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 posterior to short head on fibular head innervation tibial nerve short head of biceps femoris insertion anterior to long head on fibular head innervation common peroneal nerve Blood Supply anterior tibial artery passes just distal to PTFJ Nervous System common peroneal nerve courses laterally around the fibular neck two branches superficial peroneal nerve deep peroneal nerve Biomechanics PTFJ anterolateral and posteromedial sliding movement of PTFJ reduces torsional forces from the ankle disperses axial load while standing 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 cystic structure with thin wall gelatinous material microscopic analysis absence of true epithelial lining no nuclear atypia or mitotic activity 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 asymptomatic initial management for symptomatic lesions technique observation NSAIDs aspiration some studies show aspiration is ineffective due to risk of recurrance outcomes may spontaneously resolve but high risk of recurrence Operative surgical decompression and marginal excision indications failed nonoperative treatment progressive neurologic symptoms outcomes generally good outcomes with lower rates of recurrence (8-25%) proximal tibiofibular joint arthrodesis (PTFJ fusion) indications cyst recurrence after the first resection techniques 6.5mm partially threaded cancellous screw outcomes lowest rate of recurrence (<5%) 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 proximal tibiofibular joint arthrodesis fibular head resection 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 consider tendon transfer if complete nerve palsy Recurrence incidence 5-80% risk factors treatment with aspiration only (80% recurrence) treatment observation asymptomatic arthrodesis symptomatic 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