2026 ICD-10-CM Diagnosis Code M66.321Spontaneous rupture of flexor tendons, right upper arm

ICD-10-CM Codes›M00–M99›M65-M67›M66

ICD-10-CM M66.321
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

M66.321 is a billable ICD-10-CM diagnosis code for spontaneous rupture of flexor tendons, right upper arm. 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. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Tendon and synovial disorders.

Code Identity

ICD-10-CM Code
M66.321
Billable Status
Yes — Valid for Submission
Code Describes
Spontaneous rupture of flexor tendons, right upper arm
Short Description
Spontaneous rupture of flexor tendons, right upper arm
Same as the full description in the CMS dataset.
Parent Code
M66.32
Spontaneous rupture of flexor tendons, upper arm

Code Classification

ChapterM00–M99Diseases of the musculoskeletal system and connective tissue
SectionM65-M67Disorders of synovium and tendon
CategoryM66Spontaneous rupture of synovium and tendon
This CodeM66.321Spontaneous rupture of flexor tendons, right upper arm

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

Arm Injuries and Disorders

Of the 206 bones in your body, three of them are in your arm: the humerus, radius, and ulna. Your arms are also made up of muscles, joints, tendons, and other connective tissue. Injuries to any of these parts of the arm can occur during sports, a fall, or an accident.

Read the full article at MedlinePlus

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

Convert M66.321 to ICD-9-CMHistory

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

ICD-9-CM
727.62 Biceps tendon rupture
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 M66.321Overview

Is M66.321 (Spontaneous rupture of flexor tendons, upper arm) a billable code?

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

What MS-DRG does M66.321 group to?

When spontaneous rupture of flexor tendons, right upper arm 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 M66.321?

Under the General Equivalence Mappings, spontaneous rupture of flexor tendons, right upper arm converts to ICD-9-CM 727.62 (biceps tendon rupture). 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.