IMPLANT FIXATION: THE INFLUENCE OF PATIENT AND IMPLANT CHARACTERISTICS IN TOTAL KNEE ARTHROPLASTY
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Radiostereometric analysis (RSA) is a stereo imaging technique that permits evaluation of tibial component fixation in total knee arthroplasty (TKA) post-operatively. The overall aim of this thesis was to examine the influence of patient and implant factors on migration and inducible displacement measured with RSA in a combined dataset of 418 primary TKAs, with an overall goal of improving knowledge of how RSA-based measures can be used for clinical evaluation of patients post-operatively. Three connected studies are presented, which were used to address this global aim. In the first study, migration patterns of cemented TKA fixation and uncemented fixation were compared, determining that while uncemented components had significantly higher one year migration, they achieved equivalent stability between one and two years post-operatively. This suggests that an initial period of settling does not compromise long-term fixation for uncemented components. In the second study, longitudinal data analysis methods were used to further investigate differences in migration over two years for tibial components, examining the influence of implant factors (fixation, tibial component area), as well as patient characteristics (age, sex, body mass index, smoking status). Uncemented tibial components had higher magnitudes of migration in the first two post-operative years, and this difference was even more pronounced in female patients. Overall migration with cemented fixation was not different by sex. Analyzing uncemented tibial components separately by sex revealed that the effect of smoking was opposite in men and women. In women, smoking was associated with higher migration magnitudes. In men, smokers had lower migration compared to non-smokers, although there were relatively small proportions of smokers. Additionally, for uncemented tibial components in women, increasing age, especially above age 60 was associated with higher magnitude migration. For cemented components, the only significant factor was tibial component area, with larger sizes associated with greater migration in female patients. The third study investigated the utility of inducible displacement data over ten years of follow-up from loaded single leg stance RSA exams as an alternative assessment method to migration data. Inducible displacement was significantly different for cemented and uncemented components, but not sensitive to patient factors. Uncemented components demonstrated higher early inducible displacement within the first three post-operative years, but lower late inducible displacement at ten years compared to cemented implants. The correlation between migration and inducible displacement was greatest for uncemented components in the first year post-operatively. Inducible displacements were significantly higher for continuous migrators as well, especially for uncemented components. The overall findings of this thesis support the use of uncemented fixation of tibial components in TKA. While demographic factors influenced implant migration of female and male patients differently, there is no evidence that uncemented fixation is compromised in female patients. In addition to migration, inducible displacements are a viable metric to quantify the stability of implant fixation and have significant potential as an early screening tool. Future collaborative research has the potential to substantially enhance understanding around implant fixation as it will permit the study of increasingly specific groups.