USING SHAPE MODELS TO ANALYZE KNEE SHAPE CHANGES AND SURGICAL OUTCOMES IN ARTHROPLASTY PATIENTS

Understanding changes in knee joint morphology after total knee arthroplasty (TKA) is important for evaluating surgical outcomes and guiding future interventions. While clinical measurements can provide essential insights into varus/valgus alignment, statistical shape models (SSMs) allow a more comprehensive analysis of pre- and post-operative knee shape. SSMs capture the patterns in which the overall shape of a structure varies across a population, providing a quantification of how individual shapes differ from the mean. To use SSMs to quantify the difference in knee shape after TKA, and to investigate the association between pre-operative knee shape and post-operative patient reported outcome measurements (PROMs). We retrospectively collected pre-operative and one-year post-operative radiographs for 376 TKA patients. For a subset of 369 patients, one-year post-operative function and satisfaction PROMs were available. In each radiograph, the knee joint shape was outlined using 100 points. Point positions were placed at approximately similar positions in the pre- and post-operative radiographs. We created a single SSM including both pre- and post-operative knee shapes. For each radiograph, a B-Score was computed by projecting the SSM modes along a vector representing the difference between the pre-operative and post-operative mean knee joint shape, centering the B-Score distribution on the post-operative mean. Pre- and post-operative differences in B-Scores were analysed, and associations with post-operative PROMs were assessed. The SSM included 11 modes to explain 95% of the overall shape variation across all data. The B-Score analysis revealed significant differences between pre- and post-operative knee alignment. Pre-operatively, most patients showed varus alignment, indicated by significantly higher B-Scores, with the alignment changing to a more neutral position post-operatively. The pre- and post-operative mean B-Scores were 1.7±1.3SD and 0.0±0.99SD, respectively. The 10% of lowest (valgus) and 15% of highest (varus) pre-operative B-Scores indicated good surgical outcomes, with PROMs function and satisfaction scores above 76 and 75, respectively. SSMs provide a robust, quantitative framework for analysing morphological and alignment changes in the knee following TKA. Patients with extreme pre-operative varus or valgus alignment are likely to achieve good surgical outcomes. This study highlights the potential to complement clinical alignment measurements with SSMs for evaluating surgical outcomes. Acknowledgements: This research was funded by the Wellcome Trust [223267/Z/21/Z].