ICD-10-CM · Other

M66.20

Nontraumatic, spontaneous rupture of one or more extensor tendons at a body site that is not specified in the clinical documentation.

Verified May 8, 2026 · 4 sources ↓

Status: Billable
Chapter: 13
Related CPT: 16
Region: Other

Drawn from CDCICD10DataAAPCCMS

Documentation tips

What should appear in the chart to support M66.20.

Source · Editorial brief grounded in 4 cited references ↓

Specify the anatomical region by name (e.g., right forearm, left hand, wrist) — any site identification moves the code to a more specific child code and avoids M66.20's unspecified fallback.
Document the absence of acute trauma explicitly; phrases like 'spontaneous,' 'nontraumatic,' or 'attritional rupture' are needed to distinguish M66.20 from S-code traumatic rupture entries.
Record the underlying etiology driving tendon degeneration — rheumatoid arthritis, gout, diabetes, or prior corticosteroid injections — and code those conditions as additional diagnoses to support medical necessity.
Note imaging findings (ultrasound or MRI) confirming tendon discontinuity, retraction distance, and tendon quality; this supports surgical necessity and defends against audit.
If multiple tendons are involved at more than one site, code each site separately with the appropriate site-specific M66.2x child code rather than defaulting to M66.20.

Related CPT procedures

Procedure codes commonly billed with M66.20. Linking the right diagnosis to the right procedure is what establishes medical necessity.

Source · CMS LCDs · AAOS specialty guidance · claims-pattern analysis

26410 $598.54

Surgical repair of an extensor tendon on the dorsum of the hand, primary or secondary, without a free graft — billed per tendon repaired.

26415 $833.35

Excision of an extensor tendon from the hand or finger with implantation of a synthetic rod or tube to maintain the tendon gliding channel until a staged tendon graft can be performed.

26418 $627.60

Surgical repair of a finger extensor tendon without a free graft — primary (acute) or secondary (delayed) — billed per tendon repaired.

26420 $718.12

Repair of an extensor tendon in a finger, primary or secondary, using a free graft harvested during the same operative session — billed per tendon repaired.

26426 $475.96

Secondary repair of the extensor tendon central slip at the proximal interphalangeal joint using local tissues and lateral bands, performed to correct boutonniere deformity — one finger per unit.

26428 $789.26

Secondary repair of the extensor tendon's central slip at the proximal interphalangeal joint using a free graft, performed to correct boutonniere deformity and restore active finger extension. Graft harvest is included in the code.

26432 $542.76

Closed repair of a finger extensor tendon at its distal insertion — the mallet finger procedure — using splinting with or without percutaneous pin fixation.

26433 $567.82

Open repair of the extensor tendon at its distal insertion on the finger, performed without a graft, either as a primary repair or secondary revision.

26434 $693.40

Open repair of the extensor tendon at its distal finger insertion, using a graft — primary or secondary — for conditions such as mallet finger deformity that have failed conservative management or require surgical reconstruction.

26437 $649.65

Surgical repositioning of a displaced or misaligned extensor tendon in the hand to restore normal finger extension; reported per tendon realigned.

27385 $588.52

Primary open suture repair of a ruptured quadriceps or hamstring muscle; acute, first-time repair without graft or secondary reconstruction.

27386 $807.30

Secondary reconstruction of a ruptured quadriceps or hamstring muscle using a fascial or tendon graft harvested from the patient's own tissue.

28208 $498.34

Surgical repair of an extensor tendon in the foot, primary or secondary, per tendon.

28210 $597.88

Secondary repair of a foot extensor tendon using a free graft, including harvesting the graft, per tendon.

73130 $38.08

Radiographic examination of the hand requiring a minimum of three views.

73221 $205.08

MRI of any upper extremity joint — shoulder, elbow, or wrist — performed without contrast material.

Common coding pitfalls

The recurring mistakes coders make with M66.20 and adjacent codes.

Source · Editorial brief grounded in CDC ICD-10-CM tabular guidance, AAOS coding references, and cited references ↓

Using M66.20 when site is documented: any laterality or region in the note mandates a site-specific code (e.g., M66.211 for right shoulder); M66.20 is only valid when the site is truly unspecified.
Confusing spontaneous rupture with traumatic rupture: extensor tendon injuries caused by lacerations, falls, or direct trauma are coded with S-codes, not M66.20.
Failing to append comorbidity diagnoses that could qualify as MCCs, which drops the facility claim from MS-DRG 557 to the lower-weighted MS-DRG 558.
Coding M66.20 as a default when the chart actually documents the rupture site but the coder missed it in the body of the note — always review the operative report and imaging read, not just the assessment line.

