Degenerative joint disease of both thumb basal joints (the first carpometacarpal joints, bilaterally) caused by a secondary etiology other than post-traumatic injury — such as inflammatory arthritis, crystal deposition, or metabolic joint disease.

Documentation tips

What should appear in the chart to support M18.4.

Source · Editorial brief grounded in 4 cited references ↓

• Explicitly name the secondary cause (e.g., rheumatoid arthritis sequela, gout, pseudogout, hemochromatosis) — 'secondary OA' alone satisfies the code but the underlying condition should also be coded separately.
• Confirm bilateral involvement in the note; if only one thumb CMC is examined or treated, M18.4 is not appropriate — use M18.51 or M18.52.
• Record imaging findings that support bilateral degenerative changes: joint space narrowing, subchondral sclerosis, osteophytes at the trapezium-metacarpal articulation on both hands.
• Document that the etiology is non-traumatic; if there is any history of prior thumb fracture or dislocation, the provider must clarify whether that trauma is the cause — if yes, switch to M18.2.
• If an underlying systemic condition (e.g., gout, M10.x; rheumatoid arthritis, M05-M06) is the driver, code it first or as an additional code per Code Also instructions at the M18 parent level.

Related CPT procedures

Procedure codes commonly billed with M18.4. Linking the right diagnosis to the right procedure is what establishes medical necessity.

Source · CMS LCDs · AAOS specialty guidance · claims-pattern analysis

Common coding pitfalls

The recurring mistakes coders make with M18.4 and adjacent codes.

Source · Editorial brief grounded in CDC ICD-10-CM tabular guidance, AAOS coding references, and cited references ↓

• Using M18.4 when the chart documents a prior thumb injury as the cause — that scenario requires M18.2 (bilateral post-traumatic), not M18.4.
• Defaulting to M18.9 (unspecified) because the etiology feels unclear — if the provider documents 'secondary OA' of both CMC joints from any non-traumatic cause, M18.4 is appropriate and more specific.
• Selecting M18.0 (bilateral primary OA) when the record documents an underlying systemic disease contributing to the joint destruction — primary OA is idiopathic only.
• Appending a 7th-character extension (A, D, S) to M18.4 — M-codes do not use encounter-type extensions; the code is complete as five characters.
• Failing to code the underlying secondary condition alongside M18.4 — the secondary etiology should appear as an additional diagnosis to tell the full clinical story and satisfy payer medical necessity requirements.

Clinical context

Source · Editorial summary grounded in 4 cited references ↓

Sibling codes

Other billable codes under M18 (laterality / anatomic variants).