Outcomes following TKA often are good, but patients sometimes lack adequate range of motion and strength. Reasons for these deficits may include instability and the loss of cruciate ligament function. One approach to TKA design is to retain the PCL, and configure the TKA surfaces to approximate the function of the ACL. This can be accomplished by having a lateral surface that controls tibiofemoral motion near extension, but allows femoral rollback with flexion. We have been using such a fixed-bearing TKA design since 2001. The purpose of this study was to determine if an ‘ACL-substituting’ arthroplasty design provides clinical and functional results comparable to traditional PCL-retaining arthroplasty designs.

This series consists of 407 consecutive knees in 185 male and 222 female patients (73±9 years, 28±5 BMI) operated from November 2001 to August 2006. All patients underwent TKA by the same surgeon using PCL-retention and implantation of the same cemented ‘ACL-substituting’ TKA design. Clinical outcomes were evaluated using Knee Society Scores and radiographic review for the first 100 TKA with minimum 2 year follow-up. A subset of patients participated in IRB-approved protocols to quantitatively evaluate TKA motion and strength. Functional outcomes were assessed during gait, stair-climbing and curb step-over tasks for 10 unilateral TKA using a motion capture system, force platforms and inverse dynamics to measure the dynamic knee joint flexion moment. Kinematic outcomes were studied during kneeling for 20 TKA using fluoroscopy and shape matching techniques.

Knee Society Scores averaged 96+7 (pain) and 95+12 (function) at an average of 3.2+0.7 (range, 2 to 5) years follow-up. Passive flexion averaged 122°±10°, with 70% of the TKA achieving > 120° flexion. Radiolucent lines (2–4 mm wide) were observed in 7 TKA. Peak flexion moments (dynamic strength) for the TKA averaged 79%, 80% and 85% of the patients’ contralateral normal knees during the gait, stair-climbing and step-over tasks, respectively. In maximum kneeling, knees averaged 131°±13° flexion, 10° ±4° tibial rotation, and 2mm/10mm posterior position of the medial/lateral condyles.

This series’ early clinical follow-up was comparable to any well performing TKA. Knee flexion during passive examination and kneeling were comparable to the best reported results for PCL-retaining and PCL-substituting TKA. Peak knee flexion moments, a measure of functional strength, were comparable to the strongest knees reported in the literature. These early results suggest a fixed-bearing prosthesis with ‘ACL-substitution’ can provide patient performance comparable to the best performing designs.

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.

Achieving normal strength after total knee arthroplasty (TKA) remains a major challenge, with recent reports suggesting strength following TKA averages 65% of normal. Gait lab studies have reported the greatest strength (80%–84% of normal) in knees with intrinsic stability, i.e. knees where the implant surfaces or retained ligaments provide definitive control of tibiofemoral motions such that dynamic muscle stabilization is not required. Superior results have been reported for bicruciate retaining arthroplasty, posterior-stabilized arthroplasty with early-engaging cams, and single radius highly congruent posterior-stabilized arthroplasty. The goal of this study was to determine if knees with an intrinsically stable posterior cruciate ligament (PCL) retaining implant design showed strength comparable to these other intrinsically stable types of arthroplasty.

Ten patients with unilateral intrinsically stable PCL-retaining knee arthroplasty were studied using full-body motion capture, force platforms and electro myography while they stepped onto and over a 20cm box. The implant design includes an asymmetric tibial bearing surface with a fully congruent lateral articulation (0°–70° flexion). Subjects were recruited on the basis of combined KSS scores greater than 180 one year after surgery. Peak knee flexion torques, normalized by body weight and height, are used as a measure of functional strength.

Knees with posterior cruciate retaining, intrinsically stable TKA exhibit functional strength comparable to other intrinsically stable TKA designs and superior to strength in less stable TKA designs. These knees show some reduction of knee flexor activation, indicating that antagonist coactivation is not required for joint stability. Stable joints permit more optimal muscle activation, making patients effectively stronger while reducing loads at the joint.

Contemporary knee implants use a variety of methods to control tibiofemoral motions. Posterior stabilized implants have a post and cam to force the femur posterior with flexion. Most posterior cruciate retaining designs rely solely on this ligament and symmetric tibial surfaces to control tibiofemoral translations. However, many studies have demonstrated poor control of tibiofemoral motion in PCL retaining knees. One strategy to augmenting PCL function is to provide a gait-congruent lateral articulation providing definitive stability in extension while allowing lateral condylar translation in deep flexion. It is unknown whether this design strategy, essentially substituting for the ACL, allows the PCL to function more normally.

Fifteen knees in ten patients with a fixed-bearing, PCL retaining, lateral pivot arthroplasty were observed during maximum flexion kneeling and lunging using fluoroscopy. The tibial insert provides a fully conforming lateral articulation from 0°–70° flexion, allowing lateral AP translation at greater flexion. Recruited on the basis of combined KSS scores > 180 points, patients averaged 72 years, 27.5 BMI, and 12 months post-op. Shape matching techniques were used to determine the 3D pose of the implant components.

Skeletal flexion during kneeling averaged 134° (117°–156°) with 11° tibial internal rotation. Medial condylar contact was 3mm posterior, and lateral contact was 11 mm posterior to the tibial AP midpoint. Skeletal flexion during lunging averaged 122° (106°–146°) with 11° tibialinternal rotation. Medial condylar contact was 1mm posterior, and lateral condylar contact was 9mm posterior to the tibial AP midpoint.

Knees with lateral pivot arthroplasty exhibited flexion comparable to the best reported results in North American patients. Tibial rotation was statistically greater than has been reported for symmetric posterior stabilized or PCL retaining implants for the same activities. Posterior translation of the condyles with flexion beyond the range of full articular congruity is consistent with relatively normal PCL function.