The second goal of kinematically aligned TKA is to restore the native knee and limb alignments [3, 4, 15, 17]. Several studies support correction to the native or “constitutional” alignment when performing TKA as opposed to creating mechanical alignment to neutral (Fig. 8.2) [15, 18–20]. Creating mechanical alignment in patients with constitutional varus and valgus alignment is unnatural and causes greater strain deviations in the medial and lateral collateral ligaments from the native knee [15, 18, 21, 22]. Patients with preoperative varus have better clinical and functional outcome scores and the same implant survivorship at 7 years when the alignment is left in the native varus, as compared with patients overcorrected to neutral [19]. At a mean of 6 years after kinematically aligned TKA, restoration of the native alignments of the knee, limb, and tibia did not adversely affect implant survival and resulted in high function, which supports the consideration of kinematic alignment as an alternative to mechanical alignment when performing primary TKA [4].

Current evidence suggests that the native alignment of the limb does not cause osteoarthritis of the knee. The clinical findings of bilateral osteoarthritis with a varus deformity in one knee and a valgus deformity in the other (“wind swept”), and the lack of osteoarthritis in the majority of elderly Asian patients with severe constitutional varus suggest that native alignment plays little role in the development of osteoarthritis. Instead, the onset of osteoarthritis is associated with known changes in cartilage metabolism that occur with aging. Articular cartilage is a mechanosensitive tissue that, when healthy, increases anabolic activity and thickens when loaded. Chondrocytes experience age-related declines in their anabolic activity and thickening response which causes osteoarthritis as the ability to respond and compensate for high loads from activity and obesity is gradually lost [23].

The third goal of kinematically aligned TKA is to restore the native laxities of the knee, which are tighter at 0° of flexion than at 45° and 90° of flexion [16, 24] (Fig. 8.3). At 0° of flexion the native tibia-femoral joint behaves as a rigid body since the average varus (0.7°), valgus (0.5°), internal (4.6°), and external (4.4°) rotations of the tibia on the femur are negligible under applied loads that just engage the soft tissue restraints [16, 24, 25]. At 45° and 90° of flexion, the mean laxity is fivefold greater in varus (3.1°) rotation; fourfold greater in distraction; threefold greater in valgus (1.4°), internal (14.6°), and external (14.7°) rotation; and twofold greater in anterior translation than at 0° of flexion [16, 24]. The maintenance of these native differences in laxities between positions of knee flexion requires the maintenance of the native resting lengths of the collateral ligaments, posterior cruciate ligament, and retinacular ligaments. The alignment goal of gap balancing a TKA overtightens the laxities of the flexion gaps at 45° and 90° of flexion to match those at 0° of flexion, which patients may perceive as pain, stiffness, and/or limited flexion [6, 16].

Restoring the native laxities of knee at 0° of flexion requires removal of all osteophytes, extending the knee to 0°, and adjusting the varus-valgus angle and thickness of the tibial component until the varus, valgus, internal, and external rotational laxities are negligible [3]. Flexing the knee to 90° and adjusting the anterior-posterior slope and thickness of the tibial component until the offset of the anterior tibia from the distal medial femoral condyle measured at the time of exposure matches the knee with the trial components, and the internal and external rotation of the tibia approximately 14° restores the native laxities of knee at 90° of flexion (Fig. 8.4) [3]. The ability of kinematically aligned TKA to restore the native knee and limb alignments and the laxities of the knee may explain the reports from a randomized clinical trial and a national, multicenter study that showed patients with a kinematically aligned TKA reported better pain relief, better function, better flexion, and a more normal-feeling knee than patients with a mechanically aligned TKA [2, 5].

Kinematic alignment sets the femoral component at the native angle and level of the distal (0°) and posterior (90°) joint line. The surgical technique begins by using an offset caliper to measure the anterior-posterior offset of the anterior tibia from the distal medial femur with the knee in 90° of flexion (Fig. 8.4). Two millimeters is subtracted from the offset measurement if cartilage is missing on the distal medial femoral condyle. The measured offset is subsequently used as the reference for completing Quality Assurance Step 5 to restore the native F-E angle or slope of the tibial joint line. Once the knee is fully exposed, the locations of cartilage wear are assessed on the distal femur. A ring curette is used to remove any partially worn cartilage. The flexion-extension position of the femoral component is set by the insertion of a positioning rod 8–10 cm through a drill hole placed parallel to the anterior surface of the distal femur and perpendicular to the distal articular surface (Fig. 8.5). The use of a starting hole midway between the top of the intercondylar notch and the anterior cortex reduces the risk of flexing the femoral component more than 5° from the anatomic axis of the femur, which is associated with patellofemoral instability [26]. Minimizing flexion of the femoral component completes Quality Assurance Step 1.

The varus-valgus rotation and proximal-distal translation of the femoral component are set by using a disposable distal referencing guide that compensates 2 mm when there is cartilage wear on the distal medial femoral condyle in the varus knee and 2 mm when there is cartilage wear on the distal lateral femoral condyle in the valgus knee. The distal resections are measured with a caliper. The anterior-posterior translation and internal-external rotation of the femoral component are set by placing a 0° rotation posterior referencing guide in contact with the posterior femoral condyles (Fig. 8.6). The positioning of the posterior referencing guide infrequently requires correction because complete cartilage loss on the posterior medial and posterior lateral femoral condyles is rare in most varus and valgus osteoarthritic knees [27]. The posterior resections are measured with a caliper. Correction for bone wear is rarely needed at 0° and 90° of flexion even in the most arthritic knees [3, 27]. Adjusting the thickness of the distal and posterior resections to match the thickness of the femoral component after compensating for cartilage wear and the kerf of the saw blade completes Quality Assurance Step 2.

Kinematically aligned TKA sets the tibial component at the native internal-external, varus-valgus, flexion-extension, and proximal-distal positions of the articular surface of the tibia with use of an extramedullary tibial guide (Figs. 8.7, 8.8, and 8.9) [3]. The internal-external rotation of the tibial component is set parallel to the flexion-extension plane of the knee with use of either the major axis of the lateral tibial condyle or a kinematic tibial baseplate method [3, 28, 29]. When the major axis of the lateral tibial condyle method is used, the elliptical-shaped boundary of the articular surface of the lateral tibial condyle is identified, and the major axis is drawn (Fig. 8.7) [3, 28, 29]. A guide is used to drill two holes into the medial articular surface parallel to the major axis drawn on the lateral tibial condyle. After the tibial resection is made, the anterior-posterior axis of the tibial component is aligned parallel to these two holes. This technique uses a rationale similar to Cobb’s method, which finds the flexion-extension plane of the knee by fitting circles to the medial and lateral tibial condyles [30]. In contrast to mechanically aligned TKA where the medial border and medial one-third of the tibial tubercle are considered useful landmarks, a study of a case series of kinematically aligned TKAs showed that aligning the tibial component to the medial border or medial one-third of the tibial tubercle would have malrotated the tibial component 5° or more from the flexion-extension plane of the knee in 70% and 86% of the knees, respectively [3, 31, 32]. The use of the major axis of the lateral tibial condyle is a reproducible method as shown by a negligible bias (−1° internal) and an acceptable precision (± 5.4°) between the anterior-posterior axis of the tibial component and the flexion-extension plane of the knee and minimal malrotation of the tibial component on the femoral component [28, 29]. Aligning the anterior-posterior axis of the tibial component parallel to the flexion-extension plane of the knee completes Quality Assurance Step 3 [28, 29, 32].