Morton's Neuroma is a painful condition
of the forefoot that is caused by the entrapment of the common intermetatarsal nerve as it passes
through the forefoot to the toes. This condition was first described by
Dr. Morton, a Viennese physician, in 1876. The most common location for this
entrapment is between the third and fourth toes. The second most common
location is between the second and third toes.
To help understand this
condition a bit better, let's break down the word neuroma into its' root
form. Neuro relates to the nervous system and in this case, we are
describing a portion of the peripheral nervous system. The suffix 'oma'
is the Latin term that defines a tumor or swelling that is of primary
origin. Put the terms together and what is described is a tumor or
swelling of a peripheral nerve. Interestingly, a Morton's neuroma is not
truly a tumor, but more accurately, a nerve entrapment. Although
the term neuroma is somewhat inaccurate in describing this condition,
the term neuroma is still used today to describe this unique nerve
entrapment.
What causes Morton's Neuroma? Why does the intermetatarsal
nerve become entrapped? Clinicians and surgeons recognize a number of
factors that may aggravate Morton's neuroma, but the primary cause of Morton's
neuroma remains elusive. Shoes that are tight in the forefoot will contribute to the symptoms of
Morton's Neuroma by binding the forefoot and compressing the common
intermetatarsal nerve. High
heels will also act to increase the ground reactive forces. Ground reactive
force is the amount of force generated as the foot pushes against a fixed
surface like the floor. With high heels, the amount and focus of ground
reactive force increases since weight bearing is focused in a smaller area (just
the forefoot). A higher heel also puts the common intermetatarsal nerve
under tension, making it more prone to injury. Activities such as squatting will increase the
ground reactive force applied to the
plantar foot and aggravate the symptoms of Morton's neuroma. And finally,
clinician also agree that hypermobility of the forefoot can contribute to the
formation of Morton's neuroma.
Treatment of Morton's neuroma
It's interesting to note that until the early 1990's, we
treated Morton's neuroma in same way as described by Dr. Morton some 100
years ago. But over the past ten years, our understanding and
treatment of neuromas has changed dramatically. Our understanding of
Morton's neuroma as an entrapment and not a tumor can be attributed to
the work of Steve Barrett, DPM. Dr. Barrett was the first to take a
critical look at Morton's neuroma and describe it as an entrapment
rather than a tumor.
What Dr. Barrett recognized was
that the common intermetatarsal nerve is sometimes prone to becoming entrapped as it passes beneath the
intermetatarsal ligament. This was a new concept for us in light
of the fact that we had considered Morton's neuroma a tumor. In fact, Dr.
Barrett's findings enabled us to recognize
Morton's Neuroma to be similar to other nerve entrapments such as carpal
tunnel syndrome or tarsal tunnel syndrome. Subsequently, the treatment of Morton's Neuroma
has been slowly changing over the last ten years as the result of a new
endoscopic surgical procedure first described by Dr. Barrett.
Diagnostic testing to evaluate Morton's neuroma includes plain
x-rays, diagnostic ultrasound and MRI. Plain x-rays are not actually used
to visualize the nerve, but are use rather to screen for bone and joint
pathology adjacent to the nerve. Metatarsal fractures, Freiberg's
infraction and osteoarthritis are common conditions that can influence the
behavior of Morton's neuroma and need to be evaluated with x-ray.
Several authors have suggested that the efficacy of MRI and
ultrasound as diagnostic tools are comparable when evaluating patients for
Morton's neuroma. Diagnostic ultrasound is significantly less expensive
and much more readily available compared to MRI. Kankanala et. al described a
91.48% pre-op predictive value for diagnostic ultrasound when screening for
Morton's neuroma.
Conservative care of Morton's neuroma can be quite successful. 70% or more of new Morton's
neuroma patients respond to simple changes in shoes such as a wider toe
box. Shoe padding can also help treat Morton's neuroma.
Metatarsal pads are an
important tool for patients with Morton's neuroma symptoms. A
metatarsal pad is a small lift that is positioned in the shoe just
proximal (behind) the weight bearing surface of the metatarsal
bones. A metatarsal pad lifts and separates the metatarsal
bones thereby decreasing the pressure on the intermetatarsal
nerve. Some
prefabricated arch supports come with a metatarsal pad
already seated in the correct position. Using inserts with a metatarsal
pad is sometimes the easier way to use a met pad because they can be
easily moved from
shoe to shoe. Also, by using an insert with a fixed metatarsal
pad, the position of the met pad is always in the correct location.
Other non-surgical
methods of treating Morton's neuroma include injectable cortisone and
chemical sclerosis of the intermetatarsal nerve. Cortisone has
been used successfully for years in treating Morton's neuroma.
Although the use of cortisone does not actually treat or change the
entrapment of the intermetatarsal nerve, cortisone can decrease
inflammation and swelling of the nerve, resulting in a decrease in pain.
Care should be exercised when using cortisone injections noting that
excessive cortisone injections can thin the plantar fat pad of the foot.
Sclerosis of the nerve (also called chemical neuro-ablation or
chemical neurolysis) can be performed in the
office using a number of different solutions, most commonly dilute (4%) alcohol. Multiple
sclerosing injections are used to destroy the contents of the peripheral nerve.
A series of injections are employed, each injection separated by a period of
7-10 days. The total number of injections may vary from 3 to 7. The success rates of injectable sclerosing solutions have been reported to be as high as
60-90%. Chemical neurolysis is also a great tool for failed
neuroma surgeries where a stump neuroma has formed.
