Abstract
Introduction
Recent studies have challenged the concept that a single ‘correct’ alignment to standardised anatomical references is the primary driver of TKA performance with regards to patient satisfaction outcomes. Patient specific variations in musculoskeletal anatomy are one explanation for this. Virtual simulated environments such as rigid body modelling allow for the impact of component alignment and variable patient specific musculoskeletal anatomy to be studied simultaneously. This study aims to determine if the output kinematics derived from consideration of both postoperative component alignment and patient specific musculoskeletal modelling has predictive potential of Patient Reported Outcomes.
Method
Landmarking of key anatomical points and 3D registration of implants was performed on 96 segmented post-operative CT scans of TKAs. Both femoral and tibia implant components were registered. Acadaver rig validated platform for generating patient specific rigid body musculoskeletal models was used to assess the resultant motions and contact forces through a 0 to 140 degree deep knee bend cycle. Resultant kinematics were segmented and tested for differentiation with and correlation to a 12 month postoperative Knee injury and Osteoarthritis Outcome Score (KOOS).
Results
Significant negative correlations (p<0.05) were found between the postoperative KOOS symptoms score and the rollback occurring in midflexion, quadriceps force in mid flexion, patella shear force and patella tilt at 90 degrees of flexion. A significant positive correlation was found between lateral shit of the patella through flexion and the symptoms score. (p<0.05) When segmenting those KOOS scores performing in the lowest 20% of patients, both rollback and the three patella measurements have statistically significantly different means (t test, p<0.05).
There were other trends present that are discernible but do not have linear correlations, as they are cross-dependant on other kinematic factors or are not linear in nature. When segmenting the varus/valgus angular change into those with a varus angular change from extension to full flexion between 0 and 4 degrees (long leg axis, not implant to implant) and those with either further varus change or a valgus change, a statistically significant difference of 7 points (p<0.05) in the postoperative KOOS pain score is observed. Likewise, measured rollback of no more than 6mm without roll forward scored 10 points higher (p<0.05) in the postoperative KOOS score. These two parameters form a ‘kinematic safe zone' of outcomes in which the postoperative KOOS score is 12 points higher (p<0.05).
Conclusions
The study showed statistically significant correlations between kinematic factors in a simulation of postoperative TKA and post-operative KOOS scores. The kinematic factors so captured are the result of both the variation in implant position and the subject specific, variable musculoskeletal anatomy. The presence of a ‘kinematic safe zone' in the data suggests a subject specific optimisation target for any given individual patient and the opportunity to preoperatively determine a subject specific implant position target.
