ICD-10 M17.4: Other bilateral secondary knee osteoarthritis

Key Takeaways

M17.4 is the billable ICD-10-CM code for other bilateral secondary osteoarthritis of knee, valid for HIPAA-covered transactions from October 1, 2015.

The word ‘other’ in M17.4 specifically excludes post-traumatic bilateral OA, which is coded separately under M17.2.

M17.4 requires documented evidence of a secondary cause (e.g., obesity, prior inflammatory arthritis, metabolic disease) to distinguish it from primary bilateral OA under M17.0.

Practice management software like Pabau includes claims management tools that help orthopedic and physical therapy practices apply accurate M17.x coding, reducing claim errors across bilateral knee OA encounters.

ICD-10 code M17.4 is the billable diagnosis code for other bilateral secondary osteoarthritis of knee: osteoarthritis in both knees driven by an identifiable secondary condition rather than ordinary age-related wear.

It is one code within the M17 family that covers bilateral osteoarthritis of the knees, and it is also one of the most under-coded. Clinicians reach for M17.9 (unspecified) or M17.0 (primary bilateral) out of habit, even when documentation points to a secondary cause. That default costs practices in audit exposure and claim accuracy.

ICD-10 Code M17.4 fills a precise slot in the M17 code family: it captures bilateral knee OA that is secondary in origin but not post-traumatic. This distinction matters clinically and administratively.

ICD-10 Code M17.4: Clinical description

Secondary osteoarthritis of the knee differs from primary osteoarthritis in one fundamental way: there is an identifiable underlying cause driving cartilage degeneration beyond normal age-related wear. In bilateral knee OA, “secondary” means both knees have been affected by the same systemic or mechanical driver, whether that is inflammatory arthritis, gout, obesity-related joint loading, metabolic disease, or prior surgical intervention that was not traumatic in nature.

The parent code M17 is non-billable. M17.4 is the specific child code that carries billable status, confirmed by ICD-10-CM reference for M17.4 and the AAPC Codify ICD-10-CM reference. Submitting M17 alone on a HIPAA-covered claim will result in rejection. Clinicians and coders must always drill down to the full, specific code.

Code details at a glance

Billable status and the M17 code family

The correct ICD-10 code for osteoarthritis of bilateral knee depends entirely on the cause. The M17 family splits bilateral disease across three codes, one each for primary, post-traumatic, and other secondary origins, so knowing where M17.4 sits prevents the most common coding error: defaulting to a non-specific or incorrect sibling code. The CDC/NCHS ICD-10-CM web tool and CMS ICD-10 codes page both confirm M17.4’s position in the current tabular list.

For practices managing high volumes of musculoskeletal claims, claims management software that supports accurate M17.x code selection reduces the audit exposure that comes from defaulting to M17.9. Coding specificity is not a technicality; it directly affects medical necessity justification for subsequent procedures.

When to use M17.4 vs M17.0 and M17.2

This is where most coding errors originate. Three codes cover bilateral knee OA, and each occupies a distinct clinical lane. Using the wrong one triggers medical necessity reviews and increases denial risk.

M17.4 vs M17.0: Primary vs secondary

M17.0 applies when bilateral knee OA is idiopathic: no identifiable underlying cause, degenerative in origin, typically age-related. M17.4 applies when a specific secondary driver can be documented. Common secondary causes include:

• Obesity with documented excess mechanical joint loading
• Prior inflammatory arthritis (rheumatoid arthritis, gout, pseudogout)
• Metabolic joint disease or crystal arthropathy
• Avascular necrosis affecting both knees
• Post-surgical derangement not resulting from acute trauma
• Congenital or developmental knee deformity causing secondary degeneration

The clinical note must explicitly state the underlying condition and its link to bilateral cartilage breakdown. A diagnosis of obesity (coded separately, e.g., E66.x) alongside bilateral knee OA does not automatically make the OA secondary: the clinician must document the causal relationship.

