DEEP FLEXION KINEMATICS IN PCL-RETAINING AND–SACRIFICING KNEES WITH THE SAME IMPLANT DESIGN

Abstract

There is interest to provide total knee arthroplasty (TKA) patients large ranges of functional knee flexion. Factors contributing to flexion include a posterior femoral position on the tibia, posterior condylar offset, and posterior tibial slope. These factors can be incorporated into implant designs and surgical techniques. It is useful to assess the robustness of the resulting design, that is, the consistency of kinematic or functional results when patient and surgical factors vary widely. This study evaluates in vivo flexion performance of a single implant design in patients whose posterior cruciate ligament (PCL) was either retained or sacrificed.

28 knees in 20 patients were imaged using fluoroscopy during maximum flexion kneeling and lunge activities. 20 knees (12 patients) received TKA with the PCL retained by a bone block (PCL+ group). Eight knees (7 patients) received TKA with complete PCL resection (PCL- group). All knees received a fixed-bearing TKA (3D Knee^™, Encore Medical, Austin, TX) with an asymmetric tibial bearing having a sagittally curved medial compartment and a lateral compartment fully congruous with the lateral condyle in extension (approximating anterior cruciate ligament substitution). Three-dimensional knee kinematics were determined using model-based shape registration techniques.

For the kneeling activity, mean implant flexion was 124°±11° for PCL+ knees and 121°±17° for PCL- knees (p> 0.05), mean tibial internal rotation was 10°±4° for PCL+ knees and 9°±3° for PCL- knees (p> 0.05) and tibial valgus was −1°±1° for PCL+ knees and 2°±4° for PCL- knees (p=0.003). Medial contact location averaged −2±4mm and for PCL+ knees and −1±2mm for PCL- knees (p> 0.05). Lateral contact location averaged −10±4mm for PCL+ knees and −7±1mm for PCL- knees (p> 0.05). For the lunge activity, mean implant flexion was 120°±11° for PCL+ knees and 121°±21° for PCL- knees (p> 0.05), mean tibial internal rotation was 11°±4° for PCL+ knees and 8°±3° for PCL- knees (p> 0.05) and tibial valgus was −1°±1° for PCL+ knees and 2°±2° for PCL- knees (p=0.0002). Medial contact location averaged 0±4mm for PCL+ knees and −4±3mm for PCL- knees (p=0.04). Lateral contact location averaged −8±4mm for PCL+ knees and −9±4mm for PCL- knees (p> 0.05).

There was no difference in implant flexion between PCL retaining and sacrificing TKA. Both groups had knees with more than 145° implant flexion (~155° skeletal flexion). There were no significant differences in tibial rotation or lateral condylar contact locations. There were differences in tibial valgus for both activities. PCL- knees exhibited a tendency for the medial compartment to ‘book open’ with flexion beyond 130°, consistent with loss of PCL function. Based on this small cohort comparison, it appears that robust flexion performance and knee kinematics can be obtained with a fixed-bearing TKA design.