2026 ICD-10-CM Diagnosis Code M21.261Flexion deformity, right knee

ICD-10-CM Codes›M00–M99›M20-M25›M21

ICD-10-CM M21.261
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

M21.261 is a billable ICD-10-CM diagnosis code for flexion deformity, right knee. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 564 through 566. Coders also document this condition as bilateral flexion deformity of knees. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Acquired deformities (excluding foot).

Code Identity

ICD-10-CM Code
M21.261
Billable Status
Yes — Valid for Submission
Code Describes
Flexion deformity, right knee
Short Description
Flexion deformity, right knee
Same as the full description in the CMS dataset.
Parent Code
M21.26
Flexion deformity, knee

Code Classification

ChapterM00–M99Diseases of the musculoskeletal system and connective tissue
SectionM20-M25Other joint disorders
CategoryM21Other acquired deformities of limbs
This CodeM21.261Flexion deformity, right knee

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Bilateral flexion deformity of knees
  • Flexion deformity of joint of left knee
  • Flexion deformity of joint of right knee
  • Flexion deformity of knee

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR MUS023
Acquired deformities (excluding foot)
Default principal diagnosis: inpatient Yes · outpatient Yes

Patient EducationClinical

Knee Injuries and Disorders

Your knee joint is made up of bone, cartilage, ligaments and fluid. Muscles and tendons help the knee joint move. When any of these structures is hurt or diseased, you have knee problems. Knee problems can cause pain and difficulty walking.

Read the full article at MedlinePlus

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

Convert M21.261 to ICD-9-CMHistory

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

ICD-9-CM
736.89 Oth acq limb deformity
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 M21.261Overview

Is M21.261 (Flexion deformity, knee) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report flexion deformity, right knee on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does M21.261 group to?

When flexion deformity, right knee is the principal diagnosis on an inpatient stay, it groups to MS-DRG 564, 565, 566, with relative weights from 0.7493 to 1.5436 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of M21.261?

Under the General Equivalence Mappings, flexion deformity, right knee converts to ICD-9-CM 736.89 (oth acq limb deformity). 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.