ICD-10-CM · Knee

M23.40

M23.40 identifies a loose body within the knee joint when the operative or affected side is not documented or cannot be determined.

Verified May 8, 2026 · 4 sources ↓

Status: Billable
Chapter: 13
Related CPT: 8
Region: Knee

Drawn from CDCICD10DataAAPCIcdcodes

Documentation tips

What should appear in the chart to support M23.40.

Source · Editorial brief grounded in 4 cited references ↓

Record the specific knee affected (right or left) in every encounter note — a single laterality statement unlocks M23.41 or M23.42 and avoids the unspecified flag on M23.40.
Document imaging findings that confirm the loose body: radiograph showing calcified fragment, MRI identifying an osteochondral defect with detached fragment, or arthroscopic visualization and description.
Note the mechanical symptoms driving the diagnosis — locking, catching, or giving way episodes — to support medical necessity for surgical intervention.
If the loose body is secondary to a known underlying condition (e.g., primary OA, synovial osteochondromatosis), list that condition as an additional diagnosis code to provide complete clinical picture.
When loose bodies are bilateral, code each knee separately with M23.41 and M23.42; do not default to M23.40 as a bilateral shortcut — no bilateral code exists in M23.4.

Related CPT procedures

Procedure codes commonly billed with M23.40. Linking the right diagnosis to the right procedure is what establishes medical necessity.

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

29874 $506.02

Arthroscopic knee surgery performed specifically to locate and remove loose or foreign bodies from within the joint space.

29877 $586.85

Knee arthroscopy with surgical debridement or shaving of articular cartilage (chondroplasty) — does not include meniscal work.

29881 $515.71

Knee arthroscopy with surgical removal of the medial or lateral meniscus, including any associated cartilage shaving or debridement performed in the same or a separate compartment.

27331 $459.60

Open arthrotomy of the knee joint for exploration, biopsy, or removal of loose or foreign bodies.

73564 $49.43

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

73560 $34.40

Radiologic examination of the knee joint, one or two views, unilateral.

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.

27570 $149.97

Manipulation of the knee joint performed under general anesthesia, including application of traction or other fixation devices as needed to restore range of motion.

Common coding pitfalls

The recurring mistakes coders make with M23.40 and adjacent codes.

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

Using M23.40 as a default bilateral code: no bilateral option exists under M23.4; bilateral presentations require M23.41 and M23.42 reported together.
Coding M23.40 alongside M93.2 (osteochondritis dissecans): M93.2 is an Excludes1 condition — when OCD is the confirmed source of the loose body, M93.2 takes precedence and M23.40 is excluded.
Assigning M23.40 for an acute traumatic intra-articular fragment in the same encounter as the injury: use the appropriate S80–S89 injury code instead, per the Excludes2 note on M23.
Failing to upgrade to a laterality-specific code (M23.41 or M23.42) when the operative report or imaging report clearly documents the side — leaving money and specificity on the table.

Clinical context

Source · Editorial summary grounded in 4 cited references ↓

M23.40 applies when intra-articular loose bodies are confirmed in the knee but laterality is absent from the documentation. Loose bodies are free-floating osseous, cartilaginous, or osteocartilaginous fragments that develop from conditions such as synovial osteochondromatosis, osteochondral fracture, degenerative joint disease, or prior trauma. They cause mechanical symptoms — locking, catching, giving way — and are typically identified on plain radiographs, MRI, or arthroscopic inspection.

Always attempt to assign M23.41 (right knee) or M23.42 (left knee) before defaulting to M23.40. Payers increasingly flag unspecified laterality codes during pre-authorization and post-payment audits; M23.40 should only land on a claim when the provider's documentation genuinely fails to indicate a side — not as a convenience code.

M23.40 falls under the M23 category for internal derangement of the knee. Critical Excludes1 notes mean you cannot code M23.40 alongside M24.66 (ankylosis of knee), M21.- (deformity of knee), or M93.2 (osteochondritis dissecans) — each of those conditions requires its own code family. Excludes2 notes mean that current acute injuries of the knee (S80–S89 range) should be coded with those S-codes instead when the loose body is attributable to a same-encounter injury.

Sibling codes

Other billable codes under M23.4 (laterality / anatomic variants).

M23.41

Loose body in knee, right knee

M23.42

Loose body in knee, left knee

Frequently asked questions

Source · Generated from the editorial pipeline, verified against 4 cited references ↓

01When is M23.40 appropriate instead of M23.41 or M23.42?

Use M23.40 only when the documentation genuinely omits laterality — for example, a referral note from an outside provider that specifies a loose body in 'the knee' without naming a side. If the chart, imaging report, or operative note names right or left, assign M23.41 or M23.42 accordingly.

02Can M23.40 be used for bilateral loose bodies?

No. There is no single bilateral code under M23.4. When both knees are affected, report M23.41 for the right knee and M23.42 for the left knee as separate codes on the same claim.

03What CPT codes pair most often with M23.40 in an orthopedic setting?

Arthroscopic loose body removal maps to CPT 29874. If chondroplasty is performed at the same encounter, 29877 may apply. Open removal codes such as 27331 apply when arthroscopic access is not used. Imaging CPT codes 73564 (knee X-ray, 4+ views) and 73562 (knee X-ray, 3 views) support diagnosis workup.

04Is M23.40 ever coded alongside M93.2 for osteochondritis dissecans?

No. M93.2 is listed as an Excludes1 condition under M23, meaning these codes must never be reported together. When OCD is confirmed, use M93.2; M23.40 is excluded by definition.

05What if the loose body resulted from an acute injury at the same visit?

Acute traumatic knee injuries are covered by the S80–S89 range, which is listed as an Excludes2 note under M23. When a loose body is directly attributable to a current injury being treated at the same encounter, code the injury from S80–S89 rather than M23.40.

06Does M23.40 require a 7th character extension?

No. M-codes in the musculoskeletal chapter do not use 7th-character extensions. The A/D/S encounter extensions apply to injury S-codes, not to M23.40.

07Can M23.40 be reported with a deformity of knee code such as M21.-?

No. Deformity of knee (M21.-) is an Excludes1 condition under M23, so M23.40 and M21.- codes cannot appear on the same claim for the same joint at the same encounter.

Sources & references

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

01CDC ICD-10-CM Tabular List 2026 (effective Oct 1, 2025)
02
icd10data.com
https://www.icd10data.com/ICD10CM/Codes/M00-M99/M20-M25/M23-/M23.40
03
aapc.com
https://www.aapc.com/codes/icd-10-codes/M23.40
04
icdcodes.ai
https://icdcodes.ai/icd10/M23.40

Mira AI Scribe

Mira captures the affected knee side, mechanical symptoms (locking, catching, giving way), and imaging or arthroscopic confirmation of the loose body from the provider's encounter note. That specificity drives assignment to M23.41 or M23.42 instead of the unspecified M23.40, preventing laterality-based audit flags and supporting pre-authorization for arthroscopic loose body removal.

See how Mira captures M23.40 documentation