Primary degenerative osteoarthritis of the first carpometacarpal (CMC) joint affecting one hand, with the specific side (right or left) not documented in the medical record.

Documentation tips

What should appear in the chart to support M18.10.

Source · Editorial brief grounded in 4 cited references ↓

• Always document laterality by name — 'right hand' or 'left hand' — so the coder can use M18.11 or M18.12 instead of the unspecified M18.10.
• Distinguish primary (idiopathic/degenerative) from post-traumatic or secondary OA in the note; the etiology drives the correct M18 subcategory.
• Record imaging findings that support the diagnosis — joint space narrowing, subchondral sclerosis, osteophytes at the trapezio-metacarpal articulation, or radial subluxation of the first metacarpal.
• Note any failed conservative measures (splinting, NSAIDs, corticosteroid injections) if the encounter is for surgical planning; payers often require this for procedure authorization.
• When the provider uses lay terms like 'thumb base arthritis' or 'basilar thumb,' the coder should query for laterality before defaulting to the unspecified code.

Related CPT procedures

Procedure codes commonly billed with M18.10. 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 M18.10 and adjacent codes.

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

• Using M18.10 when the note clearly names the hand — any documented laterality mandates M18.11 (right) or M18.12 (left); M18.10 is a payer audit flag.
• Coding M18.10 for post-traumatic thumb CMC arthritis — if prior trauma caused the degeneration, the correct code is M18.30 (unspecified), M18.31 (right), or M18.32 (left).
• Conflating bilateral thumb CMC OA with unilateral — when both joints are affected, use M18.0, not two instances of M18.1x.
• Mapping 'basal joint arthritis' directly to M18.10 without querying for side — the clinical term is non-lateral, but the record almost always contains laterality somewhere.
• Selecting a secondary OA code (M18.5x) when the record says 'primary' or 'degenerative' — the etiology classification in M18 is explicit and must match the documentation.

Clinical context

Source · Editorial summary grounded in 4 cited references ↓

Sibling codes

Other billable codes under M18.1 (laterality / anatomic variants).