Abstract
Introduction
Restoration of knee function after total knee arthroplasty (TKA) often entails a balance between normal kinematics and normal knee stability, especially in performing demanding physical activities. The ultra-congruent (UC) knee design prioritizes stability over kinematics through close conformity between the femoral component and the tibial insert in extension. This configuration is intended to provide AP stability in the absence of the posterior cruciate ligament during activities that would otherwise cause anterior femoral subluxation. In this study we examine the kinematics of an ultra-congruent knee design in comparison with the intact knee and with conventional articulations used in PCL-retaining (CR) and PCL-substituting (PS) TKR designs.
Materials and Methods
The 3D tibio-femoral kinematics of 6 fresh frozen cadaveric human knees were tested during loaded simulation of squatting in a computer-controlled knee testing rig. Muscle forces were simulated by loading rectus femoris and vastus intermedius (150N), vastus lateralis (100N), vastus medialis (75N), and the hamstring muscles (60N) (total: 385N). Testing was performed on the intact knee, and after implanting a standard design of total knee prosthesis with the posterior cruciate ligament intact (CR-TKA), resected (PCL-substituting insert; PS-TKA), and a UC insert (UC-TKA group). The 3D positions of the tibia and femur were tracked with a high resolution 12 camera motion analysis system (Motion Analysis Inc.) and used to position 3D CT reconstructions of each bone. The translation and rotation of the femur with respect to the tibia were calculated by projecting the femoral transcondylar axis onto a plane normal to the longitudinal anatomical axis of the tibia coincident with the transverse axis of the tibial plateau.
Results
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In full extension, the femur was displaced posteriorly by 14.2 ±7.0 mm compared to the intact knee (p<0.01). There was minimal posterior translation (±3mm) of the medial condyle with all 3 inserts designs, and minimal (0–3mm) translation of the lateral condyle from 0–90 degrees with both the UC and PS inserts.
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From 0–30 degrees flexion, the femoral component translated anteriorly by approx. 5mm without axial rotation. Beyond 30 degrees, the tibia rotated internally by a total of 11 degrees (30–120degs). This was associated with approx. 5mm of rollback of the lateral condyle and 5mm of anterior translation of the medial condyle.
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There was significant difference in tibial rotation between the UC-TKA group and the intact knee group (p<0.01 in UC-TKA group at 15, 30, 45, and 60°). The rotation patterns of the three designs of TKA were similar during flexion from 0–120 degrees. This was markedly different than the intact knees.
Conclusions
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The UC TKA demonstrated minimal AP translation with flexion averaging approx. 6mm of posterior rollback laterally, and 3mm of anterior translation medially from 0–120 degrees. This differential translation was associated with 9degrees of internal rotation pattern, similar to that of the PS insert.
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The clinical success of the UC design and its popularity with patients as an alternative to the PS-TKA suggests that AP stability in extension, and not posterior rollback in flexion, is a critical element in patient satisfaction after TKR.
