Rotation angle between the femoral and tibial components in varus/valgus stress X-rays following total knee arthroplasty

BackgroundThe impact of rotational angle between the femoral and tibial components is often overlooked in the 2D evaluation of varus/valgus stability after TKA with anterior-posterior knee X-rays. The rotation angle between the femoral and tibial components may influence the measured angle and distance between these components in 2D stress X-rays following TKA.ObjectiveThe purpose of this study was to assess the impact of the rotational angle between the femoral and tibial components on the evaluation of varus/valgus stability using stress X-rays following total knee arthroplasty (TKA).MethodsThis prospective study analyzed 48 consecutive rTKAs (three males, aged 68 ± 6.4 years; 45 females, aged 75 ± 5.9 years). Postoperative varus/valgus stress X-rays were taken at maximum manual stress during knee extension under anesthesia, and were analyzed three-dimensionally using a 2D-3D image matching technique with 3D bone and component models. The rotation angles of the components (CR angles) were assessed under conditions of no stress, valgus stress, and varus stress. Additionally, the varus/valgus angle (VV angle) between components was evaluated under the same conditions. Medial joint opening (MJO) and lateral joint opening (LJO) were also measured in both stressed and non-stressed states.ResultsThe CR angles under no stress, valgus stress, and varus stress were 9.9 ± 5.5°, 10.1 ± 6.2°, and 10.8 ± 5.1°, respectively. The VV angles under no stress, valgus stress, and varus stress were 3.6 ± 1.1°, 1.1 ± 1.4°, and 7.1 ± 1.9°, respectively. The MJO in the non-stress condition and under valgus stress were 0.0 ± 0.4 mm and 1.3 ± 1.0 mm, respectively. The LJO in the non-stress condition and under varus stress were 0.9 ± 0.9 mm and 2.9 ± 2.7 mm, respectively.ConclusionsThis prospective study revealed two key findings: (1) the CR angle in varus stress was significantly more externally rotated compared to the CR angle in the non-stress condition, and (2) no significant correlations were found between the rotational angle of the components and the VV angle, MJO, or LJO.