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Morton's Neuroma: Getting Patients Back on Track

Matthew B. Mollica, B Pod (Honors)

THE PHYSICIAN AND SPORTSMEDICINE - VOL 25 - NO. 5 - MAY 97


In Brief: Morton's neuroma, known also as intermetatarsal or interdigital neuroma, is a common foot injury that often curtails athletic activity. Nerve compression involving adjacent metatarsal heads and the transverse intermetatarsal ligament appears implicated in injury onset. Diagnosis is made clinically, and the condition typically causes initial symptoms of dull cramping or burning pain and more persistent sharp pain with nerve deterioration. Depending on injury severity, treatment is either conservative or surgical.

Morton's neuroma is a benign, perineural fibrotic lesion of a common digital nerve. It affects females more frequently than males and is seen among patients of a wide range of ages (1-4). Typically it occurs at either the second or third intermetatarsal space, but it may also occur at other intermetatarsal spaces (3,5-7). Morton's neuroma may present bilaterally, and multiple neuromas may occur within the same foot (8).

Case Study

A 22-year-old female track-and-field athlete presented for assessment of left forefoot pain of 7 months' duration. She had recovered from recent sesamoiditis of her left foot and from a right tibial stress fracture a year earlier. The current complaint was burning discomfort with periodic sharp, intense pain around the third and fourth digits. Hurdle and high jump training made her symptoms more intense; removal of athletic shoes provided relief, as did applying ice to the foot, following training.

Physical examination revealed moderate bilateral hallux valgus and hypermobile talocalcaneal and talonavicular articulations. Palpation of the third intermetatarsal space at the level of the metatarsal heads triggered acute shooting pain and enabled detection of a thickened tissue mass. Intermetatarsal neuroma was diagnosed.

Custom-made, semirigid polypropylene thermoplastic foot orthoses were prescribed, and the patient was given two 80-mg methylprednisolone injections, 7 days apart. She was advised to rest from training for several weeks. These measures did not provide complete resolution, and symptoms persisted while the patient wore dress shoes during daily office work.

She was referred for surgical assessment 10 weeks after initial presentation, and neurectomy was later performed. A dorsal longitudinal incision was used, and the third common digital nerve, along with its daughter fibers, was resected to the level of the third metatarsal diaphysis. At a 4-month postoperative review, the patient had returned to previous training load and was experiencing no recurrent symptoms or surgical complications.

Anatomy and Causes

The common digital nerves originate from the medial and lateral plantar nerves (figure 1). The medial plantar nerve divides into many branches, including the medial three common digital nerves, which in turn bifurcate, supplying cutaneous branches to the medial three-and-a-half digits. The lateral plantar nerve gives rise to two common digital nerves, supplying cutaneous branches to the lateral one-and-a-half digits. Branches from the medial and lateral plantar nerves unite around the third intermetatarsal space near the flexor digitorum brevis (3). As the common digital nerves travel distally, they pass plantar to the transverse intermetatarsal ligament (9).

[FIGURE 1]

Various factors have been implicated in the onset of intermetatarsal neuroma. The junction of the medial and lateral plantar nerves near the muscle belly of the flexor digitorum brevis is subject to increased tensile forces on the nerve fiber during digital dorsiflexion, causing greater risk of neuroma formation (9,10). In addition, some researchers suspect that this junction at the third intermetatarsal space creates greater nerve diameter, leading to increased incidence of neuroma at this site (4,10). However, no study demonstrates increased nerve fiber diameter related to neural anastomosis at this location.

Compression of digital nerves by the metatarsal heads and the transverse intermetatarsal ligament appears to be a major cause of Morton's neuroma (4,6,8,11-13). This conclusion is supported by histologic evidence and the anatomic distribution of the lesion.

Histologic studies of excised neuroma masses demonstrate "perineural fibrosis, and fibrinoid degeneration as well as demyelination and endoneural fibrosis. These findings are consistent with compression neuropathy" (14). In more chronic cases, degeneration of the axons and proliferation of blood vessels may occur about the site of neuroma formation (6).

The location of histologic changes consistent with neuroma supports the belief that compression of nerve branches by the intermetatarsal ligaments is a factor in neuroma formation. As they travel distally, common digital nerves pass plantar to these ligaments (figure 1). Typically, fibrosis and neural degeneration are evident proximal to bifurcation of the common digital nerve, immediately distal to the transverse intermetatarsal ligament (8,12).

