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Morton's Neuroma

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Morton's neuroma (also known as an intermetatarsal neuroma or interdigital neuroma) is a  common cause of forefoot pain and disability. It presents as a sharp, burning  sensation in the web-space between the toes; most typically, it is found between the 3rd and 4th toes. Note that the pathology of this condition is not a neuroma, per se (ie, it is not a "benign growth of nerve tissue" , the definition of the word "neuroma") , but is rather a perineural fibrosis, or overgrowth of the tissue around the nerve.  The precise etiology of the condition is not known, but  Morton's neuroma is associated with believed to be related to localized repetitive overload of the forefoot and it is considered a type of "metatarsalgia."  It is associated with claw toes, a tight calf muscle, and wearing shoes that have a tapered toe box or high-heels (or worse: both).  The presence of other bunions and hammertoes increase the risk for developing a Morton's  neuroma.  

 

Structure and function  

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Secondly, the space that the nerve occupies is subject to shearing forces as the relatively mobile 4th metatarsal can stress the nerve against the relatively fixed 3rd metatarsal. Thirdly, the nerve may experience compression during use of high-heeled shoe-wear. These shoes reduce the support of the the heel-rise phase of gait. When the weight-bearing portion of the foot is reduced to the forefoot , which causes hyperextension of the metatarsophalangeal joints , compressing occurs.  This compresses the common digital nerve and metatarsal heads against the tough and unyielding intermetarsal ligament.

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Most chronic pain in the forefoot is not the result of a Morton’s neuroma, but rather is from metatarsalgia. Therefore  Metatarsalgia occurs from repetitive compression loading of the structures of the forefoot and the resulting chronic tissue injury and inflammation (ex. lesser MTP joint synovitis).Therefore a careful history and physical examination must be made to exclude other pathologies in the differential diagnosis. A patient with a Morton's neuroma often complains of a burning, sharp pain located in between the third and fourth toes. This pain is often plantar at the metatarsal heads and radiates distally on either side of the toe; it can also radiate from the forefoot up the leg proximally. The pain is often exacerbated with wearing tight or constricted shoes and alleviated while walking barefoot. A patient may describe characteristically and temporally different pain. The first, intense pain lasts for about 5-10 minutes and is precipitated by direct compression of the neuroma. This pain then becomes a dull ache for the next 2-3 hours.2,3 One feature of a Morton's neuroma is that a patient may need to rest after walking, remove the shoe, and massage the area of discomfort for moderate relief.7 This may occur several times throughout the day. Patients may also complain of the sensation of a stone or pebble under the toes or forefoot when walking.

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While early studies of imaging showed that the clinical utility of these modalities is questionable,9 most studies did not evaluate a Mortons' neuroma with today's more sensitive MRI magnets and ultrasonography equipment. Owens and colleagues showed that an MRI may have a sensitivity of 97% in patients with symptomatic neuromas, but showed neuromas in 25% of patients without symptoms.10 Sharp et al evaluated 29 confirmed cases of Morton's neuroma that were evaluated with an MRI, ultrasound, and physical examination. MRI and ultrasound's accuracy was dependent on the size of the lesions. Physical exam proved to be the most sensitive and specific modality.11

Epidemiology

The Morton's neuroma is a subset of metatarsalgia and as such the epidemiology of the disease is not clearly defined, as the .  The incidence and prevalence are of pure Morton's neuroma (as opposed to metatarsalgia with a  component of Morton's neuroma) is not known.12 Typically, women ages 45-50 are affected, and although men are also affected, women account for the overwhelming majority of cases.12,13 Both feet are equally affected, although bilateral complaints are rare. It is also relatively uncommon to find two neuromas in the same foot.

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Metatarsalgia is the most common cause of chronic forefoot pain, so make sure to do a thorough history and physical exam to exclude it from the differential diagnoses.  The challenge is determining the source of the metatarsalgia and whether a Morton's neuroma is contributing to part (or all) of the metatarsalgia symptoms.

Treatment options and Outcomes

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When a diagnosis of Morton's neuroma is made, some surgeons believe that conservative management is never warranted because the pain will persist and worsen as the neuroma enlarges.2 However, several conservative treatment modalities are available. There is no agreed-upon treatment algorithm, but most believe that these treatments should be tried from 3 months to 1 year.1,3

Comfort Shoes

Firstly, a patient should wear wide, comfortable shoes with a large toes box.1 Their shoes should be low-heeled and may be supplemented with either a metatarsal pad or a custom orthotic. The pads help to relieve metatarsal head pressure. The custom orthotics seek to limit foot subtalar pronation, which should theoretically limit the mobility of the first ray and reduce forefoot abduction.14 Unfortunately, a randomized trial failed to show any subjective pain differences between the pronated orthotic group and the control group.14

Anti-Inflammatory Medications

Secondly, a person may try to decrease the neuroma pain and inflammation with non-steroidal anti-inflammatory medications.

