M21.70

Acquired discrepancy in length between paired limbs (upper or lower) when the specific anatomical site cannot be or has not been documented.

Verified May 8, 2026 · 6 sources ↓

Status: Billable
Chapter: 13
Related CPT: 10
Region: Multi-region

Drawn from CDCICD10DataAAPCIcdcodes

Documentation tips

What should appear in the chart to support M21.70.

Source · Editorial brief grounded in 6 cited references ↓

Identify whether the discrepancy is acquired (post-traumatic, post-surgical, infection-related, tumor-related) or congenital — this single distinction determines whether an M21.7x or Q-code applies.
Name the specific bone or segment that is shorter (femur, tibia, fibula, humerus, radius, ulna); doing so unlocks a more specific child code and avoids leaving the claim at the unspecified M21.70 level.
Include imaging type and findings: scanogram or full-length standing radiograph with measured discrepancy in centimeters or millimeters, noting which limb is shorter.
Per the M21.7 tabular note, the coded site must correspond to the shorter limb — document which side and segment is deficient, not the longer one.
If LLD follows arthroplasty, document the surgical history and any intraoperative or postoperative measurement so the acquired etiology is unambiguous for audit purposes.

Related CPT procedures

Procedure codes commonly billed with M21.70. Linking the right diagnosis to the right procedure is what establishes medical necessity.

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

27715 $1,362.09

Surgical osteoplasty of both the tibia and fibula to correct leg length discrepancy, either by lengthening or shortening the bones.

27709 $1,051.13

Surgical cutting and realignment of both the tibia and fibula to correct lower leg deformity, malalignment, or leg-length discrepancy.

27465 $1,293.62

Surgical shortening of the femur by removing a bone segment and stabilizing the remaining ends — performed to correct leg length discrepancy or malunion.

27466 $1,369.44

Surgical lengthening of the femur (thigh bone) via osteoplasty, typically using osteodistraction to gradually separate bone segments and stimulate new bone formation across the gap.

27470 $1,085.20

Repair of femoral nonunion or malunion (distal to femoral head and neck) using compression or other technique, without bone graft.

27472 $1,156.01

Surgical repair of femoral nonunion or malunion (distal to the head and neck) using an autogenous iliac crest or other autograft, with graft harvest included in the code.

29581 $83.50

Application of a multilayer compression bandage system to the lower leg, including the ankle and foot.

73502 $48.77

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

73522 $54.44

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

73700 $130.26

CT scan of the lower extremity performed without contrast material, producing cross-sectional images of bones, soft tissue, and other structures without injected dye.

Common coding pitfalls

The recurring mistakes coders make with M21.70 and adjacent codes.

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

Using M21.70 when the provider actually names the affected bone — if the note says 'right tibia shorter,' M21.761 is required; M21.70 is not acceptable when site is documented.
Coding M21.70 for congenital limb length discrepancy — the Excludes1 note explicitly blocks use alongside congenital deformity codes; use Q72.8 or the appropriate Q-code instead.
Billing the parent code M21.7 — it is non-billable/non-specific; M21.70 is the minimum billable level when site is truly unspecified.
Assigning M21.70 when LLD is the sequela of a documented fracture malunion without also coding the underlying condition; coding guidelines may require sequencing the malunion or late effect code first.
Confusing 'unspecified site' with 'multiple sites' — M21.70 covers unspecified site, not bilateral or multi-segment involvement; if multiple segments are documented, code each affected site separately.

Clinical context

Source · Editorial summary grounded in 6 cited references ↓

M21.70 is the fallback code within the M21.7 family when the clinician documents acquired limb length discrepancy (LLD) but does not specify the affected bone or segment. The M21.7 tabular note requires the site to correspond to the shorter limb — so if the provider names the segment (femur, tibia/fibula, humerus, radius, ulna), a more specific child code must be used instead. M21.70 is valid only when site cannot be determined or is genuinely not documented.

In orthopedic practice, acquired LLD is commonly seen after fracture malunion, total hip or knee arthroplasty, physeal arrest, infection, or tumor resection. For lower-extremity discrepancy confirmed by scanogram or full-length standing radiograph, codes M21.761 (right lower leg) or M21.762 (left lower leg) capture more specificity and better support medical necessity for interventions such as shoe lifts, epiphysiodesis, or limb lengthening procedures.

Excludes1 notes bar M21.70 from use alongside codes for acquired absence of limb (Z89.-), congenital absence (Q71–Q73), or congenital deformities/malformations (Q65–Q66, Q68–Q74). If the discrepancy is congenital rather than acquired, Q72.8 or the appropriate Q-code applies. Do not use M21.70 for acquired deformities of fingers or toes (M20.-) or coxa plana (M91.2).

Frequently asked questions

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

01When is M21.70 the correct code versus a more specific M21.7x code?

Use M21.70 only when the provider's documentation genuinely does not identify the affected bone or segment. If the note names any specific structure — tibia, femur, humerus, radius, ulna — select the corresponding child code. M21.70 is a last resort, not a default.

02Can M21.70 be used for congenital leg length discrepancy?

No. The M21 category Excludes1 congenital deformities and absence of limbs. For congenital LLD, use Q72.8 (other congenital malformations of lower limb, including congenital leg length discrepancy) or the appropriate Q-code based on documentation.

03Which site does the M21.7 tabular note say to code — the shorter or the longer limb?

The shorter limb. The ICD-10-CM tabular note for M21.7 states: 'The site used should correspond to the shorter limb.' Document and code accordingly.

04What imaging is typically needed to support M21.70 or a related M21.7x code?

A scanogram (orthoroentgenogram) or full-length standing anteroposterior radiograph with measured discrepancy is the standard. Document the measured difference and identify which limb is shorter to support medical necessity for orthotics, epiphysiodesis, or lengthening procedures.

05Is M21.70 appropriate after total hip or knee arthroplasty causes limb length inequality?

Yes, if the discrepancy is documented as acquired post-surgically and the specific shorter segment is not named. However, if the shorter segment is identifiable (e.g., femur), use the specific M21.7x code. Also consider coding the arthroplasty status (Z96.6x) as an additional code if relevant to the encounter.

06What MS-DRG groupings does M21.70 fall under?

M21.70 groups to MS-DRG v43.0 564 (Other musculoskeletal system and connective tissue diagnoses with MCC), 565 (with CC), or 566 (without CC/MCC), depending on comorbidities and complications documented in the encounter.

07Can M21.70 be reported alongside fracture sequela codes?

It can, but sequencing matters. If LLD results from a fracture malunion, the malunion or its sequela code may need to be listed first depending on the reason for the encounter. Review ICD-10-CM sequencing guidelines for late effects and document the causal relationship explicitly.

Sources & references

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

Mira AI Scribe

The Mira AI Scribe captures the etiology of the discrepancy (fracture, surgery, infection, physeal arrest), the specific bone or segment that is shorter, the laterality, and imaging data — scanogram measurement in cm/mm, Kellgren-Lawrence grade if joint degeneration coexists, and any prior corrective treatment. Capturing that detail prevents default to M21.70 (unspecified site), which can trigger medical necessity queries and underpay for limb-lengthening or corrective surgical claims.

See how Mira captures M21.70 documentation