Summary

Excellent result after TKA is predictable if three conditions are fulfilled: an optimally balanced well sized stable implant. This balance should be asymmetrical and is dependent on the implant used. The author proposes a new algorithm to balance a medial pivot implant.

Abstract

Introduction

Medially conforming or medial pivoting knee designs offer an alternative design which aim to minimize paradoxical movement thereby mimicking the native kinematics. With medially stabilized prostheses, a dedicated algorithm different from the other TKA designs is needed during surgery in order to balance the soft tissues. However, little is known which structures must be preserved during surgery with a medially stabilized implant and which structures can be safely released.

Methods

Six cadaveric fresh-frozen knee specimens were implanted with a medially stabilized implant. The surgical workflow uses the extension gap first technique. Balance of the soft tissues is achieved when the medial side is more tight than the lateral side and the flexion gap is slightly more loose than the extension gap. To achieve this balance in vivo: 1 in 4 needs a ligamentous release. Selective releases on these six cadaveric knees were performed in a progressive way by starting posteriorly in extension and then move anteriorly. We analyzed the effect of these progressive selective release of the soft tissues, both laterally and medially. Aim of the study was to investigate the role of the individual medial and lateral soft tissues of the knee in recreating this natural balance and whether this role is somehow dependent on the preoperative deformity, either varus, valgus or neutral.

Results

On the medial side, progressive medial release could be performed as long as at least the anterior fibers of superficial medial collateral ligament (sMCL) remained intact. With regard to lateral release, in all instances the progressive release of the lateral soft tissue envelope did not affect sagittal stability, which apparently relies mainly on the medial ball-in-socket design of the implant. Progressive lateral release could be performed as long as the anterior one third of the iliotibial band (ITB) remained intact.

Discussion

Additional studies will be required to further investigate the subject. First, future studies are needed to develop algorithms for progressive releases for selective soft tissue management in a medially stabilized prosthesis. Second, studies are needed to observe the effect of medial and lateral releases on the tibia rotation and natural roll back of the knee implanted with a medially stabilized prosthesis. Third, studies are needed to determine the effect of medial and lateral releases on the intra-articular pressure of the knee implanted with a medially stabilized prosthesis and to quantify the resulting forces (Newton) and gap opening (mm). Fourth, the association between laxity and functional outcome needs to be established.
Conclusion.
In conclusion, as long as the anterior fibers of sMCL and the anterior fibers of the ITB remain intact, the anterior-posterior stability of the medially conforming implant remains granted. The current study found that the sagittal stability relies mostly on the medial ball-in-socket design of the implant.