Monostotic fibrous dysplasia affecting a skeletal site not captured by the more specific M85.01–M85.07 site codes — includes bones such as the ribs, skull base, pelvis, clavicle, scapula, or sternum when the lesion is confined to a single bone.

Documentation tips

What should appear in the chart to support M85.08.

Source · Editorial brief grounded in 4 cited references ↓

• Name the specific bone affected (e.g., 'left 5th rib,' 'right ilium,' 'clavicle') — 'other site' is a catch-all; the operative note or radiology report must confirm the exact location.
• Document that the lesion is monostotic: cite the imaging modality (X-ray, CT, MRI, or bone scan) and explicitly state a single bone is involved to distinguish from polyostotic disease (Q78.1).
• Record Kellgren-Lawrence-equivalent severity or lesion size/extent on imaging when available — this supports medical necessity for surveillance imaging and pre-surgical planning.
• If biopsy was performed, link the pathology report confirming fibrous dysplasia histology to the encounter note; this anchors the diagnosis code to a confirmed finding rather than a suspected one.
• Note absence of endocrine findings or café-au-lait spots when applicable, to support monostotic classification and distance the record from McCune-Albright syndrome (Q78.1).

Related CPT procedures

Procedure codes commonly billed with M85.08. 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 M85.08 and adjacent codes.

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

• Assigning M85.08 when multiple bones are involved — polyostotic disease codes to Q78.1, not anywhere in M85.0; audit exposure is high if imaging in the record shows more than one lesion.
• Using M85.08 for jaw (mandibular or maxillary) fibrous dysplasia — an Excludes2 note at M85.0 redirects jaw lesions to M27.8; M85.08 and M27.8 may be coded together only when both a jaw lesion and a separate 'other site' lesion are confirmed monostotic.
• Defaulting to M85.00 (unspecified site) when the bone is documented — 'unspecified' is an audit flag; if the operative note or radiology report names the bone, use M85.08.
• Confusing M85.08 with M85.09 (multiple sites) — M85.09 is appropriate only when a single patient has monostotic dysplasia confirmed at more than one distinct non-contiguous bone, which by definition approaches polyostotic territory and may warrant reconsideration of Q78.1.

Clinical context

Source · Editorial summary grounded in 4 cited references ↓

Sibling codes

Other billable codes under M85.0 (laterality / anatomic variants).