• Herniation of synovium through joint capsule
• Usually present in joint of extremeties
• In spine, referred to as spinal cyst
• Called Baker cyst if in popliteal space
• May bleed and become a mass-like lesion

Baker's cyst

• Baker's cyst: palisading histiocytes and fibrinoid necrosis, similar to rheumatoid pericarditis and rheumatoid nodules

• Soft tissue mass located extradurally along medial border of degenerated facet joint
• Usually at L4-5, rarely in cervical or thoracic spine
• Filled with clear or xanthochromic fluid
• Causes back and radicular pain
• Rarely results in cauda equina syndrome (Pain Physician 2012;15:435)

Cystic mass in L2-L3 facet joint

• 14 year old girl with synovial cyst of thoracic spine (Arq Neuropsiquiatr 2007;65:838)
• 63 year old woman with intradural invasion of lumbar synovial cyst (Neurol Med Chir (Tokyo) 2012;52:234)
• 71 year man with synovial cyst of thoracic spine (BMJ Case Rep 2009;2009 Epub 2009 Feb 16)
• 72 year old woman with hemorrhagic lumbar synovial cyst (J Korean Neurosurg Soc 2012;52:567)
• 84 year old man with cervical brachyalgia (Eur Spine J 2010;19:S100)

Extradural cystic lesion

Hemorrhagic synovial cyst

Thin fibrous wall

• Lined by metaplastic synovial tissue
• May be due to local trauma

• 51 year old woman with cutaneous metaplastic synovial cyst of first metatarsal head area.(Ann Dermatol 2011;23:S165)

• Surgical excision; recurrence is rare

Solitary, well-demarcated 3 cm mass

Cystic lesion filled with gelatinous material

• Cystic structure with papillary projections lined by epithelium resembling synovium

Numerous villous-like structures

Villous structures lined by membrane of varying cellularity

Vimentin+ CD34- CD68-

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