Seven phenotypes of varus osteoarthritic knees can be identified in the coronal plane

Purpose Recommendations for resecting distal femur and proximal tibia in mechanical and anatomical alignment techniques are standardized. Kinematic alignment propagates individualizing resection planes. Whether significant variation exists, to warrant departure from standardized resection planes, has not been shown thus far in a large cohort of knees and with a wide range of varus deformity. The null hypothesis of this study was that there was no phenotypic variation in varus osteoarthritic knees. The aim of this paper was to determine whether distinct phenotypes could be identified, based on variations in coronal femoral and tibial morphology, which could aid in surgical planning and categorizing varus knees for future studies. Methods 2129 full-leg weightbearing radiographs were analyzed (1704 preoperative; 425 of contralateral arthritic knee). Measurements made were of HKA (hip-knee-ankle angle), VCA (valgus correction angle), mLDFA (lateral mechanical distal femoral angle), aLDFA (lateral anatomical distal femoral angle), MPTA (medial proximal tibial angle), MNSA (medial neck shaft angle), TAMA (angle between tibial mechanical and anatomical axes), and TPDR (percentage length of tibia proximal to extra-articular deformity). Results Seven distinct types were identified covering 2021 knees, reducible to 4 broad phenotypes: 11% were Type 1 'Neutral' knees showing values close to reported normal knees (mean VCA 5.5 degrees, mLDFA 87 degrees, aLDFA 81 degrees). 38% were Type 2 'Intra-articular varus' with medial intra-articular bone loss (mean mLDFA 90.9 degrees, MPTA 85.4 degrees, VCA of 5.7 degrees). 41% were Type 3 'Extra-articular varus' with extra-articular deformity (EAD). Type 3a had proximal tibial EAD; Type 3b had tibial diaphyseal EAD; Type 3c had femoral EAD (mean VCA 8.7 degrees, HKA 166 degrees), and severe medial bone loss (mean mLDFA 92 degrees, MPTA 83 degrees). 9% were Type 4 'Valgoid type' with features of valgus knees: Type 4a had medial femoral bowing (mean VCA 2.9 degrees); Type 4b had significant distal femoral valgus (mean mLDFA 85.3 degrees, aLDFA 78.6 degrees). Conclusions The null hypothesis that there was no phenotypic variation in varus osteoarthritic knees was rejected as considerable variation was found in coronal morphology of femur and tibia. Four broad phenotypic groups could be identified. Plane of the knee joint articular surface was quite variable. This has relevance to planning and performance of corrective osteotomies, unicompartmental and total knee arthroplasty.