Rotational alignment of the tibial component in total knee arthroplasty is better at the medial third of tibial tuberosity than at the medial border

Abstract

Background: Correct rotational alignment of the femoral and tibial component is an important factor for successful TKA. The transepicondylar axis is widely accepted as a reference for the femoral component. There is not a standard reference for the tibial component. CT scans were used in this study to measure which of 2 tibial landmarks most reliably reproduces a correct femoro-tibial rotational alignment in TKA.

Methods: 80 patients received a cemented, unconstrained, cruciate-retaining TKA with a rotating platform. CT scans were performed 5-7 days postoperatively but before discharge. The rotational mismatch between the femoral and tibial components was measured. Furthermore, the rotational variance between the transepicondylar line, as a reference for the orientation of the femoral component and different tibial landmarks, was measured.

Results: There was notable rotational mismatch between the femoral and tibial components. The median mismatch was 0 degrees (range: 16.2 degrees relative external to 14.4 degrees relative internal rotation of the femoral component).Using the transepicondylar line as a reference for femoral rotational alignment and the medial third of the tuberosity as a reference for tibial rotational alignment, 67.5% of all TKA had a femoro-tibial variance within +/- 5 degrees, 85% within +/- 10 degrees and 97.5% within +/- 20 degrees. Using the medial border of the tibial tubercle as a reference this variance was greater, only 3.8% had a femoro-tibial variance within +/- 5 degrees, 15% within +/- 10 degrees and 68.8% within +/- 20 degrees.

Conclusion: Using fixed bone landmarks for rotational alignment leads to a notable variance between femoral and tibial components. Referencing the tibial rotation on a line from the medial third of the tibial tubercle to the center of the tibial tray resulted in a better femoro-tibial rotational alignment than using the medial border of tibial tubercle as a landmark. Surgeons using fixed bearings with a high rotational constraint between the inlay and the femoral component should be aware of this effect to avoid premature polyethylene wear.

Trial registration: Clinical trials registry NCT01022099.

Figure 1

Determination of rotational mismatch between femoral and tibial component. a: Line through femoral fixation pegs for determination of the femoral component rotation, b: Angle between rotational alignment of the femoral and a line along the posterior border of the tibial component for determination of the tibial rotation.

Figure 2

Determination of the "true" rotational variance between femur and tibia: Transepicondylar axis (a) is superimposed to the tibia and a perpendicular line is drawn through the rotational centre of the tibial tray (fixation peg for rotating platform, b). These lines are superimposed to a slice where the tibial tuberosity is visible (c). The tibial tuberosity is divided into three parts (d). The angle between a line from the medial border of the tibial tuberosity to the rotational center and the line perpendicular to the transepicondylar axis is measured (e). The angle between a line from the lateral border of the medial third of the tibial tuberosity to the rotational center and the line perpendicular to the transepicondylar axis is measured (f).

Figure 3

"True" rotational variance between femur (transepicondylar axis) and different tibial landmarks (medial third or medial border of the tuberosity). (Negative values indicate femoral external rotation relative to the tibia, positive values indicate relative femoral internal rotation; Box plot horizontals indicate medians and quartiles, verticals indicate minumum and maximum observations. Circles and asterixes indicate statistical outliers).