Morton's neuroma

The lay population, as well as medical professionals, frequently, but incorrectly, refer to this problem as a neuroma or nerve tumor. This is incorrect; histology has defined the pathology to be one of perineural fibrosis. Therefore, an emphasis must be made to correctly define the terms:

Traumatic neuroma - proliferative mass of Schwann cells and neuritis that may develop at the proximal end of a severed or injured nerve.

Axontmesis - interruption of the axons of a nerve followed by completed degeneration of the peripheral segment, without severance of the supporting structures of the nerve; such a lesion may result from pinching, crushing, or prolonged pressure.

Neurotmesis - a type of axon loss lesion that results from focal peripheral nerve injury, in which, at the lesion site, the nerve stroma, axon, and myelin are damaged to varying degrees, which degenerate from that point distal; with the most severe Neurotmesis lesions, the gross continuity of the nerve is disrupted.

Neuroplasty - plastic surgery of the nerve. Typically, this surgery is performed to release the nerve from compressive structures.

Morton's foot - A foot with a second metatarsal and toe that are longer than the first toe.

Interdigital neuritis (IDN) - a term coined by Weinfeld and Myerson that represents the preferred term for this entity (compression neuropathy of the plantar digital nerves). This phrase signifies an interdigital nerve compression syndrome and more accurately represents the problem to be discussed.

By the correct application of these terms our diagnosis and treatment will be more precise and subsequently successful.

The affliction of the forefoot was first described by Durlacher in 1845. Later in the nineteenth century Morton ascribed the pain to pathology of the fourth metatarsophalangeal joint. The initial treatment was to remove the fourth metatarsal head to treat the pain. At this time our understanding of the pathology is well-defined. The Morton's neuroma is now known to be perineural fibrosis.

The plantar digital nerves are terminal branches of the medial and lateral plantar nerves. The nerves run in the deep or fourth space of the foot at the metatarsal level. The nerves are plantar to the intermetarsal ligament. Just distal to the ligament, the nerve branches to the adjacent toes to provide sensation to the plantar web space skin. A particular anatomic finding at the 3/4 metatarsal web space may be the presence of a branch from the medial and the lateral plantar nerves. It is believed that this produces a larger nerve that is tethered. These characteristics are believed to cause a greater likelihood for local trauma that can result in the greater incidence of symptoms in the 3/4 metatarsal web space. Amis et al observed that the plantar digital nerve has small branches that attach directly to the plantar skin. When the nerve is transected distal to these branches, it is tethered to the plantar skin and will not retract into the surrounding muscle bellies. This leaves the nerve exposed for further trauma that can result in a painful neuroma that creates more pain than the original problem. Not cutting these plantar skin branches is one of the causes of a so-called “recurrent neuroma”, although an adequate procedure was not originally performed. The digital artery and vein are plantar to the nerve. The neurovascular bundle is surrounded by fatty tissue that provides protection to the nerve. A small venous plexus can be seen just distal to the intermetatarsal ligament. Congestion of the veins may be another source of nerve irritation. It is important that careful hemostasis be directed to these veins. Indiscriminate cautery or ligation will result in injury to the digital vessels, whereas persistent bleeding from these veins will create a significant postoperative hematoma. The lumbrical tendon runs parallel with the neurovascular bundle. It is a narrow, shiny structure that, on occasion, will cross the neurovascular bundle. When a limited exposure is used, the tendon may be mistaken for the nerve. The intermetatarsal bursae may be thick and inflamed or relatively nonexistent. The dorsal cutaneous nerves are abundant and must be protected. The dorsal skin may be contracted when associated with claw toes. The plantar fat pad will be retracted distally with the toes. This will add further trauma to the metatarsal.

The pathology of removed nerves demonstrates the following histologic changes:

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  Perineural, epineural, and endoneural fibrosis
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  Degenerative vascular changes
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  Axonal demyelination and branching in organized and disorganized patterns
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  Fibrinoid degeneration
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  Arrested axonal nerve endings that generate impulses spontaneously
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  An increase in sympathetic nerve fibers intermingled with other axons

The typical patient is a woman in her fourth to sixth decade; men and young, active individuals can be affected as well. The primary complaint is intermittent burning or electric, shock-like pain that occurs in the forefoot. Most patients state that the pain is worsened by walking in their bare feet and can be alleviated by wearing dress shoes. Although wearing tight shoes seems to be a paradox, anesthesia may be created by further nerve compression. Other patients remove their shoes to alleviate the symptoms. The most common location for the pain is between the third and fourth toes and the next common location is between the second and third toes. It is very unusual to have these symptoms at the first/second and fourth/fifth interspaces. Some individuals will have pain at the second/third/fourth interspaces, however. Some patients describe a crepitant sensation when walking. Others will describe a painful numbness to the toes. The history should be directed to the length of time that the patient has had symptoms. Individuals with symptoms for more than 1 year will have a decreased chance for improvement with nonoperative treatment. A notation should be made if the patient has swelling to the small joints of the hands or feet. Other peripheral nerve complaints in the opposite foot or the hands should alert the practitioner to a peripheral neuropathy. Medications, such as antineoplastics, can be causative agents of neuropathies and should be directly questioned. The family history should note if rheumatoid or another inflammatory arthritis has occurred. Diabetics can be prone to having a Morton's neuroma, in addition to diabetic neuropathy.

