2026 ICD-10-CM Diagnosis Code M66.359Spontaneous rupture of flexor tendons, unspecified thigh

ICD-10-CM Codes›M00–M99›M65-M67›M66

ICD-10-CM M66.359
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

M66.359 is a billable ICD-10-CM diagnosis code for spontaneous rupture of flexor tendons, unspecified thigh. 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 nontraumatic rupture of flexor tendon of thigh. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Tendon and synovial disorders.

Code Identity

ICD-10-CM Code
M66.359
Billable Status
Yes — Valid for Submission
Code Describes
Spontaneous rupture of flexor tendons, unspecified thigh
Short Description
Spontaneous rupture of flexor tendons, unspecified thigh
Same as the full description in the CMS dataset.
Parent Code
M66.35
Spontaneous rupture of flexor tendons, thigh

Code Classification

ChapterM00–M99Diseases of the musculoskeletal system and connective tissue
SectionM65-M67Disorders of synovium and tendon
CategoryM66Spontaneous rupture of synovium and tendon
This CodeM66.359Spontaneous rupture of flexor tendons, unspecified thigh

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Nontraumatic rupture of flexor tendon of thigh

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

Leg Injuries and Disorders

Your legs are made up of bones, blood vessels, muscles, and other connective tissue. They are important for motion and standing. Playing sports, running, falling, or having an accident can damage your legs. Common leg injuries include sprains and strains, joint dislocations, and fractures (broken bones).

Read the full article at MedlinePlus

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

Convert M66.359 to ICD-9-CMHistory

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

ICD-9-CM
727.69 Nontraum tendon rupt 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 M66.359Overview

Is M66.359 (Spontaneous rupture of flexor tendons, thigh) a billable code?

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

What MS-DRG does M66.359 group to?

When spontaneous rupture of flexor tendons, unspecified thigh 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 M66.359?

Under the General Equivalence Mappings, spontaneous rupture of flexor tendons, unspecified thigh converts to ICD-9-CM 727.69 (nontraum tendon rupt 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.