Knee and Hip Osteoarthritis ICD-10 Codes 2026 (M16 and M17): Coding for Medical Necessity, Prior Auth, and Arthroplasty

Knee osteoarthritis is coded in ICD-10-CM category M17 and hip osteoarthritis in category M16. The correct code depends on three facts the surgeon must document: whether the arthritis is primary or secondary (and if secondary, the cause, such as trauma or dysplasia), whether it is unilateral or bilateral, and which side is affected. The most common primary knee codes are M17.11 (right knee), M17.12 (left knee), and M17.0 (bilateral). The most common primary hip codes are M16.11 (right hip), M16.12 (left hip), and M16.0 (bilateral). Unspecified codes such as M17.9 and M16.9 should be avoided, because payers increasingly deny or pend arthroplasty and arthroscopy requests that lack specificity.

Osteoarthritis is the diagnosis behind most total joint replacements, and it is the diagnosis payers scrutinize when they decide whether to approve surgery. Getting the code right is necessary but not sufficient: the documentation behind it has to meet medical necessity criteria for the procedure. This guide covers both.

Why Osteoarthritis Coding Matters More in 2026

Three developments make OA coding a revenue issue for orthopaedic practices this year.

First, prior authorization scrutiny is rising. Medicare Advantage plans denied 12% of standard prior authorization requests in 2025, according to KFF's August 2026 analysis of payer-published data. Joint replacement is one of the most reviewed service categories.

Second, CMS's WISeR model, which began January 1, 2026 in six states (Arizona, New Jersey, Ohio, Oklahoma, Texas, and Washington), added prior authorization review for select services in Traditional Medicare, including knee arthroscopy for patients with knee osteoarthritis. For the first time, Traditional Medicare patients in those states face prior review tied directly to an OA diagnosis.

Third, the CY2027 Physician Fee Schedule proposed rule would cut work RVUs for total knee and total hip arthroplasty significantly. With less margin per case, a denied or delayed arthroplasty costs more.

Knee Osteoarthritis ICD-10 Codes (M17)

Category M17 classifies osteoarthritis of the knee by etiology and laterality.

‍Primary osteoarthritis of the knee

• M17.0 Bilateral primary osteoarthritis of knee
• M17.10 Unilateral primary osteoarthritis, unspecified knee
• M17.11 Unilateral primary osteoarthritis, right knee
• M17.12 Unilateral primary osteoarthritis, left knee

‍Post-traumatic osteoarthritis of the 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

‍Other secondary osteoarthritis of the knee

• M17.4 Other bilateral secondary osteoarthritis of knee
• M17.5 Other unilateral secondary osteoarthritis of knee

‍Unspecified

• M17.9 Osteoarthritis of knee, unspecified

Note that M17.5 does not carry a laterality character. When a patient has secondary OA in one knee, document the side in the note even though the code itself does not capture it, because payers and auditors will look for it.

Hip Osteoarthritis ICD-10 Codes (M16)

Category M16 follows a similar structure but adds dysplasia as a distinct etiology.

‍Primary osteoarthritis of the hip

• M16.0 Bilateral primary osteoarthritis of hip
• M16.10 Unilateral primary osteoarthritis, unspecified hip
• M16.11 Unilateral primary osteoarthritis, right hip
• M16.12 Unilateral primary osteoarthritis, left hip

‍Osteoarthritis resulting from hip dysplasia

• M16.2 Bilateral osteoarthritis resulting from hip dysplasia
• M16.30 Unilateral osteoarthritis resulting from hip dysplasia, unspecified hip
• M16.31 Unilateral osteoarthritis resulting from hip dysplasia, right hip
• M16.32 Unilateral osteoarthritis resulting from hip dysplasia, left hip

‍Post-traumatic osteoarthritis of the hip

• M16.4 Bilateral post-traumatic osteoarthritis of hip
• M16.50 Unilateral post-traumatic osteoarthritis, unspecified hip
• M16.51 Unilateral post-traumatic osteoarthritis, right hip
• M16.52 Unilateral post-traumatic osteoarthritis, left hip

‍Other secondary osteoarthritis of the hip

• M16.6 Other bilateral secondary osteoarthritis of hip
• M16.7 Other unilateral secondary osteoarthritis of hip

‍Unspecified

• M16.9 Osteoarthritis of hip, unspecified

Five Coding Rules That Prevent OA Denials

1. Code bilateral only when both joints are affected and documented

Use the bilateral code (M17.0, M16.0) when the surgeon documents OA in both joints, even if surgery is planned on only one side. Do not report two unilateral codes (for example, M17.11 and M17.12) for the same encounter; the bilateral code exists for that situation.

2. Match laterality to the procedure

A right total knee arthroplasty billed with a left knee OA code, or with an unspecified laterality code, is one of the simplest denials to prevent and one of the most common. The diagnosis laterality must match the procedure's RT or LT modifier.

3. Capture the etiology when OA is secondary

Post-traumatic OA (M17.2 to M17.32, M16.4 to M16.52) and dysplasia-related hip OA (M16.2 to M16.32) should be coded when the surgeon documents the cause, such as a prior tibial plateau fracture or developmental dysplasia. These codes add clinical context that supports medical necessity, especially in younger patients where payers question arthroplasty.

4. Avoid unspecified codes

M17.9, M16.9, and the "unspecified knee" or "unspecified hip" codes rarely belong on a surgical claim. The surgeon knows which joint and what type. If the documentation does not say, query the surgeon before the claim is submitted.