Treatments that have failed must be taken into account, whether these were tried by the patient or directed by another health care provider. Nonsteroidal medications alone are of limited benefit. Shoewear modifications need to be identified. Some individuals will have custom foot orthoses placed into a shoe that is already of inadequate size. These will only increase the crowding on the toes. The effect to the mobile first and fifth rays is to elevate heads. This creates an even greater mechanical loading to the central metatarsal heads. Corticosteroid injections infrequently will be curative. Many patients will have a series of three or more corticosteroid injections into the web space. Multiple injections can create a variety of local problems that will exacerbate the patient's symptoms. Atrophic and blanched skin may occur; this is particularly noticeable in darkly-pigmented skin. Toes that are clawed may dislocate secondary to rupture of the collateral ligaments or the plantar plate. Systemic steroid effects can occur, depending upon the concentration of the steroid used. In diabetics, the blood sugars can elevate significantly. The most damaging local effect is atrophy of the forefoot fat pad. This problem is particularly pronounced in perimenopausal women. When the cushioning provided by the forefoot pad is decreased, this exacerbates the local trauma to the nerves and surrounding tissues. The only alternative is to provide external cushion with orthotics and soft-soled shoes.

Work issues must be clearly defined. Some patients state that their symptoms are caused by long hours of standing on concrete or terrazzo floors and insist that they should be taken off work for an extended time. It is unlikely that this work environment is the cause of their complaints. Workers who have to use their feet repetitively on stiff pedals can injure their forefeet. Professional dancers, whether classical ballet, ballroom, or flamenco, can traumatize their feet create IDN. Professional and competitive athletes can damage the forefoot through the constant use and abuse to the foot in their respective activities.

A general musculoskeletal examination is essential. The key areas to note are generalized ligamentous laxity and signs of synovitis, particularly to the small joints. The lower extremity range of motion must be recorded. If a patient has an equinus contracture it will increase the forefoot loading. The skin is evaluated for a discrete callosity or intractable plantar keratosis and central metatarsal head callousing. Epidermoid inclusion cysts and plantar verruca can be a source of forefoot pain. Atrophic skin and forefoot padding, pulses, capillary refill, and sensation should be documented.

The posture of the toes, clawed or hammered, will distally displace the forefoot fat pad and may stretch the neurovascular bundle on the intermetatarsal ligament. A bunion deformity, when associated with a hypermobile medial column, will add additional loading to the central metatarsals. When the toes are splayed, this indicates inflamed and swollen intermetatarsal bursae. The shuck test, or anteroposterior instability to the metatarsophalangeal joints, is seen with chronic synovitis. Synovitis and intermetatarsal bursitis can be seen individually or combined with IDN. The “click”, described by Mulder, occurs when dorsiflexing a metatarsal while plantarflexing the adjacent metatarsal. This is seen in only 20% of patients with symptomatic IDN. Direct palpation of the plantar web space, so that the digital nerve is compressed between the metatarsal heads, will usually reproduce the pain. A cavovarus foot will load the forefoot abnormally and may be a contributing factor to the development of forefoot pain. Foot posture, cavus or planus, although it may be a contributing factor, has not been implicated as the sole causative factor of IDN.

Observation of previous surgical scars is helpful. Is the scar dorsal or plantar? If a keyhole dorsal incision was previously used, visualization can be limited with inadequate nerve resection or release. Metatarsal osteotomies can create unequal loading or transfer metatarsalgia, as well as adherent scar formation. The dorsal cutaneous nerves can be the causative agent. If reflex sympathetic dystrophy is present, then diagnosis of the patient will be challenging. Secondary gain has to be gauged when dealing with postoperative and work-related problems. These patients are typically manipulative and can show malingering tendencies.

Standing posture and gait should be examined for abnormalities, particularly abnormally long periods on the forefoot. However, some will exhibit abnormally short time period at toe off. The foot is typically externally rotated during stance phase.

