Juvenile osteochondrosis affecting the hip and/or pelvis on the right side, where the specific subtype (e.g., Legg-Calvé-Perthes, coxa plana, pseudocoxalgia) has not been documented or cannot be determined.

Documentation tips

What should appear in the chart to support M91.91.

Source · Editorial brief grounded in 4 cited references ↓

• Document laterality explicitly as 'right' in the clinical note — 'right hip osteochondrosis' locks in M91.91 and prevents a drop to the unspecified M91.90.
• Specify the subtype whenever possible (Legg-Calvé-Perthes, coxa plana, pseudocoxalgia, coxa magna); M91.91 is a fallback for genuinely unspecified cases only.
• Record the patient's age or skeletal maturity status to confirm appropriateness of a juvenile osteochondrosis code versus an adult avascular necrosis or degenerative diagnosis.
• Document relevant imaging findings — plain radiograph findings (Catterall stage, Herring lateral pillar classification for Perthes), MRI signal changes, or bone scan results — to support medical necessity and the most specific code.
• If conservative care has been trialed (activity restriction, physical therapy, bracing), document the duration and response, as this supports medical necessity for advanced imaging or surgical intervention.

Related CPT procedures

Procedure codes commonly billed with M91.91. 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 M91.91 and adjacent codes.

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

• Assigning M91.91 when a specific subtype is documented: if the record says Legg-Calvé-Perthes, right side, the correct code is M91.11, not M91.91.
• Using M91.91 alongside an M93.0- code for slipped upper femoral epiphysis — the Excludes1 note at category M91 prohibits this combination.
• Defaulting to M91.90 (unspecified leg) when laterality is actually documented in the record; always read the full note before selecting unspecified.
• Applying M91.91 to adult patients with avascular necrosis of the femoral head — juvenile osteochondrosis codes apply to skeletally immature patients; adult AVN maps to M87 series codes.
• Failing to code bilateral disease separately: if both hips are affected, M91.91 and M91.92 should each be assigned rather than using M91.90 alone.

Clinical context

Source · Editorial summary grounded in 4 cited references ↓

Sibling codes

Other billable codes under M91.9 (laterality / anatomic variants).