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Introduction

Tingling, burning pain that shoots to the tips of two adjacent toes is a classic indication of a neuroma. Neuromas are a common foot problem that can affect anyone at anytime. Though neuromas frequently resolve with conservative (non-surgical) therapy, surgery may ultimately become necessary to alleviate the pain.

What is Morton's Neuroma?

Morton's neuroma is defined as an irritative process involving a nerve that supplies adjacent toes in the foot (figure 1 below). The major nervous supply to the bottom of a person's foot comes from the inside of the ankle. As the nerve passes to the toes, it begins to break into smaller branches. The bottom of each toe will receive two small nerve branches, which provide feeling to the skin. The nerves are relatively close to the plantar skin, and are often nestled between the metatarsal heads. The metatarsals are the long bones located just behind each toe. The metatarsal head is the rounded far end of the bone that makes contact with the small bones in the toes. As the nerve passes between adjacent metatarsal heads, it will branch into small nerves that enter the toes. Neuromas typically occur just prior to this branching, between the metatarsal heads.

Another important structure is a ligament that holds the metatarsal heads together (called the deep transverse intermetatarsal ligament). It is located between the metatarsal heads and just above the nerves on the bottom of the foot. Neuromas are the result of an anatomical "mistake" in which a nerve becomes squeezed between two bones (metatarsal heads) on either side. Chronic irritation of the nerve from repetitive trauma or constriction causes nerve thickening and damage resulting in scar tissue around the nerve. The close proximity of adjacent metatarsals and a deep ligament that passes between the two metatarsals results in a confinement of the plantar nerve. This results in trauma to the nerve. Slowly, the nerve becomes thicker and begins to degenerate.

A neuroma occurs most commonly between the third and fourth toes, but can present anywhere along the metatarsal heads. The term "neuroma" is actually a misnomer. This condition does not involve a tumorous growth; rather it is a reaction of normal nerve cells to some sort of traumatic event. This reaction involves enlargement of the nerve and improper function of the nerve itself.

Who's Morton?

The 'Morton' in "Morton's Neuroma" refers to T.G. Morton, the physician who described a similar condition back in 1876. Although some argue that he was not the first to identify the neuroma specifically, his name is synonymous with the neuroma nonetheless. Morton's name only applies to neuromas found between the third and fourth toes (the third interspace), as this was the location Dr. Morton described. Neuromas found between other toes are named after those who first described them (starting from the inside of the great toe to the outside of the fifth, the names are Joplin, Hauser, Heuter, Morton, and Islen).

Who Gets Neuromas?

Neuromas most frequently affect persons between the ages of 18 and 60 years old [1]. It is rare to find this condition in persons under the age of 18. Although both males and females can be affected, females appear to be at greater risk-one study found 78% of patients presenting with neuroma pain to be female [2]. This may be attributed to the pointed and high-heeled shoes women are sometimes compelled to wear (as discussed in the section "What causes neuromas?"). In addition, neuromas generally affect a single interspace on a single foot.

Signs and Symptoms

Persons suffering from Morton's neuroma often complain of numbness, burning, or throbbing pain localized between two adjacent toes. The symptoms are described as "having a hot poker thrust into their foot" or "electric shocks starting between my toes shooting to the tips". It may also feel like "a bunched up sock under the foot." Movement of toes, weight bearing activities, and tight shoes aggravate the pain. The afflicted person will frequently have the desire to remove their shoe and massage the affected area. Removal of shoegear and rest generally relieve the pain associated with a neuroma. Direct observation of a person with a neuroma may reveal increased space between adjacent toes as compared to the other foot (forming a 'V', where the point of the 'V' points to the origin of pain). In addition, a mass may be felt at the base of the toes.

What Causes Neuromas?

There are many factors that may cause or aggravate a neuroma. First, there are several foot structural pathologies that may contribute to neuroma pain. An abnormally pronated foot structure (flat foot) can be attributed to neuroma formation. A flat foot structure leaves the forefoot unstable when pushing off the ground while stepping forward. This instability causes increased rubbing forces that can irritate the plantar nerves involved in neuroma formation. It is also important to note that during propulsion, while the toes flex, the nerve is pulled taut against the deep transverse intermetatarsal ligament. This may irritate and worsen neuroma symptoms. Earlier theories suggest that compressive forces between the third and fourth metatarsal heads may result in the chronic traumatic changes that occur within the nerve.

Certain types of shoes may also contribute to nerve irritation. In particular, many women's shoes leave a foot susceptible to neuroma pain. High-heeled shoes keep toes in a flexed position, pulling the plantar nerves taut against the overlying deep transverse intermetatarsal ligament. In addition, weight forces through the foot are more vertical; this greatly increases the pressure applied to these nerves. Narrow and/or pointed shoes may also result in compression of the metatarsal heads, trapping the plantar nerve. All of these factors lead to nerve irritation, inflammation and subsequent neuroma formation.