5. Report related conditions that affect care

Secondary diagnoses can strengthen medical necessity or explain complexity. Examples include obesity with a BMI code (Z68 category) when documented by the provider, long-term use of medications, and joint deformity. Report them when they are documented and relevant, following ICD-10-CM Official Guidelines.

Other Diagnoses That Are Often Confused With OA

Several conditions overlap with osteoarthritis clinically but have their own codes:

• Avascular necrosis of the femoral head is coded in M87 (for example, M87.051 for idiopathic aseptic necrosis of the right femur), not M16.
• Chondromalacia patellae is coded in M22.4-, not M17.
• Rheumatoid arthritis of the knee or hip is coded in M05 or M06, not in the OA categories.
• Primary OA of other joints, such as the shoulder (M19.01-) or ankle (M19.07-), falls in M19, not M16 or M17.

Coding these conditions as OA can trigger medical necessity denials, because payer policies for arthroplasty and arthroscopy often apply different criteria.

The Documentation Payers Look For Before Approving Arthroplasty

The ICD-10 code tells the payer what the patient has. The documentation tells the payer why surgery is necessary now. Commercial and Medicare Advantage medical policies for total knee and total hip arthroplasty vary, but most look for the same elements:

• Radiographic evidence of advanced disease. Joint space narrowing, osteophytes, subchondral sclerosis, or cysts, ideally with a grading system such as Kellgren-Lawrence noted in the imaging report or surgeon note.
• Failed conservative treatment. The type, duration, and outcome of non-operative care, such as physical therapy, NSAIDs, activity modification, bracing, and injections. Many policies expect at least three months.
• Functional limitation. Specific effects on walking, stairs, work, sleep, or activities of daily living, rather than a general statement of pain.
• Physical exam findings. Range of motion, deformity, effusion, and gait.
• Contraindications to surgery addressed. Some payer policies include BMI thresholds, glycemic control, or smoking status. When these apply, document how they have been evaluated.

Because Medicare Advantage plans must follow Traditional Medicare coverage criteria under CMS-4201-F, and Medicare Administrative Contractors publish LCDs and articles for many musculoskeletal services, the strongest prior authorization request quotes the applicable criteria and maps the chart to each element.

Knee Arthroscopy and OA: A Special Case

Arthroscopic lavage and debridement for knee osteoarthritis is a long-standing coverage problem. Medicare's National Coverage Determination 150.9 does not cover arthroscopic lavage or arthroscopic debridement for patients presenting with knee pain only, or for patients with severe osteoarthritis. The WISeR model now subjects knee arthroscopy for OA patients to prior review in the six participating states.

When a knee arthroscopy is performed in a patient with OA for a separate indication, such as a displaced meniscal tear with mechanical symptoms, the documentation must clearly establish that indication. Code the mechanical pathology (for example, the specific meniscal tear code) as the primary diagnosis when it is the reason for surgery, and document the mechanical symptoms and exam findings that support it.

How AI Coding Helps With OA Diagnosis Accuracy

OA coding errors are small, repetitive, and expensive: a missing laterality, an unspecified code, a bilateral case coded as unilateral, or a diagnosis that does not match the procedure modifier. These are exactly the errors automation catches reliably.

Maia's AutoCoder reads the surgeon's note inside the EHR and recommends the specific ICD-10 code, laterality, and supporting clinical justification before a coder touches the chart. When documentation is missing a detail a payer will look for, such as the side or the etiology, Maia flags it for the surgeon. Maia helps practices build documentation templates that capture conservative treatment history and functional limitation in a consistent way. Orthopaedic groups use Maia inside Athena, eClinicalWorks and coming soon, Epic, ModMed, NextGen, and Tebra.

Frequently Asked Questions

What is the ICD-10 code for primary osteoarthritis of the right knee?

The ICD-10-CM code is M17.11, unilateral primary osteoarthritis, right knee. The left knee is M17.12, and bilateral primary knee OA is M17.0.

What is the ICD-10 code for bilateral hip osteoarthritis?

Bilateral primary osteoarthritis of the hip is M16.0. Bilateral OA from hip dysplasia is M16.2, bilateral post-traumatic hip OA is M16.4, and other bilateral secondary hip OA is M16.6.

Should I code bilateral OA if surgery is only on one knee?

Yes, if the surgeon documents osteoarthritis in both knees. Use M17.0 for bilateral primary knee OA rather than two unilateral codes. The RT or LT modifier on the procedure identifies which knee was operated on.

When should post-traumatic osteoarthritis codes be used?

Use post-traumatic OA codes (M17.2 to M17.32 for the knee, M16.4 to M16.52 for the hip) when the surgeon documents that the arthritis resulted from a prior injury, such as a fracture. The codes add clinical context that often supports medical necessity for younger patients.

Does Medicare cover knee arthroscopy for osteoarthritis?

Under NCD 150.9, Medicare does not cover arthroscopic lavage or debridement for patients presenting with knee pain only or with severe osteoarthritis. Arthroscopy for a separate indication, such as a meniscal tear with mechanical symptoms, can be covered when documented. In WISeR model states, knee arthroscopy for OA patients is also subject to prior review.

What documentation do payers require for total knee replacement approval?

Most payer policies require radiographic evidence of advanced OA, documentation of failed conservative treatment (often at least three months), specific functional limitations, and physical exam findings. Requirements vary by payer, so check each plan's medical policy.

See how Maia's AutoCoder handles this automatically for orthopaedic practices. Book a demo at usemaia.com.