Morton’s plantar neuralgia

Morton's plantar neuralgia is a historical and descriptive term for a condition of severe, throbbing nerve pain in the forefoot. It is now more commonly known as Morton's neuroma. The term specifically describes the neuralgia (pain arising from a nerve) that occurs when a nerve in the sole (plantar surface) of the foot becomes compressed and irritated.

The condition is named for Dr. Thomas G. Morton, an American surgeon who published an influential paper on the condition in 1876.

Anatomical Basis

The pain of Morton's neuralgia originates from the compression of a common plantar digital nerve, which provides sensation to the toes. While any of these nerves can be affected, the condition most frequently occurs in the third intermetatarsal space, affecting the nerve that supplies the area between the third and fourth toes.

This particular nerve is anatomically unique and more susceptible to compression for two reasons:

Anastomotic Branch: It is formed by an anastomotic (communicating) branch that connects the medial and lateral plantar nerves. This confluence makes the nerve naturally thicker at this point compared to the other interdigital nerves.
Mechanical Entrapment: The nerve runs in a narrow channel beneath the deep transverse metatarsal ligament and is positioned between the heads of the metatarsal bones. During walking, especially with improper foot mechanics, this thickened nerve can become pinched between the bony structures, leading to chronic irritation.

This repetitive mechanical stress causes the nerve and its surrounding tissues to thicken and develop fibrous scarring, a condition known as perineural fibrosis. This thickening further exacerbates the compression, creating a vicious cycle of irritation and pain.

Symptoms

The term "neuralgia" aptly describes the primary symptoms, which are directly related to nerve irritation:

Severe, throbbing, or burning pain in the ball of the foot, often described as feeling like walking on a marble or a crumpled sock.
Pain that can radiate from the ball of the foot into the adjacent toes.
Sharp, shooting sensations, tingling (paresthesia), or numbness in the affected toes.
Symptoms are typically worsened by activity, particularly walking or running, and by wearing tight, narrow, or high-heeled footwear.
Relief is often found by stopping the activity, removing the shoe, and massaging the forefoot.

Causes and Risk Factors

The underlying cause of the neuralgia is the mechanical compression of the nerve. Factors that contribute to this include:

Footwear: Shoes with a narrow toe box or high heels force the metatarsal bones together, compressing the nerve.
Foot Biomechanics: Structural issues such as bunions, hammertoes, flatfeet, or high arches can alter the mechanics of the foot and increase pressure on the nerve.
Repetitive Stress: High-impact sports and occupations that involve prolonged standing can cause repetitive trauma to the forefoot.

Diagnosis and Treatment

Diagnosis is primarily based on the patient's history and a physical examination. A key diagnostic sign is Mulder's sign, where the examiner squeezes the metatarsal heads together while applying pressure to the sole of the foot; a painful click can indicate the presence of the neuroma. Imaging tests like ultrasound or MRI may be used to confirm the thickened nerve and rule out other conditions.

Treatment focuses on relieving the pressure on the nerve:

Conservative Management: The first line of treatment involves switching to footwear with a wide toe box and low heels. Orthotic devices, such as a metatarsal pad placed just behind the ball of the foot, can help spread the metatarsal bones and reduce compression.
Injections: Corticosteroid injections can reduce local inflammation and swelling, providing temporary or sometimes long-term relief.
Surgery: If conservative measures fail, surgery may be recommended. This can involve either decompressing the nerve by cutting the overlying ligament or removing the affected segment of the nerve (neurectomy).