Post-traumatic osteoarthritis localized to the left first carpometacarpal (CMC) joint, occurring as a unilateral condition attributable to prior trauma rather than primary degeneration.

Documentation tips

What should appear in the chart to support M18.32.

Source · Editorial brief grounded in 6 cited references ↓

• Provider must explicitly link the current CMC arthritis to a specific prior traumatic event — document the nature of the injury (e.g., Bennett fracture, CMC dislocation) and approximate date.
• Specify laterality by name ('left') in every encounter note; do not rely on operative or injection site documentation alone to establish side.
• Record imaging findings that confirm degenerative change: joint space narrowing, subchondral sclerosis, osteophyte formation, or post-fracture deformity at the left CMC joint.
• If the patient has concurrent primary OA of other joints, code each site separately — M18.32 is joint- and etiology-specific and does not cover adjacent hand joints.
• Document conservative care history (splinting, NSAIDs, prior injections) to support medical necessity for surgical or procedural interventions billed alongside this diagnosis.

Related CPT procedures

Procedure codes commonly billed with M18.32. 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 M18.32 and adjacent codes.

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

• Using M18.32 when the provider documents OA of the thumb without specifying post-traumatic etiology — default to M18.12 (primary, left) unless trauma causation is explicitly stated.
• Selecting M18.32 for right-hand CMC arthritis; the '2' sixth character is left only — right hand is M18.31.
• Billing M18.32 with amniotic or placental-derived injection CPT/HCPCS codes — CMS policy A59764 lists this code as not supporting medical necessity for those products, creating an automatic denial risk.
• Defaulting to M18.32 when both CMC joints are affected; bilateral post-traumatic CMC OA requires M18.2, not two unilateral codes.
• Confusing post-traumatic (M18.32) with other secondary OA (M18.52) — use M18.52 only when the secondary cause is something other than direct trauma (e.g., inflammatory arthropathy, crystal deposition).

Clinical context

Source · Editorial summary grounded in 6 cited references ↓

Sibling codes

Other billable codes under M18.3 (laterality / anatomic variants).