David R. Richardson
DEFINITION
A primary interdigital (Morton's) neuroma is in fact not a neuroma as it does not involve the haphazard proliferation of axons seen in a traumatic nerve injury.
Instead, this condition is best described as an interdigital perineural fibrosis.
It was first described in 1845 by Lewis Durlacher, a chiropodist to the Queen of England.
Recurrent neuromas are true histopathologic (haphazard proliferation of axons) amputation stump neuromas.
Eighty-five to 90% of nontraumatic neuromas are found in the third web space. The rest are found in the second web space.
ANATOMY
The medial plantar nerve supplies sensation to the first, second, and third digits and the medial aspect of the fourth digit. It emerges plantar and medial to the flexor digitorum brevis, coursing obliquely across the plantar surface of the muscle.
The lateral plantar nerve supplies sensation to the lateral half of the fourth and the fifth digit.
Both are branches of the tibial nerve and terminate with digital branches that course plantarly deep to the transverse metatarsal ligament (FIG 1).
The lumbrical tendon appears lateral and superficial to the digital nerve as it attaches to the medial aspect of the extensor expansion of the digit and may be mistaken for nerve.
In a cadaveric study, Levitsky et al12 found that 27% of specimens had a communicating branch connecting the medial and lateral plantar nerves. They also noted that the second and third interspaces were significantly narrower than the first and fourth.
Changes in the nerve itself involve perineural fibrosis, demyelinization and degeneration of nerve fibers, endoneural edema, and the absence of inflammatory changes.
Plantar-directed nerve branches may tether the common digital nerve to the plantar skin.
Theses nerve branches are present up to 4 cm proximal to the transverse metatarsal ligament.
PATHOGENESIS
All histologic changes in a primary interdigital neuroma occur distal to the transverse metatarsal ligament, as shown in studies by Lassmann11 and Graham et al.7
The cause is unclear but is thought to evolve as an entrapment neuropathy.
The second and third intermetatarsal spaces are narrower than the first and fourth.
Mobility between the medial three rays and the lateral two rays may contribute to the high number of primary neuromas in the third interspace.
In a limited number of patients (about 27%) the common digital nerve to the third interspace consists of branches from the medial and lateral plantar nerves, which perhaps increases the size of the nerve and predisposes it to entrapment (FIG 1).
A “recurrent interdigital neuroma” may be due to several factors, including failure to make the correct diagnosis originally.
Neurogenic pain may be due to causes other than perineural fibrosis, such as neuropathy and radiculopathy. Also, neuromalike symptoms may be due to nerve irritation from local synovitis or bursitis.
Beskin and Baxter3 found that in patients with recurrent symptoms of interdigital neuroma, about two thirds presented within 12 months and one third had recurrence 1 to 4 years after primary surgery.
Those with “recurrence” within the first 12 months probably represent patients who were originally misdiagnosed.
FIG 1 • Course of medial and lateral plantar nerve. A communicating branch of the lateral plantar nerve occurs in about 27% of patients.
FIG 2 • A. Standing palpation of the web space. B. Metatarsophalangeal joint plantarflexion stress test. C. Mulder test: The examiner places the thumb on the dorsal surface and the index finger on the plantar surface in the affected web space and applies gentle pressure. D. With the opposite hand the examiner applies a gentle squeeze to the forefoot in a mediolateral direction. A clicking sensation that reproduces the patient's pain will often be appreciated.
Those presenting after 12 months probably represent patients with a true bulb neuroma at the cut end of the common digital nerve. It probably requires at least this length of time for a neuroma to grow big enough to cause symptoms.
Formation of a recurrent neuroma after primary surgery is usually due to inadequate resection.
Plantar-directed nerve branches may tether the common digital nerve to the plantar skin and not allow for retraction of the nerve after it is cut. These nerve branches may occur up to 4 cm proximal to the transverse metatarsal ligament.
NATURAL HISTORY
Interdigital neuromas occur more commonly in females.
The primary symptom of an interdigital neuroma is pain, most often described as burning, aching, or cramping.
The pain often radiates to the toes or proximally along the plantar aspect of the foot.
Relief usually occurs with removing narrow toe-box shoes.
Walking barefoot on soft surfaces often produces no symptoms.
PATIENT HISTORY AND PHYSICAL FINDINGS
In patients with an interdigital neuroma, the most common complaint is plantar pain, which is often increased by walking.
Pain is often relieved by resting and removing shoes.
Often there are no symptoms with barefoot walking on a soft surface.
About half of patients describe pain radiating to the toes.
