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Disorders of the Nerves
Anatomy and Definition of Injury
The peripheral nerve is a complex structure with three layers surrounding the axons. The endoneurium is a connective tissue matrix that surrounds individual axons. The perineurium binds the axons into fascicles and it provides tensile strength and elasticity to the nerve. The epineurium is in the most peripheral layer, binds fascicles into a nerve, and provides protection to the nerve fibers. Although its elasticity contributes to the slack within the nerve, the epineurium poorly resists traction forces.
Two classification systems are commonly used to describe the nerve injury. Seddon divided the injury into three stages. Stage I is neuropraxia , defined as a focal axonal block secondary to a focal demyelination. The clinical deficit is temporary. Stage II is axonotmesis , defined as isolated axonal degeneration with maintenance of the three surrounding layers. The clinical presentation includes complete loss of motor and sensory functions along the distribution of the injured nerve. Stage III is neurotmesis , defined as complete discontinuity of the entire nerve. This has the worst prognosis, and the clinical presentation is similar to stage II. Sunderland classified the injury into five stages. Stage II (axonotmesis) was divided into three stages based on the extent of injury.
The nerve injury could be due to compression, traction, ischemia, or laceration. In foot and ankle, the most common etiologies of nerve disorder include, besides traumatic laceration, nerve compression/entrapment and neuroma formation. In a compression/entrapment scenario, the endoneurium plays the role of a shock absorber. Nerves with little endoneurium are more prone to injury. Traction injury could be iatrogenic during surgery or related to a sports injury, such as a stretch or traction superficial peroneal nerve injury after an ankle sprain.
The nerve recovery depends on the stage of injury. In neuropraxia, focal remyelination begins as soon as the causal agent is removed, and recovery could range from hours to months. In axonotmesis, recovery depends on the degree of axonal injury. If more than 90% of axons are injured, Wallerian degeneration occurs in the distal fragment and regeneration begins from the site of injury. The distal muscle’s motor end plate remains viable for 18 to 24 months post injury. There is no limit for sensory nerve recovery.
Electrodiagnostic Study
Electromyography (EMG) and nerve conduction study (NCS) are used to confirm the diagnosis of a nerve injury, localize the lesion, and determine its extent. The needle EMG records motor unit electrical signals. The waveforms are classified as voluntary, due to voluntary muscle contraction, and spontaneous, recorded upon insertion of the needle, with needle movement and at rest. The electrodiagnostic test could be performed at different timing based on the clinical presentation. Immediate testing (7–10 days) can localize the lesion and determine whether there is an underlying chronic injury (i.e., baseline neuropathy). Early testing (3–4 weeks) provides much more detail on the injury as compared to immediate testing and determines if the lesion is complete or incomplete. Intermediate testing (2–6 months) provides information on the progression of healing and prognosis. Late testing (6–9 months) is useful to monitor recovery. Abnormal EMG findings could persist after clinical recovery. Thus, it should not be used as a return to play criteria in athletes.
Interdigital Neuroma/Neuralgia
Neuroma is a misnomer since this entity is due to compression and irritation of a plantar nerve with or without neoplastic proliferation (neuroma) of the nerve. The plantar nerve between the second and third metatarsals is the most affected nerve. Different terminology has been used to describe this painful compressive neuropathy: Morton metatarsalgia, Morton entrapment, interdigital neuroma, neuralgia, nerve compression syndrome, and intermetatarsal neuroma. Neuralgia may more accurately describe the pain generated by compression on a nerve, rather than inflammation or formation of a neuroma.
The history of interdigital neuroma has been detailed by Kelikian and is briefly summarized here. The condition was first described in 1845 by the Queen’s Surgeon-Chiropodist Lewis Durlacher. He described a “form of neuralgic affection” involving “the plantar nerve between the third and fourth metatarsal bones.” In 1876, T.G. Morton related the problem to the fourth metatarsophalangeal (MTP) joint and suspected a neuroma or some type of hypertrophy of the digital branches of the lateral plantar nerve (LPN). In 1877, Mason reported a case of pain around the second MTP joint and suspected involvement of a digital branch of the medial plantar nerve (MPN). In 1893, Hoadley actually explored the digital nerves under the painful area, “found a small neuroma,” excised it, and claimed that he obtained a “prompt and perfect cure.” In 1912, Tubby reported observing on two occasions that the plantar digital nerves were congested and thickened. In 1940, Betts stated that, “Morton’s metatarsalgia is a neuritis of the fourth digital nerve.” In 1943, McElvenny stated that it was caused by a tumor involving the most lateral branch of the MPN. In 1979, Gauthier speculated that the condition was a nerve entrapment, an idea supported anatomically by others.
Etiology
Pathophysiology
Microscopic evaluation in Graham and Graham studies showed that distinctive nerve variations occurred distal to the intermetatarsal ligament. These changes included more blood vessels per fascicle, along with increased nerve diameter, perineural width, and fascicle diameter.
Lassmann studied 133 patients with Morton neuroma syndrome. Light and electron microscopic evaluation showed that in early stages of the disease the histologic findings are dominated by alterations of the nerve independent of the interdigital vessel alterations. The observed alterations included sclerosis and edema of the endoneurium, thickening and hyalinization of the walls of the endoneurial vessels caused by multiple layers of basement membrane, thickening of the perineurium, deposition of an amorphous eosinophilic material built up by filaments of tubular structures, demyelinization and degeneration of the nerve fibers without signs of Wallerian degeneration, and local initial hyperplasia of unmyelinated nerves, followed by degeneration. These authors concluded that Morton syndrome was probably caused by an entrapment neuropathy, predominantly characterized by the deposition of an amorphous eosinophilic material, followed by a slow degeneration of the nerve fibers.
Bourke compared the third interdigital nerve in 17 autopsy specimens to nerve specimens obtained from 16 patients undergoing a Morton neuroma excision. The authors found a significant increase in the amount of demyelination in the Morton neuroma group as compared to the control group, concluding that Morton neuroma does not have a recognizable morphologic substrate and demyelination might be the only typical histological change.
Giannini et al treated 63 patients with Morton neuroma via a dorsal approach to excision. In all cases, histopathologic findings included a thickened interdigital nerve, deposition of amorphous material around the nerve that was adherent to blood vessels and subcutaneous fat, thickening, and fibrosis of the perineum and epineurium, 73% with sclerohyalinosis, concentric edema in 40%, and degeneration of myelinated fibers in 43% of cases. Additional findings included hyperplasia of the muscle layer, a prominent internal elastic lamina, and small vessel proliferation in the muscle and adventitial layers in the vessels in 70% of cases. The etiology of Morton neuroma and whether the nerve and vessel changes are primary or secondary remains debatable. In addition to entrapment neuropathy at the transverse metatarsal ligament, other factors should be considered in the origin of this entity.
Anatomic Factors
The MPN has four digital branches ( Fig. 18-1 ). The most medial branch is the proper digital nerve to the medial aspect of the great toe. The next three branches are the first, second, and third common digital nerves and are distributed to both the medial and the lateral aspects of the first, second, and third interspaces, respectively. The LPN divides into a superficial branch, which splits into a proper digital nerve to the lateral side of the small toe and a common digital nerve to the fourth interspace. The common digital nerve often has a communicating branch that passes to the third digital branch of the MPN in the third interspace.
Communicating nerve and accessory branches
The role of the communicating nerve ( Fig. 18-2 ) branch in the development of Morton neuroma is unclear. Govsa et al in an anatomic dissection of 50 adult male cadavers observed the presence of communicating branches between the third and fourth plantar nerves in 28% of specimens.
Levitsky et al in a similar dissection of 71 feet observed this communicating branch in only 19 (26.8%) specimens. The communication was present from the fourth to the third webspace common digital nerve (LPN to MPN) in 11 specimens, and a reverse communication from the third to fourth webspace common digital nerve (MPN to LPN) was present in 8 specimens. The third common digital nerve was not thicker because of the communicating branch; however, the intermetatarsal head distances and ratios to digital nerve diameter were smaller in the second and third webspaces as compared to first and fourth webspaces. The authors concluded that the propensity of Morton neuroma presentation in the second and third webspaces could be explained on a mechanical basis rather than a result of a communicating branch.
Small accessory nerve trunks have been reported. These nerves pass underneath the metatarsal head and join the common plantar digital nerve distal to the deep transverse metatarsal ligament. If these nerves are inadvertently cut during surgery or the accessory or communicating branches are in a position to hinder retraction of the main nerve stump after excision, a postoperative recurrent neuroma could be formed.
