M66.0 classifies spontaneous rupture of a popliteal (Baker's) cyst — the dissection or decompression of a synovial fluid-filled cyst in the posterior knee space that occurs when normal force is applied to already-weakened tissue.
Verified May 8, 2026 · 4 sources ↓
- Status
- Billable
- Chapter
- 13
- Related CPT
- 5
- Region
- Knee
Documentation tips
What should appear in the chart to support M66.0.
Source · Editorial brief grounded in 4 cited references ↓
- State explicitly that the cyst has ruptured, not merely that it is present — 'ruptured popliteal cyst with fluid dissecting into the calf' supports M66.0; 'Baker's cyst' alone maps to M71.2x.
- Record the affected side (right, left, or bilateral) in the note; while M66.0 has no laterality subcode, documentation supports audit defense and clinical continuity.
- Include the imaging modality and findings (e.g., ultrasound or MRI confirming posterior knee cyst with fluid tracking distally into the gastrocnemius-soleus plane) to substantiate the rupture diagnosis.
- Document that DVT was considered or excluded if calf swelling is the presenting complaint — this supports clinical decision-making and justifies any associated diagnostic workup.
- Note the underlying joint pathology (e.g., knee OA, meniscal tear, rheumatoid arthritis) that predisposed to cyst formation; code those conditions as additional diagnoses to complete the clinical picture.
Related CPT procedures
Procedure codes commonly billed with M66.0. 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 M66.0 and adjacent codes.
Source · Editorial brief grounded in CDC ICD-10-CM tabular guidance, AAOS coding references, and cited references ↓
- Using M71.21 or M71.22 (intact Baker's cyst) when the cyst has ruptured — these codes carry a Type 1 Excludes note blocking their use with M66.0 for the same episode.
- Coding M66.0 for a cyst that is large or symptomatic but not confirmed ruptured — if rupture is not documented or confirmed by imaging, default to the appropriate M71.2x laterality code.
- Confusing M66.0 with M66.1x (rupture of synovium, other joints) — M66.1 carries a Type 2 Excludes specifically excluding rupture of popliteal cyst, which belongs only at M66.0.
- Failing to code the underlying knee condition (e.g., M17.11 primary OA right knee, M23.x meniscal tear) as an additional diagnosis, which leaves the clinical picture incomplete and may trigger medical necessity questions.
Clinical context
Source · Editorial summary grounded in 4 cited references ↓
Use M66.0 when the popliteal cyst has actually ruptured — fluid has dissected into the calf — not simply when a Baker's cyst is present and intact. An intact or unruptured Baker's cyst codes to M71.21 (right) or M71.22 (left). The ICD-10-CM Tabular List places a Type 1 Excludes note at M71.2 pointing to M66.0 precisely to enforce this distinction: you cannot use M71.2x and M66.0 together for the same cyst episode.
M66.0 carries no laterality subcode — it is the only billable code in the M66.0 slot and does not extend to a 5th or 6th character for right/left/bilateral. Approximate synonyms in the index do include rupture of right, left, and bilateral popliteal cysts, so laterality should still be documented in the note even though the code itself is not laterality-differentiated.
Clinically, a ruptured popliteal cyst can mimic deep vein thrombosis (calf swelling, tenderness, erythema). If DVT has been ruled out and imaging confirms cyst rupture — typically via ultrasound or MRI — M66.0 is the appropriate primary diagnosis. The condition sits under category M66 (Spontaneous rupture of synovium and tendon), which covers ruptures that occur when normal forces act on tissue with less-than-normal structural integrity. It groups to MS-DRG 557/558 (Tendonitis, myositis and bursitis with/without MCC).
Sibling codes
Other billable codes under M66 (laterality / anatomic variants).
Frequently asked questions
Source · Generated from the editorial pipeline, verified against 4 cited references ↓
01Does M66.0 have a right/left/bilateral breakdown?
02Can I use M66.0 and M71.21 or M71.22 together for the same knee?
03What is the difference between M66.0 and M66.1x?
04Should I code the underlying knee condition separately?
05Which MS-DRGs does M66.0 group to?
06Is M66.0 appropriate when a popliteal cyst rupture is suspected but not yet confirmed by imaging?
07Does CMS recognize M66.0 as supporting medical necessity for any specific procedures?
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)
- 02icd10data.comhttps://www.icd10data.com/ICD10CM/Codes/M00-M99/M65-M67/M66-/M66.0
- 03aapc.comhttps://www.aapc.com/codes/icd-10-codes/M66.0
- 04cms.govhttps://www.cms.gov/medicare-coverage-database/view/article.aspx?articleId=56766&ver=24
Mira AI Scribe
Mira AI Scribe captures the posterior knee cyst history, confirmation of rupture (ultrasound or MRI findings showing fluid dissection into the calf), laterality, and any underlying joint pathology driving cyst formation. This prevents the encounter from defaulting to the unruptured Baker's cyst codes (M71.21/M71.22), which are excluded when rupture is confirmed, and avoids a medical necessity gap when DVT workup is also documented.
See how Mira captures M66.0 documentation