Tibial component rotation: The inveterate problem

The concept of component mal-rotation contributing to poor outcomes following total knee arthroplasty (TKA) is not new. Indeed, sentinel work on the subject by Berger and coauthors that introduced both a method for evaluating component rotation on axial computed tomography (CT) images, and demonstrated the association between component mal-rotation and patello-femoral problems after TKA was published over 15 years ago [

]. Subsequently, further important contributions by investigators from Europe, Asia and North America have increased our knowledge and confirmed an association between mal-rotation and post-operative pain, stiffness, and potentially tibial component loosening [

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]. Additional reports have also demonstrated that revision for isolated femoral and/or tibial component mal-rotation is successful and compares favorably to revision due to other causes [

,

]. Despite the effort expended to define acceptable quantitative parameters for what constitutes optimal rotation, many gaps still exist in our knowledge, particularly regarding the tibial component. On the femoral side, while the method for attaining optimal rotation intra-operatively is still debated, consensus opinion recognizes the trans-epicondylar axis as the optimal rotational landmark. In distinction, considerable debate continues regarding whether an anatomic landmark, or a functional approach should be used for determining tibial component orientation. Even amongst those who favor referencing a fixed anatomic landmark, there is no consensus about which one is best with advocates for the posterior tibial axis, the antero-posterior axis of the tibia, the medial-lateral axis of the tibia, the junction of the medial and middle thirds of the tubercle, the medial border of the patellar tendon and others [

,

,

,

]. Part of this controversy may be related to the influence of tibial component rotation on both femoro-tibial kinematics and patellar kinematics [

]. Indeed, it seems that some trade-off may exist and simultaneous optimization of both kinematic goals may be difficult [

]. Rather than defining a single optimal axis, a safe zone for tibial component rotation has been proposed by Lawrie and coauthors that achieves an adequate balance between the competing kinematic needs [

]. The uncertainty in the optimal rotational goal for tibial component positioning manifests as significantly greater variation in post-operative tibial component rotation than femoral component rotation with studies demonstrating a range of tibial component mal-positioning exceeding 30° [

,

]. Certainly part of the knowledge gap for the tibial component may be related to the more laborious process of measuring tibial component rotation on CT images originally described by Berger et al. [

]. Numerous lines must be transposed from one axial image to another for the tibial component whereas a single image can be quickly reviewed in the evaluation of the femoral component. However, new methods for accurately determining rotation from 3D axial images seem to remove this obstacle [

]. In addition to methodological issues that may contribute to this knowledge gap for the tibia, other barriers likely are also responsible. These include asymmetric and mobile bearing tibial components. Asymmetric components are difficult to evaluate even on 3D images using rules defined for symmetric components. Adding to the confusion regarding the evaluation of asymmetric tibial component rotation, some asymmetric baseplates have asymmetric polyethylene inserts that match the orientation of the baseplate, whereas others use polyethylene inserts that are not right or left specific; consequently, the rotational axis of the insert for asymmetric designs may be different than for the baseplate, and may not be consistent between designs. Mobile bearing designs may also be subjected to different rules than fixed bearing designs, as the mobile bearing may negate some of the effect of baseplate mal-positioning; however, this point has not been adequately determined and may be influenced by the actual motion realized in individual patients. Each of these special circumstances will need to be investigated and guidelines defined accordingly. Indeed, prosthesis design features such as the conformity of the bearing surface, and shape of the post in posterior-stabilized implants may be important and may ultimately determine different acceptable ranges for different knee systems.

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