ICD-10 Code M18.0: Bilateral primary osteoarthritis of the thumb (CMC) joints

Key Takeaways

M18.0 is the ICD-10-CM code for bilateral primary osteoarthritis of the first carpometacarpal (thumb basal) joints

Use M18.0 only when both thumb CMC joints are involved. Unilateral primary thumb CMC osteoarthritis uses M18.11 (right hand) or M18.12 (left hand)

Documentation must confirm primary etiology (not post-traumatic), bilateral first CMC joint involvement, and physician attestation before submitting M18.0

Practice management software like Pabau helps hand surgery and hand therapy practices attach the correct diagnosis code and reduce claim denials

ICD-10 Code M18.0 is a billable ICD-10-CM code for bilateral primary osteoarthritis of the first carpometacarpal joints, the saddle-shaped joints at the base of each thumb. It describes wear-and-tear (idiopathic) arthritis in both thumb basal joints, with no prior injury or secondary cause.

This reference covers the code’s billable status, hierarchy, inclusion terms, related-code comparisons, the ICD-9 crosswalk, paired CPT codes, and documentation requirements for hand surgery and hand therapy billing.

ICD-10 Code M18.0: Quick reference

M18.0 is a billable ICD-10-CM code for bilateral primary osteoarthritis of the first carpometacarpal (thumb basal) joints. It sits within the Diseases of the musculoskeletal system and connective tissue chapter and requires no additional character extensions. The table below summarizes the key code attributes coders need before submitting a claim.

What ICD-10 Code M18.0 means: Bilateral primary thumb CMC osteoarthritis

M18.0 means primary osteoarthritis affecting the first carpometacarpal joint of both thumbs. The first carpometacarpal (CMC) joint sits between the first metacarpal and the trapezium bone at the base of the thumb. Because it is a saddle joint that carries load with almost every pinch and grip, it is the most common site of hand osteoarthritis.

Primary means the degeneration comes from age-related wear rather than a prior injury, inflammatory disease, or other secondary cause. You will also see this joint called the thumb basal (or basilar) joint and the trapeziometacarpal joint, and the condition itself referred to as rhizarthrosis in European literature. All describe the same diagnosis M18.0 captures when both thumbs are involved.

Primary vs secondary: Why etiology changes the code?

Primary osteoarthritis of both thumb CMC joints has no identifiable cause beyond aging and repetitive loading. Secondary osteoarthritis follows a specific trigger such as a prior fracture, dislocation, or an underlying inflammatory or metabolic condition. That distinction changes the code within the M18 family, so confirm the etiology before selecting M18.0. The thumb CMC options are:

• Bilateral primary thumb CMC OA: M18.0 – no prior traumatic or systemic cause, both thumbs
• Unilateral primary thumb CMC OA: M18.10 (unspecified hand), M18.11 (right hand), or M18.12 (left hand)
• Bilateral post-traumatic thumb CMC OA: M18.2 – documented prior injury, both thumbs
• Unilateral post-traumatic thumb CMC OA: M18.30 (unspecified hand), M18.31 (right hand), or M18.32 (left hand)
• Other secondary thumb CMC OA: M18.4 (bilateral) or M18.51 (right hand) / M18.52 (left hand) / M18.50 (unspecified hand)
• Unspecified thumb CMC OA: M18.9 – side or etiology not documented in the record

For an authoritative check on musculoskeletal diagnostic code selection, see the CDC/NCHS ICD-10-CM web tool, which reflects the official annual tabular list.

Is M18.0 a billable ICD-10-CM code for FY2026?

Yes. ICD-10 Code M18.0 is a fully billable, reportable code under ICD-10-CM. It does not require a seventh character extension and is accepted for claim submission by Medicare, Medicaid, and most commercial payers as of FY2026, effective October 1, 2025.

The CMS ICD-10 codes page publishes the annual update files confirming code validity. No deletion or revision was noted for M18.0 in the FY2026 update tables. Coders can submit this code without appending additional specificity characters, unlike the unilateral M18.1x and M18.3x subcodes, which require a laterality digit for the right or left hand.

M18.0 code hierarchy: Parent codes and category structure

Understanding where M18.0 sits in the ICD-10-CM hierarchy helps coders navigate related thumb CMC codes and avoid over- or under-specificity. The full hierarchy is shown below.

The parent category M18 covers osteoarthritis of the first carpometacarpal joint only. M18.0 is one of several subcodes within M18, each differentiated by laterality (one thumb or both) and etiology (primary, post-traumatic, or other secondary).

Because M18.0 is a complete, specific code at the fourth-character level, no further extension is required for reporting. The unspecified end of this same family, M18.9, follows the same hierarchy logic when laterality or etiology isn’t documented in the record.

