M23 vs M24.66

M23 (Internal derangement of knee) compared with M24.66 (Ankylosis, knee), from the official CMS tabular data. Do not report these codes together: the official tabular list marks them Excludes1.

Can these codes be reported together?

No — do not report together. M23 carries an Excludes1 note covering M24.66: “ankylosis (M24.66)”

Sole exception: when the two conditions are documented as unrelated to each other (ICD-10-CM Official Guidelines, Section I.A.12.a).

Side by side

	M23Internal derangement of knee	M24.66Ankylosis, knee
Billing status	Non-billable header Report instead: M23.000, M23.001, M23.002, M23.003, M23.004	Non-billable header Report instead: M24.661, M24.662, M24.669
Classification	M00-M99 — Diseases of the Musculoskeletal System and Connective Tissue (M00-M99)	M00-M99 — Diseases of the Musculoskeletal System and Connective Tissue (M00-M99)
Definition	Internal derangement of knee is a non-billable ICD-10-CM category code (M23).	Ankylosis, knee is a non-billable ICD-10-CM category code (M24.66).

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Derived from the official CMS ICD-10-CM tabular data (FY2026). Not billing advice; verify sequencing rules for the encounter. All data sources