M85.40 identifies a solitary bone cyst at an unspecified skeletal site — a benign, fluid-filled unilocular lytic lesion with well-defined endosteal margins, reported when the operative or imaging report does not specify which bone is affected.

Documentation tips

What should appear in the chart to support M85.40.

Source · Editorial brief grounded in 5 cited references ↓

• Name the specific bone affected — proximal humerus, proximal femur, tibia, etc. — so coders can assign a site-specific M85.4x code instead of the unspecified fallback M85.40.
• Record laterality explicitly (right or left) in every encounter note and operative report; M85.4x codes offer right/left options at the 6th character.
• Document imaging modality and findings: confirm unilocular appearance, fluid content, and narrow endosteal margins on X-ray or MRI to support the solitary bone cyst diagnosis over other lytic lesions.
• If aspiration or steroid injection (CPT 20615) was performed, link the procedure note to the diagnosis note so the cyst site is consistently documented across both.
• Distinguish from jaw cysts in the record; a solitary cyst of the jaw requires M27.4, not M85.40, and mixing the two will trigger an Excludes2 conflict.

Related CPT procedures

Procedure codes commonly billed with M85.40. Linking the right diagnosis to the right procedure is what establishes medical necessity.

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

Common coding pitfalls

The recurring mistakes coders make with M85.40 and adjacent codes.

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

• Defaulting to M85.40 when the operative or radiology report clearly names the bone — always check for a site-specific M85.4x child code before using the unspecified fallback.
• Coding a jaw cyst to M85.40 instead of M27.4; the Excludes2 note at M85.4 explicitly carves out solitary cyst of jaw.
• Confusing solitary bone cyst with fibrous dysplasia (M85.0x) or aneurysmal bone cyst — these are distinct pathologies with distinct codes; rely on pathology or imaging confirmation before assigning.
• Applying M85.40 to conditions in the M85 Excludes1 list (osteogenesis imperfecta, osteopetrosis, osteopoikilosis, polyostotic fibrous dysplasia) — those are genetic/systemic conditions coded to the Q78 range and cannot be reported alongside M85.40.
• Omitting laterality when a site-specific code is available; M85.40 does not carry laterality granularity, so using it when the bone is documented sacrifices specificity that payers and auditors expect.

Clinical context

Source · Editorial summary grounded in 5 cited references ↓

Sibling codes

Other billable codes under M85.4 (laterality / anatomic variants).