M91.41 identifies coxa magna of the right hip — an abnormal enlargement of the femoral head and neck occurring as a sequela of juvenile osteochondrosis (typically Legg-Calvé-Perthes disease) of the right side.

Documentation tips

What should appear in the chart to support M91.41.

Source · Editorial brief grounded in 5 cited references ↓

• Explicitly state 'right hip' in the assessment — M91.41 requires documented laterality; vague terms like 'affected hip' force a drop to M91.40 (unspecified).
• Document the etiology when known (e.g., prior Legg-Calvé-Perthes disease) to support medical necessity and distinguish coxa magna from other femoral head deformities.
• Record imaging findings: AP pelvis and frog-leg lateral views showing femoral head enlargement, head-neck ratio, acetabular coverage, and any associated joint space narrowing.
• Note the patient's age and skeletal maturity — coxa magna is a juvenile osteochondrosis sequela, so documenting that growth plates are closed or the condition is a residual deformity helps auditors understand the coding logic.
• If femoroacetabular impingement or early osteoarthritis coexists, document each condition separately in the assessment so additional codes can be assigned without inference.

Related CPT procedures

Procedure codes commonly billed with M91.41. 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 M91.41 and adjacent codes.

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

• Coding M91.11 (active Legg-Calvé-Perthes, right) when the encounter is for the residual coxa magna deformity — use M91.41 once remodeling has produced the structural enlargement and the active osteochondrosis phase has resolved.
• Using M91.40 (unspecified hip) when laterality is clearly documented in the record — always match the 6th character to the documented side (1 = right, 2 = left).
• Assigning M91.41 alongside an M93.0- slipped upper femoral epiphysis code — the Type 1 Excludes under M91 prohibits this combination.
• Confusing coxa magna (enlarged femoral head, M91.4-) with coxa plana (flattened femoral head, M91.2-) — both are Perthes sequelae but map to different codes; rely on the radiographic description.

Clinical context

Source · Editorial summary grounded in 5 cited references ↓

Sibling codes

Other billable codes under M91.4 (laterality / anatomic variants).