2026 ICD-10-CM Diagnosis Code M21.161Varus deformity, not elsewhere classified, right knee

ICD-10-CM Codes›M00–M99›M20-M25›M21

ICD-10-CM M21.161
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

M21.161 is a billable ICD-10-CM diagnosis code for varus deformity, not elsewhere classified, 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. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Acquired deformities (excluding foot).

Code Identity

ICD-10-CM Code
M21.161
Billable Status
Yes — Valid for Submission
Code Describes
Varus deformity, not elsewhere classified, right knee
Short Description
Varus deformity, not elsewhere classified, right knee
Same as the full description in the CMS dataset.
Parent Code
M21.16
Varus deformity, not elsewhere classified, knee

Code Classification

ChapterM00–M99Diseases of the musculoskeletal system and connective tissue
SectionM20-M25Other joint disorders
CategoryM21Other acquired deformities of limbs
This CodeM21.161Varus deformity, not elsewhere classified, right knee

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Acquired deformity of left knee
  • Acquired deformity of right knee
  • Acquired genu valgum of bilateral knees
  • Acquired genu varum
  • Acquired varus deformity of joint of lower limb
  • Acquired varus deformity of left knee
  • Acquired varus deformity of right knee
  • Bandy legged
  • Bilateral acquired deformity of knee regions
  • Bilateral acquired varus deformity of knees

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 M21.161 to ICD-9-CMHistory

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

ICD-9-CM
736.42 Genu varum
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 M21.161Overview

Is M21.161 (Varus deformity, not elsewhere classified, knee) a billable code?

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

What MS-DRG does M21.161 group to?

When varus deformity, not elsewhere classified, 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 M21.161?

Under the General Equivalence Mappings, varus deformity, not elsewhere classified, right knee converts to ICD-9-CM 736.42 (genu varum). 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.