ICD-10-CM · Spine

M99.42

Narrowing of the thoracic spinal neural canal caused by connective tissue proliferation or fibrosis, classified as a biomechanical lesion under M99.

Verified May 8, 2026 · 5 sources ↓

Status: Billable
Chapter: 13
Related CPT: 12
Region: Spine

Drawn from CDCICD10DataCMSAAPC

Documentation tips

What should appear in the chart to support M99.42.

Source · Editorial brief grounded in 5 cited references ↓

Specify 'connective tissue' or 'ligamentous hypertrophy' as the mechanism of canal narrowing — generic 'thoracic stenosis' does not support M99.42 over M48.24.
Identify the structure narrowed as the neural canal, not the intervertebral foramen; foraminal connective tissue stenosis maps to M99.72.
Record imaging findings that confirm soft-tissue/connective tissue origin of stenosis: MRI signal changes in ligamentum flavum, epidural fibrosis on post-contrast MRI, or operative notes describing fibrotic tissue.
Document that no more specific classification applies — the M99 category note requires ruling out codes elsewhere in ICD-10-CM before defaulting to M99.42.
If thoracic myelopathy or radiculopathy is present, document it separately so it can be sequenced appropriately alongside M99.42.

Related CPT procedures

Procedure codes commonly billed with M99.42. Linking the right diagnosis to the right procedure is what establishes medical necessity.

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

63045 $1,249.53

Open cervical laminectomy with facetectomy and foraminotomy at a single vertebral level to decompress nerve roots or the spinal cord.

63048 $187.38

Add-on code for laminectomy, facetectomy, and foraminotomy at each additional cervical, thoracic, or lumbar vertebral segment beyond the primary segment.

72081 $44.09

Single-view radiologic examination of the entire spine, capturing thoracic and lumbar regions and optionally including cervical, skull, and sacral segments — typically ordered for scoliosis evaluation or global spinal alignment assessment.

72082 $71.81

Radiologic examination of the entire thoracic and lumbar spine, capturing 2 or 3 views; skull, cervical, and sacral spine included when performed.

72083 $79.83

Radiologic examination of the entire thoracic and lumbar spine using four or five views, with optional inclusion of skull, cervical, and sacral spine regions — typically ordered for scoliosis evaluation or global spinal alignment assessment.

63051 $1,574.85

Cervical laminoplasty with spinal cord decompression across two or more vertebral segments, including posterior bony element reconstruction with bridging bone graft and non-segmental fixation devices such as wire, suture, or mini-plates.

63075 View procedure details

63077 View procedure details

72072 View procedure details

72074 View procedure details

72084 View procedure details

63050 View procedure details

Common coding pitfalls

The recurring mistakes coders make with M99.42 and adjacent codes.

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

Using M99.42 when M48.24 (spinal stenosis, thoracic region) is more appropriate — M99.42 is reserved for biomechanical/connective tissue etiology when no other classification fits.
Confusing neural canal stenosis (M99.42) with intervertebral foraminal stenosis (M99.72) — these are anatomically distinct structures requiring different codes.
Assigning M99.42 based on a generic 'thoracic stenosis' diagnosis without documentation that connective tissue is the specific mechanism.
Overlooking the M99 category note that prohibits use of these codes when the condition can be classified elsewhere — failure to check leads to audit exposure.
Coding only M99.42 when an associated thoracic myelopathy or cord compression is documented and should be coded and sequenced additionally.

Clinical context

Source · Editorial summary grounded in 5 cited references ↓

M99.42 captures connective tissue stenosis of the neural canal specifically at the thoracic level. It sits under parent code M99.4 (Connective tissue stenosis of neural canal) and is reserved for cases where fibrotic or connective tissue changes — rather than bony overgrowth (M99.32) or disc intrusion (M99.52) — are the documented mechanism narrowing the thoracic spinal canal. The M99 category carries an important restriction: it should not be used if the condition can be classified elsewhere. Before assigning M99.42, confirm there is no more specific thoracic stenosis code that applies (e.g., M48.24 for spinal stenosis of the thoracic region due to other spondylopathy).