Mobility
The medial three rays are well-fixed at the midfoot, while the lateral two rays are more mobile. This difference leads to increased mobility across the third webspace, and it has been postulated that this increased motion could lead to repetitive trauma to the nerve or development of an enlarged bursa and secondary nerve compression. However, the incidence of Morton neuroma in the second webspace and the lack of incidence at the fourth webspace sheds some doubt on this theory.
Local trauma due to footwear (i.e., high heels, tight shoes, thin soles) is considered a strong factor in Morton neuroma development. That would also explain the propensity of this entity in females compared to males (8–10 times more common). In theory, dorsiflexion of the MTP joints along with the plantar fascia causes plantar flexion of the metatarsal heads that risks tethering the nerve beneath the transverse ligament (entrapment neuropathy) ( Fig. 18-3 ).
Traumatic Causes
Traumatic events are uncommon causes of Morton neuroma. Persons who engage in certain athletic endeavors (e.g., running, dancing) place an increased amount of repetitive pressure over the distal metatarsal area; however, an increased incidence of Morton neuroma has not been demonstrated in this population. Furthermore, patients with fat pad atrophy rarely report localized neuritic symptoms.
Extrinsic Factors
Intermetatarsal bursa
Bossley et al injected and dissected the intermetatarsophalangeal bursa. In the second and third webspaces, the bursa lies superior to the transverse ligament and extends distally to it, close to the neurovascular bundle. An inflamed bursa could account for the histopathological findings in this entity (neurofibrosis), but the authors did not believe that the bursa could be a source of entrapment. In the fourth webspace, the bursa does not extend beyond the ligament and is not in contact with the neurovascular bundle.
Awerbuch et al reported inflamed intermetatarsal bursa in patients with rheumatoid arthritis that may cause an increased pressure on the normal digital nerve and thickening of the connective tissue within and around the nerve.
Metatarsophalangeal joint deviation
Deterioration of the MTP joint capsule could lead to deviation of the proximal phalanx, and commonly the third toe deviates in a medial direction. That results in third metatarsal lateral deviation and compression of the third webspace bursa superior to the transverse ligament and subsequently the underlying nerve. The deviation of the toe or instability of the MTP joint could also result in traction on the nerve, which could lead to disease ( Fig. 18-4 ). Clinical symptoms of a neuroma are observed in approximately 10% to 15% of patients with deviation of the MTP joint, which may be accounted for by this anatomic malalignment ( Fig. 18-5 ).
Metatarsal fracture
Nerve compression could be a result of a metatarsal fracture. Typically, a malunion with shortening and/or angulation can cause traction or compression on the nerve from the bone itself or any anchored soft tissues. Malunion or callus formation could affect the tendon’s function and lead to lesser toe deformity with development of nerve symptoms. Direct nerve insult during the index fracture can further complicate this presentation.
Ischemic Factors
Nissen observed degenerative changes in the plantar digital artery, including disruption of the arterial wall, thrombosis, and incomplete recanalization. He suggested that intraneural vessels undergo changes due to the plantar digital artery narrowing. However, other authors noted similar arterial narrowing to be commonly present in control patients and histologic studies, as discussed above, and found entrapment and nerve demyelination to be the main factors in Morton’s neuroma formation.
Other extrinsic factors for compression include transverse metatarsal ligament thickening with an aberrant band, a ganglion or synovial cyst from the MTP joint as a primary condition or secondary to plantar plate degeneration, and a plantar lipoma.
Clinical Presentation
Patients with Morton’s neuralgia are commonly middle-aged women. Mann and Reynolds performed a critical analysis of 56 patients in whom 76 neuromas were excised. There were 53 women and 3 men. This represents a greater female-to-male ratio than that observed by Bradley et al, who reported a ratio of 4:1. The average age of the patient in the Mann and Reynolds series was 55 years (range, 29–81 years).
The neuroma is commonly unilateral, although bilateral neuromas were seen in 20% of patients. The distribution of neuroma between second and third webspaces is debatable, with the majority of the literature demonstrating that the neuroma usually involves the third interspace. Kasparek and Schneider found that the third webspace is affected in 66% and the second in 32% of cases. These findings were supported by Graham et al. The fourth webspace is rarely affected and first webspace neuroma probably does not exist as a clinical entity. Multiple locations in the same foot is also a rare presentation.
The most common symptom is pain localized to the plantar aspect of the forefoot, typically between the metatarsal heads of the affected webspace ( Table 18-1 ). The pain is commonly described as burning, tingling, stabbing, or electric in nature. It may radiate distally to the toes of the involved webspace in 60% of cases. Patients may also appreciate the sensation of “walking on a marble” with an acute sharp pain. This is probably due to the trapped nerve residing close to the metatarsal head. Rarely, the pain radiates to the dorsum of the forefoot or more proximally on the plantar side of the foot (i.e., Valleix phenomenon).
Table 18-1
Preoperative Symptoms of Interdigital Neuroma
Data from Mann RA, Reynolds JD: Interdigital neuroma: a critical clinical analysis. Foot Ankle 3:238, 1983.
| Symptom | Patients Affected (%) |
|---|---|
| Plantar pain increased by walking | 91 |
| Relief of pain by resting | 89 |
| Plantar pain | 77 |
| Relief of pain by removing shoes | 70 |
| Pain radiating into toes | 62 |
| Burning pain | 54 |
| Aching or sharp pain | 40 |
| Numbness in toes or foot | 40 |
| Pain radiating up foot or leg | 34 |
| Cramping sensation | 34 |
Pain is aggravated when the patient wears a narrow toe box or high-heeled shoes and relieved by removing the shoe and massaging the foot. Patients often voluntarily curls the toes and avoid rolling through the ball of the foot during gait to decrease pain with weight bearing. Patients are often pain free when walking barefoot on a soft surface and some would wear broad, soft walking shoes to limit their pain during walking or jogging.
Diagnosis
Morton neuroma is a clinical diagnosis based on the patient’s history and physical examination. The paraclinical studies help rule out other conditions. Radiographs are helpful to identify an underlying bone or joint disease. Magnetic resonance imaging (MRI) is useful to rule out other conditions (e.g., plantar plate tear, synovitis, stress fracture, mass) but certainly not necessary to establish the diagnosis ( Fig. 18-6 ). Ultrasound is helpful in certain situations; however, the test is operator dependent with a high rate of false-positive and negative results. Electrodiagnostic studies are not useful for diagnosis and could be used if peripheral neuropathy or radiculopathy are suspected.
Physical Examination
The physical examination begins with inspection of the foot, with the patient in a standing position, to look for deviation of the toes, presence of hammer or claw toes, or evidence of webspace fullness. The contralateral foot should be examined for comparison. Then the physician proceeds with an assessment of the MTP joints’ range of motion and stability followed by palpation of the MTP joints, metatarsal bones, and webspaces.
MTP joints
The MTP joints are carefully palpated on their dorsal and plantar aspects to look for evidence of synovitis of the joint, deformity, pain in the plantar pad area indicating early degeneration of the plantar pad, and evidence of pain around the joint. The patient with an interdigital neuroma does not have pain over the dorsal metatarsal heads. Typically, there is no pain over the plantar metatarsal head, but the presence of some tenderness here should be considered relative to the tenderness in the webspace. Thus, if there is more tenderness between the metatarsals, especially if it reproduces the character of the patient’s symptoms of burning, shooting, radiating pain, then a webspace neuroma is highly suspected.
Webspaces
Palpation of the webspace should start just proximal to the metatarsal head and continue distally into the actual webspace. When assessing each webspace, one hand is used to stabilize the foot and the other hand is used to palpate the webspace plantarly with a single finger ( Fig. 18-7 ). The goal here is to avoid inadvertent compression of the dorsal tissues. In a patient with a symptomatic Morton neuroma, this will usually reproduce the patient’s symptoms and often generate radiating into the associated toes.
The webspaces are then examined with pressure applied in a mediolateral direction around the forefoot with one hand (forefoot squeeze) to increase the pressure on the tissue between the metatarsal heads. This maneuver often results in a significant crunching or clicking feeling (the Mulder sign), most often positive in the third webspace and occasionally in the second webspace. The maneuver can reproduce the patient’s pain and, if so, is diagnostic of an interdigital neuroma.
The presence of only a crunching or clicking feeling in the webspace without reproduction of the patient’s pain does not indicate a neuroma. A painless crunching can be demonstrated in a normal foot. This test is not as helpful after a prior neuroma resection. Sometimes, a mass could be palpated on the plantar aspect of the forefoot and can be rolled beneath the examiner’s fingers. If pressure against the mass reproduce the patient’s symptoms, then the mass is associated with a webspace neuroma.