M17.4 vs M17.2: Secondary vs post-traumatic

M17.2 covers bilateral post-traumatic OA: both knees damaged by a prior injury or injuries. The critical distinction is whether documented trauma (fracture, significant ligament rupture, meniscal tear) is the primary driver of degeneration. M17.4, by contrast, covers every other secondary mechanism that is not post-traumatic.

A patient with bilateral knee OA following bariatric surgery complications belongs under M17.4, not M17.2. A patient with bilateral OA after documented ACL injuries to both knees belongs under M17.2. Where a patient has both a traumatic history and a secondary systemic condition, documentation should support whichever etiology is primary for that encounter.

Practices using physical therapy EMR platforms should ensure treatment notes capture this distinction clearly, because rehabilitation claims for M17.4 and M17.2 can attract different payer scrutiny.

Documentation requirements for M17.4

Accurate documentation protects against audit risk and supports medical necessity when billing associated procedures. ICD-10 Code M17.4 requires more than a bilateral knee pain entry in the problem list.

What the clinical note should include

• Laterality confirmed: Documentation that both knees are symptomatic or have objective signs of OA (crepitus, limited range of motion, radiographic changes).
• Secondary etiology named: The specific underlying condition driving the OA, with a statement of causal relationship.
• Comorbidity coding alignment: If obesity is the secondary cause, an E66.x code should appear alongside M17.4. If rheumatoid arthritis, the relevant M06.x code.
• Severity grading (where applicable): Kellgren-Lawrence grading from imaging strengthens medical necessity for procedures like viscosupplementation or referral for arthroplasty.
• Treatment history summary: Conservative management attempted (physical therapy, NSAIDs, weight management) supports step-therapy requirements from most payers.

Conservative management for bilateral knee OA often also includes an off-loading brace, such as HCPCS L2397, documented alongside physical therapy and NSAID use before payers will approve injections or surgery.

Orthopedic and sports medicine software platforms that include structured clinical note templates reduce the likelihood of missing these documentation elements at the point of care. Practices managing high volumes of knee OA patients benefit from structured encounter forms that prompt documentation of secondary etiology before the encounter closes.

For practices running physiotherapy clinic management workflows, applying M17.4 consistently across bilateral knee OA rehabilitation episodes also ensures accurate episode-of-care coding when multiple visits are billed under the same diagnosis.

Approximate synonyms and alternate terms

Clinical documentation often uses language that maps to M17.4 without using the code’s exact description. Bilateral knee degenerative joint disease (DJD) is the most common informal equivalent. Patients and referrers may describe it as arthritis in both knees, while coders often look the diagnosis up as bilateral osteoarthritis of the knees or osteoarthritis of bilateral knee. The following terms are recognized approximate synonyms:

• Bilateral secondary osteoarthritis of both knees
• Bilateral knee DJD (degenerative joint disease) secondary to a named condition
• Secondary gonarthrosis, bilateral
• Bilateral knee OA due to metabolic/inflammatory/mechanical cause
• Chondromalacia-related bilateral knee OA (where not post-traumatic)

CPT codes commonly paired with ICD-10 Code M17.4

M17.4 as the diagnosis code supports a range of procedures. The CPT codes below are those most commonly paired with bilateral knee osteoarthritis in orthopedic, rheumatology, and physical therapy settings. Payer coverage and prior authorization requirements vary: confirm with the CMS Physician Fee Schedule lookup before billing. Pairing M17.4 with these CPT codes does not guarantee reimbursement; medical necessity must be supported by the clinical note.

For straightforward follow-up visits that don’t require the physician’s full attention, some practices bill CPT 99211 instead of a higher-level established-patient E/M code.

For bilateral procedures (such as bilateral 20610 injections), Medicare and most commercial payers require a -50 bilateral modifier. Failing to append -50 when billing injections into both knees on the same date of service is a common claim error. For 27447 bilateral arthroplasty staged on separate dates, use separate claim lines with the appropriate RT/LT modifiers.

Practices operating compliance-focused physiotherapy workflows should build modifier guidance into their billing review process, particularly for high-volume bilateral knee injection encounters.

