Abstract
Introduction:
Appropriate transverse rotation of the tibial component is critical to achieving a balance of tibial coverage and proper tibio-femoral kinematics in total knee replacement (TKR), yet no consensus exists on the best anatomic references to determine rotation. Historically, surgeons have aligned the tibial component to the medial third of the tibial tubercle1, but recent literature suggests this may externally rotate the tibial component relative to the femoral epicondylar axis (ECA) and that the medial border of the tubercle is more reliable2. Meanwhile, some TKR components are designed with asymmetry of the tibial tray assuming that maximizing component coverage of the resected tibia will result in proper alignment. The purpose of this study was to determine how different rotational landmarks and natural variation in osteoarthritic patient anatomy may affect asymmetry of the resected tibial plateau.
Methods:
Pre-operative computed-tomography scans were collected from 14,791 TKR patients. The tibia and femur were segmented and anatomic landmarks identified: tibial mechanical axis, medial third and medial border of the tibial tubercle, PCL attachment site, and the surgical ECA of the femur. Virtual surgery was performed with an 8-mm resection (referencing the high side) made perpendicular to the tibial mechanical axis in the frontal plane, with 3° posterior slope, and transversely aligned with three different landmarks: the ECA, the medial border, and medial third of the tubercle. In each of these rotational alignments, the relative asymmetry of the medial and lateral plateaus was calculated (Medial AP/Lateral AP) (Fig. 1).
Results:
Rotational alignment of the tibial component to the ECA, medial border, and medial third of the tubercle resulted in progressive external rotation of the tibial tray on the bone. Alignment to the medial border and medial third of the tubercle resulted in average 0.9° ± 5.7° and 7.8° ± 5.3° external rotations of the tray relative to the ECA, respectively (Fig. 2). Greater external rotation of the tibial implant relative to the bone increased the appearance of tibial asymmetry (Fig. 3). Referencing the medial border and medial third of the tubercle resulted in apparent tibial bone asymmetry of 1.10 ± 0.10 and 1.12 ± 0.10, respectively.
Discussion:
Assuming the ECA is the appropriate rotational reference to re-establish appropriate kinematics2, alignment to the medial border of the tubercle resulted in the most favorable tray alignment. However, there was a great deal of variation between the relative position of the ECA and the tubercle across the patient population. Rotational alignment to either the medial border or medial third of the tubercle resulted in external tray alignment relative to the ECA of greater than 3 degrees for 36% and 84% of patients, respectively. In addition, increased tray asymmetry (broader medial plateau) necessitates relative external rotation of the tray on the bone reducing the flexibility of intra-operative rotational adjustment. Tray asymmetry greater than 1.10 (the asymmetry of the resected tibia when aligned to the ECA) may result in external mal-rotation for a significant portion of the patient population.
