M18.0 identifies degenerative joint disease of the first carpometacarpal (CMC) joint — the thumb basal joint — affecting both hands, with no traumatic or secondary cause documented.

Documentation tips

What should appear in the chart to support M18.0.

Source · Editorial brief grounded in 5 cited references ↓

• Explicitly state 'bilateral' and 'primary' (or 'degenerative') in the assessment — vague terms like 'thumb arthritis' leave etiology and laterality ambiguous.
• Record imaging findings for both hands: joint space narrowing, subchondral sclerosis, osteophyte formation, or Eaton-Littler classification stage at the first CMC joint.
• Document prior conservative treatment on both sides (splinting, NSAIDs, corticosteroid injections) to support medical necessity for advanced interventions such as trapeziectomy.
• Note absence of prior trauma or inflammatory arthritis to substantiate the 'primary' classification and rule out M18.2 or M18.4.
• If treating one hand this encounter while the contralateral side is also affected, document both sides in the assessment and apply M18.0 — not two unilateral codes.

Related CPT procedures

Procedure codes commonly billed with M18.0. 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 M18.0 and adjacent codes.

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

• Assigning M18.11 + M18.12 instead of M18.0 when bilateral OA is documented — per ICD-10-CM guidelines, use the bilateral code when it exists.
• Using M18.0 when the chart documents a prior CMC fracture or ligament injury as the cause — that etiology requires M18.2 (bilateral post-traumatic).
• Coding M18.9 (unspecified) when the provider documents 'bilateral thumb basal joint arthritis' — specificity is present in the note and must be captured.
• Omitting a separate code for associated trigger finger or flexor tenosynovitis when those conditions are treated at the same encounter — they are not bundled into M18.0.
• Confusing the first CMC joint with the MCP or IP joint of the thumb — M18.0 applies only to the trapeziometacarpal articulation, not more distal thumb joints.

Clinical context

Source · Editorial summary grounded in 5 cited references ↓

Sibling codes

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