Secondary osteoarthritis of the left thumb carpometacarpal (CMC) joint arising from a cause other than post-traumatic origin — classified as unilateral, affecting the left hand only.
Verified May 8, 2026 · 5 sources ↓
- Status
- Billable
- Chapter
- 13
- Related CPT
- 11
- Region
- Hand
Documentation tips
What should appear in the chart to support M18.52.
Source · Editorial brief grounded in 5 cited references ↓
- Specify laterality by name — 'left hand' or 'left thumb CMC joint' — in the assessment or impression; 'non-dominant' alone is insufficient for coding.
- Identify the underlying secondary etiology (e.g., prior inflammatory arthritis, metabolic disorder, joint hypermobility) to distinguish M18.52 from primary OA (M18.12) or post-traumatic OA (M18.32).
- Record imaging findings that confirm CMC joint involvement: joint space narrowing, subchondral sclerosis, osteophyte formation, or subluxation on AP and stress views.
- Document history of conservative treatment tried and failed (splinting, corticosteroid injections, occupational therapy) if the encounter involves surgical planning — supports medical necessity.
- If the patient has a concurrent systemic condition driving the secondary OA (e.g., rheumatoid arthritis), code that underlying condition separately and sequence appropriately per the Chapter 13 guidelines.
Related CPT procedures
Procedure codes commonly billed with M18.52. 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 M18.52 and adjacent codes.
Source · Editorial brief grounded in CDC ICD-10-CM tabular guidance, AAOS coding references, and cited references ↓
- Assigning M18.52 when the secondary OA follows a documented traumatic injury — that is post-traumatic OA and belongs in M18.32 (left hand), not M18.52.
- Using M18.52 for primary (idiopathic) thumb CMC OA — primary left-hand CMC OA is M18.12; secondary etiology must be documented to justify M18.52.
- Defaulting to M18.50 (unspecified hand) when the operative note or imaging report clearly documents the left side — a laterality query can prevent this downcode.
- Coding M18.52 for OA of the IP or MCP joints of the thumb — those joints are not the first CMC and fall under M19, not M18.
- Omitting the underlying systemic diagnosis when one drives the secondary OA; ICD-10-CM guidelines generally require coding the underlying condition as well.
Clinical context
Source · Editorial summary grounded in 5 cited references ↓
M18.52 identifies secondary osteoarthritis of the first CMC joint (the basal thumb joint) in the left hand when the etiology is neither post-traumatic nor primary idiopathic. 'Secondary' means the degeneration is attributable to an underlying condition — such as prior inflammatory arthritis (rheumatoid, psoriatic, gout), metabolic disease, or joint hypermobility — rather than wear-and-tear alone. The 'other' designation within M18.5 separates it from post-traumatic secondary OA (M18.3x), which has its own laterality-specific codes.
Use M18.52 only when documentation explicitly identifies the left hand and specifies a secondary etiology that is not post-traumatic. If the underlying cause is undocumented but secondary OA is noted without further specification, M18.52 still applies per the 'Secondary osteoarthritis of first CMC joint NOS' annotation under parent M18.5. If laterality is missing, drop to M18.50 (unspecified hand). For bilateral involvement, use M18.4x codes instead.
Spine OA is excluded from M18 entirely — code to M47. Additionally, osteoarthritis of other hand joints (non-CMC) falls under M19, not M18. Confirm the joint is the first CMC before assigning any M18 code; documentation that says only 'thumb arthritis' without joint specification warrants a query.
Sibling codes
Other billable codes under M18.5 (laterality / anatomic variants).
Frequently asked questions
Source · Generated from the editorial pipeline, verified against 5 cited references ↓
01What distinguishes M18.52 from M18.12?
02When should I use M18.32 instead of M18.52?
03Can M18.52 be used if only 'secondary CMC arthritis NOS' is documented?
04What CPT codes are commonly paired with M18.52?
05Should I code the underlying condition separately when using M18.52?
06Is M18.52 valid for bilateral thumb CMC OA?
07Does M18.52 require a 7th character extension?
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-/M18.52
- 03aapc.comhttps://www.aapc.com/codes/icd-10-codes/M18.52
- 04aapc.comhttps://www.aapc.com/codes/icd-10-codes/M18.5
- 05cms.govhttps://www.cms.gov/medicare/coordination-benefits-recovery/overview/icd-code-lists
Mira AI Scribe
Mira captures the affected side (left), joint location (first CMC/basal thumb), etiology class (secondary, non-traumatic), imaging results (joint space narrowing, osteophyte formation), and any underlying systemic condition from the encounter note. That documentation prevents a fallback to unspecified M18.50, blocks misassignment to post-traumatic M18.32, and ensures the secondary etiology is coded alongside any driving systemic diagnosis.
See how Mira captures M18.52 documentation