Inclusion terms: How M18.0 appears in documentation?

Physicians rarely write “M18.0” in the note. They describe the diagnosis in clinical language, and the coder maps it to the code. The following terms all point to M18.0 when both thumbs are involved and the cause is primary:

• Bilateral primary trapeziometacarpal osteoarthritis
• Bilateral thumb basal (or basilar) joint arthritis, primary
• Bilateral primary rhizarthrosis
• Idiopathic degenerative joint disease of both first CMC joints

If a physician documents “bilateral basal joint arthritis of the thumbs” or “primary trapeziometacarpal OA, both hands” without naming a code, the coder may assign M18.0 based on these equivalent terms. The documentation does not need to use the word “osteoarthritis” verbatim, as long as the clinical meaning and the bilateral primary pattern are clear.

When not to use ICD-10 Code M18.0?

Do not reach for M18.0 by default whenever a chart says “osteoarthritis.” The single most common error with this code is coding the wrong joint entirely. Reserve M18.0 for the bilateral primary thumb CMC scenario and route everything else to its correct code.

M18.0 is the thumb, not the knee

Bilateral knee osteoarthritis is not M18.0. Knee osteoarthritis lives in category M17 (for example, M17.0 for bilateral primary osteoarthritis of the knee). If the chart documents both knees, you are in the M17 family, not M18.

Getting this wrong produces a code that describes the wrong body part, which fails medical necessity against any hand or knee procedure on the same claim.

A knee arthroscopy claim carries its own anesthesia code, such as CPT 01382, entirely separate from the M18 thumb family.

Other codes to route away from M18.0

• Osteoarthritis of other hand joints or generalized hand OA: use the M15 (polyosteoarthritis) or M19 categories, not M18
• One thumb only: use M18.11 (right hand) or M18.12 (left hand), not M18.0
• Post-traumatic thumb CMC OA: use M18.2 or M18.3, even when both thumbs are affected
• Side or cause not documented: query the provider before defaulting to the unspecified M18.9

Coders should verify borderline cases against the current tabular list using the AAPC Codify ICD-10-CM lookup. For HIPAA-compliant billing documentation practices, see our guide on HIPAA-compliant billing workflows.

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M18.0 vs M18.1 vs M17: Choosing the right osteoarthritis code

The M18 subcodes overlap enough that coders regularly submit the wrong one, and the neighboring M17 (knee) category adds to the confusion. The key differentiators are the joint (thumb CMC vs knee), laterality (both thumbs vs one), and etiology (primary vs post-traumatic vs other secondary). The table below resolves the most common decision points.

The practical rule: use M18.0 when the record documents primary osteoarthritis of both thumb CMC joints. If only one thumb is involved, switch to M18.11 or M18.12. If a prior thumb injury is the origin, move to M18.2 or M18.3. When the side or cause isn’t documented at all, the correct code is M18.9.

ICD-9-CM to ICD-10-CM crosswalk for M18.0

For practices still managing legacy claims, audits, or historical documentation comparisons, the approximate ICD-9-CM equivalent for M18.0 is 715.94 (Osteoarthrosis, unspecified whether generalized or localized, involving hand). This mapping comes from the Centers for Medicare and Medicaid Services’ General Equivalence Mappings (GEM) files.

GEM mappings are approximate. The ICD-9-CM code 715.94 is far less specific than M18.0 because ICD-9-CM could not distinguish the first carpometacarpal joint, laterality, or primary versus secondary etiology within the hand.

Going the other direction, 715.94 actually forward-maps to the unspecified M18.9, not M18.0, so do not assume historical hand-OA claims re-code cleanly to M18.0. Clinical record review is required to confirm both thumbs, primary etiology, and the CMC joint specifically. The ICD-9 to ICD-10 converter from ICD10Data provides a free lookup for confirming these mappings.

CPT codes commonly billed with ICD-10 Code M18.0

Diagnosis code M18.0 appears alongside a range of CPT procedure codes depending on the treatment pathway, from conservative splinting and injections through to surgical reconstruction of the thumb basal joint. The table below lists the most common pairings. Payer-specific medical necessity criteria apply. Confirm with the applicable Local Coverage Determination (LCD) before submitting.

For CPT payment amounts and relative value units associated with these codes, use the CMS fee schedule lookup. Practices using claims management software can attach these CPT-ICD pairings to encounter templates, reducing manual code selection errors at the point of billing.

CPT 25447 and CPT 25448 are mutually exclusive for the same CMC arthroplasty. Report only one per joint per operative session. Never bill 25448 in conjunction with 25447 for the same procedure, since 25448 already includes the interposition component that 25447 bills on its own.

