Description
Morton's neuroma (also known as an intermetatarsal or interdigital neuroma) is a common cause of forefoot pain. 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 (that is, Morton's neuroma is not a "benign growth of nerve tissue" , as the definition of the word "neuroma" would imply); rather what is seen is an overgrowth of the tissue around the nerve: perineural fibrosis. The etiology of the condition is believed to be related to localized repetitive overload of the forefoot and wearing shoes that have a tapered toe box or high-heels.
Structure and function
The medial and lateral plantar nerves arise from the posterior branch of the tibial nerve and innervate the sole of the foot. The medial plantar nerve is the larger of the two and supplies the great toe, the second and third toes, and the medial side of the fourth. This corresponds to the distribution of the median nerve in the hand. The lateral plantar nerve supplies the fifth toe and lateral half of the fourth toe, just as the ulnar nerve does with the fingers. These nerves split into common digital nerves as they course distally. Just proximal to the web space (that is, near the metatarsal head), the nerves bifurcate into interdigital nerves, one branch medially and one laterally, coursing out to the distal aspect of the two toes. Unlike the hand, where anastomoses between the median and ulnar nerves are rare, in the foot, the third interdigital nerve is composed of confluent fibers from both the medial and lateral plantar nerves. In about 85% of all cases, Morton's neuroma affects this third nerve, likely owing to its size (among other considerations). Thus, there are symptoms in the third and fourth toes. In the remaining 15% of cases, it is the second common digital nerve in the second webspace that is affected.
FIGURE: need a good line drawing of nerve anatomy relative to ligament. here is a not so good rough draft (jb)
The neuroma is usually densely adherent to the intermetarsal ligament. The tissue is usually a pale yellow soft mass. Histologically, there is evidence of fibroblast and Schwann cell proliferation within extensive perivascular and subintimal fibrosis. Additionally, demyelination, axonal damage and hyalinized nodules (so called Renaut bodies) under the perineurium are present.
The exact etiology of Morton's neuroma is not known. The eponymous Morton himself suggested that capsulitis of the metatarsophalangeal joint was the source of pain. Betts speculated that the contraction of the flexor digitorum brevis caused the nerve to shear against the intermetarsal ligament, which in turn causes inflammation. Another theory is that bursal enlargement produces an ischemic effect. Last, normal biomechanics alone may be responsible: shearing forces are produced when the relatively mobile 4th metatarsal moves against the relatively fixed 3rd metatarsal, and the nerve may be compressed against the intermetarsal ligament during the heel-rise phase of gait.
Patient presentation
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 two qualitatively and temporally distinct pains. The first is an intense pain that lasts for about 10 minutes and is precipitated by direct compression of the neuroma. This pain then gives way to a second type, a dull ache that is present for a few hours and abates with rest, removal of one’s shoe and direct massage. Patients may also complain of the sensation of a stone or pebble under the toes or forefoot when walking.
The patient's foot often appears to be unremarkable, without signs of intermetatarsal bursitis or dorsal bulging. On palpation, the usual location of pain is at the interspace between the metatarsal heads. Interdigital skin sensation is often decreased.
A useful test for Morton's neuroma is the "lateral squeeze test": when the forefoot is compressed by the examiner’s hands a painful or palpable "Mulder's click" might be produced by the subluxation of the neuroma between the metatarsal heads.
Objective Evidence
Morton's neuroma is usually a clinical diagnosis made on a thorough history and physical examination. Imaging, however, may be necessary to rule out the differential diagnosis. Occasionally, an X-ray may show a faint radiopaque mass and lateral toe deviation. MRI and ultrasound may also confirm a Morton's neuroma. An injection of a local anesthetics may confirm the diagnosis of a Morton's neuroma by producing an immediate (though temporary) amelioration of the symptoms.
Epidemiology
The incidence of Morton's neuroma is not known. What is known is the incidence is about 5 times higher in women than in men, but the left and right feet are equally affected. The typical patient is about 45 years old.
Differential diagnosis
Most chronic pain in the forefoot is not the result of a Morton’s neuroma. Other more common sources of metatarsalgia (the medical term for forefoot pain) are peripheral neuropathy (from diabetes most likely); stress fractures of the metatarsals; synovitis or other inflammation; and trauma. Also to be considered are Freiberg's infraction (avascular necrosis of the metatarsal head), tarsal tunnel syndrome, infection, and tumors. A thorough physical examination is essential in differentiating among these possible diagnoses.
Red flags
Foot pain in patients with diabetes can be the harbinger of complications. A history of diabetes, thus, should motivate an especially detailed examination.
Treatment options and Outcomes
Non-operative treatment is the best initial approach. The patient should be instructed to wear shoes with a large toes box and low heels; a metatarsal pad or a custom orthotic can be used to relieve pressure as well. The use of anti-inflammatory medications can be justified empirically, for pain relief. Physical therapy modalities such as ultrasound or electrical stimulation might help but studies demonstrating their effectiveness are lacking. An intermetatarsal injection that perfuses the neurovascular bundle can, with the addition of local anesthetic, help confirmed the diagnosis, and a corticosteroid in the cocktail may help produce more enduring relief.
If a patient fails to improve with the passage of time and non-operative measures, and if all other sources of the pain have been considered and eliminated as diagnostic possibilities, surgery may be indicated. This treatment involves resecting the nerve proximal to the area of fibrosis. Another option is the surgical release of the intermetatarsal ligament and removal of scar tissue. Resecting is thought to be more definitive but cutting the nerve produces toe numbness, so there is some disagreement which operation is best.)
Potential operative complications include not only the usual suspects of all foot surgery (delayed healing, infection, deep vein thrombosis to name but three) but in the case of Morton’s neuroma surgery, the persistence of the pain is not rare. Complex regional pain syndrome (formerly known as reflex sympathetic dystrophy) can appear in rare instances.
Figure . Location of nerve right under the skin (between base of 3rd and 4th toes)
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 their use should be avoided or at least limited
Miscellany
According to Stigler's Law of Eponymy states that no scientific discovery is named after its original discoverer. Indeed, Stigler's Law was itself ,described with no credit by Robert K. Merton (as the law implies), This law applies here too in a way: the condition is not named for its original correct discoverer. Although Morton has his name attached to this disorder (by being first to write about the symptoms) it was Betts who first correctly described the pathology. Complicating the eponymy, the Dr. Morton in question was a Thomas Morton, not Dudley Morton, author of the landmark text, "The Human Foot: It's Evolution, Physiology, and Functional Disorders".
Key terms
Medial and lateral plantar nerves, digital nerves, intermetarsal ligament, perineural fibrosis, metatarsalgia
Skills
Perform a lateral squeeze test and elicit a Mulder’s click, described under patient presentation.
