Post-traumatic osteoarthritis of the first carpometacarpal (CMC) joint of the right hand, arising unilaterally as a sequela of prior trauma to the thumb base.
Verified May 8, 2026 · 6 sources ↓
- Status
- Billable
- Chapter
- 13
- Related CPT
- 10
- Region
- Hand
Documentation tips
What should appear in the chart to support M18.31.
Source · Editorial brief grounded in 6 cited references ↓
- Explicitly document the index trauma (e.g., 'Bennett fracture 2018') and connect it to the current arthritis in the assessment — imaging alone does not establish post-traumatic etiology.
- Record laterality by name ('right hand') in the diagnosis; M18.31 is right-specific and will not map correctly from a laterality-neutral note.
- Include imaging findings that confirm CMC joint degeneration: joint space narrowing, subchondral sclerosis, osteophytes at the trapezio-metacarpal articulation.
- Document the stage or severity of functional impairment (pinch strength, grip, ADL limitations) to support medical necessity for surgical or injection procedures.
- If the patient has prior records of the traumatic event, reference them in the note to substantiate the causal link for payer audit purposes.
Related CPT procedures
Procedure codes commonly billed with M18.31. 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 M18.31 and adjacent codes.
Source · Editorial brief grounded in CDC ICD-10-CM tabular guidance, AAOS coding references, and cited references ↓
- Assigning M18.11 (primary OA, right CMC) when the record documents a prior thumb fracture or dislocation — always review the history section before defaulting to primary.
- Using the non-billable parent code M18.3 instead of the laterality-specific M18.31; M18.3 is not valid for claim submission.
- Coding M18.31 without any documented traumatic history — post-traumatic etiology requires explicit physician linkage, not coder inference from imaging.
- Confusing M18.31 with M18.51 (other unilateral secondary OA, right hand) — inflammatory or metabolic secondary arthritis goes to M18.51, not M18.31.
- Failing to add a separate code for the index injury sequela if required by the payer or registry; some trauma registries require an S-code with 7th character 'S' to reflect the sequela relationship.
Clinical context
Source · Editorial summary grounded in 6 cited references ↓
M18.31 applies when a patient develops osteoarthritis at the right first CMC joint — the basilar thumb joint — and the provider has documented a causal relationship to prior trauma (e.g., a Bennett fracture, Rolando fracture, CMC dislocation, or repeated ligamentous injury). This is a distinct etiology from primary/degenerative CMC arthritis (M18.11) and must be supported by both the traumatic history and current imaging or clinical findings consistent with joint degeneration.
The M18 category separates CMC osteoarthritis by etiology and laterality. For right-hand post-traumatic disease: M18.31. Left hand: M18.32. Unspecified laterality: M18.30. Bilateral post-traumatic: M18.2 (a single code, no laterality suffix needed). If the etiology is primary (degenerative, no trauma history), use M18.11 for the right hand instead. If secondary but not post-traumatic (e.g., inflammatory, metabolic), use M18.51.
Post-traumatic CMC arthritis commonly presents years after the index injury. The provider note must explicitly link the arthritis to the prior trauma — a vague note of 'thumb arthritis' defaults to primary unless otherwise specified. Coders should not infer post-traumatic etiology from imaging alone; the physician must establish the causal relationship in the record.
Sibling codes
Other billable codes under M18.3 (laterality / anatomic variants).
Frequently asked questions
Source · Generated from the editorial pipeline, verified against 6 cited references ↓
01What is the difference between M18.31 and M18.11?
02Can I use M18.31 if the trauma occurred decades ago?
03What code do I use if both CMC joints are post-traumatic?
04Is an external cause code required alongside M18.31?
05What if the provider documents 'basilar thumb arthritis' without specifying etiology?
06Does M18.31 require a specific imaging study to be valid?
07Which CPT codes are most commonly billed with M18.31?
Sources & references
Editorial content was developed using the following public sources. Last verified May 8, 2026.
- 01CDC ICD-10-CM Tabular List 2026 (effective Oct 1, 2025)
- 02icd10data.comhttps://www.icd10data.com/ICD10CM/Codes/M00-M99/M15-M19/M18-/M18.31
- 03aapc.comhttps://www.aapc.com/codes/icd-10-codes/M18.31
- 04icd10data.comhttps://www.icd10data.com/ICD10CM/Codes/M00-M99/M15-M19/M18-
- 05cms.govhttps://www.cms.gov/medicare/coding/icd10/downloads/icd10clinicalconceptsorthopedics1.pdf
- 06handsurgeryresource.nethttps://www.handsurgeryresource.net/taxonomy/term/175
Mira AI Scribe
Mira AI Scribe captures the right-hand laterality, the documented prior traumatic event (fracture, dislocation, or ligamentous injury at the thumb CMC), and current imaging or clinical findings confirming joint degeneration — together establishing the post-traumatic etiology required for M18.31. This prevents default assignment to primary OA (M18.11), which would misrepresent the etiology and potentially misalign surgical authorization criteria.
See how Mira captures M18.31 documentation