Interdigital Neuritis
Interdigital neuritis, which is known more commonly as Morton neuroma, is a condition that results from entrapment of a plantar interdigital nerve as it passes under the transverse metatarsal ligament.52 This condition was named after Thomas Morton, who reported "a peculiar and painful affection of the fourth metatarso-phalangeal articulation" in 1876.53 The condition has since been defined as a perineural fibrosis of the involved interdigital nerves, and true neuromatous involvement is not seen.
Other terms used for this condition include interdigital neuroma, Morton metatarsalgia, and interdigital nerve compression.
This condition is most common in the second or third interspace, but occurrences in the first and fourth interspace have been reported. Double neuromas occurring in the second and third interspace are not uncommon. Morton neuromas are more common in women than in men; they can occur in persons of all ages, but they are most common in middle-aged individuals.
Anatomy
The common digital nerves originate from the medial and lateral plantar nerves. The medial plantar nerve divides into 3 common digital nerves, which in turn bifurcate, supplying cutaneous branches to the medial 3.5 digits. The lateral plantar nerve gives rise to 2 common digital nerves, supplying cutaneous branches to the lateral one and a half digits. As the common digital nerves travel distally, they pass plantar to the transverse intermetatarsal ligament.
Substantial variability has been noted in the cutaneous innervation of the webspace and in the plantar aspect of the foot adjacent to the webspace.
Etiology
The most direct cause of entrapment of the interdigital nerve is compression of the nerve as it passes under the transverse intermetatarsal ligament. As weight is transferred to the ball of foot when the toes are dorsiflexed during the last phase of stance, the interdigital nerve is compressed between the plantar foot and the distal edge of the intermetatarsal ligament. Many theories exist about the pathophysiology of this compression, including ischemia, inflammation, soft-tissue trauma, tumor, muscle imbalance, and fibrous ingrowth.
Edema of the endoneurium, fibrosis beneath the perineurium, axonal degeneration, and necrosis often are seen; their presence suggests that nerve damage occurs secondary to mechanical impingement. Using electron microscopy, Ha'Eri and colleagues saw lesions consisting of a progressive fibrosis that enveloped and disrupted nerves and arteries.54 They did not see nerve-tissue proliferation or inflammation. The authors concluded that repeated trauma leads to reactive overgrowth and scarring that disrupts the nerves and the arteries.
Typically, these changes are evident proximal to bifurcation of the common digital nerve, immediately distal to the transverse intermetatarsal ligament. In more chronic cases, degeneration of the axons and proliferation of blood vessels may occur about the site of an enlarged nerve. Shereff and Grande described the presence of Renaut bodies, which are densely packed whorls of collagen, in the supraneural space.55 These bodies are characteristically present only in peripheral neural entrapment.
However, Morscher and coauthors, in a histomorphologic study, found only a diameter difference between biopsy results from patients with typical symptoms of Morton neuroma and nerves examined from autopsies of persons without forefoot problems.56 In addition, some have implicated the inter-MTP bursa as the main cause of Morton neuroma.
Forefoot deformities, such as hammertoe, can further aggravate the nerve because of dorsal subluxation of the proximal phalanx, which stretches the interdigital nerve over the intermetatarsal ligament. In addition, there are a number of external conditions that have been noted to contribute to the development and aggravation of this compression. High-heeled shoes put the feet in chronic dorsiflexion, forcing one's weight onto the forefoot; and tight shoes, which compress the foot further, limit the intermetatarsal space.
Multiple investigations have looked into the underlying reason for the common locations in the second and third webspace. Levitsky and colleagues demonstrated that the relative space in the metatarsal head/transverse metatarsal ligament is smaller in the second and third webspaces, where the condition is more common, than it is in the other webspaces, therefore supporting mechanical factors as the underlying pathophysiology.57 They also dismissed the common theory that the cause relates to the occurrence of a lateral and medial plantar nerve combination and to the anatomical thickness of the nerve. They reported neuromas in third webspaces in which the plantar nerve did not arise from a combination of medial and lateral plantar nerves. The intermetatarsal head distances in the second and third interspace have been noted to be significantly less than are those in the first and fourth intermetatarsal space.