The patient's shoes must be evaluated. Many women have shoe requirements that are related to their occupation. Numerous women have a social preference to wear incorrectly fitted shoes. There is excellent documentation that 90% of women are wearing shoes that are too small for their feet (The American Orthopaedic Foot and Ankle Society (AOFAS) position statement: women's shoes and foot problems. http://www.aofas.org/womensfoot.asp). The sole of the shoe is typically thin and the shoe's only mechanism to stay on the foot is by tightly compressing the toes. A simple method of checking shoe sizing is to trace the foot while the patient is standing, and place the shoe over the tracing. The observation is made about how many “toes” are not contained in the toe box. This should always be pointed out to the patient. Improper shoewear is a contributing factor in the development and perpetuation of IDN.

An injection is often performed for diagnosis and treatment of IDN. The injection is performed through the dorsal skin. The injected solution consists of 1 to 1.5 mL of a local anesthetic without epinephrine and is combined with a soluble corticosteroid. The solution can be mixed or the corticosteroid can be injected separately. Care must be taken to keep the corticosteroid away from the skin. The intermetatarsal ligament must be perforated to place the solution adjacent to the neurovascular bundle. This can be confirmed by observation of the skin protruding plantarly, or by palpation of the plantar fullness during the injection. The intermetatarsal bursae and the metatarsophalangeal joint can be injected through the same puncture. At times, patients can be confused by the diagnostic component of the injection. Written guidelines are made so that the following questions are answered:

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  Was there resolution of the pain? If so, what percent?
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  What was the length of time that the symptoms resolved?
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  Were there any adverse events related to the injection?

The patient is instructed to call within 3 to 5 days from the injection and the data are stored in the chart. All attempts should be made to delay surgery, if possible, to minimize possible soft tissue problems after a corticosteroid injection.

By following the above history and physical examination, most patients with IDN are diagnosed. Some patients can present a confusing picture, particularly those who have had failed surgery on the symptomatic or the adjacent interspace. When a diagnostic dilemma is encountered, it may be appropriate to obtain an ultrasound or an MRI. The ultrasound, although accurate, is operator-dependent. The MRI can be overly sensitive and asymptomatic neuromas may be diagnosed and resected.

Radiographs are not diagnostic of this soft tissue condition. They can provide valuable secondary information and help to diagnose other conditions that create forefoot pain. The following features need to be surveyed:

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  Erosive changes at the metatarsophalangeal joints
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  Elongated central metatarsals compared with the first metatarsals
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  Thickened cortices of the central metatarsals. This is indicative of a stress reaction and is caused by excessive force.
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  Stress fractures to the metatarsals
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  Freiberg's infraction or avascular necrosis of the metatarsal head
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  Enthosopathic changes or ossification of tendon and ligament insertions

Some patients may have what appear to be abutting metatarsal heads. Surgery should be performed to allow more space for the nerve through partial condylectomies. This approach is reminiscent of Morton's original treatment. There are no convincing data in the literature that the apparent abutment is any more than a radiographic variant. Usually MRI and ultrasonography provide little additional information for classic patients and the cost is rarely justified.

All surgical procedures can have unanticipated outcomes. It is imperative that patients be given reasonable expectations and that all failures are scrutinized carefully. Management of a patient with a failed procedure will need to be handled with skill. Honest questioning of the patient will be needed. We must clearly identify the source of the dissatisfaction.

• Did we have the correct diagnosis?
• Was the surgery performed correctly?
• Did this patient have realistic or unrealistic expectations?
• Did the patient follow the postoperative regimen? What type of shoewear is currently being used?

The diagnosis must be clearly defined. For revision surgery the testing will need to change. MRI, ultrasonography, serologic evaluation for inflammatory arthritis, and electrodiagnostic testing may now be pertinent. If the surgery was done elsewhere, an attempt should be made to obtain a pathology report. It is imperative to determine that nerve was removed, as well as the length of the specimen. If only a short segment of nerve was excised then a neuroma entrapment is the usual cause of the failure. When the decision to operate has been made, the tactics are clear. Resect the nerve to a more proximal level and bury the stump neuroma in the intrinsic musculature. This may leave a broader zone of anesthesia with trophic changes to the skin and subcutaneous fat. In revision surgery, the incision can again be either dorsal or plantar, but most surgeons choose the latter. The plantar approach gives direct access to the nerve and its branches. The plantar branch must be protected longer than the dorsal approach, however. Sometimes the previous dorsal scar is retracted and creates toe deformities. In this case, the authors approach the nerve from the dorsal side and address the toe deformity simultaneously.

Morton's neuroma is a common problem. Progress has been made in the understanding of this frequent problem since Morton's original description and treatment. Today, we accept a failure rate of 15% to 20%, even in the best of series. We must ask ourselves if this is good enough. What can we do to achieve an acceptable failure of 5% or less? How can we improve? Only through an honest analysis and discussion can we improve the care that we deliver.