Disorders of the Nerves

The diagnosis and treatment of diseases of the peripheral nerves in the foot and ankle usually are quite straightforward, but, at other times, they present a very complex clinical and surgical problem. Nerve problems can result from overuse injuries (because of repetitive stresses) or from one-time traumatic events. Nerve disorders in many patients resolve uneventfully and do not require much, if any, treatment. Some nerve disorders resolve as a more recognizable condition receives treatment, such as in a patient with a grade 3 ankle sprain and a mild traction injury to the superficial peroneal nerve.

The more challenging diagnostic cases are dynamic and occur only with mechanical stresses, such as walking, running, or wearing certain shoes. Some nerve disorders occur as a double-crush syndrome. In the double-crush syndrome, a more proximal nerve dysfunction (radiculopathy), which may be subclinical, causes impairment of axoplasmic flow and diminishes the threshold for distal nerve symptoms from focal disease.41,212 The role of metabolic, endocrinologic, chemical, pharmacologic, or rheumatologic conditions in nerve physiology and function must be recognized as creating or enhancing nerve dysfunction. Occasionally, the clinical problem has been aggravated by surgery, and the preoperative condition must be distinguished from the postoperative condition. The foot and ankle specialist is challenged to elucidate the differences between the symptoms from mechanical nonneurologic symptoms and those that arise from the nerves. Awareness of these features and manifestations of these conditions will help the specialist design an effective treatment plan and anticipate the outcome of the interventions.

Interdigital Neuroma

The history of the interdigital neuroma has been long and colorful. Kelikian’s detailed chronology of the history ³¹ is briefly summarized here. The condition was first described in 1845 by the Queen’s Surgeon-Chiropodist Lewis Durlacher.¹⁶ He described a “form of neuralgic affection” involving “the plantar nerve between the third and fourth metatarsal bones.” In 1876 T.G. Morton³⁹ 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 Mason³⁷ reported a case of pain around the second MTP joint and suspected involvement of a digital branch of the medial plantar nerve. In 1893 Hoadley²⁷ 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 Tubby⁵⁵ reported observing on two occasions that the plantar digital nerves were congested and thickened. In 1940 Betts⁸ stated that “Morton’s metatarsalgia is a neuritis of the fourth digital nerve.” In 1943 McElvenny³⁸ stated that it was caused by a tumor involving the most lateral branch of the medial plantar nerve. In 1979 Gauthier¹⁹ speculated that the condition was a nerve entrapment, an idea supported anatomically by others.^22,32

Etiology

In discussions of interdigital neuroma, it must first be pointed out that the subject is not a neuroma per se, but rather a painful clinical entity involving the common digital nerve. Thus the best terminology for the condition should perhaps be interdigital neuralgia based on the complexity and variability of the cause. The term neuritis is a less favorable descriptor because the suffix “-itis” implies inflammation. Because the condition can occur as an entrapment without inflammatory cells or mediators, the suffix “-algia,” used to describe pain, is more accurate.

Studies by Lassmann ³² and Graham and Graham²² have demonstrated that most, if not all, of the histologic changes in the nerve occur beyond the transverse metatarsal ligament and that the nerve proximal to the ligament appears to be normal. These authors demonstrated quantitatively that there was a decrease in the number of thick myelinated fibers, decreased diameter in the individual nerve fibers as a result of attenuation of their myelin sheaths, and increased nerve and vesicular width. Alterations in the interdigital arteries could not be correlated with the alterations found in the nerves. Hyalinization of vessel walls has been observed in patients with interdigital neuroma, but it has also been observed in control material in patients without interdigital neuroma. In a study of 133 patients with a clinical diagnosis of Morton’s syndrome, light and electron microscopic investigations revealed that in the early stages of the disease the histologic findings are dominated by certain alterations of the nerves independent of alterations of the interdigital vessels. These alterations include 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’s syndrome is 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.

Giannini et al ²⁰ reported on the histopathology in 63 cases and found intraneural (epineurium and perineurium) fibrosis and sclerohyalinosis and an increase of the elastic fibers in the stroma. They also found hyperplasia of the muscle layer, very evident internal elastic lamina, and proliferation of small vessels in the muscle layer and adventitia in the vessels.

