M91.20 classifies coxa plana of an unspecified hip — a flattening deformity of the femoral head resulting from prior juvenile osteochondrosis (Legg-Calvé-Perthes disease), where the affected side is not documented.

Documentation tips

What should appear in the chart to support M91.20.

Source · Editorial brief grounded in 5 cited references ↓

• Specify laterality (right or left) in every encounter note — use M91.21 or M91.22 instead of M91.20 whenever the affected side is documented.
• Record the patient's childhood history of Legg-Calvé-Perthes disease or juvenile osteochondrosis to justify the M91.2x code family under the 'Applicable To' note.
• Document imaging findings that confirm femoral head flattening: AP and frog-leg lateral radiographs showing coxa plana morphology, Stulberg classification grade, or MRI findings of residual deformity.
• Note functional impairment — gait abnormality, leg-length discrepancy, range-of-motion restriction — to support medical necessity for imaging and surgical referral.
• If osteoarthritis has developed as a sequela, code it separately (e.g., M16.1x for primary unilateral hip OA) as an additional diagnosis; M91.20 alone does not capture secondary arthritic change.

Related CPT procedures

Procedure codes commonly billed with M91.20. 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 M91.20 and adjacent codes.

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

• Defaulting to M91.20 when the chart clearly names the affected side — laterality-specific codes M91.21 (right) and M91.22 (left) are always preferred over the unspecified variant.
• Confusing M91.2x (coxa plana, a residual deformity) with M91.1x (active Legg-Calvé-Perthes juvenile osteochondrosis) — use M91.1x for the acute/active phase, M91.2x only for the established deformity after disease resolution.
• Coding slipped upper femoral epiphysis (SUFE) under M91.2x — SUFE maps to M93.0–, which is an Excludes1 condition and cannot be coded with M91.2x.
• Omitting secondary diagnoses such as hip OA or leg-length discrepancy when they are documented and clinically relevant; M91.20 does not capture downstream joint pathology.

Clinical context

Source · Editorial summary grounded in 5 cited references ↓

Sibling codes

Other billable codes under M91.2 (laterality / anatomic variants).