2026 ICD-10-CM Diagnosis Code M21.731Unequal limb length (acquired), right ulna

ICD-10-CM Codes›M00–M99›M20-M25›M21

ICD-10-CM M21.731
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

M21.731 is a billable ICD-10-CM diagnosis code for unequal limb length (acquired), right ulna. 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 deformity of right forearm. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Acquired deformities (excluding foot).

Code Identity

ICD-10-CM Code
M21.731
Billable Status
Yes — Valid for Submission
Code Describes
Unequal limb length (acquired), right ulna
Short Description
Unequal limb length (acquired), right ulna
Same as the full description in the CMS dataset.
Parent Code
M21.73
Unequal limb length (acquired), ulna and radius

Code Classification

ChapterM00–M99Diseases of the musculoskeletal system and connective tissue
SectionM20-M25Other joint disorders
CategoryM21Other acquired deformities of limbs
This CodeM21.731Unequal limb length (acquired), right ulna

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Acquired deformity of right forearm
  • Acquired forearm deformity
  • Acquired unequal arm length
  • Deformity of ulna
  • Unequal upper limb length due to acquired shortening of right ulna

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

Arm Injuries and Disorders

Of the 206 bones in your body, three of them are in your arm: the humerus, radius, and ulna. Your arms are also made up of muscles, joints, tendons, and other connective tissue. Injuries to any of these parts of the arm can occur during sports, a fall, or an accident.

Read the full article at MedlinePlus

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

Convert M21.731 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 M21.731Overview

Is M21.731 (Unequal limb length (acquired), ulna and radius) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report unequal limb length (acquired), right ulna on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does M21.731 group to?

When unequal limb length (acquired), right ulna 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 M21.731?

Under the General Equivalence Mappings, unequal limb length (acquired), right ulna 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.