Clinical
Many patients with Morton metatarsalgia present with an intermittent dull ache or cramping sensation on the plantar aspect of either the second or third interspace. Many patients present with a vague discomfort in the involved toes, and some may feel numbness or burning, with occasional shooting pain. Some patients notice spreading of the involved toes, and others may notice symptoms only with certain shoes. Symptoms usually worsen with walking, particularly with bare feet on hard surfaces, and sudden, sharp pain may result from various activities, such as such as sprinting, jumping, squatting, or repeated hopping, as well as with wearing high-heeled or tight shoes. Symptoms improve with rest, and night pain is rare. With progression of the condition, pain may radiate proximally. In chronic cases, patients may sense a mass or a stone bruise in the ball of the foot.
Nearly equal involvement of the second and third interspace has been reported in the literature. Involvement of the first and fourth space also has been reported, albeit rarely.
Simultaneous tenderness in the second and third interspace is not rare. Bilateral cases have been reported but are uncommon. Coexistence of other pathologies of the forefoot, such as instability of the second MTP joint, is not uncommon. Coughlin and colleagues reported that 20% of their patients originally had concomitant instability of an adjacent MTP joint.
Clinically, dorsoplantar compression of the second or third intermetatarsal space reproduces pain that may radiate to the toes or proximally along the course of the affected nerve. The patient may display relative paresthesia of the webspace supplied by the injured nerve, although this is often difficult to ascertain.
The Mulder click is a useful diagnostic test. The forefoot is held in one hand, and the metatarsal heads are squeezed while the other hand places direct pressure on the plantar aspect of the interspace. As the metatarsal heads are compressed, the enlarged nerve is pushed plantar and away from the metatarsal heads, and a click is palpated with the digit in the plantar web space just distal to the transverse intermetatarsal ligament. This is often uncomfortable for the patients but does not usually reproduce their symptoms. This palpated mass can be pushed up again in the interspace with manual pressure, while partial compression is maintained on the metatarsal heads. Many times, 1 digit is noted to move suddenly.
A range of conditions may mimic Morton neuroma, including metatarsal stress fracture, MTP joint synovitis, intermetatarsal bursitis, extensor tendon tenosynovitis, tumor, and more proximal nerve injury. Metatarsal stress fracture will present with bony tenderness and pain upon palpation of the metatarsal shaft, rather than the common digital nerve. MTP joint synovitis will often prove painful during active or passive joint motion.
The diagnosis of Morton neuroma is based primarily on clinical findings. Careful clinical examination usually will reveal other conditions that can mimic or coexist with intermetatarsal neuritis. In complex cases, immediate, temporary resolution in response to a local anesthetic block proximal to the involved area below the intermetatarsal ligament can confirm the diagnosis.
Ultrasonography and MRI are helpful in rare, complex situations.58 The accuracy of these studies, however, varies significantly and depends on multiple factors, including the MRI machine, the technician and the technique, and the interpreting radiologist or orthopedic surgeon.
Ultrasonography reveals a hypoechoic, ovoid mass parallel to the long axis of the metatarsal. Ultrasonography can also be used to diagnose other pathologic conditions in the forefoot, such as bursitis and MTP joint effusion. Reed and colleagues reported this study to be 95% sensitive in detection of webspace abnormality, but Morton neuroma could not be distinguished clearly from an associated mass, such as mucoid degeneration in the adjacent loose connective tissue.
Quinn and coauthors found that ultrasonography revealed the diagnosis in 85% of cases in which it was suspected.59 They reported limitation in detecting neuromas shorter than 5 mm. Terk and colleagues reported on MRI with T1- and T2-weighted sequences, along with a combination of fat suppression and the administration of gadopentetate dimeglumine.60 Williams and colleagues showed that T1-weighted axial and coronal images obtained with an axial, fast spin-echo (FSE), T2-weighted sequence depict neuromata more consistently than do other methods.
