M85.559 Aneurysmal bone cyst, unspecified thigh

ICD-10-CM M85.559: Aneurysmal bone cyst, unspecified thigh

ICD-10-CM code M85.559 designates an aneurysmal bone cyst of an unspecified thigh. This active diagnosis concept is classified under musculoskeletal system disorders and represents a rare, benign, blood-filled cystic bone tumor. The code is considered billable and lacks any listed child codes or additional instructional notes in the referenced terminology records. 1 2 3

Plain-language overview

An aneurysmal bone cyst is a rare, locally destructive, blood-filled, benign cystic bone tumor that may be discovered incidentally on radiographs or prompted by a fracture. 2

What this code represents

The active ICD-10-CM code M85.559 represents the diagnosis of an aneurysmal bone cyst in an unspecified thigh, classified under the semantic types of Anatomical Abnormality and Disease or Syndrome. 1

M85.559 is a billable and specific code with no child codes, no inclusion terms, no code first instructions, and no use additional code instructions listed in the referenced April 2026 reference data. 3

Coding details

Code M85.559 falls under Chapter 13, Diseases of the musculoskeletal system and connective tissue, within category M85 for other disorders of bone density and structure, and subcategory M85.55 for aneurysmal bone cyst of the thigh. 3

The parent concept for M85.559 is M85.55, aneurysmal bone cyst of the thigh, and the terminology record indicates that M85.559 has narrower concepts within the full hierarchy. 1

Documentation considerations

The ICD-10-CM code tree lists separate specific codes for aneurysmal bone cysts of the right thigh, left thigh, and unspecified thigh, providing distinct options for documenting laterality. 4

What the sources add

While the official terminology record establishes the active status and parent hierarchy of M85.559, the reference data source adds operational context by confirming its billable status and absence of instructional notes. 1 3

A clinical reference source describes aneurysmal bone cysts as benign tumors that can show quiescent, active, or aggressive forms of evolutivity, adding clinical context to the diagnostic code. 2

Questions to verify

The ICD-10-CM code tree includes distinct codes for right thigh, left thigh, and unspecified thigh aneurysmal bone cysts, which can be reviewed alongside clinical documentation. 4

Sources

1. ICD-10-CM M85.559 terminology record — National Cancer Institute Enterprise Vocabulary Services; accessed 2026-07-27.
2. Bookshelf — ncbi.nlm.nih.gov; accessed 2026-07-27.
3. ICD-10-CM Code M85.559 | Vero — veroscribe.com; accessed 2026-07-27.
4. ICD-10-CM 2024 | M85.5 - Aneurysmal bone cyst — unboundmedicine.com; accessed 2026-07-27.