M17.11 — 片側性原発性変形性関節症，右ひざ

The code means the patient has primary osteoarthritis in their right knee, and it's only affecting that one side.

Primary osteoarthritis happens naturally over time, usually due to aging, while secondary osteoarthritis is caused by something else, like an injury or another condition.

M17.12 means the person has primary osteoarthritis in their left knee, and it's limited to that one knee.