The duration of pain varies from a few weeks to many years.
Plantar tenderness in the web space is the most common physical examination finding.
The examiner should inspect for deviation or subluxation of the toes or fullness of the web space. This is best done with the patient standing (FIG 2A).
Palpating the web space proximal to the metatarsal heads and proceeding distally will usually reproduce the patient's symptoms.
It is often difficult to differentiate adjacent metatarsophalangeal (MTP) joint synovitis from a neuroma.
Plantarflexion of the corresponding MTP joint may help with the diagnosis (FIG 2B). This maneuver often causes little increased pain in those with an interdigital neuroma but is quite painful in those with MTP joint synovitis.
Difficulty in making a diagnosis may arise when primary synovitis causes secondary neuritic symptoms.
The Mulder test is also useful.
Pain may be present on the asymptomatic contralateral side but is usually not as painful and the “click” not as striking.
This test is best performed with the patient lying prone and the knee flexed 90 degrees. The examiner places the thumb on the dorsal surface and the index finger on the plantar surface in the affected web space and applies gentle pressure (FIG 2C). With the opposite hand the examiner applies a gentle squeeze to the forefoot in a mediolateral direction (FIG 2D). A clicking sensation that reproduces the patient's pain will often be appreciated.
IMAGING AND OTHER DIAGNOSTIC STUDIES
The diagnosis of an interdigital neuroma is most often made solely on the basis of the history and physical examination.
Standing AP, lateral, and oblique radiographs are necessary to exclude osseous pathology and to assess the MTP joint.
The use of nerve conduction testing has not been shown to be beneficial, as findings often are abnormal in patients without symptoms of an interdigital neuroma.
Studies differ as to the benefit of ultrasonography or MRI. If necessary, ultrasonography appears to be more useful than MRI in cases with a questionable diagnosis.
A diagnostic injection may be helpful, although other pathology in the area may improve with this local anesthetic.
2 cc of lidocaine is placed in the symptomatic web space through a dorsal approach.
The needle must be plantar to the transverse metatarsal ligament.
DIFFERENTIAL DIAGNOSIS
Adjacent web space neuroma
MTP joint synovitis
Freiberg osteochondrosis
Stress fracture of the metatarsal neck
Tarsal tunnel syndrome
Peripheral neuropathy
Lumbar radiculopathy
Unrelated soft tissue tumor (eg, ganglion, synovial cyst, lipoma)
NONOPERATIVE MANAGEMENT
Although reported results of conservative treatment vary, it is still worthwhile to try, as 30% to 40% of patients may avoid surgery.
The patient should be fitted with a wide, soft, laced shoe with a low heel.
A soft metatarsal support should be added just proximal to the metatarsal heads (FIG 3A).
An injection of steroids with anesthetic may be both diagnostic and therapeutic. For there to be diagnostic value, however, the anesthetic must be directed to the common digital nerve in the affected web space and not into the MTP joint. A combination of 40 mg Depo-Medrol and 1 cc 0.25% Marcaine is used for the injection (FIG 3B). Thirty percent of patients may have relief for 2 years or longer. Steroids should be used with caution as fat pad atrophy, skin discoloration, or MTP joint capsule laxity may result and create a new problem for the patient.
FIG 3 • A. Soft inserts and metatarsal support should be the first line of treatment. B. Steroid injection may improve symptoms and help with diagnosis.
FIG 4 • A neuroma retractor may help with exposure during surgery.
SURGICAL MANAGEMENT
The indication for surgery is failure of conservative treatment in a patient who is healthy enough to undergo forefoot surgery and who has appropriate vascular status.
Preoperative Planning
A forefoot or ankle block may be used. Twenty to 30 cc of a 50% mixture of a short- and long-acting anesthetic (eg, lidocaine and Marcaine) without epinephrine is recommended.
An examination under anesthesia allows for better appreciation of an interspace mass and often will produce a more striking Mulder click.
Instruments needed include a Weitlaner or neuroma retractor (FIG 4), small tenotomy scissors, a Senn retractor, and a Freer elevator.
An ankle tourniquet is used with cast padding and an Esmarch bandage.
If a plantar approach is being used (recurrent neuroma), the surgeon should palpate and outline with a sterile marker the metatarsal heads corresponding to the web space being explored.
Positioning
The patient is placed supine with a 3-inch bump under the distal leg just proximal to the heel. The heel should be floating just off the bed.
FIG 5 • A. Surgeon position for primary neuroma excision. Magnifying loupes are beneficial. B. Surgeon position for revision neuroma excision.
