2026 ICD-10-CM Diagnosis Code M21.961Unspecified acquired deformity of right lower leg

ICD-10-CM Codes›M00–M99›M20-M25›M21

ICD-10-CM M21.961
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

M21.961 is a billable ICD-10-CM diagnosis code for unspecified acquired deformity of right lower leg. 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. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Acquired deformities (excluding foot).

Code Identity

ICD-10-CM Code
M21.961
Billable Status
Yes — Valid for Submission
Code Describes
Unspecified acquired deformity of right lower leg
Short Description
Unspecified acquired deformity of right lower leg
Same as the full description in the CMS dataset.
Parent Code
M21.96
Unspecified acquired deformity of lower leg

Code Classification

ChapterM00–M99Diseases of the musculoskeletal system and connective tissue
SectionM20-M25Other joint disorders
CategoryM21Other acquired deformities of limbs
This CodeM21.961Unspecified acquired deformity of right lower leg

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Acquired deformity of bilateral ankles
  • Acquired deformity of bilateral calcaneus
  • Acquired deformity of calcaneus
  • Acquired deformity of left ankle
  • Acquired deformity of left calcaneus
  • Acquired deformity of left knee
  • Acquired deformity of left lower leg
  • Acquired deformity of right ankle
  • Acquired deformity of right calcaneus
  • Acquired deformity of right foot
  • Acquired deformity of right knee
  • Acquired deformity of right lower leg
  • Bilateral acquired deformity of feet
  • Bilateral acquired deformity of knee regions
  • Bilateral acquired deformity of lower legs
  • Deformity of calcaneum
  • Deformity of right foot

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

Leg Injuries and Disorders

Your legs are made up of bones, blood vessels, muscles, and other connective tissue. They are important for motion and standing. Playing sports, running, falling, or having an accident can damage your legs. Common leg injuries include sprains and strains, joint dislocations, and fractures (broken bones).

Read the full article at MedlinePlus

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

Convert M21.961 to ICD-9-CMHistory

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

ICD-9-CM
736.70 Acq ankle-foot def NOS
Approximate The match is approximate rather than exact.
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 M21.961Overview

Is M21.961 (Unspecified acquired deformity of lower leg) a billable code?

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

What MS-DRG does M21.961 group to?

When unspecified acquired deformity of right lower leg 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 M21.961?

Under the General Equivalence Mappings, unspecified acquired deformity of right lower leg converts to ICD-9-CM 736.70 (acq ankle-foot def NOS) and 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.