2027 ICD-10-CM Diagnosis Code M66.339Spontaneous rupture of flexor tendons, unspecified forearm

ICD-10-CM Codes›M00–M99›M65-M67›M66

ICD-10-CM M66.339
CMSSource: CMS FY 2027 ICD-10-CM dataset · Effective Oct 1, 2026 – Sep 30, 2027

M66.339 is a billable ICD-10-CM diagnosis code for spontaneous rupture of flexor tendons, unspecified forearm. It is valid on HIPAA claims for fiscal year 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 557 through 558. Coders also document this condition as nontraumatic rupture of flexor tendon of forearm. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Tendon and synovial disorders.

Code Identity

ICD-10-CM Code
M66.339
Billable Status
Yes — Valid for Submission
Code Describes
Spontaneous rupture of flexor tendons, unspecified forearm
Short Description
Spontaneous rupture of flexor tendons, unspecified forearm
Same as the full description in the CMS dataset.
Parent Code
M66.33
Spontaneous rupture of flexor tendons, forearm

Code Classification

ChapterM00–M99Diseases of the musculoskeletal system and connective tissue
SectionM65-M67Disorders of synovium and tendon
CategoryM66Spontaneous rupture of synovium and tendon
This CodeM66.339Spontaneous rupture of flexor tendons, unspecified forearm

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Nontraumatic rupture of flexor tendon of forearm
  • Nontraumatic rupture of flexor tendon of wrist region
  • Nontraumatic rupture of tendon of forearm
  • Rupture of flexor tendon of wrist

Instructional NotesGuidance

Instructions from the official ICD-10-CM Tabular List that apply to M66.339: its own notes plus those printed at M66 and Chapter 13. A note printed at a category, block or chapter applies to every code under it.

Includes

Further defines, or gives examples of, the content of the category.

From M66 Spontaneous rupture of synovium and tendon applies to 103 codes
  • rupture that occurs when a normal force is applied to tissues that are inferred to have less than normal strength

Excludes2

Not included here: the excluded condition is not part of this code, but a patient may have both, so both codes may be reported.

From M66 Spontaneous rupture of synovium and tendon applies to 103 codes
  • rotator cuff syndrome (M75.1-)
  • rupture where an abnormal force is applied to normal tissue - see injury of tendon by body region
From Chapter 13 (M00-M99) Diseases of the musculoskeletal system and connective tissue applies to 6,687 codes
  • arthropathic psoriasis (L40.5-)
  • certain conditions originating in the perinatal period (P04-P96)
  • certain infectious and parasitic diseases (A00-B99)
  • compartment syndrome (traumatic) (T79.A-)
  • complications of pregnancy, childbirth and the puerperium (O00-O9A)
  • congenital malformations, deformations, and chromosomal abnormalities (Q00-Q99)
  • endocrine, nutritional and metabolic diseases (E00-E88)
  • injury, poisoning and certain other consequences of external causes (S00-T88)
  • neoplasms (C00-D49)
  • symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified (R00-R94)

Notes

General instructions on how these codes are used.

From Chapter 13 (M00-M99) Diseases of the musculoskeletal system and connective tissue applies to 6,687 codes
  • Use an external cause code following the code for the musculoskeletal condition, if applicable, to identify the cause of the musculoskeletal condition

Source: CMS ICD-10-CM Tabular List. How to read instructional notes.

Referenced in Other NotesGuidance

Instructional notes printed at other codes that name M66.339, its category, or a range that includes it.

Excludes2 1

These codes carry an Excludes2 note naming M66.339: its condition is not part of those codes, and both may be reported when the patient has both.

  • M62.1 Other rupture of muscle (nontraumatic)
    rupture of tendon (M66.-) names M66.-, which includes this code

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 M66.339 to ICD-9-CMHistory

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

ICD-9-CM
727.64 Rupt flexor tendon hand
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 2017–2026No changes
FY 2027CurrentCurrent code set, no changesEffective October 1, 2026 through September 30, 2027.

Questions About M66.339Overview

What is the ICD-10 code for spontaneous rupture of flexor tendons, unspecified forearm?

The ICD-10-CM code for spontaneous rupture of flexor tendons, unspecified forearm is M66.339 (sometimes written as M66339). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.

Is M66.339 (Spontaneous rupture of flexor tendons, forearm) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report spontaneous rupture of flexor tendons, unspecified forearm on HIPAA-covered claims from October 1, 2026 through September 30, 2027.

What MS-DRG does M66.339 group to?

When spontaneous rupture of flexor tendons, unspecified forearm is the principal diagnosis on an inpatient stay, it groups to MS-DRG 557, 558, with relative weights from 0.8873 to 1.4883 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of M66.339?

Under the General Equivalence Mappings, spontaneous rupture of flexor tendons, unspecified forearm converts to ICD-9-CM 727.64 (rupt flexor tendon hand). The mapping is approximate, so confirm the match fits the documentation.