Enlargement of the left femoral head and neck as a sequela of juvenile osteochondrosis of the hip, classified under the chondropathies block of ICD-10-CM Chapter 13.
Verified May 8, 2026 · 4 sources ↓
- Status
- Billable
- Chapter
- 13
- Related CPT
- 9
- Region
- Hip
Documentation tips
What should appear in the chart to support M91.42.
Source · Editorial brief grounded in 4 cited references ↓
- Explicitly document 'left hip' — M91.42 requires confirmed laterality; 'bilateral' or unspecified routes to M91.40, not here.
- Record the clinical basis for coxa magna: enlarged femoral head and/or neck on imaging (AP pelvis X-ray, MRI), with reference to prior Perthes disease or juvenile osteochondrosis history when known.
- Distinguish coxa magna from coxa plana (flat head deformity, M91.22) and pseudocoxalgia (M91.32) in the assessment — different structural findings, different codes.
- If femoroacetabular impingement or secondary osteoarthritis is also documented, code those conditions additionally; M91.42 alone does not capture downstream joint pathology.
- Note patient age and any prior treatment history (bracing, Perthes management) to support medical necessity when this diagnosis drives imaging orders or surgical planning.
Related CPT procedures
Procedure codes commonly billed with M91.42. 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 M91.42 and adjacent codes.
Source · Editorial brief grounded in CDC ICD-10-CM tabular guidance, AAOS coding references, and cited references ↓
- Assigning M91.42 alongside M93.0 (slipped upper femoral epiphysis, nontraumatic) — the Type 1 Excludes note prohibits simultaneous use; the two conditions are mutually exclusive at the category level.
- Defaulting to M91.40 (unspecified hip) without querying the provider — most operative or imaging reports clearly identify left vs. right, making unspecified unnecessary and audit-prone.
- Coding M91.42 for coxa plana findings — a flattened femoral head is M91.22 (left), not coxa magna; confirm the deformity type in the radiology or operative report before selecting the code.
- Using M91.42 as a primary diagnosis for a claim when the presenting problem is femoroacetabular impingement or hip OA — sequence the active condition first if that drove the encounter.
Clinical context
Source · Editorial summary grounded in 4 cited references ↓
M91.42 codes coxa magna of the left hip — a deformity in which the femoral head and neck become abnormally enlarged, typically as a late consequence of Legg-Calvé-Perthes disease or other juvenile osteochondrosis of the hip. The structural enlargement can lead to femoroacetabular impingement, reduced range of motion, and early-onset hip osteoarthritis. Use this code when documentation explicitly identifies the left hip and the deformity is consistent with a coxa magna pattern rather than coxa plana (flattened head, M91.22) or pseudocoxalgia (M91.32).
This code sits under parent category M91.4 (Coxa magna), which carries a Type 1 Excludes note for slipped upper femoral epiphysis (nontraumatic) — M93.0. Never assign M91.42 alongside M93.0; if SUFE is the documented condition, use the M93.0 series instead. If laterality is not documented, drop to M91.40 (unspecified hip), but push back to the provider for side clarification before accepting unspecified.
In orthopedic practice, M91.42 most often appears in the records of adolescent or young adult patients being evaluated for hip pain, gait abnormality, or impingement after a childhood history of Perthes disease. It may support medical necessity for hip preservation surgery, periacetabular osteotomy, or total hip arthroplasty in younger patients where the deformity has progressed to joint destruction.
Sibling codes
Other billable codes under M91.4 (laterality / anatomic variants).
Frequently asked questions
Source · Generated from the editorial pipeline, verified against 4 cited references ↓
01What is the difference between M91.42 (coxa magna) and M91.22 (coxa plana) for the left hip?
02Can M91.42 and M93.0 be coded together on the same claim?
03When should I use M91.40 instead of M91.42?
04Does M91.42 require a 7th character extension?
05Is M91.42 appropriate for an adult patient with a childhood history of Perthes disease?
06Should secondary hip osteoarthritis be coded separately when M91.42 is the underlying cause?
Sources & references
Editorial content was developed using the following public sources. Last verified May 8, 2026.
Mira AI Scribe
The Mira AI Scribe captures left-side laterality, the structural deformity type (enlarged femoral head/neck consistent with coxa magna), relevant imaging findings (AP pelvis X-ray, MRI), and any documented history of Legg-Calvé-Perthes disease or prior juvenile osteochondrosis treatment. That documentation prevents downcode to unspecified M91.40 and blocks erroneous assignment of the coxa plana (M91.22) or pseudocoxalgia (M91.32) codes at audit.
See how Mira captures M91.42 documentation