Rotational alignment of the femoral component has a major influence on the outcome of TKA. Incorrect rotational alignment leads to mediolateral imbalance in extension and flexion, resulting in problems such as postoperative restriction of motion and patellofemoral joint incompatibility [3, 4]. It may also lead to excessive soft tissue release in an attempt to achieve adequate soft tissue balance. There are several methods for determining the rotational alignment of the femoral component [2, 5, 6]. Among them, the use of the surgical epicondylar axis (SEA) has the advantage of not being influenced by deformity of the articular surface and condyles. In addition, because it crosses the femoral and tibial axes at right angles in both flexion and extension, the correct balance between the tibial osteotomy surface and soft tissues can be achieved more easily [2]. Various methods of identifying the SEA have been proposed, such as estimation from preoperative images. While intraoperative identification of the SEA has been reported to be difficult [7, 8], we consider that identification by palpation during surgery is the most practical method. In fact, postoperative measurement of the difference of rotation between the SEA and the femoral component on CT scans reveals a difference within ±1° in approximately 80% of our patients. The medial epicondyle can be identified by inspection and palpation as the site of attachment of the medial collateral ligament or as a sulcus at the center of the horseshoe-shaped bony protrusion (Fig. 5.2a). On the other hand, it is difficult to visually identify the lateral epicondyle, because it is the site of attachment of the lateral collateral ligament, and the soft tissues are thicker than at the medial epicondyle. To perform palpation, the patella is retracted, and the top of the bony elevation is identified as the lateral epicondyle (Fig. 5.2b). After identification of the medial and lateral epicondyles, the SEA jig (SA original) is fixed to the epicondyles by pins (Fig. 5.3a). Then the anterior surface of the femur is resected in parallel with the SEA by the anterior reference method (Fig. 5.3b). This surface is used as the reference for every subsequent osteotomy and also for determining the appropriate size of the femoral component by anterior reference.

The anteroposterior (AP) size of the medial condyle is different from that of the lateral condyle when measured at the anterior osteotomy surface of the femur parallel to the SEA (reference osteotomy surface). Thus, sizing of the medial and lateral condyles is done individually, and the component is selected for CR-TKA with smaller AP size between that of the medial and lateral condyles (Fig. 5.4).

In CR-TKA, a sagittal posterior slope is added during osteotomy because this increases flexion after surgery. As a result, the bone defect at the posterior part of the tibia can become smaller, and the PCL must be preserved together with the protecting bone in a manner mentioned later. The anteroposterior axis of the tibia must be determined carefully, because the posterior tibial slope influences varus and valgus tilt of the osteotomy surface. Several reports have been published concerning determination of the anteroposterior axis of the tibia, but this issue is still controversial [9, 10]. To maintain patellofemoral tracking, it is essential to avoid positioning the tibial component in internal rotation. We determine the AP axis of the tibia by adjusting the line that connects the center of the PCL attachment site to a point slightly lateral to the inner margin of the tibial tubercle depending on the degree of rotational freedom of the implant. Final rotational position is determined so that the trial component remains within the acceptable range in knee extension. Osteotomy of the proximal tibia is performed with an angel fin-shaped jig as close to the damaged surface as possible and within 8 mm of the normal articular surface (Fig. 5.5). If there is a residual defect after resection within 8 mm, autologous bone grafting is performed.