ICD-10 Diagnosis Code for Internal Derangement of the Knee
The diagnosis code for internal derangement of the knee is M23.9 (Internal derangement of knee, unspecified) when the specific structure involved is not documented, or more specific codes from the M23 series when the exact pathology is identified.
Understanding Internal Derangement of the Knee
Internal derangement refers to disruption of the normal anatomical relationships within the knee joint, most commonly involving:
- Meniscal tears (medial or lateral) 1
- Cruciate ligament injuries (ACL or PCL) 1
- Collateral ligament tears 1
- Loose bodies within the joint 1
- Articular cartilage damage 1, 2
The majority (93.5%) of patients presenting with acute knee injuries have soft-tissue injuries rather than osseous injuries, making internal derangement a common diagnosis 1, 3.
Specific ICD-10 Coding Structure
The M23 code series provides specificity based on the anatomical structure involved:
- M23.2-: Derangement of meniscus due to old tear or injury
- M23.3-: Other meniscus derangements
- M23.5-: Chronic instability of knee
- M23.6-: Other spontaneous disruption of ligament(s) of knee
- M23.8-: Other internal derangements of knee (includes loose bodies) 1
- M23.9: Internal derangement of knee, unspecified
Each code requires additional characters to specify laterality (right, left, or bilateral) and in some cases, the specific meniscus or ligament involved.
Diagnostic Pathway to Determine Specific Code
Initial Clinical Assessment
Key clinical findings that indicate internal derangement include: 1
- Knee pain, swelling, locking, catching, popping, or giving way
- Joint effusion on examination
- Loss of motion or crepitus
- Positive McMurray test or Apley grinding test for meniscal tears 2
- Positive drawer test for cruciate ligament injury 2
Imaging to Confirm Diagnosis
Radiographs should be obtained first (AP, lateral, sunrise/Merchant, and tunnel views) to evaluate for fractures and underlying joint pathology 1.
MRI without IV contrast is the definitive diagnostic modality when internal derangement is suspected, with 92-96% sensitivity and 96-100% specificity for detecting meniscal tears 1, 3, 2. MRI accurately identifies:
- Meniscal tears with high diagnostic accuracy (92.6-95.1%) 2
- Cruciate ligament injuries 1, 2
- Articular cartilage damage 2, 4
- Bone marrow contusions 1
- Associated pathology including synovial thickening 2
Clinical examination alone has low diagnostic benefit in acute knee trauma, with higher-than-suspected incidence of injuries found on MRI 1, 3.
Common Coding Pitfalls to Avoid
Do not use M23.9 (unspecified) when specific pathology is documented. If MRI or arthroscopy identifies a specific meniscal tear, ligament injury, or loose body, use the appropriate specific code from the M23 series 2, 4.
Do not delay MRI when indicated. In patients <40 years old with acute knee injury and knee effusion >10 mm on lateral radiograph, immediate MRI consideration can decrease delayed diagnosis and improve outcomes 1, 3.
Distinguish internal derangement from other knee pathology:
- Isolated patellofemoral degeneration has different associations (maltracking, synovial shelves) and may require different coding 5
- Post-surgical complications (e.g., loose methylmethacrylate fragments) require complication codes, not simple internal derangement codes 6
Ultrasound has limited utility for definitive diagnosis of internal derangement, though it can detect knee effusion with 79.1% sensitivity, and presence of effusion has 90.9% positive predictive value for internal derangement 7. However, MRI remains the gold standard for characterizing specific pathology 2, 4.
Documentation Requirements for Specific Coding
To assign the most accurate code, documentation should specify:
- Laterality (right, left, bilateral)
- Specific structure involved (medial meniscus, lateral meniscus, ACL, PCL, etc.)
- Nature of injury (tear, rupture, degeneration)
- Acuity (acute traumatic vs. chronic degenerative) 2, 5
When arthroscopy is performed, it serves as the gold standard for confirming internal derangement and should be used to refine the diagnosis code 1, 2.