In this study, we investigated the relationship between MRI findings and knee symptoms in women with early KOA. The most important findings of this study were that MRI findings in early KOA, diagnosed by the newly established criteria were proven, and that imaging evidence of various types of knee pain was demonstrated. Although MRI evaluation showed that cartilage damage (53%) was the most common finding in women without radiographic knee abnormalities, there was no significant association between cartilage damage and knee symptoms at an early phase of the disease. On the other hand, the prevalence of BMLs (33%), bone attrition (8%), meniscal lesions (16%), and synovitis (27%) was higher in patients with early KOA. Meniscal lesions and synovitis were positively associated with knee symptoms, and key factors for various types of knee pain. These results suggested that meniscal lesions and synovitis have the greatest impact on symptoms of patients with early KOA; these should be detected and appropriate intervention applied in order to prevent disease progression.
Few studies have investigated the prevalence of abnormalities detected by MRI specifically in a population without radiographic KOA. Guermazi et al. reported that osteophytes were the most common abnormality (74%), followed by cartilage damage (69%), BMLs (52%), synovitis (37%), and bone attrition (32%) in participants without radiographic evidence of KOA22. Javaid et al. found that osteophytes (99%), cartilage damage (70%), and BMLs (59%) were common in participants without knee symptoms and radiographic KOA23. In this study, the same tendency, i.e., that many participants had cartilage damage, osteophytes, and BMLs, was observed. However, the prevalence of each abnormality was lower than that in previous reports. This may be because the participants in this study were younger, had a lower BMI, and might be at a lower risk of KOA than in previous studies.
Many previous studies have reported an association between knee effusion and knee symptoms. Torres et al. noted that synovitis or effusion detected on MRI correlated best with knee pain measured on a visual analog scale24. Chiba et al. investigated suprapatellar effusion using quantitative measurement of ultrasonography and concluded that knee effusion was associated with the symptoms as evaluated by KOOS15. Moreover, several studies have shown that knee effusion was associated with KOA disease stage and progression14,25,26. Although there have been few studies of effusion in early KOA, Harkey et al. suggested that effusion precedes the onset of accelerated KOA and may be a prognostic biomarker27. In this study, synovitis was the most strongly associated with knee symptoms and was considered to be the most important clinical finding indicating the need for intervention, in agreement with previous reports.
Meniscal lesions and BMLs were also associated with knee symptoms and disease progression in previous studies28–31. In addition, there have been reports that meniscal lesions, BMLs, and synovitis are associated with each other32–34. However, van Oudenaarde et al. have suggested that the discriminative power of single MRI features is insufficient to be useful as predictors of KOA35. In this study, although meniscal lesions, together with BMLs, bone attrition, and synovitis, were significantly associated with the presence of knee symptoms, the adjusted regression model showed that this association was attenuated after adjustment for age and BMI. In the early KOA population, synovitis would contribute more to knee symptoms than meniscal lesions.
BMLs reflect bony damage, which is associated with knee symptoms in KOA patients and bone metabolism, such as bone absorption. In recent studies, the presence of BMLs has been associated with knee pain and predicted cartilage loss in patients with established KOA36–38. Antiresorptive drugs (e.g., bisphosphonate) were found to reduce the size of BMLs and the risk for total knee arthroplasty in KOA patients39,40. In early KOA, lower bone mineral density and higher levels of some bone markers were associated with the presence of BMLs16. In the present study, BMLs were associated with pain in relatively loaded motions, such as bending and straightening of knees, walking, and climbing stairs. BMLs may be induced by bone fragility, resulting in early subchondral changes, such as microcracking before radiologic osteoarthritic findings become definitive, and causing pain on loading.
In many previous studies, although cartilage damage was associated with bone marrow lesions41,42, meniscal damage43, and synovitis or effusion44, cartilage damage was not always associated with the severity of knee pain22. Since articular cartilage is not innervated, the association between cartilage and pain severity may be due to other structural abnormalities associated with KOA45. In this study, cartilage damage was the most common finding, but it was not significantly associated with pain. This result suggested that minor cartilage damage detected by MRI was not associated with knee symptoms in accordance with established KOA.
This study had several limitations. First, all participants in this study were women. Although the prevalence of KOA is higher in women than in men, men may be more likely to experience knee trauma, which may affect the prevalence of cartilage damage, BMLs, and meniscal lesions. Second, laboratory data, including inflammation, cartilage/bone metabolic markers, were not evaluated. Serum hyaluronic acid, as a marker of synovitis was assessed in many previous studies14,46. Third, synovial thickening and joint effusion were not distinguished from each other, since knee effusion was not evaluated by contrast-enhanced MRI. Due to the possible complications and increased cost, we did not use contrast-enhanced MRI. Fourth, the findings of this study were based on cross-sectional data; therefore, future longitudinal studies are needed to investigate whether patients with knee effusion are likely to develop KOA.