KINEMATIC COMPARISON OF THE NORMAL KNEE AND A SECOND-GENERATION BCS TKA DURING A DEEP KNEE BEND

Background The Bi-Cruciate Stabilized (BCS) total knee arthroplasty (TKA) incorporates two cam-post mechanisms in order to replicate the functionality and stability provided by the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) in the native knee. Recently (2012), a second generation BCS design has introduced femur and tibial bearing modifications that are intended to delay lateral femoral condyle rollback and encourage more stable positioning of the medial femoral condyle to more closely replicate normal knee kinematics. The purpose of this study was to compare the kinematics of this TKA to the normal knee during a weight bearing flexion activity. Methods In vivo kinematics were derived for 10 normal non-implanted knees and 40 second generation BCS TKAs all implanted by a single surgeon. Computed tomography (CT) scans were obtained for each normal patient, and 3D reconstruction of the femur, tibia/fibula, and patella was performed. Fluoroscopic images were captured at 60 Hz using a mobile fluoroscopic unit that tracked the knee while patients performed a deep knee bend (DKB) from full extension to maximum flexion. A 3D-to-2D image registration technique was used at 30° increments to determine the transformations of the segmented bones or TKA components. The anterior-posterior motion of the lateral femoral condyle contact point (LAP) and the medial femoral condyle contact point (MAP), as well as tibio-femoral axial rotation, were measured at 30° increments from full extension to maximum flexion. Statistical analysis was conducted at the 95% confidence level. Results From full extension to 120° of knee flexion the lateral condyle contact point translated posteriorly by 14.55 mm ± 5.11 mm and 10.47 mm ± 3.14 mm in the Normal and BCS groups respectively (p=0.1984). Over the same range of motion, the anterior-posterior motion of the medial condyle contact point in the Normal and BCS groups was −5.05 mm ± 2.91 mm and −10.66 mm ± 4.46 mm respectively (p=0.0433). Significant differences in LAP and/or MAP position existed at each flexion increment from 0–120° of flexion (Figure 1). The Normal group exhibited 19.85° ± 6.92° of axial rotation from full extension to 120° of flexion, while the BCS rotated 7.36° ± 4.31° (p=0.0085). Significant differences in femoral rotation with respect to the tibia existed at full extension as well as at 30° and 60° of knee flexion (Figure 2). Conclusions Like the normal knee, the BCS experiences larger amounts of posterior motion in the first 30 degrees of knee flexion, compared to its mid-flexion phases (30°–90°). After 90 degrees the posterior motion in the BCS continues to increase, likely in part due to posterior cam-post engagement as intended. In this sample of normal knee subjects, very little posterior motion of either femoral condyle happens between 90 and 120 degrees of knee flexion although significant amounts of rollback are expected thereafter as reported in the literature. The axial rotation experienced by the BCS group is quite less than that experienced by the Normal group, however it is quite comparable to other TKAs, if not greater.