Summary Proximal tibiofibular joint ganglion cyst is a rare, mucin-filled synovial cyst. They typically are seen in adults and present with lateral knee pain, fullness, and neurological symptoms due to compression of the common peroneal nerve. Diagnosis is often initially made based on typical MRI findings and is confirmed histologically. Nonoperative treatment is indicated for asymptomatic lesions. Operative treatment is indicated for persistent symptoms or associated neurological deficits. Epidemiology Incidence <1% of patients undergoing MRI for knee pain Demographics age range: 20-76 years old median age: 67 years old male (90%) Location more commonly found in right leg (~70%) extraneural (80%) > intraneural (20%) Etiology Pathophysiology mechanism of injury repetitive microtrauma degenerative pathoanatomy mucin-filled synovial cell lined sac without a true epithelial lining cell biology walls contain sheets of collagen fibers mucinous material is highly viscous due to high concentration of hyaluronic acid and mucopolysaccharides Associated conditions osteoarthritis Anatomy Osteology fibular head sits in groove behind lateral tibial ridge limits anterior fibular movement with knee flexion Arthrology proximal tibiofibular joint articulation of the lateral tibial plateau of the tibia and fibular head Ligament PTFJ capsule stabilizers anterior and posterior tibiofibular ligaments lateral collateral ligament popliteus biceps femoris tendons Muscles long head of biceps femoris insertion login to view 1 more bullet innervation login to view 1 more bullet short head of biceps femoris insertion login to view 1 more bullet innervation login to view 1 more bullet Blood Supply anterior tibial artery passes just distal to PTFJ Nervous System common peroneal nerve courses laterally around the fibular neck two branches login to view 2 more bullets Biomechanics PTFJ anterolateral and posteromedial sliding movement of PTFJ login to view 2 more bullets Classification Location of tumor intraneural within epineurium extraneural outside of epineurium Presentation Symptoms usually asymptomatic if symptomatic pain palpable mass fullness paresthesias Physical exam inspection mass near lateral aspect of knee palpation freely mobile, circumscribed palpable mass neurological exam foot drop due to common peroneal nerve compression Imaging Radiographs recommended views AP and lateral of knee and tibia findings typically unremarkable may see sclerotic lesions on lateral aspect of proximal tibia MRI indications persistent symptoms neurological deficits findings well-circumscribed cystic homogenous structure low signal on T1 high signal on T2 Ultrasound indications unable to undergo an MRI findings fluid filled cyst with thin wall Studies Labs typically unremarkable Invasive studies Histology gross anatomy login to view 2 more bullets microscopic analysis login to view 2 more bullets EMG and NCS adjunct to MRI to determine the extent of sensory and motor dysfunction Differential Differential diagnosis Aneurysmal bone cyst Chondroblastoma Giant-cell tumor Juxta-articular myxomas Lumbar disc herniation Pigmented villonodular synovitis Solid nerve tumors Diagnosis Diagnostic criteria MRI is diagnostic tool of choice if typical features and enhancement patterns are present Histological analysis can be used to confirm diagnosis Treatment Nonoperative observation, NSAIDS, aspiration indications login to view 2 more bullets technique observation NSAIDs aspiration login to view 1 more bullet outcomes may spontaneously resolve but high risk of recurrence Operative surgical decompression and marginal excision indications login to view 2 more bullets outcomes login to view 1 more bullet proximal tibiofibular joint arthrodesis (PTFJ fusion) indications login to view 1 more bullet techniques login to view 1 more bullet outcomes login to view 1 more bullet Techniques Nonoperative technique symptomatic treatment aspiration to decompress complications high rate of recurrence with aspiration alone (80%) Decompression and Marginal Excision technique hockey-stick lateral approach dissect cyst from common peroneal nerve and perform neurolysis excise stalk from joint additional options login to view 2 more bullets complications recurrence (8-25%) neuropraxia perineural fibrosis PTFJ arthrodesis indications failed nonoperative management recurrent cyst technique hockey stick lateral approach removal of PTFJ cartilage arthrodesis with 6.5mm partially threaded cancellous screw complications recurrence (<5%) Complications Peroneal nerve dysfunction indicence up to 50% risk factors intraneural cyst treatment surgical decompression login to view 1 more bullet Recurrence incidence 5-80% risk factors treatment with aspiration only (80% recurrence) treatment observation login to view 1 more bullet arthrodesis login to view 1 more bullet Hardware irritation risk factor arthrodesis treatment hardware removal Prognosis Natural history of disease / Prognosis without treatment may resolve but higher likelihood of recurrence Prognostic variable negative prolonged duration of neurologic symptoms Survival with treatment good outcomes with low rate of recurrence