Chapter 186. Neuroma Management

Morton's neuroma, also referred to as an interdigital neuroma, is one of the most common painful disorders of the forefoot. It was first described in 1845 by Dulacher. It is named after Thomas Morton, who presented a case series of patients afflicted with this disorder in 1876. Patients with an established Morton's neuroma are usually cared for by a Podiatrist or an Orthopedist. They may present to the Emergency Department with a previously undiagnosed neuroma or with a painful exacerbation of a previously diagnosed neuroma.

The term neuroma is actually a misnomer. Histologic investigation does not reveal the typical proliferation of axons found in true neuromas. Instead, there is a fibrosis and thickening of the perineural tissue with corresponding degeneration of the underlying nerve in a Morton's neuroma.1 This most commonly affects the third plantar common digital nerve located in the third interspace, between the third and fourth metatarsal heads. It may also occur less commonly in the second interspace, between the second and third metatarsals. A neuroma is rarely seen in the first or fourth interspaces, between the first and second or the fourth and fifth metatarsals, respectively.

Morton's neuroma most commonly affects women between their fourth and sixth decades.2 It is especially common in those who wear high-heeled shoes, poor fitting shoes, worn shoes, shoes with poor or no padding, or shoes that are narrow at the forefoot. Persons with pronated or pes cavus feet are similarly at risk.3 Neuromas do not usually become symptomatic until their transverse diameter reaches more than 5 mm.4

Neuromas form just proximal to the bifurcation of the plantar common digital nerves (Figure 186-1) and below the deep transverse intermetatarsal ligament (Figure 186-2). The deep transverse intermetatarsal ligament connects the metatarsal heads on the plantar aspect of the foot (Figure 186-2). The neuroma is made up of branches from both the medial and lateral plantar nerves (Figure 186-1). Most commonly affected is the third interdigital nerve. It is the largest of the interdigital nerves and may explain the increased frequency of neuroma formation in this location.

Morton and others postulated that the increased mobility of the fourth and fifth metatarsal heads relative to the more fixed medial portion of the foot results in disproportionate trauma to the third interdigital nerve. These mechanical factors, combined with the impingement and stretching from a tight transverse intermetatarsal ligament, result in repetitive microtrauma. Histologic evaluation reveals perineural fibroma formation consistent with compression-induced trauma.2 Injury begins with edema of the endoneurium, followed by fibrosis beneath the perineurium, axonal degeneration, and finally neuronal necrosis.

Some authors believe that a more significant contributor to neuroma formation is enlargement of the interphalangeal component of the intermetatarsophalangeal bursa, leading to microvascular trauma.5 Movements of the bursa result in minor compressive effects on the adjacent digital arteries, leading to ischemia of local neural tissue.

The diagnosis of a Morton's neuroma is usually made clinically, based upon classic historical features and physical examination findings.1 The pain of a neuroma usually begins intermittently, becoming more frequent with time and eventually constant. The patient may complain of pain, burning, electric shocks, or tingling over the involved intermetatarsal space. Attacks typically occur suddenly after a period of walking, running, or standing.5 Some patients may complain of pain disturbing their sleep.5 Hyperesthesia or hypoesthesia in the involved toes and web space is common. Patients often complain of the unilateral feeling of “walking on a lump.” Symptoms are aggravated by walking, particularly in narrow shoes, and relieved by rest and shoe removal.

Physical examination localizes the pain to the involved interspace with minimal involvement of the adjacent metatarsal heads. Moderate pressure applied proximally in the affected web space reproduces the pain. A small mass, representing the neuroma, can be palpated in approximately one-third of the cases. A positive Mulder's sign is diagnostic.6 This is a click felt in the interspace with medial and lateral compression of the metatarsal heads during simultaneous palpation of the interspace (Figure 186-3).

Radiographic imaging may be helpful in confirming the diagnosis and exploring alternative etiologies. Plain radiographs will not demonstrate a neuroma. They may reveal splaying of the involved toes when the neuroma is especially large or alternative diagnoses such as stress fractures or arthritis. Ultrasound represents the most efficient and effective imaging modality.2,7 An experienced operator may identify hypoechoic neuromas as small as 2.9 mm.8 Magnetic resonance imaging (MRI) is quite effective at identifying the presence of a neuroma, but cost and access issues make it a less practical choice, especially in the Emergency Department.1 Computed tomography (CT) may be useful if an MRI is contraindicated or not available. The sensitivity of CT scanning is less than that of MRI or ultrasound.2

Injection of the presumed neuroma will provide significant relief to the patient and confirm a Morton's neuroma as the etiology of the patient's complaints. Injection is indicated after less invasive methods have failed.9,10 These include foot elevation and rest, nonsteroidal anti-inflammatory drugs, arch supports, orthoses, and changing footwear to prevent forefoot compression (i.e., low heels and wide toe boxes). Corticosteroid injections can provide partial or complete relief in up to 80% of patients.9,11

The primary contraindication to injection of a Morton's neuroma is failure to make a correct initial diagnosis. The differential diagnosis for forefoot pain includes a wide variety of pathologies, many of which may closely mimic the presentation of a neuroma. Alternative diagnoses include tarsal tunnel syndrome, peripheral neuropathy, capsulitis, bursitis, rheumatoid arthritis, foreign bodies, avascular necrosis, stress fractures, and peripheral vascular insufficiency. A careful history and physical examination with discretionary use of imaging modalities will lead to a correct diagnosis.

