A Closer Look At Endoscopic Nerve Decompression For Morton’s ‘Neuroma’
The interdigital neuroma is a common source of forefoot pain that podiatric foot and ankle surgeons see in their practices every day. The characteristic symptoms have been well known since Morton first described them in 1876.1 Even though Morton believed the problem was located at the fourth metatarsophalangeal articulation, his name has been associated with the pathology for over a century. Peripheral nerve surgeons have chosen to avoid the term “neuroma,” opting instead for “interdigital neuritis,” which better describes the problem.
The plantar digital nerves are the terminal branches of the medial and lateral plantar nerves. The medial plantar nerve has four digital branches. The most medial branch represents the proper digital nerve to the medial aspect of the great toe. The next three branches represent the first, second and third common digital nerves. These three nerves are distributed to the medial and lateral aspects of the first, second and third interspaces respectively.
The superficial branch of the lateral plantar nerve has two digital branches. The more lateral branch represents the proper digital nerve to the lateral aspect of the fifth toe and the other branch represents the fourth common digital nerve for the fourth interspace.
Along their distal course, the interdigital nerves are accompanied by the digital arteries and veins as well as the lumbrical tendons. At the level of the lesser metatarsal heads, the neurovascular bundle and the lumbrical tendons enter the “metatarsal tunnel,” which is formed by the lesser metatarsophalangeal joint (MPJ) capsules, the deep transverse intermetatarsal ligament, and the transverse fibers of the plantar aponeurosis.2 The neurovascular bundle and the lumbrical tendons pass beneath the deep transverse intermetatarsal ligament. Just distal to the deep transverse intermetatarsal ligament, the interdigital nerves branch to the adjacent toes to provide sensation.
Gauthier initially supported the theory that the interdigital neuroma is a form of entrapment neuropathy.3 He meticulously described the mechanism of the interdigital nerve entrapment as it passes under the deep transverse intermetatarsal ligament. During the last stages of the stance phase of the gait cycle, the loads transmit to the metatarsal heads and the toes dorsiflex. The interdigital nerve subsequently compresses between the plantar soft tissues and the unyielding, anterior edge of the transverse metatarsal ligament. Fashionable shoes that are tight in the forefoot area and have increased heel height further aggravate the compression.
The diagnosis is typically a clinical one with pain upon palpation of the involved interspace with or without side-to-side pressure applied to the metatarsal heads. Sometimes, the physician will note a palpable click. Patients complain of burning pain that often radiates to the toes and occasionally to the calf. Some patients describe the classic “rolled up sock” sensation in the forefoot and many patients report that removing their shoes and rubbing the forefoot helps to relieve the pain.