Accurate rotational positioning of the femoral component is one of the most important factors for successful total knee arthroplasty (TKA) because femoral component malrotation may cause patellofemoral complication, mid-flexion instability, and polyethylene or post wear due to imbalance of flexion and extension gap [[1], [2], [3], [4], [5], [6]]. According to the measured resection technique, the goal of femoral component rotational alignment is to align the femoral component mediolateral (ML) axis parallel to the transepicondylar axis (TEA) in the axial plane [[6], [7], [8]].

It has been generally accepted that in the native knee, the posterior condylar line (PCL) is usually 3° internally rotated relative to the surgical transepicondylar axis (sTEA) and 5° internally rotated relative to the clinical transepicondylar axis (cTEA) [3,4,9]. The angle formed by the TEAs and the PCL is called the posterior condylar angle (PCA) [10]. Many surgeons rotate the femoral component 3–5° externally relative to the PCL, and the currently used TKA cutting guides for distal femur anteroposterior (AP) cutting allow 3 or 5° external rotation relative to the PCL [5,7,11]. Nevertheless, several comparative studies have demonstrated the difficulties in achieving the ideal femoral component rotation and shown outliers with abnormal external or internal rotation of the femoral component ML axis from the patients’ native TEA [12]. One of the factors for these outliers might be the individual difference of the PCL, which can be attributed to asymmetrical cartilage erosion of the posterior aspect of the medial or lateral condyle [13,14]. This change in distal femoral geometry can be associated with malalignment of the whole limb due to osteoarthritis. Previous studies have shown that the PCL is more internally rotated from the TEA, with increasing PCA in valgus-aligned osteoarthritic knees than those of normal or varus-aligned osteoarthritic knees, due to hypoplasia of the lateral femoral condyle [1,15,16]. However, there is a relative lack of studies investigating the relationship between axial plane distal femoral condyle phenotype and varus alignment [1,[16], [17], [18]]. Consequently, the effect of varus deformity severity on distal femoral condylar geometry in the axial plane remains unclear.

Therefore, the purpose of this study was to investigate the relationship between varus deformity severity and the change in distal femoral posterior condyle phenotype. We hypothesized that severe varus deformity would cause hypoplasia of the medial femoral posterior condyle, resulting in less internal rotation of the PCL from the TEA.