Neurologic examination
The motor strength of the foot and ankle should be assessed, along with palpation of all the webspaces and a peripheral nerves exam. The tibial nerve should be palpated at the tarsal tunnel, posterior leg, popliteal space, and behind the femur. Additionally, the sural, saphenous, deep, and superficial peroneal nerves are examined. In certain situations, a straight leg raise (i.e., Lasegue test) should be performed to rule out lumbosacral nerve roots irritation and, if warranted, deep tendon reflex testing completed. The patient’s hand examination could reveal evidence of systemic neurologic or neuromuscular disease, such as intrinsic muscle atrophy or tremor.
It is also important to assess the tightness of calf muscles since such tightness would reduce the pressure in the heel and transfer pressure towards the plantar side of the metatarsal heads.
Serial examination
If the patient’s examination is inconclusive, the patient should be serially reexamined to ensure a correct diagnosis. Several conditions can coexist with a Morton neuroma, including plantar fat pad atrophy, plantar plate tear, metatarsal stress fracture, and MTP joint synovitis. Careful examination is helpful to localize the point of maximal tenderness. For example, when performing a vertical Lachman’s test ( Fig. 18-8 ) to evaluate for a plantar plate tear, the physician must not press on the nerve while translating the toe dorsally.
Differential Diagnosis
There are several structures in this region of the forefoot within a few millimeters of each other. Thus, a variety of other conditions can mimic an interdigital neuroma. Box 18-1 provides a differential diagnosis to assist physicians regarding this condition.
Box 18-1
Differential Diagnosis of Interdigital Neuroma
Arthrosis of metatarsophalangeal (MTP) joint
Degeneration of plantar pad or capsule
Degenerative disk disease
Freiberg infraction
Lesion of medial or lateral plantar nerve
Lesions of plantar aspect of foot
Metatarsal stress fracture
MTP joint disorders
Pain of neurogenic origin unrelated to interdigital neuroma
Peripheral neuropathy
Soft tissue tumor (e.g., lipoma)
Soft tissue tumor not involving MTP joint (e.g., ganglion, lipoma, synovial cyst)
Subluxation or dislocation of MTP joint
Synovial cysts
Synovitis of MTP joint caused by nonspecific synovitis or rheumatoid arthritis
Tarsal tunnel syndrome
Tumor of metatarsal bone
Diagnostic Studies
Radiographs
Weight-bearing radiographs of the foot are essential to assess osseous or articular abnormalities that may be present, such as MTP joint subluxation or angulation, arthritis, Freiberg disease, toe deformities, fracture, bone lesion, or the presence of foreign body. Occasionally, radiographs show a distal intermetatarsal space widening with splaying of the toes, known as Sullivan’s sign (see Fig. 18-5 ), which can also just be appreciated clinically. Rarely, a shadow of the neuroma is seen on anteroposterior film.
This being a clinical diagnosis, routinely obtaining any further imaging beyond radiographs is not indicated. However, in the setting of an unclear clinical presentation, in the need to rule out other diagnoses, when more than one webspace appears to be affected, or in the case of neuroma recurrence, more advanced imaging can be helpful.
Ultrasound
The use of ultrasound in the evaluation of a patient with a potential Morton neuroma is well supported in the literature, with an equivalent to superior accuracy as compared to MRI. It is highly operator dependent; hence, it could be considered the imaging modality of choice in the hands of a skilled sonographer. Ultrasound is an inexpensive tool, widely available, fast, noninvasive, radiation free, and well tolerated by patients. However, use of ultrasound can lead to a high false-positive rate of identifying an asymptomatic interdigital nerve enlargement.
Sharp et al compared the accuracy of clinical assessment, ultrasound, and MRI in the diagnosis of Morton neuroma in 29 cases. Clinical assessment was the most sensitive and specific modality. Ultrasound was inaccurate for small lesions and the authors did not find any correlation between the size of the lesion and either the preoperative or postoperative pain score. Small (<6 mm) or larger lesions would cause similar symptoms. Reliance on either ultrasound or MRI would have led to an inaccurate diagnosis in 18 of 29 cases.
On ultrasound, the neuroma appears as a well-circumscribed, ovoid hypoechoic lesion in the intermetatarsal space that reflects the perineural enlargement and surrounded by hyperechoic fatty tissues. The deep transverse metatarsal ligament is not seen on ultrasound, thus it might be difficult to distinguish the neuroma from intermetatarsal bursal tissue, thus the mass diagnosed on ultrasound is typically larger than the actual neuroma size.
MRI
MRI provides static images that are non-operator dependent, allowing for visualization of all of the surrounding soft tissues, that can be interpreted by radiologists and surgeons. MRI is more expensive and time consuming, as compared to ultrasound. Routine use of MRI in diagnosing this condition is not indicated.
The Morton lesion appears within the intermetatarsal space, centered in the neurovascular bundle on the plantar side of the deep transverse ligament, with low to intermediate signal intensity on fluid sensitive sequences, which helps to distinguish a neuroma from other lesions such as bursae, ganglion cysts, or other masses. On T1-weighted images, the lesion appears isointense to skeletal muscle and well-demarcated from the surrounding hyperintense fat tissue.
Careful interpretation of MRI findings and correlation to clinical symptomatology is required. Bencardino found a 33% prevalence of Morton neuroma on MRI in patients with no clinical evidence of this entity. Weishaupt et al showed significantly improved visualization of the Morton neuroma when the MRI was performed in a prone position as compared to the supine position. The authors attributed their findings to the reduced motion artefact and the influence of the plantar and flexed position of the MTP joints associated with prone positioning.
Previously, Terk et al supported a combination of contrast enhancement and fat suppression for improved MR imaging; however, more recent studies showed no typical enhancement pattern of Morton neuroma after intravenous gadolinium administration. Furthermore, when enhancement is present, it is usually due to the bursal tissue surrounding the neuroma, hence gadolinium is not required for the diagnosis of a Morton’s lesion on MRI.
Although Sharp et al did not find a correlation between the neuroma size on imaging and clinical outcomes, Biasca et al reported a more favorable clinical outcome in 19 patients after surgical neurectomy when the neuroma had a transverse measurement larger than 5 mm on MRI.
Electrophysiologic testing
Electrodiagnostic studies have been used, in the setting of a Morton neuroma, to identify a more proximal nerve entrapment, radiculopathy, or underlying neuropathy. Recent modifications of the electrodiagnostic technique have increased the accuracy of these studies. The essential modification was in the use of different latencies between the two separate nerves that innervate a single toe, instead of the sensory conduction velocity of these nerves. Promising results were reported with abnormal values in patients with Morton neuroma; however, the clinical role of this technique has yet to be determined.
Diagnostic injection
Lidocaine can be injected into the suspected, symptomatic webspace for diagnostic purposes. The recommended dose is 1 or 2 mL of anesthetic placed below the transverse metatarsal ligament. Although complete symptom relief can be obtained, caution is advised in interpreting this as confirming an interdigital neuroma because this injection may also relieve pain from other local conditions, such as degeneration of the joint capsule or plantar plate.
Younger and Claridge performed a study to determine the role of a diagnostic block in predicting the results of surgery. In 37 patients with 41 excisions (7 patients with revisions), 24% of the primary procedures were failures despite relief with a block, and 43% of the 7 revision procedures were failures.
Hembree et al injected radiopaque contrast (1 mL and 2 mL) into the third webspace of 49 cadaveric specimens and then obtained computed tomography (CT) images. All of the injections were accurate; however, a large amount of contrast was seen in the second (70%) and fourth (30%) webspaces in both injection volume groups. Extravasation was significantly greater with the 2 ml injection.
Local injection of cortisone can be useful in about one third of patients, but cortisone can cause deterioration of the nearby MTP joint capsule or possibly the plantar fat pad. Only infrequently should more than one cortisone injection be made into the webspace. In some patients, medial or lateral deviation of the adjacent MTP joint has subsequently developed following multiple steroid injections, presumably from damage to a collateral ligament ( Fig. 18-9 ). Decreasing the injection volume can help to diminish the risk of damage to the MTP joint capsule, plantar plate ligament, or flexor tendon. This can be accomplished by injecting proximal to the MTP joint, below the intermetatarsal ligament, as opposed to more distally. In addition, if during the injection the toes begin to spread apart, the injection should be stopped. If the deviation is noted after an injection, the toes should be supported by taping.
Conservative Treatment
Footwear
Conservative treatment consists of modifying footwear to a wide and soft, round toe box shoe to reduce intermetatarsal head pressure and allow the forefoot to be spread out. High heels and fashionable pointed shoes should be avoided. Thicker compressible rubber-soled shoes can also make a big difference by providing shock attenuation to the forefoot region.
