M71.22 identifies a Baker's cyst (synovial cyst of the popliteal space) specifically located at the left knee — a fluid-filled sac that forms behind the left knee joint when excess synovial fluid accumulates in the popliteal fossa.

Documentation tips

What should appear in the chart to support M71.22.

Source · Editorial brief grounded in 5 cited references ↓

• Specify laterality by name — 'left knee' — in the assessment; don't rely on the body diagram or flow sheet alone to establish side.
• Record the imaging modality and key finding: e.g., 'MRI confirms a 3 cm fluid-filled cyst in the left popliteal fossa consistent with Baker's cyst.'
• Document any associated intra-articular pathology (OA, meniscal tear, inflammatory arthritis) so you can code the underlying cause alongside M71.22.
• Note whether the cyst is intact or ruptured — rupture changes the code to M66.0 and is a Type 1 Excludes conflict with M71.2.
• If aspiration or injection was performed, document the exact site (left popliteal bursa) and the substance injected to support CPT and HCPCS modifier selection.

Related CPT procedures

Procedure codes commonly billed with M71.22. 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 M71.22 and adjacent codes.

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

• Using M71.22 for a ruptured Baker's cyst — M66.0 is required for rupture; the Tabular List excludes ruptured popliteal synovial cysts from the M71.2 category.
• Defaulting to M71.20 (unspecified knee) when the note clearly documents 'left knee' — specificity is available and required when laterality is documented.
• Confusing a popliteal Baker's cyst with a ganglion cyst (M67.4) or other bursal cyst (M71.3x) — location in the popliteal space is the distinguishing feature for M71.22.
• Failing to code the underlying intra-articular condition (e.g., osteoarthritis, meniscal tear) as an additional diagnosis, which can reduce clinical justification for advanced imaging or procedural authorization.
• Billing M71.22 for bilateral Baker's cysts without adding M71.21 for the right side — bilateral presentations require both laterality-specific codes; there is no single bilateral code in the M71.2 subcategory.

Clinical context

Source · Editorial summary grounded in 5 cited references ↓

Sibling codes

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