[Kinematic alignment-management of deformities and flexion contractures].

BACKGROUND: Kinematic alignment (KA) total knee arthroplasty (TKA) has proven to be a viable alternative to mechanical alignment (MA) TKA. Like any technique, its validity has to encounter challenges. Two of these are flexion contracture and large deformities of the knee. FLEXION CONTRACTURE: Flexion contracture is commonly treated with posterior capsular release. Often, however, this technique is not sufficient. Many surgeons accustomed to MA techniques and guided by traditional femoral instruments manage the problem by proximalizing the distal femoral cut. However, this has proven to be a surgical mistake that often leads to mid-flex knee instability. KA rules limit this mistake but leave the problem of flexion contracture. In these cases, the surgeon acts on the tibia distalizing the cut and, so as not to create instability in flexion, decreasing the slope. The technique is effective to obtain full extension but often leaves instability at 90° of knee flexion. To avoid this, it is useful to use the so-called "virtuous mistake" strategy, which strictly follows the KA guidelines but undercuts the posterior femoral condyles by 2 mm. DEFORMITIES: Another problem that often limits the use of KA is large deformities. It is necessary to distinguish whether they are intra or extra-articular. In the first case, it is possible to decide whether to use the pure KA technique or to restrict the indication at the level of the tibial cut. In the latter, extraarticular osteotomies or combined procedures (osteotomy plus KA) have to be performed.