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.

Numerous fluoroscopic studies of total knee arthroplasty (TKA) kinematics have shown that many contemporary TKA designs exhibit abnormal tibiofemoral translations during activities like gait and stair climbing. One reason for these abnormal motions is the absence of the anterior cruciate ligament (ACL) in the vast majority of knees with TKA. The purpose of this study was to analyze knee kinematics during gait and stair activities in patients with a new design of TKA, incorporating a lateral compartment which is fully congruent in extension, but lax in flexion approximating the function of the anterior cruciate ligament. Our goal was to determine if such ACL-substitution results in more normal weight-bearing kinematics during gait and stair activities.

Thirteen ACL-substituting TKAs (AS knees) in 8 patients were observed using fluoroscopy during treadmill gait (1 m/s) and stair stepping. Model-image registration was used to determine the 3D knee kinematics. These kinematics were compared with those from 5 knees with posterior cruciate preserving TKA (PCL Group) and 7 knees with ACL-intact bi-unicondylar arthroplasties (bi-UNI Group). AS Group subjects were 12±6 months post-op. Control groups (PCL Group/bi-UNI Group) subjects were 72±6/15±6 months post-op.

During gait, the AS knees showed 1.6±0.4mm medial condyle posterior translation from heel strike to the middle of stance phase and 2.6±0.3mm posterior translation during swing phase. A similar pattern was observed in the bi-UNI knees. The lateral condyle translated posteriorly 2.1±0.2mm from heel strike to terminal stance phase, similar to the PCL knees and the bi-UNI knees. The center of rotation was predominantly lateral (19% lateral) from heel strike to mid-stance and then moved medially (16% medial) in swing phase. AS knees showed 3.4°±2.4°of internal tibial rotation from mid-stance to terminal stance, similar to the bi-UNI knees. During the stair activity, medial/lateral condylar AP translation in the AS Group was 1.6±0.1mm/2.0±0.3mm from extension to flexion, similar to the bi-UNI knees. The AS knees showed 5.9°±2.4° of internal tibial rotation from 20° to 80° during stair activity, similar to the bi-UNI knees.

Substitution of the ACL by a lateral compartment which is conforming in extension may provide more natural stability and function with knee arthroplasty. Medial condylar translations and axial rotations were similar to those observed in ACL-intact bi-unicondylar knees. Gait kinematics were similar to those reported for healthy natural knees [Koo S and Andriacchi TP, J Biomechancs, 2008]. The long-term success of TKA depends not only on kinematic factors, such as those reported here, but also on polyethylene wear and patellar complications. A longer-term clinical study will be required to determine if ACL-substituting TKA represents an overall functional and clinical improvement compared to more traditional designs.