Different options could be used to further decrease the pressure on the nerves including, a soft metatarsal pad ( Fig. 18-10 ) placed just proximal to the metatarsal heads of the involved web space, a metatarsal bar, or a rocker bottom sole. Occasionally, a soft metatarsal support can be added to a high-heeled shoe, provided sufficient room exists in the toe box area for both the support and the foot. In cases of secondary neuralgia due to MTP joint deviation, instability, or synovitis, a Budin splint (see Fig. 18-9 ) can be useful.
Mann and Reynolds reported substantial relief with conservative treatment; however, the localized plantar pain persisted. Since the majority of patients were females and not satisfied with shoewear modifications, 70% of them proceeded with surgical excision.
Webspace Injections
Anesthetic injection
The use of a local anesthetic injection for diagnostic purposes has been discussed earlier in this chapter. The use of a small volume of local anesthetic and careful injection around the common digital nerve is acceptable and useful. However, the clinician should be mindful of the low specificity of such injection and, in equivocal cases, correlation with the clinical presentation and imaging would be prudent.
Corticosteroid injection
A combination of corticosteroid and local anesthetic can be injected into the affected webspace, with or without ultrasound guidance. Greenfield et al in a study of 76 cases of suspected Morton neuromas, noted significant short-term relief with a series of corticosteroid injections, but only 30% of patients had relief that lasted more than 2 years. Markovic et al, in a study of 39 Morton neuroma patients treated with a single steroid injection, found that 26 (66%) had a positive outcome (satisfied or satisfied with minor reservations) at 9 months. Mahadevan et al in a randomized controlled trial (RCT) found that the corticosteroid injection remained effective in 49% of the patients at 1 year follow-up.
In a prospective, midterm follow-up study of a prior RCT of 45 neuromas in 36 patients treated with a single corticosteroid injection with or without ultrasound guidance, Hau et al reported no significant difference between the two techniques. The original injection remained effective in 16 patients (36%) for a mean of 4.8 years, and 20 patients (44%) eventually underwent surgical excision.
Three types of corticosteroids are typically used, including methylprednisolone, triamcinolone, and betamethasone. The latter is a long-acting agent with a half-life of 36 to 72 hours, and the first two are intermediate-acting agents, each with a half-life of 12 to 46 hours.
A dorsal approach is commonly used for the injection; however, a plantar approach has been described and utilized in a few studies. We recommend a dorsal approach for the injection, since it is generally less painful and better tolerated by patients, and we feel can decrease the risk of fat pad atrophy.
Local corticosteroid injection for Morton neuroma can cause a few potential problems, including local skin depigmentation (3%), skin atrophy (5%), and fat pad atrophy (0.93%). One of the more serious issues, albeit rare, is iatrogenic compromise or disruption of the nearby MTP joint capsules, with resultant deviation of the toe/s, depending on the extent of involvement. This can be an unfortunate consequence of corticosteroid injections and one that can then generate a new problem for the patient.
Other injections, radiofrequency ablation, and other treatments
Radiofrequency ablation (RFA) and alcohol injections have been proposed as less traumatic, more conservative methods of treating neuromas. Recent approaches have combined MRI, electroneurography, and fluoroscopy to localize the Morton lesion prior to treatment. In the case of alcohol injections, various alcohol concentrations have been used in the literature (4%–70%). Pabinger et al used 2.5 ml of 70% ethanol to inject 33 patients that had a symptomatic Morton neuroma. At a mean follow-up of 5 years, the authors reported an 82% success rate after a single ethanol injection. Temporary numbness and mild swelling were seen for a few weeks postoperatively and 27% of patients still reported mild pain during running.
Fanucci et al reported their results with 40 interdigital neuroma injections using alcohol (70% carbocaine-adrenaline and 30% ethyl alcohol) performed under ultrasound guidance. Complete or partial symptomatic relief was obtained in 90% of cases without complications. There was temporary plantar pain reported in 15% of patients. In contrast, Espinosa et al reported symptomatic relief in only 7 of 32 (22%) patients with an MRI-confirmed interdigital neuroma treated with alcohol sclerosing therapy. Further investigation is certainly warranted to standardize the technique and to clarify the indications, if any, for alcohol-based injections.
RFA is used for various medical indications, including cardiac arrhythmias, treatment of solid organ tumors, and facet joint arthritis in the spine. The role of RFA in the treatment of patients with Morton neuroma is less defined. An RF generator creates an electric current that acts on charged ions within the tissue, creating an oscillatory motion and friction heating. When the tissue temperature exceeds 70° C, thermocoagulation necrosis occurs. The optimal role and long-term effectiveness of RFA in Morton neuralgia treatment remains unclear. In a study of 28 patients with Morton neuroma treated with RFA, Brooks et al found, at a mean follow-up of 1 year, that 88% of patients were very or moderately satisfied with their outcome and that three cycles may be superior to two cycles.
Hyaluronic acid is a nonsulphated linear glycosaminoglycan. It is a major component of synovial fluid. Its effectiveness in the treatment of knee osteoarthritis and tendinopathy has been widely investigated and utilized in clinical practice. Several studies support its use to enhance nerve regeneration and reduce perineural scar formation. However, the role of hyaluronic acid in Morton neuroma treatment is unknown. In a study of 83 patients treated with ultrasound-guided hyaluronic acid injections (administered weekly for 3 weeks) for third webspace neuromas, Lee et al found significant improvements in visual pain scores and functional outcomes at 2 months continuing through 12 months postinjection.
Other nonsurgical treatments for Morton neuroma include botulinum toxin A ; oral vitamin B 6 ; antiinflammatory medications; tricyclic antidepressants (TCAs) such as imipramine, nortriptyline, desipramine, and amitriptyline; selective serotonin reuptake inhibitors (SSRIs) such as sertraline, paroxetine; other antidepressant medications such as venlafaxin and duloxetine; and antiseizure medications, such as gabapentin, pregabalin, topiramate, and carbamazepine. The use of these drugs for this indication is off-label, but they have been shown to lessen the severity of nerve-related symptoms. For a more complete discussion of the various medications for chronic nerve pain, consult the American Chronic Pain Association’s Resource Guide to Chronic Pain Medication and Treatment.
While many patients will show some positive, initial response to the above conservative treatments for Morton neuroma, around 60% to 70% will ultimately elect to have excision of the symptomatic neuroma. Not infrequently, the choice to undergo surgical management is made because of the patient’s desire to be able to continue with their usual daily pattern of living, whether that is walking barefoot or wearing fashionable, high-heeled shoes, even though they are often comfortable in more supportive shoewear.
Surgical Treatment
Neurectomy
Surgical excision of the affected, common digital nerve should be performed as proximal as possible, allowing the resection stump to retract into the intrinsic muscles. This technique can help to avoid the development of a symptomatic amputation neuroma residing underneath the metatarsal head or scarred into the plantar fat pad.
Two general approaches are described, dorsal and plantar. A longitudinal dorsal incision is the most common surgical approach ( Fig. 18-11 ). It was described in 1943 by McElvenny and provides several advantages over the plantar approach, including less scar tissue formation, more aesthetic surgical scar without the risk of developing a painful scar on the weight-bearing forefoot, and avoiding the need to dissect through the plantar fat pad, which can lead to atrophy. For these reasons, when performing a plantar approach, the incision must be accurately placed in the interspace between the respective metatarsal heads, for optimal visualization and to avoid a scar directly under a metatarsal head that can become symptomatic.
Plantar approaches include a more standard longitudinal incision made between the respective metatarsal heads or a transverse incision made proximal to the associated metatarsal heads. Exposure of the nerve through a plantar approach is fairly direct but certainly comes with a learning curve to become familiar with the plantar anatomy, particularly in a revision setting. If a painful keloid results from a plantar incision, this can become quite limiting to a patient and honestly, quite difficult to rectify or even minimize. Attempts at scar massage, local compression, topical therapies, and various injections may need to be utilized in order to improve symptoms. Thus, for the treatment of a primary interdigital neuralgia, the dorsal incision tends to be the preferred approach.
Yanez Arauz described an incision in the affected commissural space. In a study of 108 patients with Morton neuroma, the author reported significant improvements in visual analogue pain scores postoperatively with good to excellent patient satisfaction in 93% of cases at a mean follow-up of 121 months. Postoperative complications included one painful neuroma stump that required reoperation via a plantar approach and two wound dehiscences that healed by secondary intention and oral antibiotic therapy in one case and local antimycotic therapy in the second case.
