2027 ICD-10-CM Diagnosis Code M66.18Rupture of synovium, other site
ICD-10-CM Codes›M00–M99›M65-M67›M66
- Billable — Valid for Submission
- Not Chronic
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 2027 (October 1, 2026 through September 30, 2027) 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
Code Classification
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
Instructional NotesGuidance
Instructions from the official ICD-10-CM Tabular List that apply to M66.18: its own notes plus those printed at M66.1, 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.
- 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.
- rotator cuff syndrome (M75.1-)
- rupture where an abnormal force is applied to normal tissue - see injury of tendon by body region
- 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.
- 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 M66.18, its category, or a range that includes it.
Excludes1 1
These codes carry an Excludes1 note naming M66.18: they are not reported together with it, unless the two conditions are unrelated.
Index to Diseases and InjuriesGuidance
Alphabetical index entries that point to this code.
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
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.
Code HistoryHistory
Questions About M66.18Overview
What is the ICD-10 code for rupture of synovium, other site?
The ICD-10-CM code for rupture of synovium, other site is M66.18 (sometimes written as M6618). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
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, 2026 through September 30, 2027.
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 0.8873 to 1.4883 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.
