Two types of transepicondylar axes have been described [7]: the surgical and clinical transepicondylar axis (Fig. 3.1). A line connecting the tip of the lateral epicondyle to the medial epicondylar sulcus is defined as the surgical transepicondylar axis, whereas a line connecting the tip of the lateral epicondyle to the medial epicondylar tip is defined as the clinical transepicondylar axis. The surgical transepicondylar axis is typically used to determine femoral rotation [7]. However, the identification of both axes is difficult during surgery, and no instrument relies directly on these axes. The posterior condylar line is, on average, internally rotated 3° from the surgical transepicondylar axis [8]. Thus, standard instruments use 3° of external rotation from the posterior femoral condyles to set the femoral rotation parallel to the surgical transepicondylar axis. The location of the collateral ligaments can help to determine the medial and lateral epicondyles and to identify the clinical transepicondylar axis [9]. Several authors [7, 10] suggested preoperatively identifying the surgical transepicondylar axis on a special computed tomography (CT) scan [7]. However, studies demonstrated it was difficult to find the medial epicondylar sulcus on axial CT scans and, therefore, difficult to identify the surgical transepicondylar axis [10]. In these cases, 3° of the clinical transepicondylar axis could be subtracted to determine the surgical transepicondylar axis [10]. The surgical transepicondylar axis was shown to be the optimal flexion axis [11]. However, recent studies focused on more kinematic three-dimensional models to describe the optimal femoral rotation [8, 12, 13]. These studies described the kinematic axes (the cylindrical axis [12] and femoral transverse axis [8]), which did not coincide with the surgical transepicondylar axis.

Whiteside et al. described the anteroposterior (AP) trochlear line as a reliable landmark for femoral component rotation. This line is defined as a line connecting the deepest point of the trochlea to the center of the notch [2]. In a cadaver study of 30 femora, 5 observers identified the AP trochlear line more accurately than the transepicondylar axis [14]. The AP trochlear line is, on average, rotated 90° from the surgical transepicondylar axis. Thus, both landmarks may be used to double-check during surgery. Several other studies confirmed the AP trochlear line as a reliable landmark for femoral rotation [2, 15, 16]. However, in cases in which severe anterior and posterior deformity is present or in cases with a dysplastic trochlea, the AP line may be difficult to identify. One study demonstrated that the AP trochlear line was less reliable in knees with medial femorotibial osteoarthritis [17].

Several resection instruments use the posterior condylar line plus a few degrees (0–10°) of fixed external rotation to determine femoral rotation. Several authors concluded that the average value of the angle between the surgical transepicondylar axis and the posterior condylar line is −3° in the “standard” osteoarthritic varus knee [8]. However, it is critical to always use +3° external rotation of the femoral cutting guide with respect to the posterior femoral condyles. In up to 30 % of cases, the femoral rotation alignment will not be correct, especially in the valgus deformed knees [18].

In the flexion gap balanced technique, the flexion gap is balanced by laminar spreaders or a tensioning device after ligamentous release in extension [19]. With the knee in flexion, the femoral cutting block is rotated by the ligaments parallel to the 90° proximal tibia cut. Thus, a rectangular flexion gap is created. However, a perfect 90° proximal tibia, a perfectly balanced extension gap, and a perfect 90° distal femoral cut are key for proper femoral rotation and offer several possibilities for failure. Studies have shown there are difficulties in obtaining proper femoral rotation using only the soft tissue as a reference [20–22]. Surgeons who prefer the flexion gap balanced technique may use bony landmarks to verify their femoral rotation.

A variation of the flexion gap balanced technique is the tibial shaft axis method [23]. Here, the tibia shaft axis is used to balance the flexion gap, rather than the soft tissue [23]. Studies have shown a perpendicular relation of the transepicondylar axis to the longitudinal axis of the lower extremity (tibia shaft axis) [23, 24]. Therefore, the tibial shaft axis can be used to determine the femoral rotation and balance the flexion gap [23].

The anterior surface of the femur was described as a possible landmark for femoral component rotation [4]. The anterior surface of the femur was cleared and used directly as a platform to determine femoral component rotation [4]. The anterior surface of the femur showed a strong correlation to the anteroposterior (AP) trochlear line (Whiteside’s line) [4]. However, further studies are necessary to confirm this correlation and the practical relevance.

The external rotation of the femoral component with regard to the posterior condylar line (parallel to the transepicondylar axis) creates a morphological pattern on the anterior resected surface of the femur, known as the grand piano sign [5, 25] (Fig. 3.2). One study showed variance of the grand piano sign was related to this external rotation from the posterior condylar line [5]. However, the grand piano sign cannot be used directly to identify proper femoral rotation, given that the rotational alignment can only be estimated and verified after the anterior femoral cut.