M91.40 identifies coxa magna of an unspecified hip — pathological enlargement of the femoral head and neck occurring as a sequela of juvenile osteochondrosis of the hip and pelvis, where laterality has not been documented.

Documentation tips

What should appear in the chart to support M91.40.

Source · Editorial brief grounded in 5 cited references ↓

• Specify laterality (right or left hip) in every encounter note — it's the difference between M91.40 and a laterality-specific code that survives payer scrutiny.
• Document the underlying osteochondrosis history explicitly (e.g., prior Legg-Calvé-Perthes disease) to justify the M91 category rather than a generic hip deformity code.
• Record radiographic findings that confirm femoral head enlargement — AP pelvis X-ray findings, head-neck ratio, Kellgren-Lawrence grade if secondary arthritis is present.
• If secondary hip osteoarthritis has developed, code that separately (M16.1x for primary or M16.3x post-traumatic, as clinically appropriate) — coxa magna is the underlying structural diagnosis, not the arthritic complication.
• Note any functional limitations (range of motion deficit, gait abnormality, leg-length discrepancy) to support medical necessity for imaging or surgical procedures billed alongside this code.

Related CPT procedures

Procedure codes commonly billed with M91.40. 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 M91.40 and adjacent codes.

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

• Defaulting to M91.40 when laterality is documented in the chart — use M91.41 (right) or M91.42 (left) whenever the side is specified; unspecified codes invite payer downcoding or denial.
• Confusing coxa magna with coxa plana (M91.2x) or pseudocoxalgia (M91.3x) — each is a distinct sequela of juvenile hip osteochondrosis with separate codes at the same hierarchical level.
• Coding slipped upper femoral epiphysis under M91.40 — SUFE is explicitly excluded from M91 by a Type 1 Excludes note and must be coded to M93.0–.
• Using M91.40 as the primary diagnosis on surgical claims without supporting imaging documentation, which increases audit risk for procedures such as periacetabular osteotomy or THA.
• Overlooking the need for a separate code for secondary osteoarthritis when coxa magna has progressed to joint degeneration — the structural deformity and the arthritic complication are not captured by a single code.

Clinical context

Source · Editorial summary grounded in 5 cited references ↓

Sibling codes

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