ICD-10 Code M16.6: Other bilateral secondary osteoarthritis of hip

Key Takeaways

ICD-10 code M16.6 describes other bilateral secondary osteoarthritis of hip, valid and billable for FY2026 (effective October 1, 2025).

Secondary means the OA has an identifiable underlying cause other than dysplasia (M16.2/M16.3) or trauma (M16.4/M16.5). That cause must be documented to justify M16.6.

Bilateral means both hips are affected. If only one hip is documented, M16.7 applies instead. Laterality errors are among the most common M16 coding mistakes.

Practice management software like Pabau supports accurate ICD-10-CM diagnosis code capture and billing workflows for orthopedic and musculoskeletal practices.

ICD-10 code M16.6 is the billable diagnosis code for other bilateral secondary osteoarthritis of hip, valid for FY2026. It applies when both hip joints show osteoarthritis and physician documentation names a cause that isn’t dysplasia or a prior injury.

The catch is documentation. Most M16.6 denials come down to one missing detail: the note confirms bilateral hip OA but never names what’s driving it. Without a documented cause, the claim can’t stand as secondary, and it gets kicked back for unspecified or primary coding instead.

Getting the code right starts with knowing exactly where M16.6 sits among the other M16 subcodes. Miss one qualifier, laterality or etiology, and the claim lands in the wrong category entirely.

What ICD-10 code M16.6 means and when you can bill it

ICD-10 code M16.6 is the billable ICD-10-CM diagnosis code for “other bilateral secondary osteoarthritis of hip.” It’s valid for submission for FY2026, effective October 1, 2025, and can be used on claims submitted to Medicare, Medicaid, and commercial payers.

The WHO equivalent term is “other secondary coxarthrosis, bilateral,” reflecting the international ICD-10 terminology. Both descriptions point to the same clinical picture: osteoarthritis in both hip joints, caused by something identifiable that isn’t dysplasia and isn’t an old injury.

Where M16.6 sits inside the M16 coxarthrosis family

M16.6 belongs to the M16 coxarthrosis category, the ICD-10-CM block for hip osteoarthritis. M16 sits inside the M15-M19 osteoarthritis subblock, part of the larger M00-M99 musculoskeletal chapter. Coders using musculoskeletal practice software run into this category constantly across orthopedics, rheumatology, and physical therapy.

The M16 family breaks into nine subcode groups. Each one specifies laterality, bilateral or unilateral, and etiology: primary, dysplastic, post-traumatic, or other secondary. With laterality splits applied, that expands to 15 billable codes. M16.6 sits in the “other secondary” group, covering bilateral involvement.

M16 also carries two Excludes2 notes worth knowing. Arthrosis of the spine, such as M47.9, and hallux rigidus are both excluded from coxarthrosis coding. Excludes2 means these conditions are never coded as a form of M16, but if a patient genuinely has both, you code them separately on the same claim.

M16 is also easy to confuse with M15. M15 covers polyosteoarthrosis, arthritis in multiple joints, such as M15.9. M16 is reserved for the hip alone, and bilateral coding only happens through M16.6 or M16.7. A patient with OA in a hip and a knee needs both an M16 code and the matching knee code, not one M15 code covering everything.

Primary vs secondary hip osteoarthritis: what really separates them

The primary vs secondary distinction is the most consequential decision in the M16 family. Get it wrong, and the claim lands in a clinically inaccurate category, which can trigger a medical necessity query.

Primary osteoarthritis (M16.0, M16.10-M16.12) is idiopathic. No underlying cause is identified. It’s the most common form, developing through age-related cartilage wear without a prior condition setting it off.

Secondary osteoarthritis (M16.2-M16.7) is different. It has an identifiable cause, and ICD-10-CM splits that cause into three subgroups:

• Dysplastic (M16.2/M16.3): caused by hip dysplasia, acetabular dysplasia, or developmental deformity.
• Post-traumatic (M16.4/M16.5): follows hip fracture, dislocation, or prior surgical trauma.
• Other secondary (M16.6/M16.7): caused by any other identifiable condition such as avascular necrosis, Paget’s disease, inflammatory arthropathy, or metabolic bone disease, where the underlying condition is documented but does not fall into the dysplasia or trauma categories.