Some specialists believe injection therapy to be contraindicated in serious athletes.3 They propose that steroid injection may result in fat pad atrophy, degeneration of the volar plate, and degeneration of the collateral ligaments. A discussion of alternative therapies to injection therapy is provided later in this chapter.

• Sterile gloves
• Sterile drapes
• Povidone iodine or chlorhexidine solution
• 3 mL syringe
• 22 or 25 gauge needle
• 0.5% bupivacaine without epinephrine
• Injectable methylprednisolone or triamcinolone
• Sterile bandage

Explain the risks, benefits, complications, and aftercare of the procedure to the patient and/or their representative. Place the patient supine on a gurney with their hip and knee flexed so that the sole of the affected foot is flat on the gurney. Clean the skin overlying the neuroma of any dirt and debris. Apply povidone iodine or chlorhexidine solution and allow it to dry. Apply sterile drapes to isolate a sterile field.

Prepare the injection solution. Mix 1 mL of 0.5% bupivacaine without epinephrine in a 3 mL syringe with 10 mg of methylprednisolone or triamcinolone. Another local anesthetic agent, without epinephrine, may be used instead of bupivacaine. Long-acting local anesthetic agents are preferred as they provide the patient with longer pain relief after the injection.

Instruct the patient to moderately dorsiflex their toes to separate the metatarsal heads. Use a dorsal approach to the neuroma, as this is less painful than penetrating the sole with a needle. If the neuroma is palpable, insert the needle perpendicular into the skin overlying the neuroma. Advance the needle perpendicular to the skin and into the neuroma or into its fascial plane (Figure 186-4). Inject the affected interspace with 1 to 2 mL of the combination long-acting local anesthetic agent and steroid.

In the absence of a palpable neuroma, insert the needle approximately 1 to 2 cm proximal to the web space. Direct the needle into the facial space between the deep and superficial transverse metatarsal ligaments. Note that this space is deep to the metatarsal heads on the plantar aspect of the foot (Figure 186-2).

Various types of conservative treatment can be initiated once a Morton's neuroma is diagnosed. A wide range of success rates have been reported (20% to 80%) using a combination of injections with other conservative therapy including orthoses, metatarsal pads, shoes with wider toe boxes, and physical therapy.12,13 Orthoses are used to help control abnormal pronation, although some studies have shown this to be ineffective.13 Metatarsal pads serve to spread the metatarsal heads at the involved interspace and decrease compressive trauma (Figure 186-5). The types of physical therapy employed include massage, ultrasound, electrical stimulation, and whirlpool immersion. A trial of acetaminophen or nonsteroidal anti-inflammatory drugs may be helpful in the short term.

Two recent studies have described the injection of dilute alcohol into the site of the neuroma using ultrasound guidance. The effect is to sclerose and harden the perineural tissues. The injection is typically repeated every 1 to 2 weeks and on average 4 to 7 times, with treated patients experiencing a significant reduction in symptoms and neuroma size.14,15

Podiatrists and Orthopedists may offer more aggressive and definitive therapy when conservative measures fail. Surgical excision using a dorsal approach has success rates as high as 84% to 93%.16–18 The podiatry literature offers increasing support for endoscopic decompression of the deep transverse intermetatarsal ligament, thus leaving the nerve intact. The advantage is a shorter recovery time, which may make this especially desirable for athletes. No long-term results have been reported, but short-term results appear quite promising.3 Excision using a carbon dioxide laser is yet another treatment option that may allow for an even shorter recovery.7

Allow 10 to 20 minutes for the local anesthetic agent to take full effect. The patient will experience a rapid resolution of symptoms if the injection is successful. The relief of pain confirms that the corticosteroid has been injected into the correct location, allowing it to work on the neuroma.

Pain may be controlled with the use of acetaminophen or nonsteroidal anti-inflammatory drugs. Instruct the patient to wear flat shoes, shoes with flat and wide toe boxes, and to avoid shoes whose heel is elevated above the metatarsals (usually a heal no higher than 0.5 to 1.0 inches). The patient should avoid activities that place repeated pressure on the forefoot (e.g., bicycling, jogging, and sports). Apply a metatarsal pad to decrease compressive forces on the neuroma. Place the dome of the pad between the third and fourth metatarsals and just posterior to the metatarsal heads (Figure 186-5). Cold packs can be applied for 10 to 15 minutes every 3 hours to decrease inflammation and pain, especially after activities that aggravate the patient's symptoms. Refer the patient to a Podiatrist or Orthopedic Surgeon for follow-up, additional injection therapy, orthoses, physical therapy, and possible surgical management.

Complications associated with neuroma injection are exceedingly rare. They include the introduction of infection, damage to neurovascular structures, local fat pad and skin atrophy secondary to the effects of the steroid, skin depigmentation or hyperpigmentation, telangiectasias, and thinning of the skin.14,15 Fat pad atrophy on the dorsal foot is primarily a cosmetic issue. Fat pad atrophy on the plantar aspect of the foot can result in painful ambulation and gait disturbances.19 Skin changes and fat pad atrophy can be prevented by ensuring the injection is deep and properly placed so that the corticosteroids do not leak into the subcutaneous tissues.20 It is common for the pain to recur in days to weeks, even when the injection is successful. Single injections may not totally relieve the patient's symptoms. The patient often requires a series of injections every 1 to 2 weeks to be successful.

Morton's neuroma is one of the most common causes of forefoot pain. Injection therapy is useful in alleviating the symptoms of a neuroma and in confirming the diagnosis. Arrange Podiatric or Orthopedic follow-up for all patients given this diagnosis, as more invasive procedures may be needed for long-term resolution of symptoms.