ICD-10 Code M05.66 – Rheumatoid arthritis of knee with involvement of other organs and systems (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code M05.66 – Rheumatoid arthritis of knee with involvement of other organs and systems
What it is
This code identifies rheumatoid arthritis affecting the knee, with documented involvement of other organs or body systems. Use it when the knee is part of a systemic rheumatoid process, not isolated joint disease.
Clinical signs
Typical findings include chronic inflammatory joint pain, swelling, stiffness, and reduced knee motion. Clinical features vary; refer to documentation for evidence of extra-articular involvement such as systemic rheumatoid manifestations.
When to use this code
Use this code when the record clearly states rheumatoid arthritis of the knee and also documents involvement of other organs or systems. It fits cases where the provider links the knee arthritis to a systemic rheumatoid condition. Check documentation if the site or systemic involvement is unclear.
Do not use for
Do not use it for degenerative knee arthritis, isolated knee pain, or rheumatoid arthritis without documented systemic involvement. Do not assign it if the chart only says rheumatoid arthritis and does not specify the knee.
Coding tip
Confirm both the joint site and the extra-articular/systemic documentation before assigning M05.66.