*3.2.2. Kinematic alignment*

not releasing medial ligaments he usually needs to add external rotation to the femoral cuts. Some references help to establish the appropriate rotation: transepicondylar axis, posterior femoral condyles, Whitside's line (trochlear groove axis), tibial shaft axis, and ligament tension. In some situations, even a small degree of internal rotation could be applied. Before

The measured resection technique aims not to move the joint line position. That situation, in theory, preserves knee's anatomy, sparing the PCL, when possible. Some advocate that preserving the PCL has advantages as it is an important varus/valgus stabilizer of the knee; it can absorb stress; and it can control the movement of rolling back of the femur onto the tibia

implant is settled, the gap must be rectangular and symmetrical (**Figure 10**).

**Figure 9.** Before ligament releasing, there is usually a trapezoidal gap.

12 Primary Total Knee Arthroplasty

**Figure 10.** Rectangular gap. Before the implant is set, there must be a rectangular gap.

The refinement of surgical techniques, implant designs and an individualized tendency of surgical treatment have contributed to resurrection of the discussion about restoration of the native's knee anatomy and preservation of the articular line.

Some authors have noticed that neutral mechanical alignment does not restore biomechanics in a significant part of the population [13–15].

Kinematic Alignment (KA) aims to restore constitutional alignment, ligament tension, and the joint's line level and orientation [16, 17]. Hungerford, Kenna, and Krakow defended a slight varus alignment for the tibia in relation to the MAT. They are considered precursors on KA [18].

According to Howell et al., there are three kinematic knee axis: the primary femoral axis, wich is a transverse axis of femur around which the tibia flexes and extends; the secondary femoral axis, wiich is a traverse axis in the femur around which the patella flexes and extends; and the longitudinal tibial axis, around which the tibia internally or externally rotates on the femur [19] (**Figure 11**). Each axis is parallel or perpendicular to the natural joint line between the femur and tibia throughout the motion arc.

The preoperative plan of a kinematic TKA can be done by using MRI exams in order to estimate chondral and bone erosion [20–22]. These parameters are used to compensate implant's position that should be parallel to the primary and secondary kinematic femoral axis. A patient-specific implant or a conventional implant can be used.

The KA principle is a promising alternative for the execution of TKA. In the centers where it was adopted, the results in the short and medium term were favorable [22], with the premise of restoration of the biomechanics of the knee, which may point to a new paradigm. Nevertheless, studies on durability and long-term function are needed before universal adoption of this new methodology.

Its applicability in patients with a higher degree of bone erosion should also be evaluated cautiously because of the increased risk of malalignment, which may be caused by the difficulty in identifying the references that guide the positioning of the bone cutting guides.

**Figure 11.** Kinematics axis of the knee (from Howell's studies with authorization).