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Comorbidity coding and sequencing with M17.4

ICD-10 Code M17.4 is rarely used in isolation. Secondary OA by definition involves an underlying condition, and accurate coding requires that condition to appear on the claim as well. Sequencing follows the CMS ICD-10-CM Official Guidelines: list the code for the condition most responsible for the visit first, then the etiology code, then relevant comorbidities.

Common comorbidity codes used alongside M17.4

• E66.01 / E66.09 (Morbid/other obesity): When excess body weight is documented as the secondary cause of bilateral joint loading and OA progression.
• M06.00-M06.09 (Rheumatoid arthritis without rheumatoid factor): When inflammatory arthritis has driven secondary bilateral cartilage loss.
• M10.x (Gout): Urate crystal deposition in both knee joints driving secondary OA.
• M11.x (Other crystal arthropathies): Calcium pyrophosphate deposition disease (CPPD/pseudogout) as a secondary OA driver.
• M87.x (Avascular necrosis): When bilateral AVN has led to secondary joint degeneration.

The sequencing decision affects how payers evaluate medical necessity for associated procedures. A visit primarily for bilateral knee OA management should list M17.4 first; a visit primarily for obesity management where the bilateral OA is a documented complication might sequence E66.x first. For physical therapy practices handling musculoskeletal rehabilitation requirements, consistent sequencing across all M17.4 encounters reduces administrative friction at claim adjudication.

Medicare, Medicaid, and payer coverage considerations

M17.4 is valid for Medicare and Medicaid billing, but coverage for the procedures billed alongside it depends on medical necessity documentation. CMS Local Coverage Determinations (LCDs) for knee joint injections (e.g., viscosupplementation with hyaluronic acid products such as HCPCS J7318) require specific documentation of OA severity, conservative therapy failure, and in some policies, a Kellgren-Lawrence grade of 2 or higher.

For bilateral procedures billed on the same date, Medicare applies a 50% reduction to the lower-valued procedure when two identical procedures are performed bilaterally. Practices should verify whether the -50 modifier or separate line items with RT/LT modifiers apply to their specific MAC’s guidelines for each CPT code, checking CMS’s current fiscal year guidelines before submitting M17.4 encounters.

Physical therapy services (97110 and related codes) billed under M17.4 are subject to Medicare’s KX modifier threshold for outpatient therapy, since the hard annual therapy caps were repealed by the Bipartisan Budget Act of 2018. Practices with structured clinical documentation tools can build KX threshold tracking directly into the billing workflow, preventing claim denials from missing KX modifiers on high-utilization bilateral knee OA patients.

For practices serving patients across multiple physical therapy locations, opening a physiotherapy clinic with payer-specific billing rules built into each location’s workflow reduces the compliance risk that comes from applying generic billing defaults to bilateral OA encounters.

Practices also benefit from using digital intake forms that capture bilateral symptom history, prior injury data, and secondary condition documentation before the encounter begins. This ensures the clinical note is already structured for accurate M17.4 coding by the time the provider enters the room.

Conclusion

Bilateral secondary knee OA is a distinct clinical entity, and ICD-10 Code M17.4 is the only code that captures it accurately. Defaulting to M17.9 (unspecified) or M17.0 (primary) leaves practices exposed to medical necessity challenges, claim denials, and audit scrutiny that accurate documentation and coding can prevent.

Pabau’s claims management software helps orthopedic, rheumatology, and physical therapy practices build M17.x coding accuracy into every encounter workflow, from structured intake to claim submission. To see how Pabau handles musculoskeletal billing documentation, book a demo with our team.

Continue your research

Coding secondary arthritis outside the knee? ICD-10 Code M15.3 covers secondary multiple-site arthritis and how it’s sequenced against the underlying cause.

Assessing ligament stability alongside knee OA? The anterior drawer test template gives a structured format for documenting the exam finding.

Need the code for RA with a positive rheumatoid factor? ICD-10 Code M05.9 pairs with M17.4 when seropositive RA is the documented secondary cause.

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