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Abstract
Background: Restoring function to that of the native knee without ligament release is a goal of unrestricted kinematic alignment (KA) total knee arthroplasty (TKA). Focusing on the tibial component, two design variables that affect knee function are retention versus resection of the posterior cruciate ligament (PCL) and conformity of the medial and lateral articular surfaces to the respective condyles of the femoral component. Therefore, it is of interest to determine the effects of PCL retention versus resection and the degree of medial conformity of the tibial insert in restoring kinematics to that of the native knee after KA TKA in three different exercises of daily living and their effects on patient reported outcome scores. En route to this objective, it also was of interest to determine causes of posterior edge loading in the lateral compartment for one of the insert designs that had a high incidence.
Methods: Four cohorts of patients were involved. One cohort had a TKA featuring a tibial insert with high medial conformity (ball-in-socket medial conformity) that resected the PCL (B-in-S MC-PCL). The second had an insert with less medial conformity (termed intermediate medial conformity) which retained the PCL (I MC+PCL). The third had an intermediate medial conformity that retained the PCL (I MC+PCL) and contralateral knee had an insert with ball-in-socket medial conformity that retained the PCL (B-in-S MC+PCL). The last cohort included patients with a healthy knee. Single-plane fluoroscopy captured tibiofemoral images while patients performed weighted deep knee bend, step-up, and chair rise exercises. 3D model-to-2D image registration followed by additional processing determined the anterior-posterior (A-P) positions of the femoral condyles, which were used to compute internal tibial rotation from extension to maximum flexion during each exercise. CT images were used to investigate the potential causes of posterior edge loading in the lateral compartment. Patient reported outcome scores also were determined for each cohort.
Results: In the first comparison between the B-in-S MC-PCL and I MC+PCL inserts, the I MC+PCL insert led to marginally greater internal tibial rotation in deep knee bend and better post-operative patient reported outcome scores despite a relatively high incidence of posterior edge loading in the lateral compartment. In the I MC+PCL insert, the primary cause of posterior edge loading in the lateral compartment was the posterior position of the lateral femoral condyle at extension. The internal tibial rotation occurred naturally about a medial pivot point and moved the lateral condyle sufficiently posterior that contact occurred on the edge of the insert. In the second comparison between the I MC+PCL and B-in-S MC+PCL inserts, the greater medial conformity in the B-in-S MC+PCL led to greater internal tibial rotation in the deep knee exercise but not the other two exercises and the degree of medial conformity did not affect post-operative patient reported outcome scores. In the last comparison between the B-in-S MC+PCL and native knees, the B-in-S MC+PCL insert exhibited greater internal tibial rotation in the deep knee bend and similar rotation in the other two exercises. However, there were significant differences in patterns of AP movement in both compartments during all exercises. With the native knee, most rotation occurred in the first 30º of flexion in deep knee bend or the last 30° of extension in step up and chair rise whereas the change in tibial rotation occurred more evenly with flexion or extension in the KA TKA knees. Nevertheless, the median Oxford Knee Score was 43 out of 48 and the median Oxford Activity and Participation Score was 91 out of 100.
Conclusion: While the B-in-S MC+PCL tibial insert design proved to best restore native tibiofemoral kinematics than the other insert designs, patterns of AP movement in both compartments differed from the native knee. However, the B-in-S MC+PCL tibial insert exhibited good patient reported outcome scores at a mean follow-up time of 8 months.
