M17.12: Unilateral primary osteoarthritis, left knee

The ICD-10-CM code for primary osteoarthritis of the left knee, used when the right knee is not involved and no prior injury or secondary cause is documented.

Billable

FY2027

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At a glance

Code Family

M17: Osteoarthritis of knee.

Condition

Knee osteoarthritis

Billable

Yes

HCC Mapping (v28)

No HCC mapping. Primary knee osteoarthritis carries no risk adjustment weight under CMS-HCC v28.

Effective Date

Valid in the FY2026 code set and unchanged in the FY2027 update.

Family Specificity

Eleven billable child codes split on two axes at once. The first axis is how the osteoarthritis arose, whether primary, post-traumatic or secondary. The second axis is which knee is involved, whether right, left, both or unstated.

Which M17 code do I assign?

The M17 family splits on two facts the note must carry independently: the etiology of the osteoarthritis and the knee it affects. Etiology selects the code series, and laterality selects the final character within that series.

When documentation says…

Code

Primary osteoarthritis, left knee

M17.12

Primary osteoarthritis confined to the left knee. The note must name the left side, must record no prior knee injury, and must record no secondary cause.

Primary osteoarthritis, right knee

M17.11

Primary osteoarthritis confined to the right knee. This code differs from M17.12 only in the laterality character.

Primary osteoarthritis, both knees

M17.0

Primary osteoarthritis of both knees. A bilateral diagnosis is carried by this single code rather than by M17.11 and M17.12 reported together.

Primary osteoarthritis of the knee, side not stated

M17.10

Unilateral primary osteoarthritis of an unspecified knee. Assign it only when the record documents one knee but never names which one.

Osteoarthritis of the left knee following an earlier injury

M17.32

Unilateral post-traumatic osteoarthritis of the left knee. The note must tie the osteoarthritis to a documented prior injury.

Osteoarthritis of the left knee secondary to another condition

M17.5

Other unilateral secondary osteoarthritis of knee. The note must name the underlying cause that makes the osteoarthritis secondary.

Left knee pain, osteoarthritis not confirmed

M25.562

Pain in left knee. Assign it when osteoarthritis is suspected but no diagnosis is documented.

Tabular notes

EXCLUDES2

Osteoarthritis of spine (the M47 codes). The note sits on the M15–M19 osteoarthritis block and reaches M17.12 through it. An Excludes2 note permits both codes on the same encounter when the record documents both conditions.

FY2027: EFFECTIVE 1 OCTOBER 2026

M17.12 is unchanged in the FY2027 update, effective 1 October 2026. The FY2027 codes addenda add, delete or revise no code in category M17 or elsewhere in the M15–M19 block. The FY2027 tabular addenda revise no note in category M17 or at the block. M17.12 is in the FY2027 billable code list.

Operational & financial impact

Why specificity matters beyond the chart: risk adjustment, DRG assignment and denial exposure all follow from which code is selected.

CMS-HCC v28 (PY2024+)

M17.12 does not map to any HCC under CMS-HCC v28.

Primary knee osteoarthritis carries no risk adjustment weight under the current CMS model.

M17.12 also carried no HCC mapping under CMS-HCC v24.

MEDICARE AND COMMERCIAL PAYERS

No National Coverage Determination governs osteoarthritis diagnosis coding.

INPATIENT AND OUTPATIENT USE

M17.12 is used in the outpatient setting and in the inpatient setting.

For a total knee arthroplasty admission, M17.12 is the principal diagnosis when unilateral primary osteoarthritis of the left knee is the documented reason for surgery.

As a principal inpatient diagnosis without an arthroplasty procedure, M17.12 groups to MS-DRG 553 or MS-DRG 554 under the FY2027 MS-DRG v44 grouper. The tier is set by the presence of a major complication or comorbidity in the full code set.

WHERE SPECIFICITY IS LOST

M17.9 records osteoarthritis of the knee without either the laterality or the etiology the chart already holds.

M17.10 records a unilateral primary case without naming which knee is affected.

Specificity in laterality and in etiology supports clinical decision support, surgical documentation and quality reporting.

Documentation checklist to support M17.12

Chart elements that support code selection and reduce specificity-driven denials.

A documented diagnosis of osteoarthritis of the knee. The Index sends Osteoarthritis, knee, unilateral to the M17.1 codes without requiring the word primary.

Laterality stated explicitly as the left knee.

Unilateral involvement. Bilateral primary disease is coded to M17.0.

No documented prior knee trauma, which would direct the encounter to the M17.3- codes.

