Background and Objective: Monoarthritis is the inflammation of a single joint. There is limited data on the etiology of monorthritis in Iran. This study was aimed to evaluate epidemiologic charecteristics of the patients with monorthritis in Kashan, Iran.
Materials and Methods: This cross-sectional study was conducted on 227 patients with monorthritis referred to the rheumatology clinic of Kashan, Iran between 2018 and 2019. General, physical and rheumatological examinations were performed on all patients. synovial fluid samples were analyzed for viscosity, leucocyte count, Gram's staining, culture and microscopic evaluation of crystals.
Results: Mean age of the patients at the baseline was 56.1±17.2 years, out of whom 44.1% were male. Acute and chronic monoarthritis cases were 51.5% and 48.5%, respectively. The most common involved joint was the knee (60.7%) followed by  the ankle (32.2%). Pseudogout (39.3%,) and gout (31.6%) were the most common causes of acute monoarthritis. There was no significant difference in etiology of acute monoarthritis between male and female (P value=0.33). Osteoarthritis (53.50%) and rheumatoid arthritis (25.50%) were the most common causes of chronic arthritis. A significant correlation was obtained between sex and causes of chronic monoarthritis (P value=0.012). Patients with various etiologies of chronic monoarthritis had significantly different mean age (P value<0.001).
Conclusion: The knee joint was the most common site affected by monoarthritis. Pseudogout and osteoarthritis were the most common causes of acute and chronic arthritis, respectively.