As its name suggests, Monoarticular involves one joint.
Oligoarticular is defined as involving greater than one joint but less than 5 joints.
Polyarticular is defined as greater than or equal to 5 joints.
This pattern/ number of joint involvement is typically defined within the first 6 months of disease onset.
Macaubas C, Nguyen K, Milojevic D, Park JL, Mellins ED. Oligoarticular and polarticular JIA: epidemiology and pathogenesis. Nature reviews Rheumatology. 2009;5(11):616-626. doi:10.1038/nrrheum.2009.209.
Lin YT, Wang CT, Gershwin ME, Chiang BL. The pathogenesis of oligoarticular/polyarticular vs systemic juvenile idiopathic arthritis. Autoimmun Rev. 2011 Jun;10(8):482-9. doi: 10.1016/j.autrev.2011.02.001. Epub 2011 Feb 12.