2026 ICD-10-CM Diagnosis Code M17.12 – Unilateral Primary Osteoarthritis, Left Knee

Osteoarthritis of the knee continues to be one of the most commonly reported musculoskeletal conditions in outpatient and inpatient settings. For 2026, the ICD-10-CM diagnosis code M17.12 remains unchanged but continues to play a significant role in accurate documentation, clinical coding, reimbursement, and quality reporting.

In this guide, we break down the definition, coding guidelines, clinical examples, documentation tips, DRG impact, and real-world application for M17.12 to help healthcare professionals code confidently and correctly.

ICD-10-CM diagnosis code M17.12

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What Is ICD-10-CM Code M17.12?

M17.12 — Unilateral primary osteoarthritis, left knee
This is a billable/specific code used to report primary osteoarthritis affecting only the left knee joint.

Effective Date

The 2026 edition of ICD-10-CM M17.12 became effective on October 1, 2025.

Approximate Clinical Synonyms

·         Arthritis of left knee

·         Osteoarthritis of left knee

·         Primary localized osteoarthritis of left knee

·         Osteoarthritis of left knee – patellofemoral joint

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Understanding the Condition

Primary osteoarthritis is caused by degenerative wear-and-tear of the joint cartilage, usually due to aging or chronic mechanical stress.
Symptoms include:

·         Knee pain

·         Stiffness

·         Reduced range of motion

·         Crepitus

·         Swelling

Accurate diagnosis is essential because osteoarthritis affects treatment planning, physical therapy referrals, imaging, and surgical decisions.

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ICD-10-CM Hierarchy (Code Structure)

M00–M99 — Diseases of musculoskeletal system
M15–M19 — Osteoarthritis
M17.1 — Unilateral primary osteoarthritis of knee
M17.12 — Left knee (specific)

This hierarchical structure ensures coders select the correct laterality, which is highly important for compliant billing.

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Coding Guidelines for M17.12

1. Laterality Matters

·         Left knee → M17.12

·         Right knee → M17.11

·         Unspecified knee → M17.10 (Avoid unless documentation lacks laterality)

2. Primary vs. Secondary Osteoarthritis

Use M17.12 only when primary OA is documented.
For post-traumatic OA → M17.5X series
For secondary OA → M17.3X series

3. Do Not Code Joint Pain Separately

If OA is diagnosed, symptoms such as pain (e.g., M25.562 – left knee pain) are not coded separately unless the provider documents they are unrelated.

4. Use Additional Codes When Applicable

Examples:

·         Obesity (E66.9) – a risk factor impacting joint degeneration

·         Injury codes – if OA developed after trauma (do not use M17.12)

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Real-World Clinical Documentation Requirements

To code M17.12 correctly, provider documentation should include:

✔ Laterality (Left knee)
✔ Type of OA (Primary)
✔ Location (Knee)
✔ Severity (mild/moderate/severe – optional but helpful)
✔ Diagnostic methods (X-ray findings, exam notes)

Example of acceptable documentation:

“Primary osteoarthritis of the left knee. Patient reports chronic knee pain worsening over the past year. X-ray shows joint space narrowing and osteophytes.”

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Practical Coding Examples

Example 1: Outpatient Visit

Scenario:
Patient presents with left knee pain due to known primary osteoarthritis.

Documentation:
“Primary OA left knee — chronic.”

Correct Code:
M17.12 – Unilateral primary osteoarthritis, left knee

Do NOT code: Knee pain (M25.562) since pain is part of OA.

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Example 2: Physical Therapy Referral

Documentation:
“Severe primary OA of left knee. Refer to PT.”

Correct Code:
M17.12

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Example 3: Pre-operative Evaluation for Knee Replacement

Documentation:
“Patient scheduled for total left knee arthroplasty due to progressive primary OA.”

Correct Codes:

·         M17.12

·         Z01.818 (Pre-op exam)

·         Z96.652 (if patient already has a right knee replacement)

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MS-DRG Impact (Inpatient Coding)

ICD-10-CM M17.12 falls under the following MS-DRGs:

·         553 — Bone Diseases and Arthropathies With MCC

·         554 — Bone Diseases and Arthropathies Without MCC

This affects hospital reimbursement, particularly for surgical admissions such as total knee replacements.

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Common Mistakes to Avoid

❌ Coding M17.12 when documentation only says “knee pain”
✔ Must specify osteoarthritis

❌ Selecting unspecified laterality (M17.10)
✔ Always query the provider if laterality is missing

❌ Using M17.12 for post-traumatic or secondary osteoarthritis
✔ Use correct series based on cause

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When to Query the Provider

You must query when:

·         Laterality is missing

·         Type of OA is unspecified (primary vs. secondary)

·         Both knees are involved

·         OA is related to a previous injury or disease

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ICD-9-CM Conversion

ICD-10-CM M17.12 roughly corresponds to:
715.16 – Osteoarthrosis, localized, primary, lower leg

(Do not use ICD-9-CM for claims dated after Oct 1, 2015.)

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Why Accurate Coding Matters

✔ Ensures proper reimbursement
✔ Prevents claim denials
✔ Supports clinical accuracy
✔ Helps in quality and outcome tracking
✔ Required for risk adjustment and HCC capture when applicable
(OA itself is not an HCC, but coding supports severity documentation.)

The ICD-10-CM code M17.12 remains an essential and frequently used diagnosis code for reporting primary osteoarthritis of the left knee in 2026. Coders must focus on laterality, type of osteoarthritis, and supporting documentation to ensure accurate coding and clean claim submission.

When used correctly, M17.12 supports appropriate reimbursement, reduces audit risk, and improves data accuracy across healthcare systems.