Other factors that may cause or worsen a neuroma include inflammatory conditions such as arthritis. Also, activities that involve repetitive trauma to plantar nerves may contribute to neuroma formation. For example, repetitive pounding on a hard surface in sports like racquetball and jogging results in shearing and irritation of the nerve. Cysts, soft tissue masses, or tumors may compress the nerve and cause neuroma-like symptoms. Lastly, direct acute trauma may result in neuroma formation.

Diagnosis

The first step to the detection of a neuroma is obtaining a thorough clinical history and examination. The patient's description of the pain, the location of pain elicited upon examination, and the physical appearance of the foot all contribute to a diagnosis of neuroma. Side to side compression of the foot will generally reproduce pain. Your doctor may be able to elicit a "clicking" sound indicative of neuromas caused by the rubbing of the inflamed nerve between the two metatarsal bones pinching it. A "Sullivan's Sign" refers to the 'V' formation discussed earlier (see Signs and Symptoms) and is another clinical indication of neuroma. X-rays are often used to rule out other pathology. A diagnostic injection of local anesthetic will help your doctor determine the exact location of the neuroma and may help to rule out other pathology. Other non-routine tests such as ultrasonography, computed tomography, nerve conduction velocity, and magnetic resonance imaging may be run if the diagnosis is in question.

Conservative Treatment

Non-surgical treatment options should first be explored in an attempt to relieve symptoms. Conservative care includes everything from shoe gear modifications to injections of the nerve and is successful in about 50 to 80 percent of cases. Listed below are a few of the treatments that may be attempted by your doctor.

Padding: A strategically placed metatarsal raise pad under the metatarsals will result in separation of the metatarsal heads, thereby reducing nerve irritation. Another method involves placing a 1/8" lift under the fourth and fifth metatarsal heads. This theoretically will free the nerve from entrapment between the third and fourth metatarsal heads by separating the third and fourth metatarsal heads [3].

Orthoses: These are shoe inserts that can incorporate a metatarsal pad to separate the metatarsals. In addition, they will control excessive pronation, which stabilizes the forefoot and decreases the excessive foot motion that may contribute to neuroma formation.

Shoegear: Well-fitted athletic shoes, with special attention to the width, can help reduce current symptoms and can decrease the risk of recurrence. Narrow shoes that squeeze the forefoot together will make neuroma pain worse. Also, avoid shoes with a heel, as they tend to increase stretch and sheer forces on the nerve, which exacerbates the pain.

Try this at home: Place your foot on a blank piece of paper and outline it with a pencil. Place your shoe over the outlined foot. Can you see your pencil outline with the shoe lying over it? If you can, your shoes are too tight-this may be the cause of your neuroma.

Oral Anti-Inflammatory Medication: Non-steroidal anti-inflammatory medications (NSAIDs) like ibuprofen and naprosyn can reduce the inflammation around the nerve. These medications come in over-the-counter and prescription strengths-your doctor will discuss your appropriate dosage.

Injection Therapy: The two common agents used in injection therapy of neuroma are corticosteroid/local anesthetic and alcohol sclerosing agent. Corticosteroid is injected around the nerve to decrease the inflammation and reduce fibrosis or scaring. Greenfield found that 30% of patients undergoing corticosteroid/local anesthetic therapy had complete relief of symptoms and 50% had partial relief [4]. Alcohol sclerosing agent is a dilute mixture that includes a small percent of alcohol and a numbing medication. The alcohol is injected around the nerve in hopes of reducing its size. The therapy involves a series of 3-8 injections with 5-10 days between injections. Dockery found an 89% success rate using this method of injection therapy [5].

Surgical Treatment

When conservative attempts have failed to relieve symptoms, surgical treatment should be considered. Typically, surgical treatment involves removal of the neuroma. Other surgical treatments may include nerve repair and salvage (neurolysis) and endoscopic techniques to visualize the nerve with a camera and release the deep transverse metatarsal ligament. Plantar, dorsal, and web space incisions are the most common surgical approaches. After surgery, the patient can usually bear weight on the foot. Typical recovery can last from 3 to 6 weeks. Residual numbness between the toes can be expected. Infrequently, the nerve may attempt to grow back, resulting in the reformation of a neuroma with subsequent pain.

Conclusion

Morton's neuroma is a common condition that results in a painful, burning sensation that radiates to the toes. There are several causes, including pronated foot structure and narrow shoes. Conservative treatment options include padding, change in shoe gear, orthoses, and serial injections. When these treatments fail, surgical removal of the nerve may be an option.

References

1. Downey, MS: Surgical treatment of peripheral nerve entrapment syndromes. In: Musculoskeletal Disorders of the Lower Extremities, pp. 685-714, edited by LM Oloff, W.B. Saunders Company, Philadelphia, 1994.
2. Keh, RA et al: Long-term follow-up of Morton's Neuroma. J Foot Surgery 31(1): 93-95, 1992.
3. Hirschberg GG: A simple cure for Morton's neuralgia. JAPMA, 90(2): 100-101, 2000.
4. Greenfield, J et al: Morton's Interdigital Neuroma. Clinical Orthopedics, (185): 142-144, 1984.
5. Dockery, GL: The treatment of intermetatarsal neuromas with 4% alcohol sclerosing injections. J Foot Ankle Surg, 38(6): 403-8, 1999.

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