**3. Classification**

Many authors have proposed ways of classifying valgus deformities of the knee for the purpose of surgical correction with TKA. The idea is to stratify the patients in order to improve the surgical planning and the choice of the degree of constriction of the implants.

The Krakow classification [5] proposed in 1991 is one of the most famous classifications. It categorizes valgus knees based on the integrity of the medial soft tissues and on prior surgeries. Type 1 deformity has an intact medial collateral ligament (MCL). Type 2 has an insufficiency of the MCL with positive valgus stress test. Type 3 is a secondary valgus deformity created by an overcorrected high tibial osteotomy (HTO) in a previously varus-aligned limb.

The SOO classification, presented in 2003 (Societe d'Orthopedie de l'Ouest - Western France Orthopedics Society), recognizes four types of valgus knee, with increasing surgical difficulty. Type I can be completely reduced, without medial laxity. Type II is totally or partially irreducible, but without medial laxity. Lateral release is required, whereas Type III is reducible, but with medial distension laxity, and may require management of the medial laxity. Lastly, Type IV is irreducible, with medial distension laxity, combining the problems of types II and III [6].

Lombardi et al. in 2004 [7] proposed a slight modification of the Krakow classification, taking into account the degree of deformity, the status of the MCL, and the amount of release that must be performed. Variant I is characterized by mild deficiencies of the lateral femoral condyle and tibial plateau, with stable MCL and correction of the deformity with varus stress. In variant-II, the MCLs are intact, but they do not correct to neutral alignment with varus stress. Variant-III is distinguished

#### *Total Knee Arthroplasty in Valgus Knee DOI: http://dx.doi.org/10.5772/intechopen.109573*

by attenuation of the medial capsular ligament complex with opening of the medial joint line on valgus stress test.

Ranawat et al. in 2005 [1] added one more small modification, merging the previous classifications, adding the measure of the magnitude of the deformity to the Krakow classification. A type-I deformity has minimal valgus and medial soft-tissue stretching. A typical type-II fixed valgus deformity has a more substantial deformity (>10°) with medial soft-tissue stretching. A type-III deformity is a severe osseous deformity after a prior osteotomy with an incompetent medial soft-tissue sleeve.

Despite being widely used, the Krakow and the Ranawat classifications were designed with patients from developed countries, where most cases have minor deformities. In poorer countries, where the population has greater difficulty in accessing surgical treatment, there is a greater prevalence of severe cases and complex deformities. Therefore, new classifications have been proposed to better stratify severe cases.

In 2014, Mulaji et al. [8] proposed a classification into six types: type 1 reducible valgus, type 2 irreducible valgus, type 3 valgus associated with recurvatum, type 4 valgus associated with flexion contracture, type 5 valgus with MCL insufficiency, and type 6 extra-articular valgus.

Based on full-leg weight-bearing radiographs of 233 knees, the study of Mulaji et al. [9] identified four broad groups of valgus arthritic knees with nine phenotypes based on coronal plane variations in femoral and tibial morphology. Type 1 Neutral knees (12.5%) had almost normal values. Type 2 "Intra-articular valgus" (22.7%) showed lateral compartment bone loss. Type 3 "Extra-articular valgus" (35.2%) had extra-articular deformity: 3a showed valgus femoral bowing; 3b showed tibial valgus bowing; 3c showed tibial valgus bowing with lateral femoral condyle wear. Type 4 "Varus" type (29.6%) had features of varus knees: 4a had varus femoral bowing; distal femur in 4b was akin to varus knees with lateral tibial bone loss. 4c had varus tibial bowing and deficient lateral femoral condyle. 4d had varus tibial bowing and lateral tibial bone loss.

Yang et al. in 2021 [10] made deformity analysis on standing long-film radiographs and computed tomography (CT). Valgus deformities could be classified into five subtypes: the distal lateral femoral condyle (F1a), both distal and posterior lateral femoral condyle (F1b), the supracondylar region of the femur (F2), the tibial plateau (T1), or the metaphyseal segment of the tibia (T2). F2 and T1 (40.0% and 28.6%, respectively) were the most common two subtypes.