Physical Therapy

Physical therapy has also been used.15 Ultrasound, whirlpool, massage, and electrical stimulation are often used, although their efficacy has not been studied.

Corticosteroid Injections

Another treatment modality is an intermetatarsal corticosteroid and local anesthetic injection. It is imperative that the needle traverses the intermetatarsal ligament to enter the bursa and infiltrate the neurovascular bundle. While some studies show that 47% of people may experience relief with injections,16 others fail to show any long-term relief.17 Recently, studies have looked at injection of either phenol or alcohol. An electrode-guided injection of phenol proved to be effective in 80.3% of cases.18 Ultrasound-guided injection of alcohol gave 84% of patients total relief, with partial relief to 94%.19 However, there is evidence that this relief is not long-term, with many having surgery several years later for recurrent pain.20 The evidence for conservative therapies completely eradicating all of the symptoms of Morton's neuroma is relatively weak, however, effective non-operative management is the norm.12 If a patient fails these conservative measures, surgery is may be warranted. Relative contraindications include uncontrolled diabetes, complex regional pain syndrome, and vascular dysfunction.

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A dorsal incision is made in the interspace between the affected metatarsals. The 3- to 4-cm longitudinal incision is taken down through the skin and subcutaneous tissues. One must take care to identify and retract away the dorsal sensory branch of the intermediate dorsal cutaneous branch of the superficial peroneal nerve.1 Dissection is then bluntly continued down to the level of the intermetatarsal ligament. A small lamina spreader is inserted between the metatarsal heads to facilitate exposure and to place the ligament under tension. A Freer elevator is placed beneath the intermetatarsal ligament and transected with a scalpel or scissors, protecting the underlying neurovascular structures. The neuroma is then identified between the metatarsal heads adherent to the intermetarsal bursa. The neuroma is pulled distally such that the nerve trunk can be divided as proximally as possible with long scissors. The distal branches of the nerve are then dissected. The remaining nerve trunk may be treated with phenol, followed by isopropyl alcohol to prevent the recurrence of a neuroma. 

Figure 5. Location of nerve right under the skin (between base of 3rd and 4th toes)

The plantar approach is often used in cases of neuroma recurrence, as it provides better visualization of the proximal nerve trunk, albeit with the risk of a painful plantar scar. A plantar excision is made just proximal to the webspace and extends at least 4 cm proximally. The incision is extended between the metatarsal heads to avoid scarring the bony prominences. Strands of the fibrous plantar tissue must be divided. The neuroma is usually located subcutaneously, and is then resected as described above.

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The most common complication results from wound healing, keloid formation, or superficial infection.3 In one series, 1.1% of patients had a wound infection, while 2.2% had keloid formation over the scar. Recurrent neuroma formation is another complication. This may be the results from of inadequate proximal resection of the common digital nerve. This neuroma is then trapped by the metatarsal heads, compressed, and causes pain. In re-operation for recurrent neuroma, one may bury the stump in the intrinsic musculature of the foot. Despite the above complications, , but more commonly represents metatarsalgia symptoms from MTP synovitis (or another causes of metatarsalgia) that would not be addressed by the Morton's neuroma surgery. However, typically patients describe high satisfaction rates from (80% to 96%) following Morton's neuroma surgery.16,21

Risk factors and prevention

There are no clear risk factors since the exact etiology of is unknown. Tight ill-fitting shoes and shoes with high heels possibly contribute to the development of Morton’s neuroma and can be avoided.  Risk factors include clawing of the lesser toes, an equinus contracture, or tight, ill fitting shoes -anything that will lead to repetitive injury to the Morton's nerve. 

Miscellany

Historically, Morton's neuroma was first described in 1845 by Queen Victoria's surgeon chiropodist, Lewis Durlacher as a "form of neuralgic affection" involving the distal plantar nerves.22 Thirty years later, Thomas G. Morton of Philadelphia described this disorder (incorrectly) as a capsulitis of the 4th metatarsophalangeal joint, although he accurately described the disorder's symptomatology.23 In 1883, Hoadley was the first surgeon to actually excise the interdigital neuroma as a treatment for Morton's neuroma.24 Then in 1940, L.O. Betts confirmed that Morton's neuroma pain was attributed to a swelling of the interdigital nerve.2interdigital nerve.2  Morton's neuroma was named after Thomas G Morton who described the disorder in the 1800, not Dudley Morton who wrote a landmark book entitled "The Human Foot: It's Evolution,Physiology, and Functional Disorders" in 1935.

Key terms 

Medial and lateral plantar nerves, digital nerves, intermetarsal ligament, perineural fibrosis, metatarsalgia

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