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Foucher sign

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Foucher sign clinical examination

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Foucher sign Baker cyst mechanism

I now have a clear, well-sourced picture. My initial answer was incorrect - the Foucher sign is associated with Baker's cyst, not Dupuytren's contracture. Here is the corrected, full answer:

Foucher Sign

Foucher's sign is a clinical examination finding specific to a Baker's cyst (popliteal cyst). It describes the change in consistency and size of a popliteal cyst with changes in knee position.

What It Is

  • In full knee extension: the Baker's cyst becomes tense, firm, and prominent - it may also be tender.
  • With knee flexion to ~45 degrees: the cyst softens or disappears entirely.
This positional change in the cyst is Foucher's sign.

Mechanism

The mechanism was formally investigated by Canoso et al. (1987, Ann Rheum Dis; PMID 3579388) using echography, arthrography, and CT:
  • During knee extension, the medial gastrocnemius and semimembranosus muscles approximate one another and compress the cyst between themselves and the joint capsule, increasing intracystic pressure and making it tense.
  • During knee flexion, these muscles separate, relieving pressure and allowing the cyst to decompress and soften.

How to Elicit It

Ask the patient to stand with the knee in full extension - the mass is most visible and palpable in this position. Then passively or actively flex the knee to 45 degrees and note the softening or disappearance of the mass.

Clinical Utility

Foucher's sign is useful for differentiating a Baker's cyst from other popliteal masses such as:
  • Popliteal artery aneurysm
  • Adventitial (popliteal artery) cysts
  • Ganglia
  • Soft tissue sarcomas
These other lesions are unaffected by knee position, so a clear Foucher's sign points specifically toward a Baker's cyst.

Context: Baker's Cyst

Baker's cysts arise from distension of the gastrocnemius-semimembranosus bursa, almost always in association with an intra-articular knee disorder (osteoarthritis, meniscal tear, rheumatoid arthritis, ACL tears account for >90% of cases). The bursa communicates with the joint via a one-way valvular mechanism, allowing fluid to accumulate but not easily return.
Correction to my initial answer: I incorrectly stated this was related to Dupuytren's contracture. Foucher's sign belongs entirely to the Baker's cyst literature. I apologize for the error.
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