The impact of different alignment strategies on bone cuts in total knee arthroplasty for varus knee phenotypes

Abstract

Purpose: The purpose of this study was to visualise the influence of alignment strategy on bone resection in varus knee phenotypes. The hypothesis was that different amounts of bone resection would be required depending on the alignment strategy chosen. Through visualisation of the corresponding bone sections, it was hypothesised, it would be possible to assess which of the different alignment strategies would require the least amount of change to the soft tissues for the chosen phenotype, whilst still ensuring acceptable alignment of the components, and thus could be considered the most ideal alignment strategy.

Methods: Simulations of the different alignment strategies (mechanical, anatomical, constrained kinematic and unconstrained kinematic) in relation to their bone resections were performed on five common exemplary varus knee phenotypes. VAR_HKA174° VAR_FMA87° VAR_TMA84°, VAR_HKA174° VAR_FMA90° NEU_TMA87°, VAR_HKA174° NEU_FMA93° VAR_TMA84°, VAR_HKA177° NEU_FMA93° NEU_TMA87° and VAR_HKA177° VAL_FMA96° VAR_TMA81°. The phenotype system used categorises knees based on overall limb alignment (i.e. hip knee angle) but also takes into account joint line obliquity (i.e. TKA and FMA) and has been applied in the global orthopaedic community since its introduction in 2019. The simulations are based on long-leg radiographs under load. It is assumed that a change of 1° in the alignment of the joint line corresponds to a displacement of the distal condyle by 1 mm.

Results: In the most common phenotype VAR_HKA174° NEU_FMA93° VAR_TMA84°, a mechanical alignment would result in an asymmetric elevation of the tibial medial joint line by 6 mm and a lateral distalisation of the femoral condyle by 3 mm, an anatomical alignment only by 0 and 3 mm, a restricted by 3 and 3 mm, respectively, whilst a kinematic alignment would result in no change in joint line obliquity. In the similarly common phenotype 2 VAR_HKA174° VAR_FMA90° NEU_TMA87° with the same HKA, the changes are considerably less with only 3 mm asymmetric height change on one joint side, respectively, and no change in restricted or kinematic alignment.

Conclusion: This study shows that significantly different amounts of bone resection are required depending on the varus phenotype and the alignment strategy chosen. Based on the simulations performed, it can, therefore, be assumed that an individual decision for the respective phenotype is more important than the dogmatically correct alignment strategy. By including such simulations, the modern orthopaedic surgeon can now avoid biomechanically inferior alignments and still obtain the most natural possible knee alignment for the patient.

Keywords: Alignment; Anatomical; Arthroplasty; Bone cuts; Kinematic; Knee; Mechanical; Phenotype; Restricted; TKA; Varus.

Fig. 1

The hip–knee–ankle angle (HKA) is formed by the lines connecting the centre points of the femoral head, the knee and the talus; FMA is the angle between the mechanical axis of the femur and a tangent to the distal femoral condyles; TMA is defined as the angle between the mechanical axis of the tibia and a tangent to the proximal articular surface of the tibia. The joint line convergence angle (JLCA) is the angle between a tangent to the proximal articular surface of the tibia and the tangent of the femoral condyles

Fig. 2

The five exemplary VAR phenotypes (STATIC). The indicated prevalences refer to the proportion of phenotypes in the population with varus-aligned knees

Fig. 3

The five exemplary VAR “native” phenotypes

Fig. 4

First main VAR “native” phenotype 1 VAR_HKA174° VAR_FMA87° VAR_TMA84°

Fig. 5

Second main NEU “native” phenotype VAR_HKA174° VAR_FMA90° NEU_TMA87°

Fig. 6

Third main NEU “native” VAR_HKA174° NEU_FMA93° VAR_TMA84°

Fig. 7

Fourth main NEU “native” phenotype VAR_HKA177° NEU_FMA93° NEU_TMA87°

Fig. 8

Fifth main VAR “native” phenotype VAR_HKA177° VAL_FMA96° VAR_TMA81°