M18.0 documentation requirements for bilateral thumb CMC osteoarthritis

Submitting M18.0 without adequate supporting documentation is the leading reason hand and orthopedic claims are down-coded or denied on audit. The medical record must substantiate both bilateral first CMC involvement and primary etiology before the code is defensible.

Hand surgery and hand therapy practices managing musculoskeletal compliance requirements and documentation standards will find overlapping requirements here. For robust clinical documentation tools, see Pabau’s client record features, which support structured clinical notes and bilateral joint assessment fields.

Required documentation elements

• Bilateral thumb CMC involvement confirmed: exam findings (basal joint tenderness, a positive CMC grind test, thumb-base squaring) and/or imaging showing joint space narrowing and osteophytes at both trapeziometacarpal joints
• Primary etiology documented: physician attestation that the OA is idiopathic or age-related, without a specific precipitating injury. The absence of a documented injury is not enough. The record should affirmatively note “primary” or “idiopathic” etiology
• Symptom documentation: thumb-base pain, weak or painful pinch and grip, and difficulty with tasks like opening jars, turning keys, or writing
• Absence of secondary cause: the record should reflect that inflammatory arthritis, prior fracture or dislocation, and other secondary causes have been considered and excluded or coded separately
• Physician signature: the diagnosing physician’s attestation of the bilateral primary diagnosis is required for Medicare and most commercial payer submissions

Using digital intake forms that include bilateral thumb CMC assessment fields helps capture the documentation points above at the point of care rather than retrospectively. A signed medical release form on file also covers the records-release step auditors sometimes request separately. Pabau’s structured note templates can be configured for bilateral hand and musculoskeletal documentation.

Using M18.0 to support thumb CMC arthroplasty and trapeziectomy

When a hand surgeon bills CPT 25448 (suspension arthroplasty with interposition, the LRTI procedure) for a trapeziectomy with ligament reconstruction tendon interposition, M18.0 functions as the primary supporting diagnosis for bilateral primary thumb basal joint OA. CPT 25447 applies instead when the procedure is interposition-only, without the suspension or tendon-transfer component.

Medicare requires that the diagnosis substantiate medical necessity for the procedure. A non-specific or unspecified code can trigger a medical necessity denial even when the surgery is clinically appropriate.

Thumb CMC reconstruction is almost always staged one hand at a time, so each operative claim carries M18.0 as the supporting diagnosis for the thumb being treated that day. The operative report should confirm the primary, non-traumatic etiology to keep the M18.0 assignment defensible.

Medicare sequencing for CMC surgery with M18.0

• M18.0 should be sequenced as the principal or first-listed diagnosis code on the claim
• Secondary codes may include hand or thumb pain (for example, M79.641 for right hand pain or M79.642 for left hand pain), or relevant systemic conditions
• The operative report must confirm bilateral primary thumb CMC OA without a documented traumatic etiology to support the M18.0 assignment
• Local Coverage Determinations (LCDs) for CMC arthroplasty list the ICD-10 codes that satisfy medical necessity. Confirm M18.0 appears on the applicable LCD for your Medicare Administrative Contractor (MAC)

When LCD criteria are borderline for a specific case, get a signed Medicare waiver on file before the surgery date, so the practice isn’t exposed to a post-payment recoupment if medical necessity is later challenged.

M18.0 for hand therapy and rehabilitation billing

Hand therapists and occupational therapists use M18.0 as the supporting diagnosis when treating bilateral thumb CMC OA conservatively. The diagnosis code drives the functional goals documented in the plan of care and supports medical necessity for the therapeutic interventions billed.

For therapy practice management, attaching the correct supporting diagnosis to each encounter is a core workflow step. Pabau’s documentation tools help hand therapy practices maintain compliant records across high-volume musculoskeletal cases.

Therapy payers frequently require functional outcome measures in the plan of care when M18.0 is the supporting diagnosis. For the thumb, capture a hand-specific measure such as the QuickDASH or the Patient-Rated Wrist/Hand Evaluation (PRWHE) at evaluation and again at reassessment to defend medical necessity for continued treatment.

Practices using musculoskeletal practice software with integrated outcomes tracking face similar documentation requirements for billing.

Conclusion

Bilateral primary thumb CMC osteoarthritis is a common diagnosis in hand surgery and hand therapy billing, but M18.0 is easy to misapply, most often by defaulting to it for one-sided or post-traumatic cases without confirming both joints and the primary etiology first.

Getting the joint, laterality, and etiology right from the start, and supporting every claim with documentation that confirms all three, is what separates clean claims from remittance advice headaches.

Pabau helps hand surgery and hand therapy practices build ICD-10 code-pairing templates into their billing workflows, so the right diagnosis code follows the right procedure code automatically. To see how Pabau handles bilateral thumb CMC documentation and billing, book a demo.

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