Clinical context

Source · Editorial summary grounded in 4 cited references ↓

M66.20 applies when an extensor tendon ruptures without an acute traumatic event — the rupture occurs through underlying tendon degeneration, systemic disease (e.g., rheumatoid arthritis, gout, long-term corticosteroid use), or attritional wear — and the operative or clinical note does not specify which anatomical region is involved. This is the fallback code within the M66.2 family. The moment the chart identifies a body region, move to a site-specific child code: shoulder (M66.21x), upper arm (M66.22x), forearm (M66.23x), and so on.

Do not use M66.20 for traumatic tendon ruptures. Traumatic extensor tendon injuries are coded with S-codes and require a 7th-character encounter extension (A/D/S). M66.20 is exclusively for spontaneous, nontraumatic ruptures where the mechanism is degenerative or systemic rather than injury-driven.

On the facility side, M66.20 maps to MS-DRG 557 (Tendonitis, Myositis and Bursitis with MCC) or 558 (without MCC) under MS-DRG v43.0. Capture all comorbidities accurately — the MCC distinction directly affects DRG weight and reimbursement.

Sibling codes

Other billable codes under M66.2 (laterality / anatomic variants).

M66.29

Spontaneous rupture of extensor tendons, multiple sites

M66.28

Spontaneous rupture of extensor tendons, other site

Frequently asked questions

Source · Generated from the editorial pipeline, verified against 4 cited references ↓

01When is M66.20 the correct code versus a more specific M66.2x child code?

Use M66.20 only when the clinical documentation genuinely does not identify the anatomical site of the extensor tendon rupture. If any region is named — shoulder, forearm, hand, wrist, thigh, lower leg, ankle — select the corresponding site-specific code instead.

02Can M66.20 be used for a traumatic extensor tendon rupture?

No. M66.20 is restricted to spontaneous, nontraumatic ruptures. Traumatic extensor tendon injuries are coded with the appropriate S-code from Chapter 19, with a 7th-character extension (A for initial encounter, D for subsequent, S for sequela).

03What ICD-10-CM codes commonly accompany M66.20 as secondary diagnoses?

Secondary diagnoses that document the underlying cause of tendon degeneration — rheumatoid arthritis (M05–M06 range), gout (M10 range), or long-term corticosteroid use (Z79.52) — are appropriate secondary codes and support medical necessity.

04How does M66.20 differ from M66.9?

M66.9 (Spontaneous rupture of unspecified tendon) is used when neither the tendon type (extensor vs. flexor vs. other) nor the site is documented. M66.20 is more specific: the tendon type is known to be an extensor tendon, but the site is unspecified.

05Which MS-DRGs does M66.20 map to on the facility side?

M66.20 groups to MS-DRG 557 (Tendonitis, Myositis and Bursitis with MCC) or MS-DRG 558 (without MCC) under MS-DRG v43.0. Accurate comorbidity capture determines which DRG applies and directly affects reimbursement weight.

06Is M66.20 valid for an outpatient encounter?

Yes, M66.20 is a billable code valid for both inpatient and outpatient encounters. For outpatient visits, code only to the highest degree of specificity supported by documentation at the time of the encounter.

07What CPT procedures are commonly paired with M66.20?

Extensor tendon repair codes (e.g., 26410–26437 for hand and finger extensors, 28208–28210 for foot extensors) are typical procedural pairings, along with relevant imaging CPTs such as MRI or ultrasound of the affected region to confirm diagnosis.

Sources & references

Editorial content was developed using the following public sources. Last verified May 8, 2026.

01CDC ICD-10-CM Tabular List 2026
02
icd10data.com
https://www.icd10data.com/ICD10CM/Codes/M00-M99/M65-M67/M66-/M66.20
03
aapc.com
https://www.aapc.com/codes/icd-10-codes/M66.2
04
cms.gov
https://www.cms.gov/medicare-coverage-database/view/article.aspx?articleid=58893

Mira AI Scribe

Mira's AI scribe captures the absence of a traumatic mechanism, the specific tendon(s) and anatomical region involved, any underlying systemic condition contributing to degeneration, and imaging confirmation of rupture. That documentation prevents fallback to M66.20's unspecified site when a more precise child code is warranted, and it closes the audit gap between a spontaneous and a traumatic rupture classification.

See how Mira captures M66.20 documentation