Although these studies seem to support the theory that the main cause for an interdigital neuroma is an entrapment neuropathy beneath the transverse metatarsal ligament, other factors should be considered in the origin of this condition, including anatomic, traumatic, and extrinsic factors.

Anatomic Factors

From an anatomic standpoint, the medial plantar nerve (MPN) has four digital branches (Fig. 12-1). 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 lateral plantar nerve (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.

Figure 12-1 Illustration of nerves on the plantar aspect of the foot. Note the third toe has mixed innervation from the medial and lateral plantar nerves.

Researchers have speculated that because the common digital nerve to the third interspace consists of branches from the medial and lateral plantar nerves, the common digital nerve has increased thickness and is therefore more subject to trauma and possible neuroma formation (Fig. 12-2).³⁰ In an anatomic dissection of 71 feet, Levitsky et al³⁴ observed that a communicating branch to the nerve to the third web space was present in only 27% of specimens and was absent in 73%. This anatomic finding seems to indicate that the communicating branch has little relation to the cause of interdigital neuroma. The authors did observe, however, that the second and third interspace was significantly narrower than the first and fourth and postulated that this further indicated the possibility of an entrapment. The frequency of the communicating branch was confirmed by Govsa et al.²¹ They studied the anatomy of the communicating branches of the common plantar digital nerves between the fourth and third nerves in 50 adult male cadavers and found it was present in 28% of the feet.

Figure 12-2 Plantar nerves of the right foot.

Some variation exists in the distribution of the common digital nerves to the plantar aspect of the foot and their relationship to the metatarsal heads. At times, when an interdigital neuroma is resected, an accessory nerve branch appears to pass obliquely beneath the metatarsal head and joins the common digital nerve before its bifurcation into the digital branches. Because these accessory branches appear to come from beneath the metatarsal head region, when they are transected, the nerve end can retract beneath the metatarsal head and result in a recurrent neuroma.

The mobility that occurs between the medial three rays and the lateral two rays may be a reason for the increased incidence of neuromas in the third web space. The medial three rays are firmly fixed at the metatarsocuneiform joints, whereas the fourth and fifth metatarsals are fixed to the cuboid, which is more mobile. This anatomic fact results in increased mobility in the third web space, and the increased motion can result in trauma to the nerve or possibly the development of an enlarged bursa, which can secondarily place pressure on the nerve. The significant number of neuromas found in the second web space in part negates this as the main cause for the development of a neuroma.³⁵

During normal gait, dorsiflexion of the metatarsophalangeal (MTP) joints along with the action of the plantar aponeurosis causes plantar flexion of the metatarsal heads and, in theory, exposes the nerve to increased trauma (Fig. 12-3). The incidence of interdigital neuromas is 8 to 10 times more common in women, whose toes are often hyperextended at the MTP joint by high-fashion footwear, and this is probably a significant factor. Rather than increased trauma to the nerve itself resulting from long-term hyperextension of the MTP joints, the nerve may be tethered beneath the transverse metatarsal ligament, which results in the entrapment neuropathy previously discussed (Fig. 12-4).

Figure 12-3 The demipointe position can increase the stretch on the common digital nerve.

Figure 12-4 Illustration of the third branch of the medial plantar nerve. Note that the nerve courses plantarward, under the transverse metatarsal ligament.

Traumatic Causes

Acute trauma resulting from a fall from a height, a crush injury, or stepping on a sharp object occasionally results in a traumatic origin of an interdigital neuroma. Runners, persons who spend many hours on their feet at work, or persons who engage in certain athletic endeavors, such as racquet sports or dance, and place much stress on the metatarsal region, do not demonstrate an increased incidence of neuroma formation. The precise cause of neuroma development in one person but not in another exposed to essentially the same level of activity or inactivity remains an enigma. Even patients with atrophy of the plantar fat pad with resultant metatarsalgia rarely, if ever, report focal neuritic symptoms isolated to a single web space.