M16.6 belongs exclusively to the third group. The physician needs to document both the bilateral involvement and the specific condition driving the secondary OA before this code holds up.

When M16.6 is the right call over its neighboring codes

Picking the right M16 subcode means checking two things at once: laterality (bilateral or unilateral) and etiology (primary, dysplastic, post-traumatic, or other secondary). M16.6 applies only when both hips are involved and the cause is secondary but neither dysplastic nor traumatic.

Here’s a practical point worth remembering: if a patient has bilateral hip OA from avascular necrosis, the record needs to state both “bilateral” and the underlying condition. Miss either qualifier, and M16.6 won’t hold up.

What the chart needs before you code M16.6

Assigning M16.6 correctly means the medical record has to carry specific detail. Coders shouldn’t assign this code from symptoms or imaging alone without physician attestation. Digital intake forms and structured clinical templates help ensure every required element gets captured at the point of care.

The following elements need to show up in the chart to justify M16.6, per the ICD-10-CM Official Guidelines for Coding and Reporting (published jointly by CMS and NCHS) and standard CDI practice:

• Bilateral involvement confirmed: The provider must explicitly document that both hip joints are affected, not inferred from imaging alone. “Bilateral hip OA” or equivalent language must appear in the provider note.
• Secondary etiology identified: The underlying cause must be named. Acceptable examples include avascular necrosis, Paget’s disease of bone, rheumatoid arthritis sequelae, metabolic bone disease, septic arthritis sequelae, or other documented conditions. The phrasing “due to” or “resulting from” links the OA to the cause.
• Etiology is not dysplasia or trauma: If the cause is hip dysplasia, use M16.2. If post-traumatic, use M16.4. M16.6 is the residual category for other causes.
• Radiological evidence: Imaging (X-ray, MRI) supporting OA findings strengthens the record, though physician attestation carries the coding weight.
• Physician attestation: The attending or treating physician must document the diagnosis. Coder inference alone does not justify M16.6 assignment.

Practices running physical therapy EMR software can configure structured note templates that prompt clinicians to document bilaterality and secondary cause at every relevant encounter, closing off the missing documentation that drives most M16.6 queries. Pabau’s clinical documentation tools support structured field capture for diagnosis documentation across musculoskeletal specialties.

How providers describe M16.6 without using its name

The ICD-10-CM Alphabetic Index carries exactly two literal entries for this code: “osteoarthritis” and “secondary osteoarthritis of bilateral hips.” Beyond those two, providers use plenty of other documentation language that also maps to M16.6, even though it won’t show up verbatim in the Index.

• Bilateral secondary coxarthrosis
• Bilateral hip OA due to [named condition, not dysplasia or trauma]
• Secondary degenerative joint disease (DJD) of both hips
• Bilateral coxarthrosis secondary to [named metabolic or inflammatory condition]

Degenerative joint disease (DJD) of the hip is a broader clinical synonym for osteoarthritis. When a provider documents “bilateral DJD of hips secondary to [named condition],” this maps to M16.6, as long as the secondary cause is on record and isn’t dysplastic or traumatic in origin.

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What M16.6 replaced under ICD-9-CM

ICD-10-CM replaced ICD-9-CM for US payers on October 1, 2015. The switch pushed specificity for musculoskeletal codes up significantly. M16.6 maps back to ICD-9-CM code 715.25, per the CMS General Equivalence Mappings (GEMs). GEM mappings are approximations, and they need clinical context for an accurate translation.

ICD-9-CM never required the laterality detail that ICD-10-CM does. Practices converting legacy records should verify the laterality documentation before applying M16.6, rather than relying on the GEM forward mapping alone. The CMS ICD-10 page carries the current GEM files and annual tabular list updates.

