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ICD-10-CM 2027 diagnosis code

M66.331Spontaneous rupture of flexor tendons, right forearm

✓ Billable / specific

M66.331 is a valid, billable ICD-10-CM code for spontaneous rupture of flexor tendons, right forearm. It can be reported on claims for dates of service from October 1, 2026 – September 30, 2027.

Code unchanged since ICD-10-CM took effect on October 1, 2015. Source: official FY2027 ICD-10-CM release from CDC/NCHS and CMS. Page updated September 29, 2026. About our data

About M66.331

M66.331 is the ICD-10-CM diagnosis code for spontaneous rupture of flexor tendons, right forearm. It belongs to category M66 (spontaneous rupture of synovium and tendon), block M65-M67 (disorders of synovium and tendon) and chapter 13 (diseases of the musculoskeletal system and connective tissue). It is billable, so it can be reported on its own for dates of service from October 1, 2026 through September 30, 2027.

Within M66.33 (spontaneous rupture of flexor tendons, forearm), M66.331 is specifically for spontaneous rupture of flexor tendons, right forearm. Related codes cover left forearm (M66.332) and unspecified forearm (M66.339).

It is not a CC or MCC, so as a secondary diagnosis it doesn't change the MS-DRG severity level.

In MS-DRG v44.0, it is part of the grouping logic for DRG 557 (Tendonitis, Myositis and Bursitis with MCC, relative weight 1.489) and DRG 558 (Tendonitis, Myositis and Bursitis without MCC, relative weight 0.8875), in MDC 08 (Diseases and Disorders of the Musculoskeletal System and Connective Tissue), as a principal or secondary diagnosis. The DRG a claim lands in depends on the principal diagnosis, procedures and any CC/MCC secondary diagnoses.

It has been part of ICD-10-CM since the code set took effect on October 1, 2015 and hasn't changed since.

Before ICD-10, this condition was coded in ICD-9-CM as 727.64 (nontraumatic rupture of flexor tendons of hand and wrist).

Notes that apply from higher levels

Instructions written at a parent level also apply to M66.331.

› From M66 Spontaneous rupture of synovium and tendon
Includes
  • rupture that occurs when a normal force is applied to tissues that are inferred to have less than normal strength
Excludes2
  • rotator cuff syndrome (M75.1-)
  • rupture where an abnormal force is applied to normal tissue - see injury of tendon by body region

Broader instructions also apply from Chapter 13: Diseases of the musculoskeletal system and connective tissue.

MS-DRG v44.0 grouping

All DRGs →

Inpatient MS-DRGs this diagnosis can group to, as the principal or a secondary diagnosis, depending on the rest of the claim.

MDC 08 · Diseases and Disorders of the Musculoskeletal System and Connective Tissue

ICD-9-CM equivalent

Converter →
  • 727.64Nontraumatic rupture of flexor tendons of hand and wristapproximate

From the CMS 2018 General Equivalence Mappings (GEMs), the final GEMs release.

Code history

  1. 2016
  2. 2017
  3. 2018
  4. 2019
  5. 2020
  6. 2021
  7. 2022
  8. 2023
  9. 2024
  10. 2025
  11. 2026
  12. 2027
  • FY2016 (effective 10/1/2015): Added (first year of ICD-10-CM)
  • No changes since FY2016.

Common questions about M66.331

Is M66.331 billable?

Yes. M66.331 is a billable/specific ICD-10-CM code valid for FY2027 (October 1, 2026 – September 30, 2027).

Can M66.331 be used as a principal diagnosis?

Yes. The Medicare Code Editor doesn't restrict M66.331 as a principal diagnosis.

Is M66.331 a CC or MCC?

No. M66.331 is neither a CC nor an MCC under MS-DRG v44.0.

What DRG does M66.331 group to?

M66.331 is used in the MS-DRG v44.0 logic for MS-DRG 557 (Tendonitis, Myositis and Bursitis with MCC) and MS-DRG 558 (Tendonitis, Myositis and Bursitis without MCC). Which one applies depends on whether it is the principal diagnosis and on the rest of the claim.

What is the ICD-9 code for M66.331?

The CMS General Equivalence Mappings map M66.331 to ICD-9-CM 727.64.