Steven L. Shapiro
DEFINITION
Morton's neuroma is a nerve entrapment syndrome in which the intermetatarsal nerve in the second or third web space becomes compressed by the intermetatarsal ligament, enlarges, and undergoes perineural fibrosis.2–4
ANATOMY
The most important soft tissue structure is the transverse intermetatarsal ligament (TIML), which is a continuation of the plantar plates. This structure becomes taut during the late midstance and push-off phases of gait.
The TIML should be well visualized. It measures 10 to 15 mm long and 2 to 3 mm thick.1
The lumbrical tendon is located on the plantar lateral aspect of the TIML. It is the most likely structure to be severed during endoscopic decompression of the intermetatarsal nerve, but with proper identification it can be spared. In my experience, inadvertent severing of the lumbrical tendon, however, has not resulted in any adverse sequelae.
The plantar interossei muscles are superior to the TIML in the second, third, and fourth intermetatarsal spaces.
The intermetatarsal nerve is plantar to the TIML and should not be visualized during endoscopic division of the TIML; the nerve, however, may be seen by rotating the cannula 180 degrees, to the 6 o'clock position. With the cannula in the proper position the nerve is protected.
PATHOGENESIS
The clinical symptoms of this condition were first described by Durlacher in 1845 and later by Morton in 1876. It is Morton's name that has remained linked to this condition.
The most recent literature attributes Morton's neuroma to nerve entrapment; this has been confirmed by electron microscopy.
Perineural fibrosis has been identified at the level of nerve compression.
NATURAL HISTORY
The symptoms of Morton's neuroma are dull, aching pain in the ball of the foot, often radiating into the second, third, or fourth toes.
This may be associated with tingling, burning, or numbness.
It may occur gradually over several months or progress more acutely.
Overuse activities and compression by narrow-toed shoes and high heels have been implicated.
75% of patients are female.
The average age of onset is 54.5
Occasionally trauma can result in formation of an interdigital neuroma.
Pain is sometimes relieved by removing the shoe.
PHYSICAL FINDINGS
Classic findings include localized tenderness in the second or third web space. Subtle swelling may be present in the affected web spaces. The two adjacent toes may be slightly separated.
Mulder's click (a palpable snap) may be elicited in the affected web space.
The metatarsal compression test may be positive.
This is performed by grasping and squeezing the patient's forefoot. This maneuver is positive if it reproduces the patient's symptoms.
IMAGING AND DIAGNOSTIC STUDIES
Plain films should routinely be performed to rule out other pathologies.
If the diagnosis or correct web space is in doubt, sonographic imaging can be performed with a high degree of accuracy in experienced hands.
MRI is not operator-dependent but yields a large percentage of false-negative and false-positive findings and is also much more costly than sonography.
On ultrasound, a neuroma appears as a hypoechoic oval mass in the interspace at the level of the metatarsal heads. The size of the neuroma can be measured.5
DIFFERENTIAL DIAGNOSIS
Metatarsal stress fracture
Freiberg disease (avascular necrosis of the metatarsal head)
Synovitis
Intermetatarsal bursitis
Metatarsophalangeal synovitis
Peripheral neuropathy
Lumbar radiculopathy
Tarsal tunnel syndrome
Vascular claudication
Spinal stenosis
NONSURGICAL MANAGEMENT
Conservative treatment may include metatarsal pads, orthotics, shoes with a wide toebox, steroid injections, and, more recently, alcohol injections.
In our experience, conservative treatment has been successful in about 70% of patients.
SURGICAL MANAGEMENT
Surgery is indicated when conservative treatment has failed to relieve pain after at least 6 months.
The advantage of dividing the TIML without excising the interdigital neuroma is that there is no loss of sensation or possible formation of a stump neuroma, which may produce symptoms worse than those with which the patient originally presented. Barrett and Pignetti introduced endoscopic decompression of the intermetatarsal nerve, a procedure that offers several advantages over an open procedure, including a smaller incision, faster postoperative recovery, and a reduced incidence of hematoma and infection.1
Although these authors reported good and excellent results in 88% of patients, the original technique was difficult, with a steep learning curve.
They have since modified their technique, changing from two portals to a single portal.
Preoperative Planning
All patients should have plain films preoperatively to rule out other diagnoses, in particular stress fracture or Freiberg infraction.
In our experience, preoperative ultrasound is valuable in confirming the diagnosis.
Without ultrasound, simple palpation of the web space is typically accurate in determining which web space is most tender.
Diagnostic lidocaine injection may also pinpoint the appropriate web space. However, if both the second and third web spaces are symptomatic, the surgeon should consider endoscopy on both spaces.
Positioning
The patient should be positioned supine on the operating table.
We use a bump under the ipsilateral buttock and thigh when the leg tends to externally rotate.
The toes should extend just beyond the end of the table, with the heel firmly resting on the table.
Anesthesia may be general or regional (popliteal or ankle block).
Local anesthesia should be avoided, as it may distort the endoscopic anatomy.
Prophylactic intravenous antibiotics are given when the patient comes to the operating room.
We routinely use an ankle tourniquet inflated to 250 mm Hg. Equipment required includes the AM Surgical set and a 30degree 4-mm scope. The AM Surgical system includes an elevator, slotted cannula and obturator, locking device, and disposable knife blade.