Neurolysis (Transverse Intermetatarsal Ligament Release)
Neurectomy is the most common surgical treatment and it has been supported by numerous studies. However, some authors have reported acceptable results with simple decompression, that is, a transverse intermetatarsal ligament release without a neurectomy. In a study of 206 patients with Morton neuroma, Gauthier reported symptomatic improvement in 85% of patients undergoing decompression alone for symptomatic Morton neuroma. This was also supported by the work of Diebold and Delagoutte who reported 84% satisfactory results with transverse ligament release.
Endoscopic decompression of the intermetatarsal nerve has also been reported on by Barrett and Pignetti. The authors reported good to excellent outcomes in 88% of patients; however, the instrumentation used is delicate, three incisions are required, and the nerve cannot be directly visualized. This technique has not been embraced by other surgeons and comes with a learning curve. Shapiro adapted an endoscopic uniportal carpal tunnel release technique to perform decompression of the intermetatarsal nerve with promising early results in 40 patients.
Okafor et al in a study of 35 Morton neuroma patients noted that if the patient had no foot disorder other than the interdigital neuroma, 72% (13/18) noted 100% relief of symptoms following neurolysis. If the patient had a separate foot disorder, such as a hallux valgus or hammer toe, then 30% had complete relief, with 70% still reporting some residual symptoms. In 20 of their patients who underwent a lidocaine block before surgery with complete relief, 15 (75%) patients noted complete relief postoperatively, and 5 (25%) had significant relief.
Villas et al retrospectively reviewed 69 cases of interdigital neuroma treated surgically through a dorsal approach. In 67% of cases in which there was macroscopic nerve thickening noted, the nerve was resected. Otherwise, a neurolysis was performed (33%, 23 of 69 cases). They noted equivalent results between the two groups using this treatment approach.
Intramuscular Implantation
Intramuscular implantation is a technique by which the nerve stump is placed in a healthy, well-vascularized muscle tissue, away from the denervated region and skin, in order to decrease the neuronal growth factor release and formation of a painful amputation neuroma. In a prospective, randomized study of 23 Morton neuroma patients, Colgrove et al compared simple neurectomy to neurectomy with intramuscular nerve transposition. The postoperative pain level was initially higher in the transposition group; however, by 1 year and through 3 year follow-up, the transposition group had lower pain levels as compared to simple neurectomy. At final follow-up, the transposition group had significantly more asymptomatic patients as compared to the simple neurectomy group (96% vs. 68%). In another comparative study, similar pain and functional scores were reported by Rungprai et al between the two techniques; however, the authors found a trend towards fewer complications (i.e., persistent pain, revision rate, and painful scar) without reaching a statistically significant difference. Proponents of the transposition technique believe that a dorsal approach to explore a recurrent neuroma would be technically easier following transposition, since the nerve lies in a predictable area. Transposition requires further initial dissection to identify the interval between the transverse head of the adductor hallucis and the interossei muscles.
Adjacent Interdigital Neuromas (Second and Third Webspaces)
In patients with adjacent, symptomatic Morton neuromas (i.e., second and third webspaces), Hort and DeOrio described a single, longitudinal dorsal incision that was centered over the third metatarsal beginning at the MTP joint level, releasing both transverse intermetatarsal ligaments, and then only excising one of the neuromas (if it was large enough to indicate neuroma) (see Fig. 18-11 ). The concept of decompressing one webspace neuroma and decompressing and excising the adjacent neuroma was to avoid giving the patient a completely insensate third toe at the plantar portion. At a mean follow-up of 11 months, the authors reported complete resolution of pain in 19 patients (90%), 20 (95%) had no to minimal activity limitation, and 20 (95%) were completely satisfied with their outcome.
Benedetti et al reported on 15 patients (19 feet) who underwent simultaneous surgical excision of two primary interdigital neuromas in adjacent webspaces, with a mean follow-up of 68.6 months. Ten feet (53%) had complete resolution of symptoms, 6 feet (31%) had minimal residual symptoms, and 3 feet in two patients (16%) continued to have significant pain. The authors reported that patients experienced a dense sensory loss at the plantar aspect of the third metatarsal head to the tip of the third toe and proximal dorsal sensory loss to the second, third, and fourth toes. Although the numbness did not cause disability, the patients reported some awkwardness or difficulty with nail care. The authors used a curved dorsal incision, two longitudinal dorsal incisions, or a single transverse plantar incision with no tourniquet.
Dorsal Surgical Approach ( 
)
Neurectomy: surgical technique
The technique is as follows.
- 1.
The patient is positioned supine on the operative table with bump underneath the leg. The use of tourniquet and loupes are based on surgeon’s preference. We recommend the use of a calf tourniquet. An incision is made in the dorsal aspect of the foot, starting in the webspace between the involved toes. The incision is carried proximally for about 3 cm to the level of the metatarsal head. It is important to keep the incision directly in the midline to provide the best exposure and to avoid compromise of the traversing dorsal digital nerves, which could cause a painful neuroma ( Fig. 18-12 ).
- 2.
The incision is deepened through the soft tissue, using a tenotomy scissor, to the level of the metatarsal heads. Crossing branches of the superficial peroneal nerve may be encountered and should be carefully mobilized and protected. Dorsal fascia between the extensor tendons is seen and released. Any small crossing vessels impeding deeper visualization are cauterized. A smooth lamina spreader or a neuroma/metatarsal retractor is placed between the associated metatarsals at the head/neck region to gain optimal exposure (see Fig. 18-11 ) and tension the transverse intermetatarsal ligament. If there is difficulty inserting the retractor, a hemostat can be inserted between the metatarsals first to facilitate retractor placement.
- 3.
Use of a neurologic Freer elevator, or a tenotomy scissor, to dissect the contents of the interspace allows the transverse metatarsal ligament to be identified at its proximal and distal extent and then safely transected.
- 4.
The retractor or spreader is removed, set deeper between the metatarsal heads, and then reopened. This allows visualization of the contents of the webspace. Using blunt dissection, the common digital nerve is identified at the proximal portion of the wound, and it is traced distally to its bifurcation. Digital pressure under the web space from the plantar aspect pushes the nerve more dorsally and facilitates visualization.
- 5.
Once the bifurcation is reached, a significant amount of soft tissue, sometimes almost appearing bursa-like, may be surrounding the nerve. If possible, this tissue should be removed so that the nerve can be followed past the bifurcation. However, if the adhesions are too great, all of this material is removed along with the nerve. When the interspace is explored, the surgeon should look carefully for any accessory branches that might be coming out from beneath the adjacent metatarsal head to identify them and alter the treatment plan, if necessary. Preservation of the vascular structures is critical.
- 6.
In the proximal portion of the wound, the common digital nerve is cut proximal to the metatarsal head, dissected out distally past the bifurcation, and excised ( Fig. 18-13 ). As little plantar fat as possible should be removed. If a significant accessory nerve trunk passing to the common nerve either medially or laterally is observed, the consequences of cutting this nerve and allowing it to retract under the metatarsal head area must be considered. If the nerve trunk appears to be larger than 2 mm, rather than resecting the neuroma proximal to the metatarsal heads, the common nerve should be cut just proximal to its bifurcation, which is also just proximal to the thickening usually observed in the nerve distal to the transverse metatarsal ligament. The distal portion of the nerve is removed. The cut end is sutured to the side of the metatarsal or one of the intrinsic muscles so that it will not drop onto the plantar aspect of the foot. By placing the nerve alongside the metatarsal off the bottom of the foot when the stump neuroma forms, it will not be in a weight-bearing position. When this is carried out, it is important that the nerve not be under any tension when it is sutured.
- 7.
Tourniquet, if used, is to be deflated prior to closure. The skin is closed in a single layer and a comfortable compression dressing applied.
Neurolysis of the common digital nerve and its terminal branches
Surgical technique
The technique is as follows.
- 1.
The patient is positioned supine on the operative table with bump underneath the leg. The use of tourniquet and loupes are based on surgeon preference. We recommend the use of a calf tourniquet. An incision is made in the dorsal aspect of the foot, starting in the webspace between the involved toes. The incision is carried proximally for about 3 cm to the level of the metatarsal head. It is important to keep the incision directly in the midline to optimize exposure and avoid compromise of the traversing, dorsal digital nerves, which could cause a painful neuroma.
- 2.
The incision is deepened through the soft tissue, using a tenotomy scissor, to the level of the metatarsal heads. Crossing branches of the superficial peroneal nerve might be encountered. These branches should be mobilized and protected. Dorsal fascia between the extensor tendons is seen and released. Vessels to be cauterized. A smooth laminar spreader or a neuroma retractor is placed between the metatarsal heads to gain optimal exposure. This places the transverse intermetatarsal ligament under tension.
