2026 ICD-10-CM Diagnosis Code M21.171Varus deformity, not elsewhere classified, right ankle

ICD-10-CM Codes›M00–M99›M20-M25›M21

ICD-10-CM M21.171
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

M21.171 is a billable ICD-10-CM diagnosis code for varus deformity, not elsewhere classified, right ankle. 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 acquired varus deformity of left ankle. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Acquired foot deformities.

Code Identity

ICD-10-CM Code
M21.171
Billable Status
Yes — Valid for Submission
Code Describes
Varus deformity, not elsewhere classified, right ankle
Short Description
Varus deformity, not elsewhere classified, right ankle
Same as the full description in the CMS dataset.
Parent Code
M21.17
Varus deformity, not elsewhere classified, ankle

Code Classification

ChapterM00–M99Diseases of the musculoskeletal system and connective tissue
SectionM20-M25Other joint disorders
CategoryM21Other acquired deformities of limbs
This CodeM21.171Varus deformity, not elsewhere classified, right ankle

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Acquired varus deformity of left ankle
  • Acquired varus deformity of right ankle
  • Bilateral acquired varus deformity of ankles

Clinical ClassificationClinical

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

CCSR MUS021
Acquired foot deformities
Default principal diagnosis: inpatient Yes · outpatient Yes

Patient EducationClinical

Ankle Injuries and Disorders

Your ankle bone and the ends of your two lower leg bones make up the ankle joint. Your ligaments, which connect bones to one another, stabilize and support it. Your muscles and tendons move it.

Read the full article at MedlinePlus

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

Convert M21.171 to ICD-9-CMHistory

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

ICD-9-CM
736.71 Acq equinovarus
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 M21.171Overview

Is M21.171 (Varus deformity, not elsewhere classified, ankle) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report varus deformity, not elsewhere classified, right ankle on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does M21.171 group to?

When varus deformity, not elsewhere classified, right ankle 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 M21.171?

Under the General Equivalence Mappings, varus deformity, not elsewhere classified, right ankle converts to ICD-9-CM 736.71 (acq equinovarus). 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.