For a primary interdigital neuroma the surgeon should sit proximal to the foot with the assistant positioned at the end of the table to assist with retraction (FIG 5A).
A plantar approach is used for recurrent neuromas. The surgeon sits at the end of the table facing the plantar aspect of the foot (FIG 5B).
Approach
Primary Interdigital Neuroma
A dorsal approach is used for primary neuromas.
A dorsal incision is made 3 cm proximal to the web, extending distally to the edge of the web space (FIG 6).
The incision is slightly oblique and medial to the extensor tendons. It is important not to follow the tendons themselves, as they will take a more lateral direction.
The dissection is deepened and the dorsal sensory nerves are retracted to the side of least resistance.
The lumbrical tendon is lateral to the dissection.
The surgeon should proximally identify the dorsal interosseous fascia and muscle belly and follow it distally to the bursa overlying the transverse metatarsal ligament.
The surgeon should place a Weitlaner or neuroma retractor between the metatarsals and spread them apart.
The bursa is opened to identify the transverse metatarsal ligament.
FIG 6 • For a primary interdigital neuroma, a 3-cm incision is made in the affected web space just medial to the extensor tendons.
Web space fat is retracted using a Senn retractor and the distal aspect of the intermetatarsal ligament is identified.
A Freer elevator is placed beneath the transverse metatarsal ligament from distal to proximal, protecting the underlying structures.
The transverse metatarsal ligament is incised with a no. 15 blade knife, staying on top of the Freer elevator.
The lumbrical tendon is in the lateral aspect of the dissection just plantar to the intermetatarsal ligament.
The neurovascular bundle is identified medial and plantar to the lumbrical.
Recurrent Neuroma
PLANTAR LONGITUDINAL INCISION
A longitudinal plantar incision is made 4 cm proximal to the web, extending distally to within 1 cm of the web space.
The incision is made between the metatarsal heads (which have been identified and marked before making an incision) and proceeds just distal to this area (FIG 7).
A small Weitlaner retractor is placed to retract the fat overlying the plantar aponeurosis.
Using a no. 15 blade knife, the aponeurosis is incised in line with the skin incision.
FIG 7 • For recurrent interdigital neuromas, a 4-cm longitudinal plantar incision is made proximal to the web extending distally to within 1 cm of the web space.
A tenotomy scissors is used to bluntly spread until the common digital nerve is identified proximally.
The surgeon dissects distally to identify the stump neuroma.
PLANTAR TRANSVERSE INCISION
A 3- to 4-cm transverse plantar incision is made over the affected interspace just proximal to the weight-bearing pad and parallel to the natural crease (FIG 8).
The metatarsal heads are continually palpated to provide a reference point to the appropriate interspace to be explored.
The dissection is carefully deepened with scissors to expose the septa of the plantar fascia.
The interval between the longitudinal limbs of the plantar fascia septa is opened with scissors.
The bands of the plantar fascia are retracted medially and laterally with a Senn retractor and the interspace is carefully explored with blunt dissection to identify the common digital nerve and vessel.
The nerve (neuroma) will lie superficial (plantar) to the flexor digitorum brevis muscle or tendon and immediately deep (dorsal) to the plantar fascia.
The surgeon dissects distally to identify the stump neuroma.
The neuroma is identified and dissected proximally 1 to 2 cm.
FIG 8 • Alternatively, one may use a 3- to 4-cm transverse plantar incision. The incision is placed over the affected interspace just proximal to the weight-bearing pad and parallel to the natural crease.
TECHNIQUES
PRIMARY INTERDIGITAL NEUROMA EXCISION (DORSAL)
Once the approach has been completed the nerve should be identified in the wound. It is usually easier to identify the nerve proximally and dissect distally (TECH FIG 1A).
Manually palpate in the wound to be sure the transverse metatarsal ligament has been completely transected, as this is essential to a successful outcome.
Despite the size of the nerve or the obvious presence of a neuroma, the nerve should be resected as planned.
Structures that may be mistaken for the nerve include the lumbrical tendon, which passes to the medial portion of the adjacent proximal phalanx (extensor expansion) and therefore is lateral to the nerve. The common digital artery usually crosses proximal medial to distal lateral lying dorsally over the nerve. The artery often emerges from under the metatarsal neck and if identified needs to be dissected away from the nerve and preserved.
Using gentle traction (TECH FIG 1B), transect the nerve about 4 cm proximal to the transverse metatarsal ligament.
The transverse head of the adductor hallucis may need to be retracted dorsally to identify the plantar-directed branches of the common digital nerve. Divide these branches to allow the proximal aspect of the nerve to retract at least 1 to 2 cm proximal to the weight-bearing pad of the forefoot (TECH FIG 1C).