How M16.6 affects DRG assignment and reimbursement

In inpatient settings, M16.6 can function as a principal or secondary diagnosis that drives MS-DRG assignment under the CMS MS-DRG system. Supporting accurate claims management workflows for orthopedic practices means understanding how M16.6 interacts with DRG grouping and procedure code pairing.

M16.6 maps to MS-DRG categories for major joint procedures when it’s paired with a total hip arthroplasty (THR) ICD-10-PCS procedure code. Used on its own, for medical management without surgery, it maps to MS-DRG categories for musculoskeletal disorders.

• CC/MCC status: M16.6 isn’t a complication or comorbidity (CC) or major complication or comorbidity (MCC) designation on its own, so it doesn’t independently raise DRG severity weight. A documented comorbidity, such as ICD-10 code I15.1, can push the case into a higher-weighted DRG instead. Verify the current MS-DRG version (v43.0 for FY2026) against the current CMS grouper documentation before submitting.
• Outpatient billing: M16.6 is valid for outpatient claim submission including physician office visits, physical therapy encounters, and diagnostic imaging orders.
• Payer variation: DRG assignment and reimbursement rates vary by payer, contract, and MS-DRG version. Do not use specific dollar amounts from reference guides; verify reimbursement against your payer contract and the current Medicare Physician Fee Schedule.

Coding M16.6 for a bilateral hip replacement

M16.6 is one of the most common admitting diagnoses for bilateral total hip replacement (THR), which makes it a central code in high-acuity orthopedic billing. Coders handling surgical coding for these cases need to understand the sequencing rules that govern M16.6 in THR encounters.

When you code a THR admission with M16.6 as the principal diagnosis, the ICD-10-PCS procedure codes for bilateral total hip arthroplasty get assigned alongside it. Sequencing rules per the ICD-10-CM Official Guidelines require that the condition chiefly responsible for the admission gets listed first. If the patient was admitted specifically for surgical treatment of bilateral secondary hip OA, M16.6 is the right principal diagnosis.

• Bilateral THR in one episode: M16.6 supports the medical necessity for bilateral simultaneous or staged THR.
• Sequencing with underlying cause: Code the underlying condition causing the secondary OA (e.g., avascular necrosis) as an additional diagnosis where applicable, per ICD-10-CM multiple coding instructions.
• Revision THR: If the encounter is for revision rather than primary THR, verify whether M16.6 remains appropriate as the principal diagnosis or whether a complication code takes precedence.

Bilateral THR admissions often carry separate line items alongside the M16.6 principal diagnosis. Anesthesia gets billed under codes like HCPCS code J2704 for propofol, and post-operative wound management gets billed under CPT code 97597. Both matter for a complete claim picture on a high-acuity orthopedic case.

The coding mistakes that keep M16.6 claims stuck

Four error patterns account for most M16.6 claim issues. Each one is fixable through better documentation at the point of care rather than correcting things after the fact.

A fifth common error is carrying an ICD-9-CM code over into transition-era records. If a claim references 715.15 or a similar legacy code, it gets rejected outright. Only ICD-10-CM codes are valid for claims with dates of service on or after October 1, 2015.

Conclusion

M16.6 comes down to two documentation anchors: confirmed bilateral involvement and a named secondary cause outside the dysplasia and trauma categories. Miss either one, and the claim either drops to an unspecified code or triggers a query that holds up reimbursement.

Getting both details on record consistently is as much a documentation problem as a coding one. Practice management software like Pabau gives orthopedic and musculoskeletal practices structured templates for capturing laterality and cause at the point of care, plus claims and billing tools that catch incomplete entries before a claim goes out.

If M16.6 documentation keeps stalling your claims, book a demo to see how Pabau fits your coding workflow.

Continue your research

Need to code the supplies from a THR? HCPCS code A4452 covers waterproof tape billing for post-surgical wound dressings.

Coding anesthesia for a hip procedure? HCPCS code J2704 breaks down propofol injection billing.

Working across other ICD-10-CM diagnostic categories? Intraparenchymal hemorrhage codes show how sequencing rules apply outside orthopedics.

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