- 3.
Using a neurologic Freer elevator to bluntly dissect out the contents of the interspace, the surgeon identifies and transects the transverse metatarsal ligament.
- 4.
The retractor is removed and set deeper between the metatarsal heads and spread. This permits the surgeon to visualize the contents of the webspace. A neurologic Freer elevator is used to identify the common digital nerve in the proximal portion of the wound and trace it distally to its bifurcation.
- 5.
At this point, a neurolysis is performed, first by carefully excising any bursal material that may be present about the bifurcation of the common digital nerve. The presence of bursal material proximal to the transverse metatarsal ligament is rare. Then, any adhesions between the nerve and the surrounding tissues are carefully released. Caution is necessary, because plantar-directed nerve branches pass from the plantar aspect of the nerve into the tissue on the bottom of the foot. Transection of these nerves can result in a painful plantar neuroma.
- 6.
Once the nerve is freed from approximately 3 cm proximal to the intermetatarsal ligament and 1 cm distal to it, an adequate dissection has been achieved. If the nerve is inadvertently damaged during dissection, it should be excised in the usual manner as far proximal to the metatarsal heads as possible, dissected distally, and removed at the level of the bifurcation.
- 7.
The wound is closed in a routine manner with interrupted nylon sutures.
- 8.
A sterile compression dressing is applied, and the foot is kept wrapped for a period of 2 to 3 weeks to allow the soft tissues to heal.
- 9.
After 3 weeks, the patient can progressively increase the level of activities.
Postoperative care
The patient is permitted to ambulate in a postoperative shoe. The sutures are removed between 7 to 14 days. A compressive wrap is used for several weeks, during which the patient is encouraged to work on active and passive range-of-motion exercises of the toes.
Uncommon findings
Excision of an interdigital neuroma is typically a straightforward procedure. Occasionally, a large bursa is found between the metatarsal heads, particularly in the third interspace, and must be removed to expose and decompress the underlying nerve. This is carried out by sharp dissection, with care taken not to remove any plantar fat. An accessory nerve trunk may be encountered, as mentioned earlier; if it is greater than 2 mm in diameter, the nerve distal to it can be excised and the nerve sutured to the side of the metatarsal to prevent formation of a recurrent neuroma beneath the metatarsal head.
Occasionally, a cyst, which usually consists of material that appears to be degenerated fat, is identified adjacent to a metatarsal head. This does not appear to be a true cyst but does have a somewhat irregular lining. When present, it should be unroofed, but again, as little fat as possible is removed to prevent atrophy of the fat pad beneath a metatarsal head.
Results
An analysis of patients in a series by Mann and Reynolds following excision of an interdigital neuroma demonstrated the following results: 71% essentially asymptomatic, 9% significantly improved, 6% marginally improved, and 14% failure.
The patients with poor outcomes were carefully reevaluated, with no evidence of other pathologic findings that would account for the persistent pain, such as synovitis of the MTP joint, subluxation of the MTP joint, or the presence of a neuroma in an adjacent webspace. Obviously, a certain group of patients have a condition similar to an interdigital neuroma, which clinically is not fully understood at this time. The results for those patients in whom a neuroma was removed from the second webspace and third webspace were the same. Other follow-up studies of patients after excision of an interdigital neuroma have demonstrated similar results.
Physical examination of the satisfied patient population demonstrated the following postoperative findings: local plantar pain, 65%; numbness in the interspace, 68%; and area of plantar numbness adjacent to the interspace, 51%.
Not infrequently, although the patient believed that the neuritic pain was gone, some discomfort was still noted on the plantar aspect of the forefoot. At times, this was described as a feeling as if a sock was wrinkled under the foot or a feeling of stepping on a piece of cotton. The fact that 32% of the patients still had normal sensation in the web space after complete excision of their interdigital neuroma indicates the degree of overlap that is present in the innervation of the area. It is also important to consider this when one is evaluating a patient with a recurrent neuroma because the presence of sensation does not necessarily mean that the neuroma has not been adequately excised. About half the patients noted some numbness on the plantar aspect of the foot adjacent to the interspace. This probably results from the plantar innervation that comes off the common digital nerve, which is disrupted when the nerve is excised.
Coughlin and Pinsonneault reported on 66 patients with a mean follow-up of 5.8 years. Overall satisfaction was rated as excellent or good by 85%, but 65% were pain free, with minor or major footwear restrictions. Major activity restrictions after surgery were uncommon. Subjective numbness was present but variable in pattern in half of the patients feet. The numbness was bothersome in 4 of 71 feet. Patients with bilateral neuroma excision or adjacent neuroma resection had a slightly lower level of satisfaction, but this difference was not significant.
Giannini et al reported on 60 patients (three bilateral) who were treated with excision of interdigital neuroma. The clinical results were excellent or good in 49 (78%) feet, fair in 12 (19%), and poor in 2 (3%). Of these patients, 62% had normal sensation and no paresthesias, and the remaining 38% had numbness, 57% had no difficulty with footwear, and 40% had some limitation.
Womack et al reported on the long-term results of 120 patients who underwent surgical excision of interdigital neuroma. At a mean follow-up of 66.7 months (range, 14 to 113 months), 61 patients (51%) had good or excellent results, indicating that long-term outcomes may be less favorable than short- to mid-term outcomes. An interesting finding was that patients with neuromas in the second webspace had statistically significant worse outcomes than those with third webspace neuromas.
In a review of the long-term outcomes of 81 patients who underwent surgical excision of a Morton’s neuroma, Kasparek and Schneider reported, at a mean follow-up of 15 years, excellent results in 44 cases (45%), good in 31 cases (31%), fair in 15 cases (15%), and poor in 8 cases (8%). All patients with poor results underwent a revision surgery that showed an amputation neuroma. The authors reported that 95% of the patients would do the surgery again, even those with fair or poor outcomes since they described pain relief compared to their pre-operative state. Poor results were observed more often in cases without transection of the deep transverse ligament. Patients with single neuroma excision compared to multiple neuroma excision had significantly better outcomes scores.
A few complications deserve mention. In cases where patients had multiple steroid shots followed by surgery, there is an additional risk of wound complications, such as delayed healing or infection. When adjacent webspace neuromas have been resected, the loss of sensation in the tip of the middle toe, possibly coupled with some vascular compromise, can increase the risk of frostbite in the winter. Su et al examined 674 consecutive pathologic specimens obtained after interdigital neurectomy and found that the adjacent digital artery was resected in 39% of cases. Clinically, no adverse effects were reported. Stress fracture or damage to the MTP capsule can occur if the retraction is too vigorous or the bone or capsular tissue is incompetent. Rarely, a patient can develop complex pain syndrome type 2 (causalgia) after a neuroma resection.
Plantar Surgical Approach
Akermark et al reported, in a randomized prospective study, on the outcomes of dorsal versus plantar approach for Morton’s neuroma excision. Thirty-five patients received a longitudinal plantar incision and 41 patients received a dorsal incision. The transverse intermetatarsal ligament was sectioned with the dorsal but not the plantar approach. Both groups showed significant reduction in postoperative pain scores at final follow-up. The authors showed satisfactory outcomes (excellent or good results) in 87% of the plantar group and 83% of the dorsal group. There was no significant difference in scar tenderness between the two groups at 12 months follow-up. The patients had a significant difference at baseline in subjective sensory loss, more pronounced in the dorsal group. There was a difference in the type of complications reported in both groups. In the plantar group, there were three cases of localized hypertrophic scar formation, one case of large hypertrophic scar, all of which underwent a minor scar excision, and one case of foreign body reaction.
In the dorsal group, there was one missed nerve resection (i.e., the digital artery was resected), one wound infection, one wound dehiscence, and one deep vein thrombosis. Two patients were regarded as failures due to intolerable pain and underwent revision surgery via a plantar incision.
The choice of approach is based on surgeon preference. Some authors recommend a transverse longitudinal incision for a very proximal, focal, tender trigger point for the neuralgia. This permits a more direct exposure of the nerve that lies plantar to the intermetatarsal ligament and just beneath the plantar fascia and next to the flexor digitorum tendon. It allows the nerve resection to be performed off of the weight-bearing surface of the forefoot as well as resection more proximal to the level of the transverse metatarsal ligament. It permits identification of anomalous nerve branches that can anastomose with the site of the neuroma and provides easier access to adjacent digital nerves with minimal dissection through soft tissues. Like all plantar approaches, there is a risk for development of painful plantar scar or plantar keratosis.
