The accessory flexor digitorum longus as a cause of Flexor Hallucis Syndrome

The Flexor Hallucis Syndrome has been associated with ballet and sports activities. It has been thought to represent over use with attendant tenosynovitis of the tendon in the fibro-osseous tunnel extending from the ankle to the midfoot. We report a patient with an accessory flexor digitorum longus who presented with classical clinical features of the syndrome. The patient's disabling symptoms persisted despite a year and a half of nonoperative treatment. They were finally relieved when the accessory muscle was excised. In addition to the previously described clinical features of the Flexor Hallucis Syndrome, the physical exam included abnormal fullness between the Achilles tendon and the tibia. When the ankle was held in dorsiflexion, there was limited dorsiflexion of the lateral toes, which were tethered by the "cork-in-a-bottle" effect of the distal muscle mass of the accessory flexor digitorum longus muscle at the flexor retinaculum and fibro-osseous canal of the flexor hallucis longus. The MRI examination confirmed the presence of an abnormal muscle mass extending distal to the ankle joint with the foot in neutral. The study also demonstrated fluid in the ankle joint and fibro-osseous canal of the flexor hallucis, and marrow edema within the body of the talus consistent with chronic inflammation. Follow-up MRI six months after excision of the muscle revealed some scar formation at the site of the previously excised muscle and complete resolution of the joint effusion, fluid in the tunnel of the flexor hallucis, and marrow edema.