Description
Inverse kinematic alignment (iKA) aims to reproduce tibial varus. By adjusting the femoral component in the frontal plane, the ligament balance can be adjusted in extension. In flexion, translation and rotation balance the spaces. Internal rotation of the femoral implant (RotF) relative to the transepiphyseal axis (TEA) can be used to achieve this balance, taking care not to induce internal rotation relative to the posterior condylar axis (PCA).
Firstly, we wish to determine whether there are any particular anatomical features that require internal rotation in arthroplasties performed in reverse kinematic alignment. Secondly, whether internal rotation of the femoral component is detrimental to functional scores or patellar tilt at three months post- op.
Methods: 100 successive PTGs were performed using the Mako robotic assistance system. This series concerned primary gonarthrosis (excluding revision, osteotomy or previous fractures). Prospective data collection and retrospective analysis. 98 complete files. Single centre, single operator, Oct 2021-June 2022.
Triathlon/Stryker type PTG, CR, uncemented, systematic patellar resurfacing. Inverse kinematic alignment (iKA). No ligament release in this series. Composition of the study group (98): 66 women (67.3%), 32 men (32.7%), age 69.9±9.0 (49, 89), BMI 28.8±5.2 (19.2, 41.9).
Radiographic characteristics pre-op HKA 177.3±6.2. Pre-op MPTA. Pre-op DLFA 88.5±2.8. Pre-op JLCA 2.0±3.9 (JLCA<0 open angle medially, JLCA>0 open angle laterally). Pre-op patellar tilt 4.9±3.5
Collection of radiographic data at 3 months post-op and evaluation of PROMS (WOMAC, EVA, FJS, Kujala).
Results: Femoral component rotation (- internal rotation, + external rotation) -0.13±1.9 (-5, 4)
Post-operative patellar tilt 5.2±4.1 (ρ=0.17; p=0.121)
HKA post 178.7±2.5 (173.7, 183.7). PMTA post 87.3±2.1 (83.8, 92.1). DLFA post 88.7±1.9 (85.3, 92.3). JLCA post 0.0±0.8 (-2, 2.3).
A moderate negative correlation was found between RotF and pre-op HKA (ρ=-0.58; p<0.001). A moderate positive correlation was found between RotF and pre-op JLCA (ρ=0.56; p<0.001)
A weak negative correlation was found between RotF and PMTA pre (ρ=-0.4; p<0.001)
Kujala 70.2±15.2, Womac 22.1±15.2, VAS 1,7±1,3, FJS 36,8±23,3
A weak positive correlation was found between RotF and Kujala 3m (ρ=0.12; p=0.256), Womac 3m (ρ=0.074; p=0.497), VAS 3 months (ρ=0.014; p=0.896), FJS 3m (ρ=-0.047; p=0.67).
Conclusions: The internal rotation of the femoral component used in this series to balance the ligament balance in flexion during total arthroplasty using inverse kinematic alignment has no impact on early functional results or patellar tilt.
Valgus and a negative JLCA are predictive factors for femoral internal rotation during flexion balancing.
