What is ICD-10 code M87.366?

M87.366 is the ICD-10-CM diagnosis code for Other secondary osteonecrosis, unspecified fibula. Death of bone tissue in the fibula (smaller bone in the lower leg) caused by an underlying condition or treatment, with the side not specified. M87.366 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. M87.366 is part of the FY2026 ICD-10-CM code set published by the CMS National Center for Health Statistics and used on Medicare, Medicaid, and commercial claims in the United States.

Yes. M87.366 is a billable ICD-10-CM code that can be reported on professional and institutional claims as a primary or secondary diagnosis when the provider documentation supports the specific condition described. Billable codes in ICD-10-CM are coded to the highest level of specificity available for the documented diagnosis. M87.366 is current for fiscal year 2026 and remains in the active CMS ICD-10-CM code set published by the National Center for Health Statistics.

What is the V28 vs V24 difference for M87.366?

CMS phased in the V28 risk adjustment model over payment years 2024 (33%), 2025 (67%), and 2026 (100%), replacing V24. V28 consolidates HCC categories, removes some lower-acuity conditions, and recalibrates RAF weights across the entire model. For M87.366, V28 maps to Bone/Joint/Muscle Infections/Necrosis at RAF 0.209 while V24 mapped to Bone/Joint/Muscle Infections/Necrosis at RAF 0.482. Both models can still appear in legacy data and in payer-specific lookback periods.

What should coders know when reporting M87.366?

Use this code only when laterality cannot be determined from documentation; query provider if possible to obtain specific side Document the underlying etiology of secondary osteonecrosis in the medical record for complete coding Because M87.366 maps to a payment HCC, the documentation must also satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's risk adjustment score.

M87.366 ICD-10 Code: Other secondary

ICD-10-CM Code View

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Digital ICD-10 code-book layout with official code detail, always-visible risk models, Code Trumping, and Buddy coding guidance.

Code lookupM87.366Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Diseases of the musculoskeletal system and connective tissue (M00-M99) / Other osteopathies (M86-M90)

M87.366

Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidance

Other secondary osteonecrosis, unspecified fibula

Death of bone tissue in the fibula (smaller bone in the lower leg) caused by an underlying condition or treatment, with the side not specified.

Buddy Insight

This diagnosis represents secondary osteonecrosis of the fibula when laterality is not specified, caused by underlying conditions that may affect ankle stability and cause lateral leg symptoms.

CMS-HCC V28

Mapped

HCC 92

Bone/Joint/Muscle Infections/...

RAF 0.209

CMS-HCC V24

Mapped

HCC 39

Bone/Joint/Muscle Infections/...

RAF 0.482

ACA/HHS

Not risk-adjusted for this model/period.

RAF 0

ESRD/PACE

Mapped

HCC 39

Bone/Joint/Muscle Infections/...

RAF 0.0

RXHCC

Mapped

HCC 80

Bone/Joint/Muscle Infections/...

RAF 0.0

Code Trumping

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Compare selected patient codes to show trumped or trumps impact.

Code Book Path

Official

M87.3Other secondary osteonecrosis

M87.36Other secondary osteonecrosis, tibia and fibula

M87.366Other secondary osteonecrosis, unspecified fibula

Inclusion Terms

Official

ICD-10-CM does not list inclusion terms for M87.366 in this effective period.

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for M87.366 in this effective period.

Related Child Codes

Official

M87.361Other secondary osteonecrosis, right tibia

M87.362Other secondary osteonecrosis, left tibia

M87.363Other secondary osteonecrosis, unspecified tibia

M87.364Other secondary osteonecrosis, right fibula

M87.365Other secondary osteonecrosis, left fibula

Includes

Official

ICD-10-CM does not list Includes notes for M87.366 in this effective period.

Excludes 1

Official

ICD-10-CM does not list Excludes 1 notes for M87.366 in this effective period.

