M23.90 classifies internal derangement of the knee when neither the specific type of derangement nor the laterality (right or left) is documented in the medical record.

Documentation tips

What should appear in the chart to support M23.90.

Source · Editorial brief grounded in 6 cited references ↓

• Document the specific knee (right or left) by name at every encounter — 'left knee' in the assessment moves you immediately to M23.92 and avoids the unspecified designation.
• Record the working or confirmed derangement type (e.g., meniscal tear, chronic instability, loose body) so you can assign a more specific M23 subcategory rather than defaulting to M23.90.
• Link MRI or X-ray findings explicitly to the clinical impression — note joint space, meniscal signal, ligament integrity, or loose body presence so the derangement type is supportable on audit.
• If imaging is pending, document that laterality and derangement type are not yet confirmed; this justifies M23.90 as a temporary code and shows audit-conscious intent to update the diagnosis.
• For any subsequent encounter after imaging results are available, update the diagnosis code — leaving M23.90 on a claim for a procedure like arthroscopy is a common audit trigger.

Related CPT procedures

Procedure codes commonly billed with M23.90. Linking the right diagnosis to the right procedure is what establishes medical necessity.

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

Common coding pitfalls

The recurring mistakes coders make with M23.90 and adjacent codes.

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

• Using M23.90 on the procedure claim after MRI or arthroscopy has already confirmed laterality and derangement type — at that point a specific code is required and M23.90 will not support medical necessity.
• Confusing M23.90 (unspecified derangement, unspecified knee) with M23.91 (unspecified derangement, right knee) or M23.92 (unspecified derangement, left knee) — check the chart for any laterality documentation before defaulting to .90.
• Defaulting to M23.90 when the note says 'internal derangement' without specifying type, even though the same note documents which knee was examined — that's a laterality capture failure, not a true unspecified scenario.
• Pairing M23.90 with a laterality-specific CPT code (e.g., 29881 for right knee) — the diagnosis and procedure laterality mismatch will trigger an edit or denial.
• Failing to recode at the surgical encounter when the operative report documents the specific derangement found; leaving M23.90 on the surgical claim understates specificity and risks downcoded reimbursement or audit findings.

Clinical context

Source · Editorial summary grounded in 6 cited references ↓

Sibling codes

Other billable codes under M23.9 (laterality / anatomic variants).