of motion, the anterior-posterior motion of the medial condyle in the BCS1 and BCS2 groups was −15.8±3.03 mm and −8.46±2.35 mm respectively (p=0.001). Significant differences in LAP and/or MAP position existed at each flexion increment from 0–120° of flexion (Figure 1). The BCS1 group exhibited 12.1±6.57° of axial rotation from full extension to 120° of flexion, while the BCS2 rotated 7.36±4.31° (p=0.046). Significant differences in femoral rotation with respect to the tibia existed at full extension and 30°of flexion (Figure 2).
Discussion and Conclusion
Compared to BCS2 subjects, those implanted with the BCS1 exhibited greater magnitudes of anterior-posterior motion and more tibio-femoral axial rotation during DKB. The guided motion of the BCS1 encouraged large translations and rotations that may have led to complications such as dislocations and anterolateral knee pain. In comparison, the BCS2 exhibits attenuated and more stable anterior-posterior motions, while still maintaining sufficient magnitudes of rollback and rotation as intended.
