Posterior-cruciate ligament retaining (CR) total knee arthroplasty (TKA) designs have long been used with excellent clinical success, but have shown kinematics that are significantly different from the natural knee. Recently, variations on traditional CR designs have been introduced. The purpose of this study was to compare deep-flexion knee kinematics in patients with two types of CR-TKA: one group received a traditional non-conforming symmetric articular configuration, and one group received a design incorporating a lateral compartment which is fully congruent in extension, but lax in flexion – approximating the function of the anterior cruciate ligament.

In vivo kinematics were analysed using 3D model registration and plain radiographs of kneeling and squatting activities in 20 TKAs in 18 patients with a minimum follow-up of 12 months. Two surgeons worked together placing all components. Ten knees received a traditional CR-TKA (CR Group), and 10 knees received an ACL-substituting TKA (AS Group). CR Group subjects averaged 66.1±7.4 years and were 12.3±0.5 months post-op. AS Group subjects averaged 68.0±5.4 years and were 12.4±0.7 months post-op. True lateral radiographs were taken in 4 positions:

with the patient in a weight-bearing, single-leg stance,

Two-way repeated measure ANOVA was conducted to determine if there were effects of design or flexion angle on the AP tibiofemoral contact position. Medial and lateral sides were analyzed separately. The level of significance was set at p< 0.05.

There was no significant difference in the average post-operative Knee Society Clinical/Functional Scores between CR Group (96±2/88±11) and AS Group (94±2/92±9). Clinical ROM was recorded using a handheld goniometer. The clinical pre-operative passive ROM was 113 °±15° (80°–135°) for CR Group and 116°±20° (65°–140°) for AS Group (p=0.75). The clinical post-operative passive ROM was 117°±11° (100°–130°) for CR Group and 127°±13° (115°–160°) for AS Group (p=0.07). During squatting, the implant flexion angle was greater for AS Group (119°±15°: 101°–157°) compared to CR Group (104°±10°: 94°–123°, p=0.02). Tibial external rotation at maximum kneeling and squatting activities were significantly larger in AS Group knees (10.2°±4.8°/9.0°±3.9° versus 16.6°±4.1°/15.8°±4.1°, p=0.00/p=0.00). Average tibiofemoral contact position of the lateral condyle during squatting activity was significantly posterior in AS Group compared to CR Group (−11.2±5.6mm vs. −6.2±3.0mm, p=0.02).

Substitution of the ACL by a lateral compartment which is conforming in extension may provide more natural stability and function with knee arthroplasty. In this comparison of two small groups, knees with the ACL-substituting design exhibited femoral AP translation and rotation closer to the natural knee than did knees receiving a traditional symmetric CR prosthesis. The long-term success of TKA depends not only on kinematics factors, such as those reported here, but also on polyethylene wear and patellar complication. A longer-term clinical study will be required to determine if high flexion activity will lead to increase polyethylene wear or patellar complications.

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.