Further, the second and third intermetatarsal spaces, recognized as the most common places for neuroma formation (1,3-7,11), are also the most probable sites of nerve compression. Intermetatarsal head distances here are significantly less than those at the first and fourth intermetatarsal spaces (6). The smaller zone through which the nerve passes is likely to increase the risk of nerve compression, accounting for the prevalence of intermetatarsal neuroma at these sites. Excessive foot pronation and tight footwear may further reduce the distance between intermetatarsal heads, thus contributing to a greater likelihood of neuroma formation at these sites.

Diagnosis

Symptoms of intermetatarsal neuroma are localized to the forefoot and toes. The condition may initially present as a dull ache or cramping sensation, with associated numbness. Tingling or burning radiating to the toes along with intermittent symptoms of sharp, shooting pain are reported with neuroma formation. Progression results in increased intensity and duration of symptoms, possibly radiating proximally. In chronic cases, patients may describe sensations of a hardened mass within the foot at the site of discomfort. Digital dorsiflexion may cause pain during propulsive phases of walking or during forefoot weight-bearing activity, such as sprinting, jumping, squatting, or repeated hopping. Narrow-fitting footwear usually induces symptoms; relief is often reported with shoe removal or massage of the foot.

Clinically, dorsoplantar compression of the intermetatarsal space (figure 2) often reveals a palpable mass and usually reproduces pain that may radiate to the toes or proximally along the course of the affected nerve. The patient may display relative paresthesia at the web space supplied by the injured nerve; this may be assessed through tests of light touch perception at the web spaces of both feet. Dynamic assessment may reveal excessive foot pronation, contributing to increased mobility of the metatarsals and increased risk of neural compression.

[FIGURE 2]

Mulder's sign has been reported as a useful diagnostic aid (4,5). By applying manual pressure to the medial and lateral aspects of the forefoot, the neuroma may be compressed between two metatarsals. On subsequent palpation of the area plantar and distal to the transverse intermetatarsal ligament, a "click" may or may not be heard. This sound is said to result from the neuroma mass crossing the ligament.

Sonographic imaging has been shown to accurately portray the location and magnitude of intermetatarsal neuroma (15). Given the definitive nature of clinical diagnosis and the expense of imaging, ultrasound is often reserved for cases in which diagnosis of forefoot pain is unconfirmed or for use in a presurgery workup. Suspicion of a second neuroma within the same foot may be confirmed through sonographic evaluation. Magnetic resonance imaging may also demonstrate the presence of Morton's neuroma, particularly if lesions are multiple or recur following neuroma surgery.

Differential Diagnosis

A range of conditions may mimic Morton's neuroma, including metatarsal stress fracture, metatarsophalangeal joint synovitis, intermetatarsal bursitis, extensor tendon tenosynovitis, tumor, and nerve injury more proximally. Metatarsal stress fracture will present with bony tenderness and pain upon palpation of the metatarsal shaft, rather than the common digital nerve. Metatarsophalangeal joint synovitis will often prove painful during active or passive joint motion. Competing diagnoses may be definitively excluded through plain film radiography, bone scan, computed tomography, or magnetic resonance imaging.

Conservative Treatment

[FIGURE 3]

If Morton's neuroma is detected early, conservative measures may be reasonably successful (1,4,5). Ice application (figure 3) may provide relief after athletic activity or may enable an athlete to complete the event if the ice is applied during a break in play. A metatarsal pad or dome positioned over the heads of the central three metatarsals may reduce symptoms by helping to preserve intermetatarsal space, reducing the likelihood of neural irritation. The pad may be applied directly to the athlete's skin or to the liner of the athlete's shoe (figure 4).

[FIGURE 4]

Nonsteroidal anti-inflammatory drugs (NSAIDs) may relieve acute pain and inflammation. If NSAIDs provide insufficient relief, a local anesthetic injection (figure 5) can also relieve pain, and it also may confirm the intermetatarsal space at which a nerve is injured, if the exact location of neuroma is unclear. If excessive foot pronation is thought to contribute to the condition, arch taping may be initially employed to limit the magnitude of such motion. Wearing athletic shoes that offer suitable motion control and using foot orthoses may serve as ongoing methods of addressing foot function. The use of footwear that exacerbates symptoms should be discouraged.