There are two primary types of incisions: transverse and longitudinal. Transverse incision permits greater exposure for multiple nerve dissections. A transverse incision made 1 cm proximal to the weight-bearing region allows exposure of the adjacent interdigital nerve. It also facilitates identification of accessory or aberrant nerve branches. Avoidance of the artery, vein, and tendon is easier because the dissection is proximal to where these structures are more intermingled. The incision is within the skin fold lines, making the scar cosmetic and well tolerated. Because the exposure is proximal to the area of disease, the surgeon must be comfortable that nothing in the web space requires resection. Some experience is necessary to be comfortable with this approach and the orientation. The longitudinal incision is also reported to be cosmetic because it runs parallel to the lines of the connective tissue fibers. It is extensile and thus can be continued distally between the metatarsal heads or proximally into the midfoot or hindfoot. This permits identification and resection of distal disease as well as higher transection with or without nerve burial (transposition).
Transverse approach: surgical technique
The technique is as follows.
- 1.
The incision should be made 1 cm proximal to the weight-bearing area of the metatarsal heads ( Fig. 18-14 ).
- 2.
Dissection should be performed straight down through the subcutaneous fat and then immediately through the plantar fascia to avoid creating multiple soft tissue planes.
- 3.
The interdigital nerve will be exposed immediately deep to the plantar fascia within the fatty tissue between the flexor digitorum longus tendons.
- 4.
Aberrant or accessory nerve branches can be identified in this area.
- 5.
The nerve can be transected proximally.
- 6.
Adjacent interdigital neuromas can be approached by widening the incision ( Fig. 18-15 ).
- 7.
The wound is closed with 4-0 nylon suture.
- 8.
A compression dressing is applied for 10 to 14 days until sutures are removed.
- 9.
A wrap is recommended for 1 to 2 more weeks as the patient is allowed to progress with weight bearing.
Longitudinal approach: surgical technique
The technique is as follows.
- 1.
The incision should be centered directly over the intermetatarsal space so that any subsequent scarring will not take place directly under either metatarsal head. Typically, the incision should be made approximately 1 to 2 cm proximal to the proximal end of the metatarsal head.
- 2.
Dissection should be performed straight down through the subcutaneous fat and then immediately through the plantar fascia to avoid creating soft tissue planes.
- 3.
The interdigital nerve will be exposed immediately deep to the plantar fascia within the fatty tissue between the flexor digitorum longus tendons ( Fig. 18-16 ).
- 4.
Aberrant or accessory nerve branches can be identified in this area.
- 5.
The nerve can be transected as proximally as possible or kept slightly longer to permit transposition into muscle ( Fig. 18-17 ).
- 6.
When a transposition is performed, the end of the transected nerve’s epineurium can be held with a 4-0 Vicryl suture. The suture is fed through a straight needle (Keith needle), and the needle is passed through muscles between the metatarsal. It then penetrates the dorsum of the foot. With the nerve end within the muscle belly, the suture can be tied on the dorsum to help keep the nerve in place during the first 10 to 14 days.
- 7.
The wound is closed with 4-0 nylon suture.
- 8.
A compression dressing is applied for 10 to 14 days until sutures are removed.
- 9.
A wrap is recommended for 1 to 2 more weeks as the patient is allowed to progress with weight bearing.
Recurrent Interdigital Neuromas
A patient with a recurrent webspace neuroma can present with symptoms similar to their original Morton’s neuralgia in addition to a lumplike sensation in the forefoot. The main symptom is typically a sharp area of plantar forefoot pain. The pain is aggravated by activity and diminished by rest. Some patients who have persistent pain after excision of a neuroma report that the pain is almost identical to what they experienced before their initial surgery. Other patients note that the pain is now different, well localized, and electric-like in quality.
In a review of 39 patients with recurrent neuromas, Beskin and Baxter noted that two thirds of patients had symptoms within 12 months after the original surgery and one third had symptoms 1 to 4 years after the initial surgery.
The problem of a recurrent neuroma can be viewed, perhaps, as two different groups of patients. The first group, that is, patients in whom the clinical picture is similar to an interdigital neuroma, but unfortunately is not, and who continue to have symptoms in the foot after the initial surgery, probably represent the two thirds of the patients noted by Beskin and Baxter who had recurrence of their symptoms within 12 months after the original surgery. In most, the initial symptoms probably never completely subsided.
Wolfort and Dellon reported that 7 of the 13 patients with recurrent Morton neuroma also had proximal compression of the tibial nerve and the recurrent symptoms may be due to an undiagnosed tarsal tunnel syndrome. Electrodiagnostic testing can help differentiate the two diagnoses. The authors also reported 80% excellent and 20% good results with implantation of the nerve stump into plantar intrinsic muscle belly. Histologic examination confirmed that a nerve stump buried in a muscle will form little or no neuroma.
In the second group of patients, the symptoms are caused by a bulb neuroma that forms at the end of the common digital nerve. In most cases, it probably takes at least a year or more for the neuroma to be of sufficient size for symptoms to develop. This probably represents the one third of the patients in the Beskin series who had symptoms 1 to 4 years after the initial surgery. The symptomatic, recurrent neuroma could develop because the resection of the nerve initially was not sufficiently proximal to the metatarsal head. This could then result in a neuroma forming beneath the metatarsal head, possibly because of an aberrant branch, as noted earlier, and in some cases for reasons that are not well understood.
In discussing an anatomic basis for a recurrent neuroma, Amis et al noted that plantar-directed nerve branches tether the common digital nerve to the plantar skin. These branches are concentrated about the bifurcation of the proper digital nerve, and therefore the cut end of the nerve can fail to retract. The authors observed that no plantar branches occurred 4 cm proximal to the transverse metatarsal ligament. Therefore, an effort should be made to cut the nerve 4 cm proximal to the transverse metatarsal ligament to ensure that the nerve adequately retracts. Another way to ensure that the nerve will retract is to dorsiflex the ankle joint after the nerve has been transected, thereby pulling the nerve into a more proximal position. The surgical site can be examined to see whether the nerve has indeed retracted, and if not, a more proximal resection can be considered.
In approximately 75% of patients with a recurrent neuroma, the cause is either inadequate resection of the nerve or the formation of an adherent neuroma beneath a metatarsal head.
Johnson et al reviewed a series of 37 histologic specimens in patients with recurrent interdigital neuroma. In 21% of cases, they observed a residual primary interdigital neuroma; in 21%, a true amputation stump neuroma was seen either due to failure to resect proximal enough or due to tethering of the nerve; and 46% showed features of both primary neuroma and stump neuroma. In 12% of the cases, no neuroma-like tissue was seen, and instead reactive fibrofatty tissue was appreciated. These patients were the most satisfied after reoperation.
Nelms et al described a technique to transfer the nerve stump in cases of recurrent neuroma into a drill hole in the metatarsal shaft. The authors reported 89% good or excellent outcomes with this technique.
Diagnosis
Physical examination in this group of patients must be carefully performed in the same manner as that for a primary neuroma, perhaps with even more scrutiny and care, in the scenario that the original diagnosis was incorrect. The MTP joints should be carefully palpated to look for other disorders, along with careful palpation of the webspaces. If the index surgery was performed by another surgeon, clinical records, especially the operative report, should be obtained for review. In the setting of the initial surgery being done elsewhere, the prior incision should be examined for location and adequacy of length, such as a very short dorsal incision that may have been inadequate to achieve proper exposure and proximal resection of the nerve.
During exam, other causes of pain should be ruled out, including but not limited to: MTP joint derangement (arthritis, synovitis, instability, osteochondral lesion), Freiberg or osteonecrosis of the metatarsal head, transfer metatarsalgia, ganglion cyst, lipoma, hypertrophic bursa, plantar fat pad atrophy, plantar warts, tender scar/keratosis, more diffuse neuritis, proximal nerve compression/entrapment, and iatrogenic superficial neuroma.
As a general rule, the patient with a recurrent neuroma demonstrates a well-localized area of tenderness, usually either beneath the metatarsal head or just adjacent to it. Palpation of this plantar area by the examiner usually elicits a significant electric-like pain, and the patient states that this is similar to the symptomatic pain.
At times, the plantar pain is along the medial or lateral aspect of the metatarsal head and can represent either a stump neuroma that has become adherent to the metatarsal head or possibly an accessory branch that sometimes passes obliquely beneath the metatarsal head. It is possible that the recurrent symptoms represent an activated adjacent webspace neuroma. This can be confusing to both the surgeon and the patient based on symptoms and findings and can require a nerve block for final distinction. Unfortunately, response to a nerve block is helpful but not reliably conclusive. Rarely, the neuritic symptoms are coming from a damaged dorsal nerve branch.
