2026 ICD-10-CM Diagnosis Code M66.173Rupture of synovium, unspecified ankle

ICD-10-CM Codes›M00–M99›M65-M67›M66

ICD-10-CM M66.173
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

M66.173 is a billable ICD-10-CM diagnosis code for rupture of synovium, unspecified ankle. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 557 through 558. Coders also document this condition as lesion of joint capsule of ankle region. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Tendon and synovial disorders.

Code Identity

ICD-10-CM Code
M66.173
Billable Status
Yes — Valid for Submission
Code Describes
Rupture of synovium, unspecified ankle
Short Description
Rupture of synovium, unspecified ankle
Same as the full description in the CMS dataset.
Parent Code
M66.17
Rupture of synovium, ankle, foot and toes

Code Classification

ChapterM00–M99Diseases of the musculoskeletal system and connective tissue
SectionM65-M67Disorders of synovium and tendon
CategoryM66Spontaneous rupture of synovium and tendon
This CodeM66.173Rupture of synovium, unspecified ankle

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Lesion of joint capsule of ankle region
  • Rupture of synovium - ankle/foot

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR MUS009
Tendon and synovial disorders
Default principal diagnosis: inpatient Yes · outpatient Yes

Patient EducationClinical

Ankle Injuries and Disorders

Your ankle bone and the ends of your two lower leg bones make up the ankle joint. Your ligaments, which connect bones to one another, stabilize and support it. Your muscles and tendons move it.

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert M66.173 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
727.59 Rupture of synovium NEC
Approximate The match is approximate rather than exact.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About M66.173Overview

Is M66.173 (Rupture of synovium, ankle, foot and toes) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report rupture of synovium, unspecified ankle on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does M66.173 group to?

When rupture of synovium, unspecified ankle is the principal diagnosis on an inpatient stay, it groups to MS-DRG 557, 558, with relative weights from 0.8932 to 1.4869 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of M66.173?

Under the General Equivalence Mappings, rupture of synovium, unspecified ankle converts to ICD-9-CM 727.59 (rupture of synovium NEC). The mapping is approximate, so confirm the match fits the documentation.

Footnotes

[1] Not chronic - A diagnosis code that does not fit the criteria for chronic condition (duration, ongoing medical treatment, and limitations) is considered not chronic. Some codes designated as not chronic are acute conditions. Other diagnosis codes that indicate a possible chronic condition, but for which the duration of the illness is not specified in the code description (i.e., we do not know the condition has lasted 12 months or longer) also are considered not chronic.