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popliteal-bakers-cyst-pathogenesis-and-clinical-findings

Popliteal (Baker’s) Cyst: Pathogenesis and clinical findings Authors: Liam Thompson, Megan Ure Reviewers: M. Patrick Pankow, Tara Shannon, Reza Ojaghi, Usama Malik, Dr. Gerhard Kiefer* * MD at time of publication Repeated contraction of muscles surrounding the bursaàmicro trauma to bursa Inflammatory response Idiopathic (common: kids 4-7 years old) Bursa located between medial head of gastrocnemius and semimembranosus tendon Accumulation of inflammatory fluid into the bursa forming a cyst Meniscal Tear Tear creates a channel between joint capsule and bursa posterior to the knee joint Rheumatoid Arthritis Auto-immune mediated inflammation of knee joint Osteoarthritis Destruction of articular cartilageà inflammation within knee joint See Osteoarthritis: Pathogenesis slide Damage to joint capsule and accumulation of inflammatory fluid Proteolytic enzyme dysregulation Local muscle action influences fluid flow and pressure changes Extra- articular Popliteal Cyst Fluid filled sac behind knee Usually asymptomatic (resolves spontaneously) Synovial joint herniates into popliteal fossa (most commonly medially)àaccumulation of fluid (synovial + inflammatory) into synovial herniation, forming a cyst Knee flexion Intra-articular Popliteal Cyst Known as “Baker’s cyst” Knee extension ↑ Muscle tension around cyst Channel between knee joint and cyst gradually closes with ↑ tensionàat full extension, no fluid flow between the joint and cyst (trapping the fluid in the cyst) ↓ Muscle tension around cyst Channel between knee joint and cyst opens Knee joint pressure becomes negative ↑ Fluid flow from cyst into knee joint ↓ Fluid in popliteal cyst Knee joint pressure becomes positiveà ↑ Fluid flow from knee joint into cyst (Posterior tibial nerve located lateral to the popliteal cyst and enervates posterior knee, calf, and bottom of foot) Posterior tibial nerve becomes entrapped by mass (cyst) Enlarging mass in posterior knee Cyst reaches maximum volume Pressure within cyst exceeds tensile strength of surrounding sac Fluid drains distally into calf Popliteal vein located lateral to cyst ↑ Venous pressure distal to cyst Popliteal vein occluded by mass Venous pooling distal to site of occlusion ↓ Space in posterior calf compartmentàcompression of calf muscles and posterior tibial nerve Ankle dorsiflexion ↑ pressure in compartment Positive Homan’s sign (discomfort with passive ankle dorsiflexion) Posterior plantar numbness Pain receptors (nociceptors) activated Posterior knee pain/pressure Cyst Rupture Abrupt and intense pain Stimulates inflammatory response Fluid pushed from veins into interstitial space Swelling and erythema (redness) in distal calf and foot Pseudo-Thrombophlebitis worse with extension or physical activity (shares symptoms with deep vein thrombosis but no associated clot) Legend: Pathophysiology Mechanism Sign/Symptom/Lab Finding Complications Published Feb 10, 2018, updated Nov 19, 2022 on www.thecalgaryguide.com