Zanetti and coauthors studied 54 feet in which Morton neuroma was suspected in order to determine the effect of MRI results on diagnostic thinking and the therapeutic decisions made by orthopedic surgeons.61 The authors noted considerable change in the diagnosis, location, and number of neuromas, as well as in the treatment plans, after MRI.
In a second report, Zanetti and colleagues suggested that the diagnosis of Morton neuroma based on MRI results are relevant only when the transverse diameter of the fluid collection in the bursa is 5 mm or more and when it was correlated with the clinical findings. Fluid collections in the first 3 metatarsal bursae with a transverse diameter of 3 mm or less are probably physiologic.
In a histomorphologic study of patients and autopsies, Morscher and coauthors concluded that diagnostic MRI or ultrasonography is unnecessary for making decisions about operative treatment.56 In addition, Resch and coauthors found that MRI modalities had little or no value in the diagnosis of Morton neuroma because of the high rate of false-negative results.62
Treatment
If Morton neuroma is detected early, conservative measures may be reasonably successful. About 20-30% of patients achieve adequate relief with nonoperative management. This may be accomplished by eliminating or minimizing the external sources of compression or stretch on the interdigital nerve.
Extra-wide shoes and low heels, as well as the placement of a small metatarsal pad just proximal to the heads of the central 3 metatarsals, may reduce symptoms by increasing the intermetatarsal space, elevating the metatarsals and the intermetatarsal ligament, and reducing the likelihood of neural irritation. Accommodative orthotic devices with built-in metatarsal pads may at times be helpful, especially in patients with alignment abnormalities.
NSAIDs may relieve acute pain and inflammation. If NSAIDs provide insufficient relief, a local anesthetic injection can also relieve pain and may help to confirm the diagnosis of Morton neuroma.
Corticosteroid injections have not been shown to result in predictable or lasting relief, as reported by Mann and colleagues. Rasmussen and coauthors reported initial pain relief in 80% of patients who received a single corticosteroid injection.63 However, 47% of the 41 feet in the study ultimately required surgical excision; the majority of the remaining 53% had residual symptoms. In addition, corticosteroid injection for intermetatarsal neuroma has been associated with a number of complications, including plantar fat-pad atrophy, skin depigmentation, hyperpigmentation, and telangiectasias. Fat-pad atrophy can result in metatarsalgia and gait impairment.
A number of different surgical procedures have been used. Excision of the interdigital nerve and release of the intermetatarsal ligament is the most commonly performed procedure. A variable degree of subjective numbness occurs in the 2 toes served by that nerve, which is symptomatic in fewer than 10% of cases. Dorsal and plantar incisions have been reported in the literature.
Most surgeons in the United States perform a primary surgery through a dorsal approach. The incision is approximately 3 cm in length and is centered in the respective interspace, starting from the metatarsal head level and extending distally into the respective webspace. Blunt dissection into the webspace and placement of a lamina-style spreader reveals the intermetatarsal ligament. The proximal and distal aspects of the ligament are identified and released sharply. An enlarged nerve may be identified readily beyond the ligament. Smaller nerves are more difficult to identify. In most cases, intertwining vessels require careful dissection and protection. The digital branches are identified and are sharply amputated distally. The nerve is then followed proximally, gently pulled distally, sharply amputated as far proximally as is visible, and allowed to retract into the deep soft tissues.
Mann and Reynolds reported retrospectively on a surgical excision of 76 Morton neuromas.64 Although 65% of patients still noted some local plantar tenderness to touch, 80% were subjectively improved. Coughlin and Pinsonneault noted residual pain in either the involved or adjacent interspace, which is not uncommon.65 They also reported 85% good-to-excellent results. They noted mild or major footwear restrictions in 70% of patients.
Dereymaeker and colleagues reported on 32 feet that were treated with excision of the Morton neuroma by using a dorsal incision.66 Twenty-five of 32 patients had a macroscopically visible neuroma, and 2 patients had no evidence of a neuroma on histologic examination. After resection, 81% had a good or excellent result, and 6.5% had no improvement. The authors noted that, following the operation, 60% of their patients benefited from the use, over a long period of time, of adapted shoes or inner soles. At the final follow-up, only 30% of the patients were found to be unrestricted in their choice of shoes.