TECHNIQUES
SINGLE-PORTAL TECHNIQUE
Presented here is a technique originally designed by Dr. Ather Mirza for endoscopic carpal tunnel release. I have adapted the instrumentation for uniportal endoscopic decompression of the intermetatarsal nerve (TECH FIGS 1 AND 2).6
Make a 1-cm vertical incision in the appropriate web space and spread the subcutaneous tissue gently with blunt Stevens scissors.
Use the AM Surgical elevator to palpate and separate the TIML from the surrounding soft tissues. Scrape the elevator both dorsal and plantar to the TIML.
TECH FIG 1 • Surgical technique for uniportal endoscopic decompression of the intermetatarsal nerve. Cannula is in the interspace just plantar to the transverse intermetatarsal ligament and dorsal to the intermetatarsal (interdigital) nerve. The transverse intermetatarsal ligament is being transected from distal to proximal. (Courtesy of AM Surgical.)
Place the slotted cannula and obturator through the same path, just plantar to and scraping against the TIML. The slot should face dorsally at the 12 o'clock position (TECH FIG 3).
Remove the obturator from the cannula and remove any fat or fluid from the cannula with absorbent cottontipped applicators.
Insert a short 4-mm 30-degree scope into the cannula.
Visualize the entire TIML by advancing the scope. The ligament is dense and white. The lumbrical tendon can often be seen just lateral to the TIML.
The intermetatarsal nerve can be visualized by rotating the cannula 180 degrees so that the slot is facing plantar at 6 o'clock. The nerve can often be seen unless obscured by fat. It is often thickened distally, tapers, and becomes normal proximally (TECH FIG 4).
TECH FIG 2 • Instrumentation. From left to right: elevator, cannula and obturator, disposable knife.
TECH FIG 3 • Intraoperative view of insertion of cannula and obturator into second web space, notch at 12 o'clock, positioned to view the transverse intermetatarsal ligament.
Return the cannula to the 12 o'clock position and remove the scope from the cannula.
Slide the disposable endoscopic knife onto the locking device with the lever in the open position.
Insert the knife and locking device assembly into the scope and advance the knife blade until it nearly touches the lens. The blade should also be parallel to the lens. Push the lever of the locking device forward until finger tight (TECH FIG 5).
Advance the scope and knife assembly through the cannula. Visualize the knife blade transecting the TIML from distal to proximal (TECH FIG 6). While cutting the TIML, maintain the cannula tight against the ligament. Place more tension on the TIML by placing a finger of the nondominant hand between the adjacent metatarsal necks.
TECH FIG 4 • A. Endoscopic view of transverse intermetatarsal ligament. B. Normal interdigital nerve. C. Thickened interdigital nerve (neuroma).
TECH FIG 5 • Intraoperative view of knife mounted to scope in position in cannula ready to enter second web space and transect the transverse intermetatarsal ligament.
Withdraw the scope and knife assembly and remove the knife from the scope. Reinsert the scope to confirm complete transection of the TIML. The divided edge of the ligament can be observed to further separate by applying manual digital pressure between the adjacent metatarsal heads.
Irrigate the wound through the cannula.
Remove the cannula, insert the elevator into the wound, and palpate the interspace. The taut TIML should no longer be palpable.
Deflate the tourniquet; irrigate and close the wound with one or two interrupted mattress sutures. Apply a soft compression dressing and postoperative shoe.
If the surgeon chooses to perform a neurectomy in cases where the nerve is very large and bulbous, the incision can be extended proximally 1 to 2 cm and neurectomy can be performed in routine fashion.
TECH FIG 6 • A. Endoscopic view of transverse intermetatarsal ligament. B, C. Endoscopic views of knife blade transecting the transverse intermetatarsal ligament. D. Endoscopic view after release of transverse intermetatarsal ligament.
POSTOPERATIVE CARE
Ice and elevation are recommended for the first 48 to 72 hours.
Weight bearing as tolerated is permitted in a surgical shoe. Crutches or a walker should be provided as needed.
Sutures are removed in 12 to 14 days. A comfortable shoe or sandal may then be worn.
Vigorous activities such as running or racquet sports should be avoided for 4 to 6 weeks.
Patients should be advised that complete resolution of symptoms may take up to 4 months.
OUTCOMES
Barrett and Pignetti reported 88% good and excellent results in over 40 patients.1
In our first 24 patients, there were 82% good and excellent results at 6 months postoperatively.
COMPLICATIONS
In the first 50 patients there have been no infections.
Two wound dehiscences occurred that healed uneventfully.
The postoperative protocol was then changed from suture removal at 10 to 14 days postoperatively.
No further dehiscences have occurred.
REFERENCES
1. Barrett SL, Pignetti TT. Endoscopic decompression for routine neuroma: preliminary study with cadaveric specimen: early clinical results. J Foot Ankle Surg 1994;33:503–508.
2. Dellon AL. Treatment of Morton's neuroma as a nerve compression; the role for neurolysis. J Am Podiatr Med Assoc 1992;82:399–402.
3. Gauthier GT. Morton's disease: a nerve entrapment syndrome: a new surgical technique. Clin Orthop Relat Res 1979;142:90–92.
4. Graham CE, Graham DM. Morton's neuroma: a microscopic evaluation. Foot Ankle 1984;5:150–153.
5. Shapiro PS, Shapiro SL. Sonographic evaluation of interdigital neuroma. Foot Ankle 1995;16:604–606.
6. Shapiro SL. Endoscopic decompression of the intermetatarsal nerve for Morton's neuroma. Foot Ankle Clin North Am 2004;9:297–304.