Extrinsic Factors

Extrinsic pressure against the nerve can result from a mass above the transverse metatarsal ligament or below the ligament. Anatomically, a bursa is normally present between the metatarsal heads and is located above the transverse metatarsal ligament. Bossley and Cairney ¹⁰ studied this bursa by injecting dye in cadaver feet and demonstrated extension of the bursa distal and proximal to the transverse metatarsal ligament in the third web space. They did not observe extension of the bursa in the fourth web space. They hypothesized that the bursa can cause inflammation resulting in secondary neurofibrosis of the nerve but did not believe that the bursa caused a compressive force against the nerve. Awerbuch and Shephard³ noted similar findings of an inflamed intermetatarsal bursa in patients with rheumatoid arthritis, which they believed resulted in an alteration in the interdigital nerve, as well as some direct compressive effect on the nerve by the enlarged bursa.

Degeneration of the capsule of the MTP joint from unknown causes can result in a localized inflammatory response, but the nerve is not usually involved because the inflammation is above the transverse metatarsal ligament. Such deterioration of the capsule, however, can cause deviation of the proximal phalanx, usually of the third toe, in a medial direction. This in turn results in lateral deviation of the third metatarsal head against the fourth. The pressure can result in compression of bursal material present above the ligament and subsequently pressure against 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. 12-5). 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. 12-6).

Figure 12-5 Metatarsophalangeal instability. A, Oblique fluoroscopy view before stressing the third metatarsophalangeal joint. B, Oblique fluoroscopy view after stressing the third metatarsophalangeal joint. C, Neuroma identified next to unstable joint.

Figure 12-6 Increased pressure is created in the web space as a result of deviation of lesser toes, forcing metatarsal heads together. A, Anteroposterior radiograph shows lateral deviation of third toe, resulting in narrowing of second interspace. B, Axial view demonstrates narrowing of second interspace.

Occasionally, the transverse metatarsal ligament itself is thickened or contains an aberrant band that causes pressure against the common nerve. The authors have seen a few patients in whom this was present in the second web space but have not observed it in any other web space. In these cases, release of the transverse metatarsal ligament resulted in relief of the neuritic problem. A ganglion or synovial cyst can arise from the MTP joint, either as a primary entity or possibly in association with degeneration of the plantar plate. This lesion can cause direct pressure against the nerve as it passes beneath the transverse metatarsal ligament. A lipoma on the plantar aspect of the foot can result in similar compression against the nerve.

Fracture of the metatarsal from stress or a single traumatic event can result in altered loading of the metatarsal, with resultant interdigital neuralgia. In these cases, deviation of the malunion in any plane can cause traction or compression from the bone or any anchored soft tissues. For example, alteration in the length or function of the tendons directly from the callus or indirectly from malunion with shortening of the bone can lead to a claw toe or deviated toe with development of nerve symptoms. This can be further complicated in a traumatically induced fracture by a direct insult to the nerve by the causative mechanism (e.g., crush, twist) at the time of bone injury.

Symptom Complex

Mann and Reynolds ³⁵ 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,¹¹ who reported a ratio of 4 : 1. The average age of the patient in the Mann and Reynolds³⁵ series was 55 years (range, 29-81 years).

An interdigital neuroma is usually unilateral, although bilateral neuromas were observed in 15% of the patients. The development of two neuromas simultaneously in the same foot rarely occurs; in a review of 89 neurectomies, Thompson and Deland ⁵⁴ concluded that the incidence was probably less than 3%. The majority of the literature reports that the neuroma usually involves the third interspace, but an equal number of neuromas in the second and third interspaces have been noted. This same distribution was subsequently noted by Hauser²⁵ but was not supported by Graham et al,²³ who noted that there were twice as many neuromas in the third web space as the second. An interdigital neuroma of the first or fourth interspace is rare and probably does not exist as a clinical entity.

The most common symptom of an interdigital neuroma is pain localized to the plantar aspect of the foot between the metatarsal heads. The pain is usually characterized as burning, stabbing, tingling, or electric and radiates to the toes of the involved interspace in approximately 60% of cases. On occasion, the patient feels something “moving around” in the plantar aspect of the foot that periodically gets “caught” and results in an acute, sharp pain that radiates to the toes. This symptom complex is probably because the nerve is trapped beneath the metatarsal head, which, when body weight is applied, causes the acute pain. Occasionally, patients observe that the pain radiates toward the dorsum of the foot or more proximally along the plantar aspect of the foot.

