ICD-10-CM · Hip

M91.21

Coxa plana of the right hip — a femoral head deformity resulting from avascular necrosis and remodeling after juvenile osteochondrosis (Legg-Calvé-Perthes disease), leaving a flattened, misshapen femoral head on the right side.

Verified May 8, 2026 · 6 sources ↓

Status: Billable
Chapter: 13
Related CPT: 9
Region: Hip

Drawn from CDCICD10DataAAPCCMSAAOS

Documentation tips

What should appear in the chart to support M91.21.

Source · Editorial brief grounded in 6 cited references ↓

Explicitly document right hip laterality — do not rely on imaging report language alone; the provider's note must confirm the affected side.
State whether coxa plana is active (ongoing Perthes disease) or a residual deformity from prior juvenile osteochondrosis, as this affects clinical context and any concurrent coding.
Record femoral head morphology findings from imaging — flattening, fragmentation, or irregularity of the right femoral head supports M91.21 over an unspecified hip code.
Document the patient's age at onset of Legg-Calvé-Perthes disease when known, as this supports classification under juvenile osteochondrosis rather than an acquired adult hip disorder.
If bilateral involvement exists, code M91.21 and M91.22 separately — there is no bilateral coxa plana code in the M91.2 subcategory.
When coxa plana has led to secondary osteoarthritis of the right hip, document that causal relationship so both codes can be reported and medical necessity is clear.

Related CPT procedures

Procedure codes commonly billed with M91.21. Linking the right diagnosis to the right procedure is what establishes medical necessity.

Source · CMS LCDs · AAOS specialty guidance · claims-pattern analysis

27130 $1,162.02

Primary total hip arthroplasty replacing both the acetabular socket and proximal femoral components with prosthetic implants, with or without bone graft.

27132 $1,504.04

Conversion of a previously operated hip — any prior surgery except total hip arthroplasty — to a complete total hip arthroplasty, replacing both femoral and acetabular components, with or without bone graft.

27140 $830.35

Osteotomy and distal-lateral transfer of the greater trochanter of the femur, reported as a separate procedure.

27146 $1,169.03

Osteotomy of the iliac, acetabular, or innominate bone without open reduction of the hip joint

27151 $1,428.56

Iliac, acetabular, or innominate bone osteotomy performed in conjunction with a femoral osteotomy to correct hip joint alignment.

27156 $1,533.44

Osteotomy of the iliac, acetabular, or innominate bone with concurrent femoral osteotomy and reduction of hip dislocation

73502 $48.77

Radiologic exam of a single hip, capturing two or three views, including the pelvis when performed.

73521 $41.75

Bilateral hip X-ray examination capturing two radiographic views of both hips, including the pelvis when performed.

77080 View procedure details

Common coding pitfalls

The recurring mistakes coders make with M91.21 and adjacent codes.

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

Billing M91.2 (parent, non-billable) instead of the laterality-specific M91.21 — payers will reject a non-specific code when a billable child code exists.
Confusing Legg-Calvé-Perthes disease (M91.11, right) with coxa plana (M91.21, right) — M91.11 is the active osteochondrosis of the femoral head; M91.21 is the resulting flattening deformity. These are distinct codes and should not be used interchangeably.
Using M93.0- (slipped upper femoral epiphysis, nontraumatic) when the deformity is actually coxa plana — the Excludes1 note under M91 makes these mutually exclusive at this encounter.
Omitting a secondary osteoarthritis code (e.g., M16.11) when the provider documents that the coxa plana deformity has resulted in right hip OA — failing to capture the full diagnostic picture can undermine medical necessity for surgical intervention.
Defaulting to M91.20 (unspecified hip) when the chart clearly documents the right hip — unspecified codes invite audit scrutiny and can trigger downcoding on value-based contracts.

Clinical context

Source · Editorial summary grounded in 6 cited references ↓

M91.21 is the specific, billable code for coxa plana of the right hip, classified under juvenile osteochondrosis of hip and pelvis (M91) in the Chondropathies section of Chapter 13. Use it when the provider documents a flattened femoral head deformity on the right side attributable to prior or active Legg-Calvé-Perthes disease or other juvenile osteochondrosis. The parent code M91.2 is non-billable — always select M91.21 (right), M91.22 (left), or M91.20 (unspecified) for claim submission.

