"Grand-piano sign" as a femoral rotational indicator in both varus and valgus knees: a simulation study of anterior resection surface in total knee arthroplasty

PurposeThe "Grand-piano sign" is a popular indicator of the appropriate rotational alignment of the femoral component during total knee arthroplasty (TKA). The aim of the study was to investigate the shape of the anterior femoral resection surface of varus and valgus knees.MethodsA cohort of 80 varus knees and 40 valgus knees (hip-knee-ankle angle > 2 degrees for varus and < - 2 degrees for valgus knees) matched for age, sex, height, body weight, and KL grade was made using propensity score matching. Virtual TKA was performed using 3 patterns of components (anterior flange flexion angles of 3 degrees, 5 degrees, and 7 degrees). The anterior femoral resection surface was evaluated for 3 patterns of rotational alignments: parallel to the surgical epicondylar axis (NR, neutral rotation), 3 degrees internal rotation (IR), and 3 degrees external rotation (ER) relative to the surgical epicondylar axis. In each anterior femoral resection surface, the vertical height of medial and lateral condyles was measured, and the ratio of the medial to the lateral height (M/L ratio) was evaluated.ResultsThe M/L ratio in NR for both varus and valgus knees was 0.57 to 0.64, with no significant difference between the cohorts (p value > 0.05). The M/L ratio showed a similar pattern of increasing at IR and decreasing at ER in both varus and valgus knees. The variation in the M/L ratio with malrotation in valgus knees was smaller than in varus knees.ConclusionDuring TKA, the anterior femoral resection surface was similar in varus and valgus knees; however, the variation with malrotation was smaller in valgus knees than in varus knees. TKA for valgus knees requires precise surgical technique and careful intraoperative assessment.