Disclosures: Annika Eveland, MD: No financial relationships or conflicts of interest

Case Diagnosis: A 75-year-old female with remote history of right knee replacement who developed acute right foot drop following right ankle inversion injury.

Case Description or Program Description: Patient reported a right ankle inversion injury while walking in flip-flops after which she developed pain in the right lateral calf with numbness and right foot drop. Electromyography and nerve conduction studies revealed absent right peroneal motor conduction for deep and superficial branches and acute denervation with no recruitment of motor units in peroneal-innervated muscles distal to the knee. MRI of the right lower leg revealed an expansile mass centered within the fibular head with endosteal scalloping and extraosseous extension. The patient was seen by Orthopedic Surgery and was taken to the operating room where she was found to have an intraosseous cyst with an extraosseous component causing compression of the common peroneal nerve. Incision of the cyst revealed synovial joint-like fluid and a cortical window was made in the fibular head which revealed similar material consistent with intraosseous ganglion cyst. Pathology was negative for malignancy or infection.

Setting: Tertiary Care Hospital

Assessment/Results: The patient continued to have right foot drop 2 months post-operatively with no changes in sensation, strength or function. She was recommended to continue ankle-foot-orthosis and to work with Physical Therapy to retain range of motion of the ankle and foot.

Discussion (relevance): Intraosseous ganglion cysts are rare occurrences. This is the first reported case, to our knowledge, of an expansile intraosseous ganglion cyst leading to common peroneal nerve compression and foot drop in a patient with a prior history of knee replacement as well as acute ankle inversion injury.

Conclusions: Development of an intraosseous ganglion cyst in the fibular head with extraosseous extension may be a possible complication of prior knee replacement and its expansion may lead to compression of the common peroneal nerve through an acute ankle inversion injury.

Level of Evidence: Level V

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PM&R Meeting Abstracts - https://pmrjabstracts.org/abstract/expansile-intraosseous-ganglion-cyst-causing-common-peroneal-nerve-compression-with-foot-drop-following-ankle-inversion-injury-a-case-report/