“Grand-piano sign” has been used as a popular benchmark to facilitate correct rotational alignment during total knee arthroplasty (TKA). The purpose was to quantitatively determine morphological patterns on anterior femoral resection in mechanically aligned (MA) and kinematically aligned (KA) TKA.
Computed tomography scans of 60 TKA candidates were reconstructed into 3D models. Femurs were virtually cut with a 3D imaging program using various anterior flange flexion angles (AFFAs) of 3°, 5°, and 7°. The anterior femoral resection was performed parallel to the surgical epicondylar axis, at an external rotation and internal rotation of 3° relative to surgical epicondylar axis for MA-TKA, and parallel to the cylindrical axis, at an external rotation and internal rotation of 3° to cylindrical axis for KA-TKA. The ratio of vertical distance from the anterior margin of distal femoral resection to the most proximal peak of each medial and lateral condyle of anterior femoral resection was defined as AC/BC ratio.
The mean ratios of AC/BC were 0.57, 0.60, and 0.63 respectively, according to 3°, 5°, and 7° of AFFA with standard MA-TKA method and were 0.73, 0.74, and 0.76, respectively, according to 3°, 5°, and 7° of AFFA with standard KA-TKA method. The AC/BC ratios of malrotated planes were significantly different from those of both standard MA- and KA-TKAs (P-values < .01).
Surgeons can accessorily use the quantifying method for anterior femoral resection intraoperatively to ensure correct rotational alignment of femoral resection in both mechanically and kinematically aligned TKA.