Morton's Neuroma

Summary

Morton's Neuroma is a painful condition of the forefoot that is caused by the entrapment of the common intermetatarsal nerve as it passes through the forefoot to the toes. This condition was first described by Dr. Morton, a Viennese physician, in 1876. Morton's neuroma is commonly found between the 3rd and 4th toes and to a lesser degree between the 2nd and 3rd toes. Morton's neuroma is also referred to as intermetatarsal neuroma, Morton's metatarsalgia, Morton's neuralgia, plantar neuroma, intermetatarsal perineural fibroma or intermetatarsal nerve entrapment. Morton's neuroma is found more in women than men. The onset of Morton's neuroma is between the ages of 20 and 60.

Symptoms

• Forefoot pain described as deep, achy on both the top and bottom of the foot

• Affected digits may ache and tingle prompting patients to rub the foot to stimulate feeling

• A sensation of walking on your sock being bunched up under the forefoot

• No swelling or bruising is found

Description

As the posterior tibial nerve descends the leg, it bifurcates (splits into two parts) at the level of the medial ankle. The two branches, the medial and lateral plantar nerves, continue forward into the foot to supply motor function to the muscles of the foot and sensory innervation to the bottom of the foot. The medial and lateral plantar nerves converge at the 3rd interspace (the space between the 3rd and 4th toes) to form the 3rd common intermetatarsal nerve. As a result of this unique anatomical configuration, the nerve between the 3rd and 4th toes is bound and unable to move out of the way when the adjacent bones move in the forefoot.(1) This unique anatomical variation is what makes the 3rd common intermetatarsal nerve more prone to the formation of Morton's Neuroma. As the 3rd common intermetatarsal nerve passes distally to the toes it passes beneath the intermetatarsal ligament. This is the primary location where an entrapment of the nerve occurs, resulting in a neuroma known as Morton's neuroma.

One of the tests your doctor may use to diagnose Morton's Neuroma is called a Muldier's Sign. To elicit a Muldier's sign, your doctor will gently squeeze the foot from side to side and use his or her thumb to push up between the 3rd and 4th toes. In cases of Morton's Neuroma there will be a palpable snap as the intermetatarsal nerve moves between the adjacent metatarsal bones. Performing a Muldier's Sign mimics what takes place in the shoe with every step. Squeezing the foot simulates the tightness of the shoe and pushing up on the bottom of the foot simulates the reactive forces of the ground as it pushes against the foot with each step.

A Muldier's sign was at one time considered to be pathoneumonic in the diagnosis of Morton's neuroma. Some authors have indicated that Muldier's sign may be positive in as low as 27% of cases of Morton's neuroma that have been confirmed as present by diagnostic ultrasound.

The following pictures show the steps used in performing a Muldier's sign. The foot is squeezed from side to side to mimic the tightness of a shoe. The thumb is used in the third inner space to simulate ground reactive forces. A positive Muldier's sign is described when a click of snap sensation is found by both the patient and the examiner.

Causes and contributing factors

Clinicians and surgeons recognize a number of factors that may aggravate or contribute to Morton's neuroma, but the primary cause of Morton's neuroma remains elusive. Most clinicians agree that there are two general reasons for the onset of Morton's neuroma: the unique anatomy of the 3rd web space (forefoot) and atypical biomechanics of the foot. Hypermobility of the metatarsals seems to increase the amount of mechanical pressure applied to the nerve by the adjacent metatarsals and the intermetatarsal ligament. Shoes that are tight in the forefoot will contribute to the symptoms of Morton's neuroma by binding the forefoot and compressing the common intermetatarsal nerve. High heels will also act to increase ground reactive forces. Ground reactive force is the amount of force generated as the foot pushes against a fixed surface like the floor. With high heels, the amount and focus of ground reactive force increases since weight bearing is focused in a smaller area (just the forefoot.) A higher heel also puts the common intermetatarsal nerve under tension, making it more prone to injury. Activities such as squatting will increase the ground reactive force applied to the plantar foot and aggravate the symptoms of Morton's neuroma. Morton's neuroma is more common in flat feet and flexible feet. It is not uncommon to find Morton's Neuroma in conjunction with other foot problems such as bunions and hammer toes.

Differential diagnosis

The differential diagnosis for Morton's Neuroma includes:

• Bursitis
• Capsulitis
• Freiberg's infraction
• Metatarsalgia
• Metatarsal stress fracture
• Osteoarthritis of the forefoot
• Tarsal tunnel syndrome with isolated forefoot symptoms

Treatment

Diagnostic testing to evaluate Morton's neuroma includes plain x-rays, diagnostic ultrasound or MRI. Plain x-rays are not actually used to visualize the nerve, but rather to screen for bone and joint pathology adjacent to the neuroma. Metatarsal fractures, Freiberg's infraction and osteoarthritis are common conditions that can influence the behavior of Morton's neuroma and need to be evaluated with x-ray.

Several authors have suggested that the efficacy of MRI and ultrasound as diagnostic tools are comparable when evaluating patients for Morton's neuroma. Diagnostic ultrasound is significantly less expensive and much more readily available compared to MRI. Kankanala et. al described a 91.48% pre-op predictive value for diagnostic ultrasound when screening for Morton's neuroma.

Treatment of Morton's neuroma with conservative care can be quite successful. 70% or more of new Morton's neuroma patients respond to simple changes in shoes, such as a wider toe box. Shoe padding can also help treat Morton's neuroma. Metatarsal pads are an important tool for patients with Morton's neuroma symptoms. A metatarsal pad is a small lift that is positioned in the shoe just proximal (behind) the weight bearing surface of the metatarsal bones. A metatarsal pad lifts and separates the metatarsal bones, thereby decreasing the pressure on the intermetatarsal nerve. Some prefabricated arch supports come with a metatarsal pad already seated in the correct position. Using inserts with a metatarsal pad is sometimes the easier way to use a met pad because they can be easily moved from shoe to shoe. Also, by using an insert with a fixed metatarsal pad, the position of the met pad is always in the correct location.