TECH FIG 1 • A. The transverse metatarsal ligament must be divided. B. The neuroma is visualized and the common digital nerve transected 4 cm proximal to the transverse metatarsal ligament and allowed to retract proximal to the weight-bearing pad of the forefoot. C. After transection of the intermetatarsal ligament, the nerve is transected proximally (the transverse head of the adductor hallucis muscle often must be retracted) and dissected distally past the bifurcation. D. The specimen is sent for pathologic examination. E,F. For a primary neuroma excision, a mildly compressive dressing is placed and the patient is allowed to bear weight as tolerated in a postoperative shoe.
Use a hemostat to place the remaining nerve stump well proximal and dorsal into the interosseous muscles.
Circumferentially dissect the nerve distally to the bifurcation of the proper digital branches.
Divide the proper digital nerve just distal to the bifurcation.
Send the specimen (TECH FIG 1D) for pathologic examination.
With the Weitlaner or neuroma retractor still in place, release the ankle tourniquet. Use cautery to obtain hemostasis.
Irrigate the wound with sterile saline.
Close the wound with 4-0 nylon suture in a running locking fashion.
If subcutaneous suture is desired, use a 3-0 Monocryl, taking care not to include the dorsal sensory nerves.
Place a mildly compressive dressing over a Xeroform gauze covering the wound (TECH FIG 1E,F).
REVISION INTERDIGITAL NEUROMA EXCISION (PLANTAR LONGITUDINAL INCISION)
Once the approach has been completed, the neuroma is identified just deep to the distal extensions of the plantar fascia that fan out to attach to the plantar aspects of the MTP joints and just superficial (plantar) to the flexor digitorum brevis.
The intermetatarsal ligament is often scarred in but does not need to be transected as it is distal and dorsal to the neuroma.
Place gentle traction on the common digital nerve (TECH FIG 2A). Identify and excise the neuroma (TECH FIG 2B).
TECH FIG 2 • A. The plantar longitudinal incision is shown with gentle traction placed on the common digital nerve. B. Excision of the recurrent neuroma through a plantar longitudinal incision.
Allow the common digital nerve to retract proximally as far as possible.
Release the ankle tourniquet and obtain hemostasis.
Irrigate the wound with sterile saline.
Close the wound with interrupted 3-0 nylon suture in a vertical mattress fashion.
Place a mildly compressive dressing over a Xeroform gauze on the wound.
Place the patient in a short-leg posterior splint.
REVISION INTERDIGITAL NEUROMA EXCISION (PLANTAR TRANSVERSE INCISION)
Once the plantar transverse approach is made, the technique is exactly the same as described above for the plantar longitudinal incision.
POSTOPERATIVE CARE
For 24 hours the operative extremity is maximally elevated and the patient ambulates only for bathroom privileges.
For a primary excision (dorsal approach), the patient is then allowed to ambulate with weight bearing as tolerated in a hard-soled postoperative shoe for 4 weeks.
For a revision excision (plantar approach), the patient is kept non–weight-bearing on crutches for 2 weeks and then transitioned into a stiff-soled postoperative shoe for another 2 weeks with weight bearing as tolerated.
Sutures are removed at 2 weeks and Steri-Strips are placed on the wound.
At 4 weeks after surgery the patient is allowed into a wide toebox, soft-vamp comfortable shoe and progressed as tolerated.
OUTCOMES
Surgical excision of a primary neuroma has a reported success rate of 51% to 90%, although results tend to diminish with time. A recent study by Womack et al22 suggests longterm pain relief is not as significant as once thought.
These results seem to be similar for both second and third web space neuroma excisions.
After re-exploration for a recurrent neuroma, less-than-complete satisfaction can be expected in 20% to 40% of individuals.
COMPLICATIONS
Recurrence of symptoms: This may be due to incorrect diagnosis, incomplete resection, or true recurrence.
Recurrence of symptoms due to incorrect diagnosis and incomplete resection usually occurs within the first 12 months.
Recurrence after 1 year is more likely related to the formation of a stump neuroma.
Significant wound complications are rare, but slow wound healing and superficial cellulitis are more common.
Incisional tenderness after a plantar approach is less common than one may suppose but may occur if placed under a weight-bearing portion of the forefoot.
REFERENCES
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· Womack JW, Richardson DR, Murphy GA, et al. Long-term evaluation of interdigital neuroma treated by surgical excision. Foot Ankle 2008;29(6):574.