The symptom complex is aggravated by activities on the foot, and most often the pain occurs when the patient puts on a tight-fitting high-heeled shoe. This pain is often relieved by removing the shoe and rubbing the forefoot. Often, the patient finds that the pain during gait can be limited by voluntarily curling the toes or by avoiding rolling through the ball of the feet. The patient often notes no symptoms when walking barefoot on a soft surface, only to feel pain when a high-heeled shoe is worn again. At times, wearing a broad, soft walking or jogging shoe results in a significant decrease in the symptom complex. Nonetheless, prolonged walking can induce symptoms regardless of shoe type.

Table 12-1 lists the preoperative symptoms by percentage in patients with interdigital neuroma.³⁵

Table 12-1

Preoperative Symptoms of Interdigital Neuroma

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

Diagnosis

The diagnosis of an interdigital neuroma is based on the patient’s history and the physical findings. No electrodiagnostic studies are useful unless a peripheral neuropathy or radiculopathy is suspected. Radiographs are helpful to assess the foot in general and to identify an underlying bone or joint disease. Magnetic resonance imaging (MRI) may be useful in some cases to rule out other pathologies but is not warranted for establishing the diagnosis (Fig. 12-7). Similarly, ultrasound may be helpful in some situations, but the rate of false positives and negatives is too high to depend on the test. In some cases, it may be necessary to reevaluate the patient several times before one makes the diagnosis of an interdigital neuroma if the patient’s symptoms are atypical.

Figure 12-7 A, Coronal magnetic resonance image (MRI) of common digital nerve schwannoma. B, Sagittal MRI of schwannoma. C, Schwannoma has high signal intensity with fat-suppression sequences. D, Intraoperative exposure of schwannoma. E, Removal of tumor with intrafascicular dissection from the common digital nerve.

Physical Examination

The physical examination begins by having the patient stand, with the physician carefully observing the foot for evidence of deviation of the toes, subluxation or clawing of the toes, or evidence of fullness in the involved web space or fullness of a web space not observed in the uninvolved foot. Then the range of motion of the MTP joints is observed. The MTP joints are carefully palpated on their dorsal and plantar aspects to look for evidence of synovitis of the joint, 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 web space. Thus, if there is more pain in between the metatarsals, especially if it reproduces the character of the patient’s symptoms of burning, shooting, radiating pain, then a neuroma is suspected.

Next, the physician carefully palpates the interspaces, starting just proximal to the metatarsal heads and proceeding distally into the web space. This usually reproduces the patient’s pain, which often radiates out toward the tips of the toes. It is important when performing this test to ensure that there is no inadvertent compression of the dorsal tissues as a counterforce to the plantar pressures that are directed dorsally. The physician should press the web space with one finger plantarly, using the other hand to stabilize the foot over a broad area of the midfoot or forefoot or against the resistance of the patient’s foot alone (Fig. 12-8). Concomitant dorsal pressure can lead to discomfort from other etiologies that must be distinguished from interdigital neuralgia.

Figure 12-8 Deep palpation test for interdigital neuroma. The examiner should press from plantar to dorsal, avoiding inadvertent dorsal palpation.

The web spaces are reexamined, and pressure is applied in a mediolateral direction with one hand to increase pressure on the tissue between the metatarsal heads. This maneuver often results in a significant crunching or clicking feeling (the Mulder sign),⁴⁰ most often in the third web space and occasionally in the second web space. 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 web space without reproduction of the patient’s pain does not indicate a neuroma. Examination of a normal foot can demonstrate a crunching or clicking feeling, particularly in the third web space and occasionally in the second. It is unusual to elicit this crunch or click in the first and fourth web spaces. This test is not as helpful after a prior neuroma resection. When the plantar aspect of the foot is palpated, occasionally a mass representing a small synovial cyst or ganglion is observed. Usually, this mass is less than 1 cm in diameter and can be rolled beneath the examiner’s fingers. When the mass is associated with a web space, pressure against the mass often reproduces the patient’s symptoms. A sensory examination rarely demonstrates any deficit in the patient with a neuroma.

