Osteoarthritis of the Knee ICD-10 Codes: M17.11, M17.12, M17.9

Osteoarthritis of the knee has one of the more nuanced code families in orthopedic billing — primary versus secondary, laterality, and bilateral presentation all change which code is correct. Here's how to navigate it.

Getting knee osteoarthritis coding right starts with a distinction that's easy to overlook: whether the osteoarthritis is primary (degenerative, no clear prior cause) or secondary (resulting from a prior injury or condition).

Primary Osteoarthritis: M17.1-

M17.11 — Unilateral primary osteoarthritis, right knee

M17.12 — Unilateral primary osteoarthritis, left knee

M17.10 — Unilateral primary osteoarthritis, unspecified knee

M17.0 — Bilateral primary osteoarthritis of knee (both knees, primary)

Secondary Osteoarthritis: M17.3- / M17.4- / M17.5

Secondary osteoarthritis — resulting from a prior fracture, ligament injury, or other identifiable cause — is coded separately under M17.3- (unilateral, post-traumatic), M17.4 (bilateral, post-traumatic), or M17.5 (other unilateral secondary). Documentation should specify the underlying cause when secondary osteoarthritis is being coded, since that context supports the code selection.

When Documentation Doesn't Specify Primary vs. Secondary

M17.9 (Osteoarthritis of knee, unspecified) exists for cases where osteoarthritis is confirmed but the primary-versus-secondary distinction and laterality aren't documented. It's a valid code, but because primary/secondary status is usually knowable from the patient's history, defaulting to M17.9 often reflects a documentation gap rather than genuine clinical ambiguity.

Common CPT Pairings

Knee osteoarthritis management commonly involves weight-bearing knee X-rays (73565) to assess joint space narrowing, and — for symptomatic management — intra-articular injections (20610, major joint injection/aspiration) are frequently billed alongside the diagnosis.

Documentation Tips

Strong documentation for this code family notes laterality, whether the condition is primary or has a documented prior cause, and — for bilateral cases — confirms both knees are affected rather than defaulting to a unilateral code when the exam covers both.

Get a free practice audit and we'll review how your practice's osteoarthritis coding handles the primary-versus-secondary distinction across your orthopedic claims.

Frequently Asked Questions

What's the difference between primary and secondary osteoarthritis coding? Primary osteoarthritis (M17.1-, M17.0) has no identifiable prior cause; secondary osteoarthritis (M17.3-, M17.4, M17.5) results from a documented prior injury or condition.

Is M17.9 billable? Yes, M17.9 is billable, but it should reflect genuine documentation ambiguity rather than being used as a default shortcut.

How is bilateral knee osteoarthritis coded if one knee is primary and the other is post-traumatic? Each knee would be coded separately with its own applicable unilateral code, reflecting the distinct cause for each side.

What imaging supports a knee osteoarthritis diagnosis? Weight-bearing X-rays are the standard first-line imaging to assess joint space narrowing and confirm the diagnosis.

Are injections commonly billed with knee osteoarthritis diagnoses? Yes, intra-articular injections are a common symptomatic treatment billed alongside confirmed osteoarthritis diagnoses, particularly for patients not yet candidates for surgical intervention.