Functional knee phenotypes: a novel classification for phenotyping the coronal lower limb alignment based on the native alignment in young non-osteoarthritic patients

Abstract

PurposeThe currently used system to classify the lower limb alignment (neutral, varus, valgus) does not consider the orientation of the joint line or its relationship to the overall lower limb alignment. Similarly, current total knee arthroplasty (TKA) alignment concepts do not sufficiently consider the variability of the native coronal alignment. Therefore, the purpose of this study was (1) to introduce a new classification system for the lower limb alignment, based on phenotypes, and (2) to compare the alignment targets of different TKA alignment concepts with the native alignment of non‐osteoarthritic patients. MethodsTwo recent articles phenotyped the lower limb, the femur and tibia of 308 non‐osteoarthritic knees of 160 patients [male to female ratio = 102:58, mean age ± standard deviation 30 ± 7 years (16–44 years)]. The present study introduces functional knee phenotypes, which are a combination of all previously introduced phenotypes. The functional knee phenotypes therefore enable an evaluation of all parameters in relation to each other and thus a comprehensive analysis of the coronal alignment. The existing functional knee phenotypes in the female and male population were investigated. In addition, how many non‐osteoarthritic knees had an alignment within the range of current TKA alignment targets (mechanical, anatomical and restricted kinematic alignment) was investigated. Therefore, it was defined which functional knee phenotypes represented a target of the TKA alignment concepts and which percentage of the population had such a phenotype. ResultsOut of 125 possible functional knee phenotypes, 43 were found (35 male, 26 and 18 mutual). The most common functional knee phenotype in males was NEUHKA0° + NEUFMA0° + NEUTMA0° (19%), followed by VARHKA3° + NEUFMA0° + VARTMA0° (8.2%). The most common functional knee phenotype in females was NEUHKA0° + NEUFMA0° + NEUTMA0° (17.7%), closely followed by NEUHKA0° + NEUFMA0° + VALTMA0° (16.6%). The functional knee phenotype representing a mechanical alignment target was found in 5.6% of the males and 3.6% of the females. The phenotype representing an anatomical alignment target was found in 18% of the males and 17% in females. Five of the nine phenotypes representing a restricted kinematic alignment target were found in this population (male 5, female 4, mutual 4). They represented 31.3% of all males and 45.1% of all females. ConclusionA more individualized approach to TKA alignment is needed. The functional knee phenotypes enable a simple, but detailed assessment of a patient’s individual anatomy and thereby could be a helpful tool to individualize the approach to TKA. Level of clinical evidenceIII, retrospective cohort study.

Custom total knee arthroplasty facilitates restoration of constitutional coronal alignment

“…The theoretical benefits of custom TKA based on computed tomography (CT) reconstructions include the differentiation of constitutional versus arthritic bony deformities, as well as identification of the native femoral and tibial axes [11]. This concept has the potential to reduce boneimplant mismatch and preserve or restore CA within predetermined limits, to maintain the native overall phenotype while allowing correction of severe deformities [14][15][16]. The purpose of this study was to describe the strategy for coronal alignment using a CT-based custom TKA system and to evaluate the agreement between the planned and postoperative Hip-Knee-Ankle (HKA) angle, Femoral Mechanical Angle (FMA) and Tibial Mechanical Angle (TMA).…”

Higher satisfaction after total knee arthroplasty using restricted inverse kinematic alignment compared to adjusted mechanical alignment

“…Valgus knees have been shown to have hypoplastic lateral femoral condyles (mLDFA of 85° ± 2.3°) [15] with neutral tibiae (MPTA of 90° ± 1.5°) [10]. KA aims to resurface the Table 4 Uni-and multi-variable regression analyses to identify factors associated with OKS and VAS satisfaction after TKA femur, which might result in oblique valgus femoral resections.…”

“…In a recent computed tomography (CT) study of 308 non-arthritic knees, Hirschmann et al [10] estimated that native limb alignment in men and women, respectively, corresponds to MA in 4% and 6%, to AA in 17% and 18%, and to rKA in 45% and 51%. Although patient-specific alignment techniques would maintain native alignment of all knees, safe ranges for postoperative residual varus or valgus remain unknown [2,[8][9][10] leading some surgeons to opt for hybrid alignment [18].…”

Mechanical Alignment in Knee Replacement Homogenizes Postoperative Coronal Hip–Knee–Ankle Angle in Varus Knees: A Navigation-Based Study

“…Nevertheless, it has been described that this ideal mechanical axis does not ensure the optimal function and survival of the prosthesis. 10,42 This lack of certainty is due to wide variations between individual patients 6 and, as previously mentioned, because there are changes in the coronal mechanical axis from extension to flexion of the knee. 4,40,[43][44][45] In our series, 98.6% of the patients were within 0 AE 3 degrees of the HKA angle at full extension after surgery, with an excellent result, minimizing the outliers.…”