2026 ICD-10-CM Diagnosis Code M66.18Rupture of synovium, other site

ICD-10-CM Codes›M00–M99›M65-M67›M66

ICD-10-CM M66.18
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

M66.18 is a billable ICD-10-CM diagnosis code for rupture of synovium, other site. 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 leaking synovial cyst of knee. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Tendon and synovial disorders.

Code Identity

ICD-10-CM Code
M66.18
Billable Status
Yes — Valid for Submission
Code Describes
Rupture of synovium, other site
Short Description
Rupture of synovium, other site
Same as the full description in the CMS dataset.
Parent Code
M66.1
Rupture of synovium

Code Classification

ChapterM00–M99Diseases of the musculoskeletal system and connective tissue
SectionM65-M67Disorders of synovium and tendon
CategoryM66Spontaneous rupture of synovium and tendon
This CodeM66.18Rupture of synovium, other site

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Leaking synovial cyst of knee
  • Rupture of synovial cyst of knee
  • Rupture of synovium - knee

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Rupture, ruptured
      • synovium (cyst)
        • specified site NEC

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

Joint Disorders

Your joints are places where two or more bones come together. Your shoulders, elbows, hips, knees, and knuckles are all joints. Your spine has joints, too.

The full article covers:

  • What are joints?
  • What are joint disorders?
  • What diseases can affect the joints?
  • What types of joint disorders happen from sudden injuries?
  • What types of joint disorders happen from overuse?
  • How can I keep my joints healthy?

Read the full article at MedlinePlus

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

Convert M66.18 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 M66.18Overview

Is M66.18 (Rupture of synovium) a billable code?

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

What MS-DRG does M66.18 group to?

When rupture of synovium, other site 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 M66.18?

Under the General Equivalence Mappings, rupture of synovium, other site 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.