M21.00

M21.00 identifies an acquired valgus (outward angulation) deformity of a limb when the specific anatomical site is not documented or cannot be determined from the record.

Verified May 8, 2026 · 7 sources ↓

Status: Billable
Chapter: 13
Related CPT: 10
Region: General

Drawn from CDCICD10DataAAPCCMSAAOS

Documentation tips

What should appear in the chart to support M21.00.

Source · Editorial brief grounded in 7 cited references ↓

If a specific joint is examined and the valgus deformity is confirmed there, document the joint by name (knee, hip, elbow) and laterality (right or left) — this avoids M21.00 entirely and supports a more specific billable code.
Record whether the deformity is acquired versus congenital; congenital valgus conditions route to Q-codes and are excluded from M21.0x by an Excludes1 note.
When valgus alignment is identified on imaging, document the study type (weight-bearing X-ray, CT, MRI), the affected limb segment, and any measurable deformity angle (e.g., valgus mechanical axis deviation in degrees or Kellgren-Lawrence grade if OA is present).
If valgus deformity coexists with osteoarthritis, the provider should explicitly name both findings so the coder can assign separate codes for each — one for alignment (M21.0x) and one for arthritis (M17.x or equivalent).
Note the clinical basis for 'acquired' — prior trauma, surgical history, ligamentous laxity, or progressive degenerative change — to distinguish from congenital origin and defend medical necessity.

Related CPT procedures

Procedure codes commonly billed with M21.00. Linking the right diagnosis to the right procedure is what establishes medical necessity.

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

27447 $1,159.35

Knee replacement surgery addressing both the medial and lateral tibiofemoral compartments, with or without resurfacing of the patella.

27446 $1,047.45

Arthroplasty of the knee involving resurfacing of the condyle and tibial plateau in a single tibiofemoral compartment — medial OR lateral, not both.

27454 $1,181.39

Multiple osteotomies of the femoral shaft with realignment on an intramedullary rod — the Sofield-type procedure for severe femoral deformity correction.

27395 $822.33

Surgical lengthening of multiple hamstring tendons in both legs simultaneously, performed through open incisions along the posterior thighs.

27140 $830.35

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

73564 $49.43

Radiologic examination of the knee consisting of four or more views, including oblique and tunnel projections, for a complete diagnostic workup.

73562 $42.42

Three-view radiographic examination of the knee joint, capturing anteroposterior, lateral, and a third angle such as a sunrise or oblique view.

73521 $41.75

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

73522 $54.44

Bilateral hip X-ray examination capturing 3 to 4 views, including the pelvis when clinically indicated.

77080 View procedure details

Common coding pitfalls

The recurring mistakes coders make with M21.00 and adjacent codes.

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

Stopping at M21.00 when a site is documented: if the note says 'right knee valgus,' the correct code is M21.061, not M21.00 — unspecified-site codes are not valid substitutes when laterality and site are clearly stated.
Using M21.00 for congenital valgus conditions such as metatarsus valgus (Q66.6) or talipes calcaneovalgus (Q66.4-) — these are Excludes1 from M21.0x and cannot be coded together.
Failing to assign a separate osteoarthritis code when the provider documents both valgus deformity and OA at the same joint — M21.00 does not capture the arthritis; both diagnoses require their own codes.
Assigning M21.00 for finger or toe valgus deformities: acquired deformities of the fingers and toes fall under M20.- (Excludes2 from M21), not M21.0x.

Clinical context

Source · Editorial summary grounded in 7 cited references ↓

M21.00 is the unspecified-site fallback within the M21.0x valgus deformity subcategory. Use it only when the provider's documentation genuinely does not identify which joint or limb segment is involved. For any encounter where laterality and site are known — knee, hip, or elbow — drop to the appropriate site-specific code: M21.061/062 (knee), M21.051/052 (hip), or M21.021/022 (elbow). Payers and auditors treat unspecified codes as a documentation gap, so M21.00 should be rare in a well-documented orthopedic practice.

M21.00 covers only acquired valgus deformities. Congenital conditions are excluded by category-level Excludes1 notes: metatarsus valgus routes to Q66.6, talipes calcaneovalgus to Q66.4-, and broader congenital limb deformities to Q65–Q74. If the deformity is congenital in origin, M21.00 cannot be used — select the appropriate Q-code instead.

When valgus deformity co-exists with osteoarthritis (e.g., valgus knee OA), code both the deformity and the OA separately. The M21.0x code captures the alignment finding; the M17.x code captures the arthritis. Neither code subsumes the other under current ICD-10-CM tabular conventions.

Frequently asked questions

Source · Generated from the editorial pipeline, verified against 7 cited references ↓

01When is M21.00 actually appropriate instead of a site-specific M21.0x code?

M21.00 is appropriate only when the provider's documentation genuinely does not identify which joint or limb segment is deformed — for example, a referral note that mentions valgus deformity without specifying the site. If you can identify the joint and side from anywhere in the encounter record, use the specific code.

02Can M21.00 be used for congenital valgus deformities?

No. M21.0x covers only acquired valgus deformities. Congenital conditions such as metatarsus valgus (Q66.6) and talipes calcaneovalgus (Q66.4-) are excluded by Excludes1 notation at the category level and must be coded from the Q65–Q74 range.

03If a patient has valgus knee deformity and osteoarthritis, which code takes priority?

Neither takes priority — assign both. Code the valgus deformity with the appropriate M21.06x code and the osteoarthritis separately with the applicable M17.x code. The deformity code captures alignment; the OA code captures the arthritic process. One does not subsume the other.

04Does M21.00 require a 7th character?

No. M21.00 is a 5-character code and is complete as written. Seventh-character extensions (A/D/S) apply to injury S-codes, not to M-category chronic musculoskeletal condition codes.

05Can M21.00 be used for hallux valgus (bunion)?

No. Hallux valgus is an acquired deformity of the toe and routes to M20.1x (Hallux valgus), not M21.0x. Acquired deformities of the fingers and toes are covered under M20.- which is an Excludes2 condition relative to M21.

06Is M21.00 valid for DME or orthotic billing (e.g., custom AFO or knee brace)?

M21.00 can appear on DME claims as a supporting diagnosis, but payers may reject unspecified codes as insufficient to establish medical necessity. Use a site-specific M21.0x code whenever the documentation supports it to reduce denial risk.

07How does M21.00 differ from its parent code M21.0?

M21.0 is the non-billable parent subcategory header for all acquired valgus deformities. M21.00 is the billable child code representing the unspecified-site version. You must select M21.00 (or another 5-character child) to submit a claim — M21.0 alone is not billable.

Sources & references

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

Mira AI Scribe

Mira's AI scribe captures the affected joint and side from the provider's dictation, imaging references (X-ray angulation measurements, weight-bearing films), and any notation of acquired versus congenital origin. That prevents the encounter from landing on unspecified M21.00 when a site-specific code like M21.061 (right knee) or M21.051 (right hip) is supportable — and it keeps dual-diagnosis scenarios (valgus + OA) fully coded rather than collapsed into a single unspecified code.

See how Mira captures M21.00 documentation