Emerging evidence indicates synovitis is related to OA symptoms and progression (11
). Synovial inflammation and effusions also occur with meniscal injuries (21
), even in patients without radiographic OA. However, cellular and molecular characteristics of synovitis associated with meniscal damage have not been reported. We sought to determine the prevalence and molecular features of synovial inflammation in patients who were (i) without preexistent radiographic features of OA, and (ii) undergoing arthroscopic meniscectomy for clinically-documented traumatic knee injury with MRI evidence of meniscal tears. Specifically, we wanted to determine whether synovial inflammation correlated with symptoms.
A previous study (22
) reported similar synovial pathology in OA patients and patients with joint injury. We therefore compared histologic features of synovial inflammation in meniscectomy patients to those in patients with established knee OA (27
). Appearance of cellular infiltrates was similar, but inflammation was less prevalent and extensive in meniscectomy patients. Contrary to what we anticipated, we did not see preferential localization of inflammation in the gutter on the side of the meniscal tear. One possible explanation is we did not analyze the perimeniscal SM directly adjacent to the tear. However, of the three locations biopsied, inflammation was identified most often in the suprapatellar location (in 43% of patients) suggesting synovial inflammation occurs within the joint at sites distant from the injury and is not only localized adjacent to the injury. Our findings are consistent with a recent report of the anatomic distribution of synovitis in knee OA defined by contrast-enhanced MRI techniques (34
). In that study, the suprapatellar region was the second most common area (59.5% of patients) in which synovitis was detected. The explanation for involvement of the suprapatellar region remains unclear. We speculate certain sites within the joint may be uniquely sensitive to effects of proinflammatory factors produced in response to meniscal injury.
We investigated whether inflammation was associated with preoperative joint symptoms and dysfunction. When stratified according to presence or absence of suprapatellar inflammation, Lysholm scores were significantly lower (p<0.05) in patients with inflammation. Lower Lysholm scores indicate greater knee-related symptoms. No differences in SF-12®
or VAS pain scores were observed. The Lysholm score is a knee-specific metric of symptoms and functional disability (25
). It is a weighted score, with pain and instability-related symptoms having most weight (25 points each of 100 total). In contrast, the VAS scale only reflects knee pain, and the SF-12®
health survey is not specific for knee-related issues. The unique association of inflammation with Lysholm scores and not VAS pain suggests symptoms other than pain (e.g., instability, swelling) captured by the Lysholm scale might account for this difference. The weighting of the scale may also contribute to our observation. In the future, other knee-specific instruments such as the KOOS (29
) – in which pain, other symptoms, and function can be independently evaluated – may be helpful in addressing this question. Whether inflammation is a cause or consequence of knee-specific symptoms in these patients (such as mechanical instability introduced by meniscal damage) needs to be evaluated.
We next looked at patient characteristics (age, BMI, degree of cartilage abnormality, and time elapsed between injury and surgery) in the stratified data. Age and BMI (30
) are known risk factors for OA. In this cohort, older patients were more likely to demonstrate synovial inflammation, but BMI did not differ with inflammation. We anticipated that infiltration of cells would increase with time elapsed between injury and surgery, but this did not appear to be true. Patients with inflammation tended to have shorter time intervals between injury and surgery. A possible explanation is that increased inflammatory symptoms prompt earlier intervention; however the present analysis did not address this issue. Multivariate analysis indicated the association between inflammation and Lysholm scores is independent of age, BMI, and interval between injury and surgery.