No documented systemic or other underlying cause, which would make the osteoarthritis secondary.

An active diagnosis under current management rather than a resolved or historical one.

For an arthroplasty admission, a statement that left knee osteoarthritis is the reason for surgery.

For an outpatient encounter, a confirmed diagnosis rather than suspected osteoarthritis, which is coded to left knee pain.

FAQ

Is M17.12 a billable code?

Yes. M17.12 is billable at the highest level of specificity within category M17. No further character is required.

My patient has osteoarthritis in both knees. Which code applies?

Assign M17.0, bilateral primary osteoarthritis of knee. Do not report M17.11 and M17.12 together to describe bilateral disease.

The patient injured this knee playing football ten years ago. Is this still M17.12?

No. When the osteoarthritis is documented as post-traumatic, assign M17.32, unilateral post-traumatic osteoarthritis, left knee.

Does M17.12 map to an HCC?

No. M17.12 has no HCC mapping under CMS-HCC v28. Primary knee osteoarthritis carries no risk adjustment weight under the current model.

What separates M17.12 from M17.11?

Only the side of the knee separates the two codes. M17.12 is the left knee. M17.11 is the right knee. Both describe unilateral primary osteoarthritis.

Does the radiographic grade or severity of the osteoarthritis change the code?

No. ICD-10-CM does not build severity grading into the M17 codes. Selection turns on laterality and on the primary, post-traumatic or secondary distinction alone.

How is M17.12 sequenced for a total knee replacement admission?

M17.12 is the principal diagnosis when unilateral primary osteoarthritis of the left knee is the documented reason for surgery. The code is valid in the inpatient setting and the outpatient setting.

Did anything change for M17.12 in FY2027?

M17.12 is unchanged in the FY2027 update, effective 1 October 2026. The update leaves category M17 and its notes unchanged. No FY2026 addenda changes were identified for M17.12 or for category M17.

Related codes

Codes in the same family as M17.12. Only codes with a published page render as links.

Sibling codes in the M17 family

M17.0: Bilateral primary osteoarthritis of knee.
M17.10: Unilateral primary osteoarthritis, unspecified knee.
M17.11: Unilateral primary osteoarthritis, right knee.
M17.2: Bilateral post-traumatic osteoarthritis of knee.
M17.30: Unilateral post-traumatic osteoarthritis, unspecified knee.
M17.31: Unilateral post-traumatic osteoarthritis, right knee.
M17.32: Unilateral post-traumatic osteoarthritis, left knee.
M17.4: Other bilateral secondary osteoarthritis of knee.
M17.5: Other unilateral secondary osteoarthritis of knee.
M17.9: Osteoarthritis of knee, unspecified.

Additional codes to consider

Codes outside the M17 family considered alongside M17.12.

Companion: code alongside when documented

Z96.651: Presence of right artificial knee joint, when the right knee has already been replaced. Section I.B.13 then assigns the unilateral code for the left knee.

Consider instead

M17.11: Unilateral primary osteoarthritis, right knee.
M17.0: Bilateral primary osteoarthritis of knee.
M17.32: Unilateral post-traumatic osteoarthritis, left knee.
M17.5: Other unilateral secondary osteoarthritis of knee.
M17.10: Unilateral primary osteoarthritis, unspecified knee.
M17.9: Osteoarthritis of knee, unspecified.
M25.562: Pain in left knee, when osteoarthritis is suspected but not confirmed.

Sources

All claims on this page derive from government and professional-standards sources.

National Center for Health Statistics. (2026). International classification of diseases, tenth revision, clinical modification (ICD-10-CM) FY2027 files. https://www.cdc.gov/nchs/icd/icd-10-cm/files.html
Centers for Medicare & Medicaid Services and National Center for Health Statistics. (2026). ICD-10-CM official guidelines for coding and reporting FY2027. https://ftp.cdc.gov/pub/Health_Statistics/NCHS/Publications/ICD10CM/2027/ICD-10-CM-October-1-2026-FY27-Guidelines.pdf
Centers for Medicare & Medicaid Services. (2025). 2026 model software/ICD-10 mappings. https://www.cms.gov/medicare/payment/medicare-advantage-rates-statistics/risk-adjustment/2026-model-software-icd-10-mappings
Centers for Medicare & Medicaid Services. (2026). MS-DRG classifications and software, version 44. https://www.cms.gov/medicare/payment/prospective-payment-systems/acute-inpatient-pps/ms-drg-classifications-and-software