[FIGURE 5]

Corticosteroid injection reportedly provides relatively fast and prolonged relief from intermetatarsal neuroma (2,8). Such therapy does not necessitate significant absence from activity and may be employed when conservative treatment provides insufficient relief. Adverse effects, such as skin changes, may be minimized by using solutions of greater solubility (8). If a single injection brings only temporary relief, a second or even a third may provide complete resolution. Administration should occur at 7-day intervals.

Surgical Considerations

The length of conservative treatment may vary considerably from one athlete to another, depending on the level and schedule of competition. If a conservative trial proves unsuccessful and if symptoms continue to be severe, neurectomy or decompression of the affected common digital nerve is frequently required to provide complete relief.

These procedures, however, are associated with a variety of complications, including "loss of sensation, loss of normal sweat production, and development of an end neuroma at the nerve stump site in those patients who have undergone a neurectomy (8)." The rate of poor surgical results has been repeatedly reported to be about 20% (1,3,4,11,16).

Plantar scarring is a frequent complication of surgery involving either longitudinal or transverse plantar approaches. In light of reported levels of patient dissatisfaction with these approaches (4,17), as well as the risk of scarring and resultant skin callousing, physicians should encourage techniques incorporating a dorsal incision.

As the case report emphasizes, Morton's neuroma patients who undergo surgery do not always encounter such complications. In fact, if conservative treatment fails, surgery can return an individual to previous levels of physical activity.

The author thanks Steve McMurray for his valuable professional assistance.

References

  1. Gaynor R, Hake D, Spinner SM, et al: A comparative analysis of conservative versus surgical treatment of Morton's neuroma. J Am Podiatr Med Assoc 1989;79(1):27-30
  2. Greenfield J, Rea J, Ilfeld FW: Morton's interdigital neuroma: indications for treatment by local injections versus surgery. Clin Orthop 1984;185(May):142-144
  3. Keh RA, Ballew KK, Higgins KR, et al: Long-term follow up of Morton's neuroma. J Foot Surg 1992;31(1):93-95
  4. Youngswick FD: Intermetatarsal neuroma. Clin Podiatr Med Surg 1994;11(4):579-592
  5. Head J: Morton's neuroma in athletes. Sport Health 1994;12(3):29-31
  6. Levitsky KA, Alman BA, Jevsevar DA, et al: Digital nerves of the foot: anatomic variations and implications regarding the pathogenesis of the interdigital neuroma. Foot Ankle 1993;14(4):208-214
  7. Reddy PD, Zelicof SB, Ruotolo C, et al: Interdigital neuroma: local cutaneous changes after corticosteroid injection. Clin Orthop 1995;317(Aug):185-187
  8. Thompson FM, Deland JT: Occurrence of two interdigital neuromas in one foot. Foot Ankle 1993;14(1):15-17
  9. Moore KL: Clinically Oriented Anatomy, ed 2. Baltimore, Williams and Wilkins, 1985, pp 500-509
  10. Betts LO: Morton's metatarsalgia neuritis of the fourth digital nerve. Med J Aust 1940;1:514-515
  11. Gauthier G: Thomas Morton's disease, a nerve entrapment syndrome: a new surgical technique. Clin Orthop 1979;142(Jul-Aug):90-92
  12. Graham CE, Graham DM: Morton's neuroma: a microscopic evaluation. Foot Ankle 1984;5(3):150-153
  13. Shereff MJ, Grande DA: Electron microscopic analysis of the interdigital neuroma. Clin Orthop 1991;271(Oct):296-299
  14. Schon LC, Baxter DE: Neuropathies of the foot and ankle in athletes. Clin Sports Med 1990;9(2):489-509
  15. Shapiro PP, Shapiro SL: Sonographic evaluation of interdigital neuromas. Foot Ankle 1995;16(10):604-606
  16. Mann RA, Reynolds JC: Interdigital neuroma: a critical clinical analysis. Foot Ankle 1983;3(4):238-243
  17. Kuwada GT: Plantar incisional approaches: current perspectives. J Am Podiatr Med Assoc 1995;85(1):15-21

Mr Mollica is a private practitioner at the Windy Hill Podiatry Clinic in Melbourne, Australia. He holds an honors degree in podiatry from LaTrobe University, Victoria, Australia, and is a fellow of the Australian Academy of Podiatric Sports Medicine and an associate member of the International Federation of Sports Medicine. Address correspondence to Matthew B. Mollica, B Pod (Honors), Windy Hill Podiatry Clinic, 77 Napier St, Essendon, Victoria 3040, Australia; e-mail to [email protected].


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