Morton’s neuroma is a compressive neuropathy of the
interdigital nerve. It occurs most commonly in the third interdigital space.
Perineural fibrosis and entrapment of the interdigital nerve may be the cause
of this problem. The patient usually describes a forefoot pain on the plantar
aspect of the foot, especially during standing and walking. This is usually
associated with radiation of tingling and burning into the toes that are
affected. The pain is usually localized to a specific area, and it does not
involve the entire forefoot. It occurs predominantly between the 3rd
and 4th web space in 80% of the time and between the 2nd
and 3rd web space 20% of the time. The pain is radiating distally in
about 60% of the time and numbness occurs about 40% of the time. When you
examine the patient, the area of focal and localized tenderness is in the
plantar webspace and not over the joints. The interdigital neuroma (Morton’s
neuroma) occurs in middle aged females with forefoot pain that is worse with
shoe wear. The pain is worse with weight bearing or wearing tight shoes with
high heels. The weight transfer to the metatarsal heads will aggravate the
condition. Dorsiflexion of the toes will also aggravate the condition. The
symptoms are relieved by removing the shoes and massaging the foot. The
paresthesia is most commonly on the plantar aspect of the web space. The patient
will have chronic pain in the interdigital space between the 3rd and
4th toes that occasionally radiates down distally into the toes, and
the foot exam is normal. Compression test of the web space may be positive.
Compression of the metatarsals medially and laterally (squeezing the
metatarsals) while pushing on the plantar tissue dorsally. The bursal tissue
may crease a “click”, and if this happens it is the classic test. Ultrasound
and MRI are helpful, but it is usually not as good as the history and physical
examination. MRI has a very limited role in the diagnosis of Morton’s Neuroma.
EMG and nerve studies are of little benefit. The history and physical exam is
the gold standard for the diagnosis of interdigital neuroma. You can add
diagnostic injection to see the result. Injection of local anesthesia into the
are of the interdigital nerve can be diagnostic for Morton’s neuroma.
Differential diagnoses include metatarsalgia, stress fractures, MTP synovitis,
complex regional pain syndrome, arthritis, osteonecrosis of the metatarsal
head, neoplasm, and lumbar radiculopathy. X-rays are helpful in excluding
metatarsal stress fractures. Consider MTP synovitis especially after digital
nerve block. Treatment can be conservative or surgical. To treat conservatively,
start with shoe wear modification (no high heals or tight shoes). Conservative
treatment also includes injection of steroids. This will give relief of
symptoms in 1/3 of the patients with multiple injections. Injection is usually
done dorsally, and it may be more diagnostic than therapeutic. Surgery an be
neurectomy, done after failure of nonoperative treatment (approach the neuroma
through dorsal or plantar approach). Dorsal approach is most commonly used.
Incise the transverse metatarsal ligament. Resect the nerve 3 cm proximal to
the metatarsal heads. Bury the proximal stump within the intrinsic muscles. If
the neuroma is recurrent, then there may be a retained neuroma distal to the
metatarsal heads. In this case, you may want to do surgery through the plantar
approach, and you may want to do the research of the nerve and transpose it to
muscle on the plantar foot (there might be about 70% success rate). The most
likely cause of recurrent symptoms following excision of a 3rd web
space neuroma is a traumatic neuroma tethered by plantar neural branches. When
a recurrent neuroma occurs at the end of the resected nerve, it does not
retract far enough because the transection may not be far proximal enough or it
may be tethered by the plantar neural branches. Stump neuroma can also occur
due to inadequate resection. Causes of surgical failure include resection of
the common plantar nerve is too distal, there may be a coexisting tarsal tunnel
syndrome, wrong diagnosis, or wrong interdigital space.
