The notable results in this study were that there was a correlation between rest and moving pain VAS with a decrease in synovitis scores and PPTs on synovitis area. These results indicate synovitis after TKA exists consistently with pain. This is the first report about association between residual pain and synovitis after TKA. In osteoarthritis, synovitis and bone marrow lesions are the primary pathologic lesions that have been consistently associated with pain [7, 8, 19]. Generally the cause of residual pain after TKA is considered to be neural sensitization [10, 16]. In addition, synovitis is more strongly associated with pain sensitization than bone marrow lesion [12]. Thus synovitis after TKA is said to be another important cause of residual pain in terms of not only the direct inflammatory pain but also sensitization. Although postoperative residual pain including sensitization after TKA is multifactorial, patients with residual pain after TKA should be examined with ultrasonography to detect synovitis in addition to undergoing a physical examination and conventional radiography.
We originally devised synovitis mapping of the knee and synovitis scores because the knee is spacious compared with small joints which are generally associated with rheumatoid arthritis for which the synovitis grade was developed [15]. Synovitis scores correlated with pain, although the maximum synovitis grade did not. This result suggests the efficacy of the synovitis score for evaluation of entire knee inflammation. In a magnetic resonance imaging study on osteoarthritis patients, larger bone marrow lesions are associated with greater knee pain [4]. This study suggested the efficacy of a summation of each grade across the entire knee joint to evaluate entire knee pain.
There are some limitations to this study. First, because we used convenient samples, this study does not qualify, strictly speaking, as a cross-sectional study. Therefore it is supposed that mean moving pain VAS of our samples was over twenty, in addition prevalence rate of synovitis was as high as 89%. This study is considered preliminary, and a rigorous cross-sectional study will be necessary to reveal the epidemiology of synovitis after TKA. Secondly, although the observation methods of synovitis are generally not only ultrasonography, but contrast-enhanced magnetic resonance imaging, single photon emission computed tomography, or historical examination. Takase et. al. simultaneously assessed ultrasonography and magnetic resonance imaging in comparison with histopathological changes in the knee joints of long-lasting arthritis patients [18]. Because positive power Doppler ultrasonography findings were closely associated with all pathological comportments of synovitis including inflammatory cell infiltrates, synovial lining layer thickness and vascularity, they concluded both imaging techniques are useful for the visualization of synovitis. Thirdly, there was a lack of significant information on the possible causes of synovitis: for example, existence of preoperative synovitis, synovectomy in primary surgery, changes in synovitis activity over the course of time, soft tissue balance, or implant design. To investigate the causes of synovitis after TKA, researchers will need to conduct a longitudinal study that includes these items. To investigate the pathology of synovitis after TKA, arthroscopic synovectomy may be useful if severe synovitis is observed in ultrasonography [3].