Aneurysmal bone cyst (ABC) involving two or more distinct skeletal sites simultaneously, coded under the M85.5 family of bone density and structure disorders.

Documentation tips

What should appear in the chart to support M85.59.

Source · Editorial brief grounded in 6 cited references ↓

• Name every affected site explicitly in the assessment — 'aneurysmal bone cyst of the proximal femur and T10 vertebral body' justifies M85.59 far better than 'multiple ABCs.'
• Reference the imaging modality and key findings (fluid-fluid levels on MRI, expansile lytic lesion with thin cortical shell) for each site to support medical necessity.
• If pathology was obtained, link the biopsy result to each lesion site in the note — payers may audit multi-site claims without histologic or radiologic confirmation for each location.
• Document whether lesions are primary ABCs or secondary ABCs arising within a pre-existing lesion (e.g., giant cell tumor) — secondary ABCs may require an additional code for the underlying lesion.
• Record laterality for each site even though M85.59 itself does not carry a laterality character, because surgical and procedural codes will require it.
• Note the clinical rationale for staging if procedures will be performed at separate encounters — this supports subsequent-encounter coding and avoids the appearance of duplicate billing.

Related CPT procedures

Procedure codes commonly billed with M85.59. 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 M85.59 and adjacent codes.

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

• Assigning M85.59 when only one site is documented: if the note names a single location, use the appropriate site-specific M85.5x code instead.
• Using M85.59 for an aneurysmal cyst of the jaw — the Excludes2 note at M85.5 directs jaw lesions to M27.4, regardless of whether other skeletal sites are also involved.
• Coding M85.59 when the underlying condition is polyostotic fibrous dysplasia (Q78.1) or another Excludes1-listed disorder — those codes are mutually exclusive with M85.
• Failing to link each procedural CPT code to the correct lesion site when multiple surgeries are planned — the multi-site diagnosis alone does not automatically justify multiple same-session procedures without distinct operative documentation.
• Confusing ABC with simple bone cyst (M85.40–M85.49) or fibrous dysplasia (M85.00–M85.09) — histologic or advanced imaging confirmation distinguishes these entities and determines the correct code family.

Clinical context

Source · Editorial summary grounded in 6 cited references ↓

Sibling codes

Other billable codes under M85.5 (laterality / anatomic variants).