Baker's cyst (synovial cyst of the popliteal space) specifically involving the right knee — a fluid-filled sac arising from the semimembranous bursa or the knee joint capsule, located in the posterior popliteal fossa.

Documentation tips

What should appear in the chart to support M71.21.

Source · Editorial brief grounded in 6 cited references ↓

• Explicitly document 'right knee' or 'right popliteal space' — do not rely on laterality inferred from imaging alone; the note must state it.
• Record the imaging modality that confirmed the cyst (ultrasound, MRI), including size if available, to support medical necessity for intervention.
• If an underlying intra-articular condition (meniscal tear, OA, inflammatory arthritis) drives the cyst formation, document and code that condition separately.
• For aspiration or injection encounters, note whether ultrasound guidance was used — this determines whether 76942 is additionally billable alongside the procedure code.
• If the cyst has ruptured, document rupture explicitly so the coder can move to M66.0 rather than M71.21.

Related CPT procedures

Procedure codes commonly billed with M71.21. 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 M71.21 and adjacent codes.

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

• Coding M71.21 for a ruptured Baker's cyst: M66.0 is the correct code for rupture of the synovial membrane in the popliteal space; M71.2x is a Type 1 Excludes for M66.0.
• Using the unspecified code M71.20 when the operative report, imaging report, or provider note clearly states 'right' — always capture documented laterality.
• Assigning M67.4 (ganglion) instead of M71.21 for a popliteal synovial cyst: the Tabular excludes cyst of bursa/synovium from M67.4, making M71.21 the correct code.
• Omitting the underlying joint pathology (e.g., medial meniscus tear, knee OA) that is causally related to the cyst — these are separate billable diagnoses and support medical necessity.

Clinical context

Source · Editorial summary grounded in 6 cited references ↓

Sibling codes

Other billable codes under M71.2 (laterality / anatomic variants).