Interdigital Neuroma (Morton Neuroma)
Jeffrey S. Boberg
Cameron L. Eilts
Morton in 1876 attributed associated clinical symptoms of a neuroma to an “affection of the fourth metatarsophalangeal joint” and to injury of the common plantar digital nerve (1). This idea has been supported by characteristic histologic changes that reveal degeneration of the myelinated fibers, thickening and hyalinization of the walls of the epineural and endoneural vessels, and fibrosis of the epineurium and perineurium. These histologic findings alone, however, do not fully explain the pathophysiology. Morscher et al (2) compared 23 biopsies taken from patients with typical intermetatarsal neuroma symptoms and compared them to 25 plantar nerves taken from cadaver specimens. Examination revealed that the only difference between the two specimens was the diameter of the resected nerves. Both groups demonstrated epineural and perineural fibrosis. In addition, the myelinated fibers were reduced in number and size, which is not typical of a traumatic neuroma.
The etiology of interdigital nerve hypertrophy remains a subject for debate. Nissen (3) believed an interdigital neuroma was the result of ischemia. In 1979, Ha’ Eri et al (4) microscopically examined 106 specimens and found that an increase in connective tissue from repetitive trauma and also arterial sclerosis within the interspace resulted in nerve atrophy. Nunan and Giesy (5) postulated that either excessive pronation with hypermobility of the metatarsals or a pes cavus foot type may cause excessive stretching on the nerves causing direct trauma and symptoms.
Recent reports postulate the condition is caused by interdigital nerve impingement against the deep transverse intermetatarsal ligament (DTIL) and should be referred to as intermetatarsal compression neuritis (6). However, a new anatomic study does not support this theory. Kim et al (7) demonstrated that the bifurcation area of the common digital nerve is distal to the DTIL.
The most common symptom of an interdigital neuroma is pain in the forefoot that radiates to the digits. Pain may also radiate proximally or to the dorsal aspect of the foot. Patients typically describe a burning pain that is accompanied by the feeling of electric shocks or a painful numbness. Symptoms may increase when walking barefoot or while wearing tight or high-heeled shoes. Interdigital neuromas often affect women in their fifth decade of life. Although no specific time frame has been identified, neuroma symptoms appear to be a chronic condition.
The third common digital nerve is the most frequently affected nerve. Second-interspace pain is usually capsular in origin, and neuromas are much more likely to be misdiagnosed in this interspace. The occurrence rate of second-interspace neuromas versus third-interspace neuromas varies in the literature. Coughlin and Pinsonneault (8) excised a total of 74 neuromas from 66 patients and found that 22% of the neuromas occurred within the second interspace and 78% occurred in the third interspace. This is similar to Dockery’s (9) findings in which he reported an 80% occurrence rate in the third interspace. Womack et al (10) retrospectively reviewed 120 patients. Patients with second-interspace neuromas had significantly lower visual analog scale scores than those with third-interspace neuromas. Conversely, Keh et al (11) looked at 70 neuromas, and 67% were located within the second interspace. In 1983, Mann and Reynolds (12) found equal success rates with second-and third-interspace neuromas. A different diagnosis should be sought if symptoms occur in the first and fourth interspace.
Clinical examination reveals marked palpatory pain between the distal aspect of the metatarsal heads and the bases of the adjacent digits in the involved interspace. Pain between the plantar aspect of the metatarsal heads or with range of motion of the metatarsal phalangeal joints is not consistent with a neuroma. A “clicking” can often be palpated when pushing upward between the phalangeal bases and applying lateral compression to the metatarsal heads. This is a classic Mulder sign. Unfortunately, this test is not sine qua non for neuromas as it can often be elicited in normal interspaces as well. If examination reveals a Tinel or “electric” sensation with compression, the most likely diagnosis is a neuroma. A true Morton neuroma will also exhibit sensory changes in one or both sides of the affected interspace. Diagnostic injections are of little value in the accurate diagnosing of interdigital neuromas. Younger and Claridge (13) observed the surgical outcome of a series of patients that all had a surgical excision of an interdigital neuroma following a positive diagnostic block. The authors found that diagnostic injections do not correlate to a favorable surgical outcome.
In addition to diagnostic injections, MRI and ultrasound imaging may be utilized by the physician to achieve an accurate preoperative assessment. Sharp et al (14) found that the history and physical exam was the most sensitive and specific modality compared to MRI and ultrasound in diagnosing Morton neuroma in 29 cases. Other studies have shown ultrasound to be fairly accurate. Despite this, clinical assessment is still the most helpful in diagnosis.
Conservative treatment for an interdigital neuroma includes new shoes, metatarsal padding, custom orthotics, anti-inflammatory medication, steroid injections, and sclerosing injections. There is little evidence attesting to the efficacy of these modalities.
Greenfield et al (15) recommended therapeutic injections prior to surgery based on a retrospective study of 67 patients. Conversely, Gaynor et al found the success rate of conservative treatment of the intermetatarsal neuroma to be less than 80% and concluded surgery should be the initial treatment of choice in a group of 60 patients.
Injection therapy includes the options of corticosteroids, 4% alcohol sclerosing, vitamin B12, and phenol. Corticosteroid injection, usually by a series of injections, has shown to be of limited success according to several studies. Investigations have retrospectively examined patient success rate with varied results from 11% to 47%, with the best results achieved when given in a series (16,17,18,19 and 20).
Although there are a number of articles demonstrating good results with sclerosing thereapy, they are all low-level studies with insufficient information to make a recommendation.
When conservative treatment fails to provide relief, the patient may desire surgical excision of the interdigital neuroma. The overall success rate for surgical excision has been reported from 57% to 93% for the dorsal and plantar surgical approaches (21).
The Cochrane Database of Systematic Reviews found three studies that met their inclusion criteria and concluded as follows: There is a very limited indication that transposition of the transected plantar digital nerve may yield better results than standard resection of the nerve in the long term, no evidence exists to support the use of supinatory insoles, and there are very limited indications to suggest that dorsal incisions for resection of the plantar digital nerve may result in less symptomatic postoperative scars when compared with plantar excision of the nerve. These conclusions were based on three trials involving 121 patients; thus, the authors concluded there is insufficient evidence with which to assess the effectiveness of surgical and nonsurgical interventions for Morton neuroma (22).
Regardless of information available, intermetatarsal neuroma excision is a very common approach and the only location in the body in which nerves are commonly resected.
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