In orthopedic and spine practice, this code is most applicable when imaging or operative findings explicitly identify connective tissue or ligamentous hypertrophy — such as ligamentum flavum thickening or epidural fibrosis — as the primary cause of canal compromise at the thoracic level. Documentation must name the mechanism (connective tissue), the structure affected (neural canal, not foramen — see M99.72 for foraminal involvement), and the region (thoracic).

If stenosis involves both connective tissue and disc components at the foraminal level, consider M99.72 (Connective tissue and disc stenosis of intervertebral foramina, thoracic region). For osseous thoracic canal stenosis, use M99.32. Sequencing M99.42 as primary diagnosis requires that the connective tissue stenosis is the condition driving the encounter; if the encounter is for a related neurological deficit or radiculopathy, sequence that deficit first and M99.42 as an additional code.

Sibling codes

Other billable codes under M99.4 (laterality / anatomic variants).

M99.48

Connective tissue stenosis of neural canal of rib cage

M99.40

Connective tissue stenosis of neural canal of head region

M99.45

Connective tissue stenosis of neural canal of pelvic region

M99.43

Connective tissue stenosis of neural canal of lumbar region

M99.47

Connective tiss stenosis of neural canal of upper extremity

M99.44

Connective tissue stenosis of neural canal of sacral region

M99.46

Connective tiss stenosis of neural canal of lower extremity

M99.49

Conn tiss stenos of neural canal of abdomen and oth regions

Frequently asked questions

Source · Generated from the editorial pipeline, verified against 5 cited references ↓

01What is the difference between M99.42 and M48.24?

M48.24 codes spinal stenosis of the thoracic region broadly; M99.42 is specifically for stenosis driven by connective tissue pathology (e.g., ligamentum flavum hypertrophy, epidural fibrosis) classified as a biomechanical lesion. Use M99.42 only when documentation names connective tissue as the mechanism and no other classification applies.

02Can M99.42 be used when both connective tissue and disc changes contribute to thoracic canal narrowing?

If the foramen is involved by both connective tissue and disc, use M99.72. For the neural canal itself, if disc stenosis is separately documented, also consider M99.52 (intervertebral disc stenosis of neural canal, thoracic). Assign both codes when distinct mechanisms are documented.

03Does M99.42 require a 7th character extension?

No. M-codes in the musculoskeletal chapter do not use 7th-character extensions. 7th characters (A, D, S) apply to injury codes (S-codes), not to M99 category codes.

04When should M99.42 be sequenced as the principal diagnosis vs. an additional code?

Sequence M99.42 as principal when the connective tissue canal stenosis is the condition chiefly responsible for the encounter. If thoracic myelopathy, radiculopathy, or neurological deficit is the primary reason for the visit, sequence that condition first and list M99.42 as an additional code.

05Is M99.42 appropriate after thoracic decompression surgery to describe the pre-operative diagnosis?

Yes, if operative findings confirm connective tissue (e.g., thickened ligamentum flavum, fibrotic epidural tissue) as the cause of thoracic canal stenosis and no more specific classification applies. Document the intraoperative finding explicitly in the operative note to support the code.

06What imaging findings best support M99.42?

MRI demonstrating ligamentum flavum thickening, T2 signal changes in posterior ligamentous structures, or post-contrast enhancement suggesting epidural fibrosis supports connective tissue as the stenosis mechanism. Radiologist reports should explicitly describe soft-tissue rather than osseous or disc origin of canal compromise.

Sources & references

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

Mira AI Scribe

The Mira AI Scribe captures the mechanism of stenosis (connective tissue/ligamentous hypertrophy vs. bony or disc origin), the specific spinal region (thoracic), and the affected structure (neural canal vs. foramen) from provider narrative and imaging impressions. This prevents default assignment of the less specific M48.24 or misrouting to a foraminal stenosis code, both of which trigger audit scrutiny and can undermine medical necessity support for thoracic decompression procedures.

See how Mira captures M99.42 documentation