Fixation and load in total knee arthroplasty : a clinical, radiographic, radiostereometric and gait study

Sammanfattning: The treatment of severe gonarthrosis by total knee arthroplasty is generally successful, but tibial component loosening remains a problem. The aim in the present work was to investigate if any association between loading and insufficient tibial component fixation could be found, and if the fixation was affected by the tibial component design differences. Further, to compare the outcome measures used and the instrumentation systems used. A homogeneous group of 45 gonarthrosis patients were treated with TKA, where 3 different designs of uncemented tibial components were distributed by randomisation (Tricon-M, Tricon stem and PCA). Knee joint loading before and twice after the operation was assessed by use of gait analysis and tibial component fixation by use of radiostereometric analysis (RSA). Conventional radiography was used to assess leg alignment as the Hip-Knee-Ankle (HKA)angle and the positions of the prosthetic components. The clinical course from before the operation up to 4 (range 3-5) years of follow-up was monitored by Hospital for Special Surgery (HSS) scores and 4 different modes of pain in Visual Analogue Scales (VAS). Activity level was assessed postoperatively as well as quality of life by the Nottingham Health Profile (NHP) questionnaire.Two patients died before the 2-year control, 2 more before 3 and 4 years, respectively, otherwise all patients attended all controls. Alignment was better by use of the Tricon instrumentation. Alignment correlated with frontal plane moments. No relevant differences between the 3 tibial component designs were shown in migration. Addition of a stem in the Tricon design was therefore not proven beneficial. However, when external forces were applied at 2 years of follow-up, the Tricon prostheses rotated more than the PCA prostheses, interpreted as a consequence of the more conforming articulating surfaces. Previously, long-term follow-up with RSA have shown all loosened prostheses to have had increased early migration, and a predictive value of 80 % have been reported with 2 years of follow-up. Accordingly, a threshold of 200 µmmigration the second year was applied, and the material divided into two groups with different prognosis, regardless of prosthesis design: 28 patients with good prognosis; and 15 patients with poor prognosis with risk of future loosening. The prognosis groups tumed out to be different in several aspects. The risk group showed larger inducible displacement by external forces, the interface thereby interpreted as more compliant. The flexion moments during gait were increased in the risk group, as well as the mean moments, both before and after the operation. This finding was considered important and could imply a causal relationship between increased loading during gait and risk of tibial component loosening. The risk group had subtle symptoms after 4 (3-5) years of follow-up, only detected by the NHP, which should therefore be considered as a more sensitive, possibly also more relevant, measure of outcome than HSS and VAS.