2027 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 2027 ICD-10-CM dataset · Effective Oct 1, 2026 – Sep 30, 2027

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 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 564 through 566. Coders also document this condition as acquired deformity of left knee. 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

Instructional NotesGuidance

Instructions from the official ICD-10-CM Tabular List that apply to M21.161: its own notes plus those printed at M21.1, M21, block M00-M25, block M20-M25, 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.

From Block M00-M25 applies to 2,249 codes
  • Disorders affecting predominantly peripheral (limb) joints

Excludes1

Not coded here: the excluded code is never reported together with this one, unless the two conditions are unrelated.

From M21.1 Varus deformity, not elsewhere classified applies to 13 codes
From M21 Other acquired deformities of limbs applies to 124 codes
  • acquired absence of limb (Z89.-)
  • congenital absence of limbs (Q71-Q73)
  • congenital deformities and malformations of limbs (Q65-Q66, Q68-Q74)

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.

From M21 Other acquired deformities of limbs applies to 124 codes
  • acquired deformities of fingers or toes (M20.-)
  • coxa plana (M91.2)
From Block M20-M25 Other joint disorders applies to 719 codes
From Chapter 13 (M00-M99) Diseases of the musculoskeletal system and connective tissue applies to 6,687 codes
  • 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.

From Chapter 13 (M00-M99) Diseases of the musculoskeletal system and connective tissue applies to 6,687 codes
  • 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 M21.161, its category, or a range that includes it.

Excludes1 2

These codes carry an Excludes1 note naming M21.161: they are not reported together with it, unless the two conditions are unrelated.

  • M23 Internal derangement of knee
    deformity of knee (M21.-) names M21.-, which includes this code
  • Q66 Congenital deformities of feet
    varus deformities (acquired) (M21.1-) names M21.1-, which includes this code

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 2017–2026No changes
FY 2027CurrentCurrent code set, no changesEffective October 1, 2026 through September 30, 2027.

Questions About M21.161Overview

What is the ICD-10 code for varus deformity, not elsewhere classified, right knee?

The ICD-10-CM code for varus deformity, not elsewhere classified, right knee is M21.161 (sometimes written as M21161). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.

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, 2026 through September 30, 2027.

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, 566, 565, with relative weights from 0.7374 to 1.4909 depending on complications. Higher weights mean higher Medicare reimbursement.

Can M21.161 be reported with a code from Q66.22?

Not as a rule. The Excludes1 note at M21.1, which applies to M21.161, lists "metatarsus varus (Q66.22-)". An Excludes1 note means the excluded code is never reported together with this one; the only exception is when the two conditions are unrelated to each other (Official Guidelines, Section I.A.12.a).

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.