M18.11 identifies degenerative joint disease affecting the right first carpometacarpal (CMC) joint — the basal thumb joint — arising from primary (idiopathic) wear rather than a secondary cause such as trauma or inflammatory arthritis.
Verified May 8, 2026 · 5 sources ↓
- Status
- Billable
- Chapter
- 13
- Related CPT
- 8
- Region
- Hand
Documentation tips
What should appear in the chart to support M18.11.
Source · Editorial brief grounded in 5 cited references ↓
- Explicitly name the affected side — 'right hand' or 'right thumb CMC joint' — rather than relying on laterality implied by procedure notes.
- Distinguish primary (idiopathic/degenerative) OA from post-traumatic or secondary OA in the assessment; 'primary' is required for M18.11 and is not interchangeable with M18.51.
- Record imaging findings that support the diagnosis: joint space narrowing at the right first CMC, subchondral sclerosis, osteophyte formation on X-ray, or Eaton-Littler grading stage if documented.
- Document the history of conservative care (splinting, NSAIDs, prior injections) when billing a surgical procedure, to establish medical necessity.
- When the patient describes symptoms in both thumbs, assess and document each side independently; bilateral primary OA requires M18.0, not M18.11.
Related CPT procedures
Procedure codes commonly billed with M18.11. 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 M18.11 and adjacent codes.
Source · Editorial brief grounded in CDC ICD-10-CM tabular guidance, AAOS coding references, and cited references ↓
- Using M18.11 for post-traumatic or inflammatory arthritis at the right CMC joint — those map to M18.51 (secondary, right) or the appropriate inflammatory arthropathy code, not M18.11.
- Defaulting to the unspecified parent code M18.10 when the provider clearly documents 'right hand' — laterality is documented, so M18.11 is required.
- Coding M18.11 alongside M18.0 for the same encounter when bilateral disease exists — M18.0 (bilateral primary) is the correct single code and M18.11 should not be added as a duplicate.
- Conflating the first CMC joint with the MCP or IP joints of the thumb — M18.11 is specific to the carpometacarpal (basal) articulation, not the knuckle or interphalangeal joints.
Clinical context
Source · Editorial summary grounded in 5 cited references ↓
M18.11 applies when the provider documents primary osteoarthritis of the right first CMC joint (also called basilar thumb arthritis or basal joint arthritis) and the condition is unilateral — right side only. 'Primary' means idiopathic degenerative disease; it excludes post-traumatic or secondary OA, which maps to M18.51 (unilateral secondary OA, right hand). If bilateral CMC OA is documented, use M18.0 (bilateral primary) instead. If the provider documents left-hand involvement only, use M18.12.
Clinicians frequently describe this joint as the 'base of the thumb' or 'trapeziometacarpal joint.' Both are acceptable clinical synonyms for the first CMC joint and support M18.11 when laterality is specified as right. The code is valid for conservative management visits, corticosteroid injections, splinting encounters, and as the surgical indication for trapeziectomy (25210) or CMC arthroplasty (25447).
M18.11 groups to MS-DRG 553 (Bone Diseases and Arthropathies with MCC) or 554 (without MCC) for inpatient encounters. It is classified as a chronic condition indicator, which is relevant for quality reporting and risk-adjustment programs.
Sibling codes
Other billable codes under M18.1 (laterality / anatomic variants).
Frequently asked questions
Source · Generated from the editorial pipeline, verified against 5 cited references ↓
01What is the difference between M18.11 and M18.51?
02Can I use M18.11 if the note says 'basilar thumb arthritis' without specifying primary vs. secondary?
03What CPT codes are commonly paired with M18.11 for surgical cases?
04Does M18.11 require a 7th character extension?
05If a patient has right CMC OA and also has right wrist OA, should both be coded?
06Which MS-DRGs does M18.11 map to for inpatient encounters?
Sources & references
Editorial content was developed using the following public sources. Last verified May 8, 2026.
- 01CDC ICD-10-CM Tabular List 2026 (effective October 1, 2025)
- 02icd10data.comhttps://www.icd10data.com/ICD10CM/Codes/M00-M99/M15-M19/M18-/M18.11
- 03aapc.comhttps://www.aapc.com/codes/icd-10-codes/M18.11
- 04icdlist.comhttps://icdlist.com/icd-10/M18.11
- 05aaos.orghttps://www.aaos.org/globalassets/quality-and-practice-resources/coding-and-reimbursement/resident-guide/resident-guide_icd10.pdf
Mira AI Scribe
Mira's AI scribe captures right-side laterality, the joint descriptor ('base of thumb,' 'first CMC,' or 'trapeziometacarpal'), the characterization of arthritis as primary or degenerative, and any imaging findings (X-ray grade, joint space narrowing). That documentation locks in M18.11 rather than falling back to the unspecified M18.10 or being miscoded to a secondary OA code — preventing downcoding and payer audit flags.
See how Mira captures M18.11 documentation