The intent of chemical neurolysis is to destroy the internal
contents while preserving the external sheath of the nerve. This would be
a bit like removing the copper wire in an electrical wire while preserving the
plastic outer insulation or cover of the wire. The reason that this is
important is due to the fact that peripheral nerve will regenerate over time.
With the nerve sheath intact, regeneration of the nerve is possible in a
controlled manner utilizing the existing sheath. By contrast, removal of
the nerve by surgery results in the nerve regenerating and the formation of a
mass of scar tissue called a stump neuroma. Knowing that peripheral nerve
may regenerate also means that sclerosing injections may need to be repeated at
some point in the future. The percentage of repeat sclerosing injections
varies but is overall quite low.
Another new technique used to treat Morton's neuroma is called cryogenic neuroablation. Cryo surgery is surgery that uses extremely cold
instrumentation to selectively destroy tissue. Cryosurgery has
been used commonly to destroy superficial skin lesions such as warts and
moles. In the case of Morton's neuroma, work has recently been
done by several Michigan podiatrists that suggests cryogenic
neuroablation may have a future in treating Morton's neuroma. The
technique uses what is referred to as the Joule-Thompson effect.
The Joule-Thompson effect occurs when a gas is passed through an area
where it may expand. As the gas expands, it cools to approximately
-70 degrees centigrade . In the case of cryogenic neuroablation, the
expansion of the gas is controlled in a 5.5 mm probe that freezes and
subsequently destroys the nerve tissue.
In
the cryogenic ablation study carried out by Drs. Caporusso, Fallet and
Savoy-Moore, thirty one neuromas were treated in 20 patients. All
procedures were performed in an office setting. The procedure used a small
amount of local anesthetic to numb the skin to allow the passage of a 12-gauge
cannula through the skin. A nerve stimulator was passed through the
cannula to locate the nerve. Once the position of the nerve was
established, two three minute freeze sessions were utilized to destroy the nerve
tissue. A sterile dressing was applied to the site and the patient was
dismissed without the need for pain medication. The study cites a 65%
success rate.
Dr. Morton's original treatment plan as
described in 1876 included changes in shoes, multiple injections of
cortisone and if necessary, complete excision of the common
intermetatarsal nerve. We've mentioned before that Morton's
Neuroma is a nerve entrapment much like carpal tunnel. Now let's
see if we can apply Dr. Morton's treatment plan to any other nerve
entrapment such as carpal tunnel syndrome.
Perhaps we'd splint the wrist, try some injectable cortisone, but
completely excise the nerve? No way. But that's what's been done
for the past 100 years for Morton's Neuroma. Post-op complications
were common and included thinning of the plantar fat pad and loss of
sensation in the 3rd and 4th toes.
The introduction of
Dr. Barrett's EDIN procedure has revolutionized the treatment of
Morton's Neuroma and really represents the first unique contribution to
treating this condition in over 100 years. The EDIN procedure
stands for endoscopic decompression of the common intermetatarsal nerve. Interestingly enough, Steve describes first thinking about
this procedure as he watched another surgeon perform an endoscopic
carpal tunnel surgery. Steve recognized the problem to be the
ligament and not the nerve. The EDIN procedure selectively releases the
ligament and leaves the nerve intact.
The EDIN procedure provides
us with a new alternative. In the past we knew that the
traditional surgery used to treat Morton's Neuroma, called a neurectomy,
was destructive and carried with it a number of post-op complications.
Therefore, we would tend to use excessive amounts of cortisone to avoid
surgery. The EDIN procedure provides a new
alternative using non-invasive endoscopic techniques that usually return
patients to activities much sooner than the traditional surgery.
And, what I find most helpful is the fact that it enables us to use less
cortisone, thereby avoiding fat pad atrophy. The question remains; was
the common complication of fat pad atrophy
due to the neurectomy itself or did it result from the overuse of cortisone? The EDIN
procedure shows none of the traditional post-op complications that were
so commonly seen in
the neurectomy, therefore we can assume that fat pad atrophy was in part
due to overuse of cortisone.
The EDIN procedure has been used for at least ten years
and has shown promising results. It can be technically challenging
for some who are not familiar with endoscopic techniques. As with
other surgical procedures there are pros, cons and possible complication
that need to be discussed thoroughly with your physician prior to
surgery. The following pictures show the technique used to perform
an EDIN procedure. Image 1 shows pre-operative markings
identifying the 3rd and 4th metatarsal heads. Image 2 shows
placement of the cannula through an interdigital incision. The
cannula is much like a small 4mm drinking straw with a slot cut in one
side. The slot or open side of the cannula is placed adjacent to
the intermetatarsal ligament. The cannula passes from between the
toes to a second incision on the plantar aspect of the foot just
proximal to the weight bearing surface. The endoscope and knife
are used within the slotted cannula to identify and transect the
intermetatarsal ligament. Image 3 show the use of a blunt probe
without the cannula to verify a complete release of the intermetatarsal
ligament. In the bottom of image 3, a metatarsal spreader can be
seen. The spreader is used to separate the 3rd and 4th metatarsals
subsequently putting pressure on the intermetatarsal ligament. The
procedure takes about 20 minutes and is completed in a hospital or
surgery center. Local anesthesia with sedation is used.
Patients return to regular shoes in two days with just a band-aid on the
incisions.
Traditional neurectomy, or removal of the nerve, is used
less often due to the success of sclerosing injections and the EDIN
procedure. Neurectomy can be performed from a dorsal or plantar
approach. The advantage of a dorsal approach is that patients are
able to walk immediately following the surgery. The disadvantage
of the dorsal approach is that it requires more dissections and possible
tissue trauma. The plantar approach results in less tissue trauma
but requires that patients are non-weight bearing on the surgery foot
for 3 weeks post-op.