Coxa plana most commonly presents in children aged 4–10 and is more prevalent in boys. When the deformity is documented as a sequela of prior Perthes disease in an adult, M91.21 still applies because it captures 'hip deformity due to previous juvenile osteochondrosis' per the Applicable To note under M91.2. Do not use M91.21 for slipped upper femoral epiphysis (nontraumatic) — that is an Excludes1 condition coded to M93.0-.

MS-DRG v43.0 groups M91.21 into DRG 553 (Bone diseases and arthropathies with MCC) or DRG 554 (without MCC), so comorbidity documentation directly affects DRG assignment and reimbursement weight. When surgical correction is performed — such as femoral or pelvic osteotomy — pair M91.21 with the appropriate CPT procedure code. If the patient subsequently develops hip osteoarthritis as a result of the deformity, code the osteoarthritis additionally.

Sibling codes

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

M91.22

Coxa plana, left hip

M91.20

Coxa plana, unspecified hip

Frequently asked questions

Source · Generated from the editorial pipeline, verified against 6 cited references ↓

01What is the difference between M91.11 and M91.21?

M91.11 codes active juvenile osteochondrosis of the right femoral head (Legg-Calvé-Perthes disease in progress). M91.21 codes the resulting deformity — a flattened femoral head — after the osteochondrosis process has remodeled the bone. Use M91.11 during active disease; use M91.21 once the flattening deformity is the documented diagnosis, including residual deformity in adults.

02Can M91.21 be used for an adult patient?

Yes. The Applicable To note under M91.2 explicitly includes 'hip deformity due to previous juvenile osteochondrosis,' so M91.21 applies whenever the right hip coxa plana deformity is documented, regardless of the patient's current age.

03Is there a bilateral coxa plana code?

No. The M91.2 subcategory provides only M91.20 (unspecified), M91.21 (right), and M91.22 (left). For bilateral involvement, report M91.21 and M91.22 together on the same claim.

04What Excludes1 conflict should I watch for with M91.21?

The category M91 carries an Excludes1 for slipped upper femoral epiphysis (nontraumatic), coded to M93.0-. Excludes1 means these two conditions cannot be coded together at the same encounter — if SUFE is the diagnosis, do not also assign M91.21.

05Which MS-DRGs does M91.21 map to?

Under MS-DRG v43.0, M91.21 groups to DRG 553 (Bone diseases and arthropathies with MCC) or DRG 554 (without MCC). Accurate MCC documentation — such as significant comorbidities — directly determines which DRG and reimbursement weight applies.

06Should I code secondary hip OA separately if the provider documents it as caused by coxa plana?

Yes. If the provider documents osteoarthritis of the right hip as a consequence of the coxa plana deformity, assign the appropriate secondary or post-traumatic osteoarthritis code (e.g., M16.11 or M16.51) in addition to M91.21. Capturing both diagnoses supports medical necessity for surgical planning and reflects the full clinical picture.

07What imaging documentation strengthens M91.21 coding?

AP pelvis and frog-leg lateral radiographs documenting femoral head flattening, fragmentation, or loss of sphericity on the right side are the primary imaging support. MRI findings showing avascular changes or residual deformity also reinforce the diagnosis. The provider should reference these findings explicitly in the clinical note.

Sources & references

Editorial content was developed using the following public sources. Last verified May 8, 2026.

Mira AI Scribe

Mira's AI scribe captures right hip laterality, the provider's description of femoral head flattening or deformity, any documented history of Legg-Calvé-Perthes disease (including age at onset), imaging findings such as femoral head irregularity or joint space changes, and current functional limitations. Capturing these specifics prevents fallback to the non-billable M91.2 parent code and supports DRG comorbidity documentation for accurate MS-DRG 553/554 assignment.

See how Mira captures M91.21 documentation