Capsular stem cell function and tissue composition are associated with symptoms and radiographic severity in people with knee osteoarthritis.

BACKGROUND: Osteoarthritis (OA) is associated with lost range of motion in the affected joint(s). Evidence suggests that this may be due to increased activity of posterior capsule fibroblasts, cells in turn derived from mesenchymal stromal cells (MSCs). OBJECTIVES: To test the hypotheses that (1) MSCs are more numerous in the posterior capsule of patients with knee flexion contracture (FC) and (2) in OA participants with knee FC, the MSC population in the posterior capsule differentiates toward a fibrotic phenotype. In order to complete these objectives, we looked for associations between capsule histologic and MSC outcomes with clinical outcomes. DESIGN: Cross-sectional translational research design using data from the Ottawa Knee Osteoarthritis (OKOA) database. METHODS: A total of 71 OKOA database participants and their relevant clinical and laboratory outcomes were included. Associations were first tested with bivariate correlation, then for p < 0.10, tested using a linear model. RESULTS: No lab-based differences between FC and no-FC groups we discovered. In the posterior capsule, there was an association between knee flexion and adipogenic capacity (p = 0.001), osteogenic capacity (p < 0.001), KL grade and percent "other" (mainly neurovascular) tissue (p = 0.039), visual analog scale pain, and percent fibrous tissue (p = 0.014). For the anterior capsule, there was an association between knee flexion (p = 0.002) and extension (p = 0.005) with MSC enumeration, KL grade with MSC fibrogenic capacity (p = 0.002), and Knee Injury and Osteoarthritis Outcome Score quality of life with chondrogenic capacity (p < 0.001). CONCLUSION: Joint capsule composition, MSC enumeration, and function were associated with important clinical OA outcomes. These findings suggest that the entire joint capsule may play an important role in OA-related morbidity and progression and could represent an underappreciated target for OA treatment.