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for M87.366 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for M87.366 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for M87.366 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

Documentation of osteonecrosis affecting fibula without side specification

Identification of underlying cause (e.g., corticosteroid use, radiation therapy, sickle cell disease)

Imaging findings confirming fibular bone necrosis

Clinical symptoms such as lateral leg pain or ankle instability

MEAT Support

HCC Buddy guidance

Documentation of osteonecrosis affecting fibula without side specification

Identification of underlying cause (e.g., corticosteroid use, radiation therapy, sickle cell disease)

Imaging findings confirming fibular bone necrosis

Clinical symptoms such as lateral leg pain or ankle instability

Audit Caution

HCC Buddy guidance

Using unspecified code when laterality is documented elsewhere

Confusing fibula with tibia in documentation

Failing to identify and document the secondary cause

Not upgrading to specific laterality code when information becomes available

Common Mistakes

HCC Buddy guidance

M87.364 — Other secondary osteonecrosis, right fibula (when laterality is known)

M87.365 — Other secondary osteonecrosis, left fibula (when laterality is known)

M86.369 — Chronic osteomyelitis of unspecified fibula (infection vs necrosis)

S82.409A — Unspecified fracture of shaft of unspecified fibula (acute fracture vs necrosis)

Last updated: FY2026 ICD-10-CM Apr update, Apr 1, 2026 through Sep 30, 2026. CMS-HCC V28 is 100% phased in for payment year 2026.

Is M87.366 an HCC code?

Yes. M87.366 maps to Bone/Joint/Muscle Infections/Necrosis under the CMS-HCC V28 risk adjustment model (and Bone/Joint/Muscle Infections/Necrosis under V24).

HCC Category Mapping

V28HCC 92, Bone/Joint/Muscle Infections/Necrosis

0.209

V24HCC 39, Bone/Joint/Muscle Infections/Necrosis

0.482

ESRDHCC 39, Bone/Joint/Muscle Infections/Necrosis

0.000

RxHCCHCC 80, Bone/Joint/Muscle Infections/Necrosis

0.000

RAF weights shown are the community, non-dual, aged base weights from the CMS risk adjustment model file. Actual per-patient RAF contribution depends on member segment, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains in use during the transition and for historical data.

MEAT Criteria for M87.366

For M87.366to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically, it has to be re-documented and supported each calendar year.

MMonitor: signs, symptoms, disease progression, or lab trending documented in the note
EEvaluate: test results, medication response, or physical findings reviewed by the provider
AAssess: explicit mention in the assessment or plan with acknowledgment of status
TTreat: medication, referral, procedure, therapy, or counseling tied to the diagnosis

Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed M87.366 during that encounter, not just copy-forwarded from a problem list.

What This Code Means

M87.366 is the ICD-10-CM diagnosis code for other secondary osteonecrosis, unspecified fibula. Death of bone tissue in the fibula (smaller bone in the lower leg) caused by an underlying condition or treatment, with the side not specified. M87.366 sits in the ICD-10-CM chapter for diseases of the musculoskeletal system and connective tissue (m00-m99), within the section covering other osteopathies (m86-m90).

Under the CMS-HCC V28 risk adjustment model, M87.366 maps to Bone/Joint/Muscle Infections/Necrosis (HCC 92) with a community, non-dual, aged base RAF weight of 0.209. Under the older V24 model, M87.366 mapped to the same category but with a base RAF weight of 0.482, V28 recalibrated weights across the entire model. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition.

Use this code only when laterality cannot be determined from documentation; query provider if possible to obtain specific side. Because M87.366 maps to a payment HCC, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's Medicare Advantage risk adjustment score. When documentation is ambiguous, coders should issue a provider query rather than assume the highest-specificity variant.

HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for M87.366 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•Use this code only when laterality cannot be determined from documentation; query provider if possible to obtain specific side
•Document the underlying etiology of secondary osteonecrosis in the medical record for complete coding

Clinical Significance

This diagnosis represents secondary osteonecrosis of the fibula when laterality is not specified, caused by underlying conditions that may affect ankle stability and cause lateral leg symptoms. It requires identification of the causative factor and monitoring for progression.

Documentation Requirements

✓Documentation of osteonecrosis affecting fibula without side specification
✓Identification of underlying cause (e.g., corticosteroid use, radiation therapy, sickle cell disease)
✓Imaging findings confirming fibular bone necrosis
✓Clinical symptoms such as lateral leg pain or ankle instability
✓Functional impact on walking or weight-bearing
✓Duration and progression of condition
✓Treatment history and response
✓Reason why laterality cannot be determined or if bilateral involvement

Commonly Confused Codes

•M87.364 — Other secondary osteonecrosis, right fibula (when laterality is known)
•M87.365 — Other secondary osteonecrosis, left fibula (when laterality is known)
•M86.369 — Chronic osteomyelitis of unspecified fibula (infection vs necrosis)
•S82.409A — Unspecified fracture of shaft of unspecified fibula (acute fracture vs necrosis)
•M87.363 — Other secondary osteonecrosis, unspecified tibia (fibula vs tibia)