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.