Also known as Morton's metatarsalgia or Morton's neuroma, although Morton described neither (Morton thought this was a problem in the 4th MTP joint and Betts described the "neuroma" 70 years later).
The "neuroma" consists of degenerative and fibrotic changes in the common digital nerve near its bifurcation. However, there may be similar changes in adjacent unaffected nerves and it is not known why one becomes symptomatic. A number of causative factors have been suggested:
- entrapment by the deep transverse metatarsal ligament
- tethering of the 3rd space nerve by the anastomotic branch between medial and lateral plantar nerves
- traction on the nerve by hindfoot valgus, interdigital bursitis or forced toe dosiflexion in high-heeled shoes
The symptoms may be quite non-specific:
- neuralgic pain in a toe and/or interdigital space
- tingling of a toe
- colour changes
- numb or "dead" toe
- vague forefoot tingling
- pain usually worse on walking and sometimes at night
- relief on removing shoes
Symptoms are commonest in the 3rd interdigital space, then the 2nd. Symptoms in the 4th space are rare and should make one doubt the diagnosis. Symptoms in the first space are virtually unknown.
The condition may remain undiagnosed for many years.
Clinical assessment
The diagnosis is often strongly suspected within the first minute of the consultation. However, it may be arrived as part of the assessment of a more generalised metatarsalgia, the details of which are found on the appropriate page. In any case, a full assessment of the foot should be carried out.
Ask about:
- conditions which may cause a peripheral neuropathy, especially diabetes and chronic inflammatory disorders
- trauma to the foot
- discomfort around the ankle which may suggest tarsal tunnel syndrome
- spinal problems, especially about any history of root entrapment symptoms.
Examination should begin with assessment of any suggested nerve entrapment in the spine, proximal limb or tarsal tunnel.
The whole foot should be examined, looking for any other factors likely to produce metatarsalgia.
On local examination look for:
- local tenderness ± swelling in the intermetatarsal space
- Mulder's click on metatarsal compression
A local anaesthetic injection into the affected space may be useful - if it relieves the symptoms this is supportive of the diagnosis.
Imaging
Both ultrasound and MRI have been described for imaging a neuroma, but the evidence for their value is not strong. However, if the clinical situation is atypical an ultrasound scan may be useful. If there is a suggestion of other forefoot pathology standing AP and lateral forefoot films should be obtained.
Management
All patients should be advised on the use of shoes with adequate room in the toe-box and high heels should be avoided.
There is no proven role for orthoses.
If simple measures do not control the pain a steroid injection into the intermetatarsal space should be offered. The patient is warned that it may be quite painful for several days and they may need to rest more than usual. Also warn about the small risks of infection and cutaneous atrophy. The published results of this treatment are variable. Greenfield (1984) found that 90% of patients had little or no pain two years later, even if they got temporary or no benefit from the initial injection. Bennett (1995) found that about 50% of patients were relieved of pain by a single injection, the authors imply, but do not substantiate, that this result was maintained at follow-up 2.5-5 years later. Rasmussen, however, found that although 80% were relieved of pain by a single injection, 47% eventually had a neurectomy and most of the rest were symptomatic at review 2-6 years later.
If symptoms persist despite non-surgical treatment and the diagnosis is regarded as firm enough the patient may be offered an interdigital neurectomy.
We quote a success rate of 80% and warn patients that it may take several months to reach full benefit. I also warn them that a few patients may develop a new neuroma on the severed nerve end which may be more painful than the original problem.
The operation is done through a dorsal interdigital incision. Other surgeons prefer a plantar approach. The nerve is divided 2-3cm proximal to the bifurcation and excised. The deep transverse metatarsal ligament may be partially released but I avoid dividing this important structure. The wound is closed with subcuticular Vicryl. Post-operatively the patient mobilises fully weight bearing.
Decompression of the interdigital space with excision of the bursa, division of the deep transverse metatarsal ligament and neurolysis of the common digital nerve has been suggested but remains unproven. A randomised controlled trial against neurectomy would be helpful.
References
- Benedetti RS et al. Clinical results of simultaneous adjacent interdigital neurectomy in the foot. FAI 1996; 17:264-8
- Bennett Gl et al. Morton’s interdigital neuroma: a comprehensive treatment protocol. FAI 1995; 16:760-3
- Colgrove RC et al. Interdigital neuroma: intermuscular transposition compared with resection. FAI 2000; 21:206-11
- Coughlin MJ et al. Concurrent interdigital neuroma and MTP joint instability: long-term results of treatment. FAI 2002; 23:1018-25
- Coughlin MJ, Pinsonneault T. Operative treatment of interdigital neuroma. JBJS 2001; 83A:1321-8
- Dereymaeker G et al. Results of excision of the interdigital nerve in the treatment of Morton’s metatarsalgia. Acta Orthop Belg 1996; 62:22-5
- Greenfield J et al. Morton’s interdigital neuroma. CORR 1984;142-4
- Johnson JE et al. Persistent pain after excision of an interdigital neuroma. JBJS 1988; 70A:651-7
- Levitsky KA et al. Digital nerves of the foot: anatomic variations and implications regarding the pathogenesis of interdigital neuroma. FAI 1993; 14:208-14
- Miller SD. Technique tip: forefoot pain: diagnosing metatarsophalangeal joint synovitis from interdigital neuroma. FAI 2001; 22:914-5
- Morscher E et al. Morton’s intermetatarsal neuroma: morphology and histological substrate. FAI 2000; 21:558-62
- Okafor B et al. Treatment of Morton’s neuroma by neurolysis. FAI 1997; 18:284-7
- Rasmussen MR et al. Nonoperative treatment of plantar interdigital neuroma with a single corticosteroid injection. CORR 1996; 326:188-93
- Sharp RJ et al. The role of MRI and ultrasound imaging in Morton’s neuroma and the effect of size of lesion on symptoms. JBJS 2003; 85B:999-1005
|