Our results cannot be generalized to all patients with meniscal tears. We studied a population in which an identifiable injury precipitated symptoms, and whose injuries did not involve the vascular portion of the meniscus. Also, despite a clear history of trauma most patients exhibited complex meniscal lesions. Although we excluded patients with radiographic OA, most patients demonstrated grade 1–4 Outerbridge cartilage lesions suggesting this population is enriched for patients with pre-radiographic disease. These observations show the presence of early joint degeneration in the majority of these patients (reviewed in (31
)). Synovial inflammation is associated with progression of cartilage loss in patients with established OA (11
), thus we determined whether inflammation was related to the degree of cartilage abnormality. There was a trend toward greater inflammation in patients with cartilage abnormalities, but our multivariate model demonstrated that the association between inflammation and Lysholm scores was independent of degree of cartilage abnormality. Our finding of synovial inflammation in one of seven patients with normal cartilage suggests in some cases of meniscal injury, synovitis may pre-date cartilage changes. This finding is consistent with an earlier study noting synovial immune complex deposits in patients with normal cartilage undergoing meniscal surgery (32
). It is possible synovitis contributes to alterations in structural and mechanical properties of meniscal tissues, resulting in susceptibility to meniscal injury and increased risk for development or progression of OA.
To obtain insight into molecular mediators that contribute to synovial inflammation, we did microarray analysis of synovial RNA. Four biopsies from patients with inflammation (grade 1 or 2) and four without (grade 0) were compared. Two hundred and sixty genes were differentially expressed between these two patient groups (>2 fold change). Pathway analysis, with a focus on genes involved in innate immune responses (28
), revealed a set of chemokine and chemokine receptors among the most highly upregulated transcripts in biopsies with inflammation. Expression of these genes () within synovium may promote recruitment of inflammatory cellular infiltrates, so we focused on this gene set for validation by real-time PCR.
We chose five genes for validation by real-time PCR: IL-8, CCL5, CCR7, CCL19, and CCL21. With the exception of IL-8, these belong to the “C-C” chemokine family which influences recruitment of monocytes, lymphocytes and eosinophils. IL-8, a “C-X-C” chemokine, promotes neutrophil chemotaxis to sites of inflammation. Although first described as a T-lymphocyte recruitment factor, CCL5 (or RANTES) has pleiotropic effects on multiple leukocyte subsets. CCR7 is the cognate receptor for both CCL19 and CCL21, which are involved in T-lymphocyte and dendritic cell migration. Interaction between these chemokines and their receptor mediates homing to secondary lymphoid tissues and appropriate migration of cells within lymphoid follicles (reviewed in (33
)). Our analysis revealed that IL-8, CCR7 and CCL19 transcripts were often undetectable in specimens without inflammation (), and GEE analysis demonstrated that relative expression levels of CCL5 and CCL19 were associated with inflammation, consistent with our microarray results. Levels of CCR7 and CCL19 transcripts, which represent a ligand/receptor pair, were strongly associated with Lysholm scores (Fig 4).
There are limitations to analysis of large datasets, such as those obtained by microarray, as they are prone to false positive results and not easily replicated (35
). In the present study, our application of pathway analysis to this expression dataset lends face validity to our findings, as genes are clustered functionally as well as statistically. Furthermore, chemokines identified by this high-throughput technique were validated using the more accurate method of quantitative PCR applied to a larger set of patients. Still, the biomarker potential of the chemokine signature needs to be validated in larger, prospective studies to determine if these gene expression profiles have any diagnostic or prognostic biomarker potential. Patients enrolled in this study are being followed for 2 years, and their clinical course assessed to determine whether the inflammatory response and/or chemokine expression is associated with short- and long-term outcomes after arthroscopy. Further limitations of our present study include the small sample size and the cross-sectional design. However, given the role of these chemokines in recruitment of inflammatory cells, we speculate they may contribute to development of synovial inflammation in response to meniscal injury. Determining whether they directly affect development of pain or progression of cartilage damage will require further investigation.
Identification of cellular and molecular mechanisms associated with synovial inflammation is of considerable interest not only for development of potential diagnostic or prognostic markers in early symptomatic OA and meniscal injury, but also for development of therapeutic approaches to control clinical symptoms and potentially reduce risk of joint degeneration in patients with knee injuries. Our study provides insight into mechanisms driving inflammatory infiltration and demonstrates an association between synovial inflammation and clinical symptoms in patients with meniscal injury, irrespective of the presence of underlying cartilage degeneration.