Preoperative Planning for Primary Total Knee Arthroplasty (Mechanical, Anatomic, and Kinematic Alignment)

Proper knee alignment and soft tissue envelope integrity have been considered to be influential factors in determining the long-term outcomes and implant survival after total knee arthroplasty (TKA) [1]. Nowadays, several alignment options have been introduced in knee arthroplasty. Among them, the reference ones are mechanical alignment, anatomical alignment, and kinematic alignment [2]. In addition, the concept of hybrid alignment was recently introduced with the aim of optimizing the advantages of different types of alignment [2, 3]. Regardless of the choice of alignment, every patient requires a careful clinical and radiological assessment before knee replacement surgery; this preliminary study allows us to investigate the different characteristics and deformities presented by each patient and to be prepared for intra-operative challenges [4]. The preoperative clinical evaluation aims to analyze and quantify several parameters, such as ligamentous laxity, flexion contracture, deformity reducibility, and preoperative range of motion (ROM). The evaluation of the ligamentous laxity guides the surgeon in the choice of appropriate implant constraint [5]. For example, a valid posterior cruciate ligament will allow the implant of a cruciate retaining arthroplasty. Large varus or valgus deformities can lead to collateral ligamentous contractures on the concave side and laxity on the convex side; therefore, performing a varus–valgus stress test at 0° and 30° of knee flexion can guide the choice of implant constraint [6–8]. Flexion contractures ranging from 5° up to 20–30° can occur in case of severe osteoarthritis; these cases require precautions to be taken into account before surgery: to achieve full extension it will be necessary to plan for a larger distal femur cut. Deformity reducibility predicts the need to perform ligament release in order to obtain balanced gaps [9]. No less important is the assessment of preoperative ROM; the literature shows how this correlates with postoperative ROM [10]. Therefore, the patient should be warned about the difficulty of obtaining a full ROM in case of limited preoperative ROM. This limitation is, in fact, partly due to inveterate contracture of the extensor apparatus and joint capsule, which can only be partially corrected during surgery.