Patients who have well-localized findings and who demonstrate a Tinel sign in a small area usually respond best to revision surgery. The response to the diagnostic block is also helpful to predict a success. After the physical examination, a radiographic study of the foot should be obtained to look for any osseous pathologic conditions or changes around the MTP joints. The recurrent neuroma stump can, occasionally, be seen on MRI ( Fig. 18-18 ).
The differential diagnosis presented for a primary interdigital neuroma should be once again considered in a patient with a recurrent neuroma. Careful examination of the posterior tibial nerve and its terminal branches should be carried out to rule out a tarsal tunnel syndrome as the cause of the recurrent or persisting symptoms. A double crush phenomenon should always be considered in any patient presenting with a neuroma, particularly a recurrent neuroma in question.
Treatment
Conservative Treatment
Conservative management of recurrent neuroma is similar to that for the primary neuroma. The patient should wear a wide, soft-soled, laced shoe with a soft metatarsal support to relieve pressure on the metatarsal head region and interspace. The medications to help diminish nerve excitation (e.g., gabapentin, TCAs, antiepilepsy medications), discussed under the conservative treatment of interdigital neuromas, deserve more consideration here because a subgroup of these patients has severe symptoms and underlying nerve sensitivity. A corticosteroid or anesthetic injection may be considered for diagnostic/therapeutic purposes. If conservative measures fail and the symptoms persist, reexploration of the interspace may be indicated.
Occasionally, the use of a transcutaneous nerve stimulator or ultrasound is useful in helping to break up the patient’s pain pattern. These modalities are useful in only about 10% to 20% of cases, but merit a try nonetheless.
Surgical Excision
Before reexploration of an interspace and prior to any peripheral nerve block used for surgery, the foot should be carefully examined in the pre-operative area, in order to localize the neuroma as accurately as possible. A small dot should be placed using a surgical marker or pen, at the point of maximal tenderness or positive Tinel’s sign. This helps to direct the surgeon to the exact area where the patient experiences their symptoms and should help to facilitate the surgical exposure in finding the nerve amputation stump.
The question arises, at times, as to whether the recurrent neuroma should be explored from either a plantar or a dorsal approach. The webspace can certainly be adequately explored from a dorsal approach, although the incision must be extended proximally slightly more (1 cm) than when removing a virgin neuroma. Advocates of a plantar approach to a recurrent neuroma believe that the neuroma should be approached through a longitudinal plantar incision centered between the metatarsal heads and the nerve identified and resected proximal to the metatarsal head. They point out that it is easier to find the nerve and relevant pathology in most cases from the plantar side.
The other school of thought is that because so much scarring is present around the area where the nerve had been previously excised, it is better to approach the common digital nerve proximal to the metatarsal heads and section the nerve in this region without exploring the interspace distally. In a study with this approach, 86% of 39 patients obtained significant improvement of at least 50% of their discomfort, but fewer than half the patients were completely symptom free . Fifty-eight percent had difficulty wearing certain shoes, and 88% had discomfort in high heels.
An analysis of seven patients with 11 recurrent neuromas excised through a dorsal incision demonstrated that 81% of the patients were essentially asymptomatic, 9% had marginal improvement, and 9% had no improvement. Conversely, Bradley et al noted that only two of eight patients had improvement after revision neuroma surgery.
Stamatis and Myerson reported on reexploration in 60 interspaces (49 patients, 49 feet) for recurrence or persistent symptoms after one or more previous procedures for excision of an interdigital neuroma, with an average follow-up of 39.7 months. Ten patients had simultaneous excision of an adjacent primary neuroma, and 19 underwent additional forefoot surgery. Fifteen patients (30.7%) were completely satisfied, 13 (26.5%) were satisfied with minor reservations, 10 (20.4%) were satisfied with major reservations, and 11 (22.4%) were dissatisfied with the outcome. Twenty-nine (59.2%) had moderate or severe restriction of footwear, and 8 (16.3%) had moderate restriction of activity after revision surgery.
Wolfort and Dellon reported on their results with 17 recurrent neuromas in 13 patients. At a mean of 33.8 months, 80% had excellent relief with a plantar longitudinal approach and implantation of the nerve ending into muscle. The authors also identified deficits along the tibial nerve territory by quantitative sensory examination in 54% of their patients.
Colgrove et al performed a prospective, randomized, blinded study comparing the treatment of an interdigital neuroma by the standard resection operation with a transection distal to the neuroma and transposition into muscle. In the resection group, the average pain level was slightly lower during the first 6-month period compared to the transposition group, but at the 12-month review, the resection group had a slightly higher average pain level. At the 36- to 48-month review, the resection group again reported a greater average pain level and fewer asymptomatic patients. The authors concluded that it is unnecessary to excise the interdigital neuroma to obtain excellent relief of pain. They also concluded that intramuscular transposition of the neuroma produced significantly better long-term results than did the standard resection operation.
Overall, reexploration for a recurrent neuroma through either a dorsal or a plantar incision results in less than complete satisfaction in 20% to 40% of cases. This fact should be very carefully explained to the patient before surgery.
A surgeon’s preference and experience are important factors in deciding on the approach for the revision surgery. It is our belief that a dorsal approach should be considered in two situations: 1) if there is a small, prior dorsal incision, it is possible that the resection was not adequate and a repeat transection can be made through a longer dorsal incision, and 2) if there is an adjacent neuralgia in this scenario, either a release or resection is performed via the dorsal approach.
A plantar approach is considered if the prior incision was determined to be adequate or the primary surgeon thought that the nerve resection was taken back sufficiently. If there is an adjacent primary neuroma as well, then a plantar transverse approach is to be considered. If the same nerve is the site of the recurrent neuroma, then a plantar longitudinal incision can permit a more proximal transection and burial.
The patient must be counseled preoperatively on the limits of the operation. Although literature supports near 80% satisfaction rate, two thirds of patients experience some tenderness over the incision and up to 85% report some long-term activity and shoe wear restrictions.
Excision of recurrent neuroma
Surgical technique
The technique is as follows:
- 1.
We recommend the use of surgical magnifier loupes in all revision surgeries to improve visualization. The skin incision is made in the dorsal aspect of the webspace, usually passing through the previous scar for a distance of 4 cm. If there is evidence of entrapment of a superficial nerve adjacent to the scar, an attempt should be made to identify and excise it. The incision is carried down through the scar tissue in the webspace by staying as much in the midline as possible.
- 2.
To improve visualization, a smooth lamina spreader or a neuroma retractor is placed between the two metatarsals distally. In a recurrent neuroma, this is sometimes difficult to do until the dissection has been carried down through the transverse intermetatarsal ligament. A neurologic Freer elevator usually can be used to break up the scar tissue between the two metatarsals to identify the transverse ligament.
- 3.
Although the transverse intermetatarsal ligament was likely transected at the initial surgery, it almost invariably re-forms and needs to be identified and sectioned again.
- 4.
After the transverse intermetatarsal ligament is released, the lamina spreader should be reinserted more plantarward to further separate the metatarsal heads. If the metatarsal heads still cannot be adequately distracted, a portion of the transverse intermetatarsal ligament is still intact or there is residual scar tissue that has not yet been completely released.
- 5.
It is easier to find the common digital nerve proximally in a virgin area and then trace it distally. As mentioned, before surgery the examination should reveal the approximate location of the neuroma, and this is very helpful information at surgery. Occasionally, if the nerve cannot be identified, the skin incision is extended proximally another centimeter, and more tissue is separated in the web space to enhance the exposure.
- 6.
Careful dissection allows the common digital nerve to be identified and traced distally. Once this is accomplished and the tip of the nerve is excised from the surrounding scar tissue, the nerve is cut as far proximal to the metatarsal head as possible.
- 7.
The ankle is dorsiflexed to help pull the nerve more proximally into the foot.
- 8.
The skin is closed with interrupted sutures and a compression dressing is applied.
Interdigital Neuroma/Neuralgia - Surgical treatment
For recurrent neuroma from a plantar transverse or plantar longitudinal approach, please see techniques above under the heading for a primary resection.
Postoperative care
Postoperative management consists of re-dressing the foot with a firm compression dressing for 24 hours. The foot is kept dressed for 2 weeks, during which time the patient ambulates in a postoperative shoe. After 3 weeks, the shoe is removed and the patient is started on active and passive range-of-motion exercises; the level of activity is progressively increased as tolerated. Running is restricted for approximately 2 months after excision of a recurrent neuroma to permit adequate healing to occur.
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