Some authors advocate the plantar approach for primary resection of an intermetatarsal neuroma, as it provides more direct access to the nerve and is technically simpler. However, painful plantar scars, intractable plantar keratosis beneath an adjacent lesser metatarsal head, and wound drainage have been reported. Because of these potential complications, most surgeons reserve the plantar approach for revision cases. Karges reported on 57 plantar surgeries performed using a plantar incision.36 He reported 23% indurated plantar keratosis (IPK) after surgery, and only 7% had poor results.
Potential complications of interdigital nerve excision, including development of a stump neuroma and digital numbness, have led some authors to recommend release of the transverse metatarsal ligament with or without epineurolysis. Gauthier reported on the release of 304 intermetatarsal ligaments and epineural neurolysis.67 He stated that 83% had rapid and stable improvement, and that 15% were improved but nonetheless suffered some persistent pain. Others, such as Weinfeld and Myerson, have advocated this procedure without neurolysis and have reported good preliminary results.68 Mann and Reynolds cautioned against the use of this procedure except for Morton neuroma, as they noted reconstitution of the ligament in revision cases.64
Adjacent neuromas in the second and third interspace should be investigated. Reports of double neuromas in the literature are sparse. Benedetti and colleagues reported simultaneous excision of 2 primary Morton neuromas in adjacent webspaces and noted significant pain relief in 84% of patients, but substantial numbness involving the third toe resulted.69 Thompson and Deland described 89 adjacent neurectomies and reported similar results to those achieved with single neurectomies.70
Hort and DeOrio reported on 23 patients with adjacent intermetatarsal nerve irritation who underwent excision of the more enlarged nerve in 1 space and the release of the intermetatarsal ligament in the other.71 This allowed preservation of protective sensation. They reported 95% complete satisfaction, with no or only minimal activity limitation. Approximately 11% had persistent pain with compression of the interspace where the nerve was released. None had pain in the interspace where the nerve was excised. The investigators performed this procedure through 1 incision centered over the third metatarsal.
In summary, Morton neuroma is the result of chronic compression of the interdigital nerve as it passes under the transverse intermetatarsal ligament. It is seen most commonly in the second and third intermetatarsal space, and it causes pain that may radiate to the toes or proximally along the course of the affected nerve. Pain with compression of the intermetatarsal space and the presence of a Mulder click confirm the diagnosis.
Nonoperative management is successful in about 20-30% of all cases. Nonoperative management includes the avoidance of high-heeled and tight shoes, as well as the use of a small metatarsal pad and stiffer, rocker-soled shoes. Surgical excision of the nerve and release of the intermetatarsal ligament with a dorsal or plantar approach results in a high percentage of successful results. The dorsal approach is recommended because of fewer potential complications, and also because it allows early weight bearing. Patients should be counseled preoperatively that varying degrees of postoperative numbness are commonly associated with resection of a Morton neuroma. Other surgical options include isolated intermetatarsal ligament release.
Adjacent neuromas in the second and third interspace are not uncommon. Excision of the most involved nerve and release of the intermetatarsal ligament in the less-involved nerve lead to good relief of symptoms without significant neurologic deficit.
Patient Education
For excellent patient education resources, visit eMedicine's Foot, Ankle, Knee, and Hip Center and Procedures Center. Also, see eMedicine's patient education articles Knee Pain Overview, Bursitis, and Electromyography (EMG).
Keywords
nerve entrapment, nerve entrapment of lower extremity, iliohypogastric nerve entrapment, ilioinguinal nerve entrapment, genitofemoral nerve entrapment, lateral femoral cutaneous nerve entrapment, femoral nerve entrapment, saphenous nerve entrapment, obturator nerve entrapment, peroneal nerve entrapment, posterior tibial nerve entrapment, tarsal tunnel syndrome, superficial peroneal nerve entrapment, deep peroneal nerve entrapment, interdigital neuritis, Morton metatarsalgia, meralgia paresthetica, Morton neuroma, slimmer's paralysis, strawberry picker's palsy, Tinel sign, heel pain triad
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