Histological synovitis and radiographic damage in knee osteoarthritis: insights from a comprehensive analysis of ultrasound-guided synovial biopsies in 161 patients

Abstract

Objective: Synovial inflammation plays a crucial role in osteoarthritis (OA) by producing key cytokines that mediate synovium-cartilage interaction and drive damage progression. In this study, we aimed to evaluate relationships between histological features of synovitis, radiographic damage and patients' clinical characteristics.

Methods: This observational cross-sectional study included consecutive patients with knee OA from 2016 to 2022. Enrolled patients were aged between 40 and 90 years, had chronic knee pain lasting at least 3 months and showed ultrasound evidence of synovitis. All patients underwent a general rheumatological evaluation, including the collection of clinical and laboratory data and ultrasound (US)-guided minimally invasive synovial tissue biopsy. The severity of synovitis was assessed by histology using the Krenn Synovitis Score (KSS).

Results: A total of 161 patients were considered for the analysis. The multivariate analysis showed that both US effusion and Kellgren-Lawrence (KL) grade were positively associated with histological synovitis. In contrast, age, sex, body mass index, levels of inflammatory markers, pain intensity and cardiovascular risk factors were not associated with histological synovitis. A strong positive correlation was found between KL grades and the KSS. A moderate positive correlation emerged between KL grades and the proportion of patients with lymphocytes and plasma cells in synovial tissue.

Conclusions: More severe histological synovitis in patients with non-end-stage knee OA is associated with worse radiographic structural damage. In the advanced stages of structural damage, the likelihood of detecting a lymphoplasmacytic inflammatory infiltrate in the synovial membrane increases. US-detected effusion serves as a marker of histological synovitis.

Keywords: Osteoarthritis, Knee; Synovitis; Ultrasonography.

Figure 1. (A) Distribution of KSS in patients with knee OA. (B) Light microscope images of samples obtained from patients in our cohort with different KSS (top to bottom: KSS 0–1, KSS 2–3 and KSS 4–5). (C) Distribution of the KL score in patients with knee OA. (D) Representative radiographic images of the KL score. KL, Kellgren-Lawrence; KSS, Krenn Synovitis Score; OA, osteoarthritis.

Figure 2. Proportion of patients with synovial inflammatory features across KL grades. (A) Proportional distribution of KSS among patients for each KL grade. (B and C). Proportional distribution of the patients with synovial tissue enriched in lymphocytes (B) and plasma cells (C) for each KL grade. KL, Kellgren-Lawrence; KSS, Krenn Synovitis Score; OA, osteoarthritis.