Other non-surgical methods of treating Morton's neuroma include injectable cortisone and chemical sclerosis of the intermetatarsal nerve. Cortisone has been used successfully for years in treating Morton's neuroma. Although the use of cortisone does not actually treat or change the entrapment of the intermetatarsal nerve, cortisone can decrease inflammation and swelling of the nerve, resulting in a decrease in pain. Care should be exercised when using cortisone injections, noting that excessive cortisone injections can thin the plantar fat pad of the foot.

Sclerosis of the nerve (also called chemical neuro-ablation or chemical neurolysis) can be performed in the office using a number of different solutions, most commonly dilute (4%) alcohol. Multiple sclerosing injections are used to destroy the contents of the peripheral nerve. A series of injections are employed, each injection separated by a period of 7-10 days. The total number of injections may vary from 3 to 7. The success rates of injectable sclerosing solutions have been reported to be as high as 60-90%. Chemical neurolysis is also a great tool for failed neuroma surgeries where a stump neuroma has formed.

The intent of chemical neurolysis is to destroy the internal contents while preserving the external sheath of the nerve. This would be a bit like removing the copper wire in an electrical wire while preserving the plastic outer insulation or cover of the wire. The reason that this is important is due to the fact that peripheral nerve will regenerate over time. With the nerve sheath intact, regeneration of the nerve is possible in a controlled manner utilizing the existing sheath. By contrast, removal of the nerve by surgery results in the nerve regenerating and the formation of a mass of scar tissue called a stump neuroma. Knowing that peripheral nerve may regenerate also means that sclerosing injections may need to be repeated at some point in the future. The percentage of repeat sclerosing injections varies but is overall quite low.

Another technique used to treat Morton's neuroma is called cryogenic neuroablation or "cryo surgery." Cryo surgery is surgery that uses extremely cold instrumentation to selectively destroy tissue. Cryosurgery has been commonly used to destroy superficial skin lesions such as warts and moles. The technique uses what is referred to as the Joule-Thompson effect. The Joule-Thompson effect occurs when a gas is passed through an enclosed probe where it may expand. As the gas expands, it cools to approximately -70 degrees centigrade. In the case of cryogenic neuroablation, the expansion of the gas is controlled in a 5.5 mm probe that freezes and subsequently destroys the adjacent nerve tissue.

In the cryogenic ablation study carried out by Drs. Caporusso, Fallet, and Savoy-Moore, thirty-one neuromas were treated in 20 patients. All procedures were performed in an office setting. The procedure used a small amount of local anesthetic to numb the skin to allow the passage of a 12-gauge cannula through the skin. A nerve stimulator was passed through the cannula to locate the nerve. Once the position of the nerve was established, two three-minute freeze sessions were utilized to destroy the nerve tissue. A sterile dressing was applied to the site and the patient was dismissed without the need for pain medication. The study cites a 65% success rate.

The EDIN procedure (endoscopic decompression intermetatarsal neuroma) has been used for at least ten years and has shown promising results. The EDIN procedure is an endoscopic procedure that is used to perform a selective release of the intermetatarsal ligament. The following pictures show the technique used to perform an EDIN procedure. Image 1 shows pre-operative markings identifying the 3rd and 4th metatarsal heads. Image 2 shows placement of the cannula through an interdigital incision. The cannula is much like a small 4mm drinking straw with a slot cut in one side. The slot, or open side of the cannula is placed adjacent to the intermetatarsal ligament. The cannula passes from between the toes to a second incision on the plantar aspect of the foot just proximal to the weight-bearing surface. The endoscope and knife are used within the slotted cannula to identify and transect the intermetatarsal ligament. Image 3 shows the use of a blunt probe without the cannula to verify a complete release of the intermetatarsal ligament. In the bottom of image 3, a metatarsal spreader can be seen. The spreader is used to separate the 3rd and 4th metatarsals, subsequently putting tension on the intermetatarsal ligament. The procedure takes about 20 minutes and is completed in a hospital or surgery center. Local anesthesia with sedation is used. Patients return to regular shoes in two days with just a band-aid on the incisions.

Traditional neurectomy, or removal of the nerve for the treatment of Morton's neuroma, is still commonly used. Neurectomy can be performed from a dorsal or plantar approach. The advantage of a dorsal approach is that patients are able to walk immediately following the surgery. The disadvantage of the dorsal approach is that it requires more dissections and possible tissue trauma. The plantar approach results in less tissue trauma but requires that patients are non-weight bearing on the surgery foot for 3 weeks post-op. Traditional neurectomy for the treatment of Morton's neuroma is performed on an out-patient basis at a surgery center or hospital using a local or general anesthetic. The procedure is completed in less than 30 minutes.

When to contact your doctor

Forefoot pain that fails to respond to a two week period of conservative care should be evaluated by your podiatrist or orthopedist.

References

1. Bojsen-Moller F, Flagstad KE: Plantar aponeurosis and internal architecture of the ball of the foot. J Anat 121:599, 1976.

Author(s)

This article was written by Myfootshop.com medical advisor Jeffrey A. Oster, DPM.  

Competing Interests - None

Cite this article as: Oster, Jeffrey. Morton's Neuroma.  https://www.myfootshop.com/article/mortons-neuroma

Most recent article update: January 14, 2021.
 

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