A more complete neurologic examination should include palpation of all the web spaces and along the course of the tibial nerve branches. The tibial nerve itself should be palpated and percussed in the posterior leg, the popliteal space, and behind the femur. Other nerves should be examined, including the sural, saphenous, deep peroneal, and superficial peroneal nerves. A straight-leg maneuver should be performed to rule out radiculopathy when clinically indicated. A quick motor examination, including eversion, inversion, dorsiflexion, and plantar flexion of the toes, foot, and ankle is useful. When warranted, an examination of reflexes is performed. Checking the patient’s hands for evidence of intrinsic atrophy or tremor can help identify systemic neurologic conditions.

At times, the patient’s physical examination is not conclusive, and the patient needs to be reevaluated on several occasions to ensure that the symptom complex indeed indicates a neuroma. If there is pain around an MTP joint as well as in the web space, caution is advised in making the diagnosis of a neuroma because early degeneration of the plantar pad, early synovitis of the MTP joint, or some degeneration of the plantar fat may be present. It is useful to stress the MTP joint by checking for a dorsal drawer sign and instability of the MTP and induction of symptoms. When undertaking this maneuver, the physician must secure the base of the toe at the proximal phalanx without inadvertently pressing on the nerve. With the other hand securing the foot at the metatarsals, slight traction is applied to the toe, and an attempt is made to translate the base of the proximal phalanx dorsally. The adjacent and contralateral toes should be checked to determine the baseline joint laxity for the patient (Fig. 12-9). This test may be positive with instability or synovitis, which can coexist with neuralgia, but if the pain is not reproduced with the symptoms, then instability or synovitis is not triggering the neuralgia, or the patient’s forefoot pain may be coming from another condition.

Figure 12-9 A provocative evaluation of metatarsophalangeal joint instability or inflammation is important because these can be either contributing factors or the primary cause of the patient’s pain. The metatarsals are stabilized by one hand while the other secures the toe at the base of the proximal phalanx. A dorsal thrust in the sagittal plane can reveal subluxation or dislocation and can induce pain. The patient’s symptoms may be either mechanical or neuritic, depending on the pathology.

Diagnostic Studies

A radiograph of the foot during weight bearing should always be obtained in an evaluation of the patient with a suspected interdigital neuroma. The purpose of the radiographic study is to observe any abnormalities within the osseous structures; subluxation, dislocation, or arthritis of the MTP joint; or possibly evidence of a foreign body.

Schon’s (LCS’s) opinion is that MRI has not been demonstrated as effective in the diagnosis of an interdigital neuroma. This has been substantiated by a study by Bencardino,⁵ who found a lack of correlation of an MRI finding of a neuroma and symptoms in 33% of patients. If there is a question about the presence of other causes of the patient’s pain, an MRI may be performed in the prone position⁵⁷ using a combination of contrast enhancement and fat suppression.⁵³ One study by Biasca et al⁹ suggested that a more favorable result could be achieved after neuroma resection when the nerve has a transverse measurement larger than 5 mm on MRI scans.

Ultrasound is operator-dependent,⁴⁸ and although there are proponents of this imaging technique,^33,43,44 the authors and others do not find it a useful modality.^45,46 This impression has been substantiated in a report by Sharp et al.⁵⁰ These authors looked at the accuracy of preoperative clinical assessment, ultrasound, and MRI and compared these findings to intraoperative histology and the clinical outcome. They found the accuracy of the ultrasound and MRI was similar and dependent on lesion size. Small lesions were not well visualized on ultrasound. There was no correlation among the size, the pain score, or the change in pain score after surgery. Reliance on either ultrasound or MRI would have led to an inaccurate diagnosis in 18 of 29 cases. The predictive values attained by clinical assessment surpass imaging with one or both modalities.

No reliable electrodiagnostic studies are available to document the presence of an interdigital neuroma. However, these studies are useful to help identify a proximal entrapment, radiculopathy, or neuropathy when their presence is suspected.

Lidocaine can be injected into the suspected web space for diagnostic purposes. The recommended dose is 1 or 2 mL of anesthetic placed below the transverse metatarsal ligament. Although complete relief of symptoms can be obtained, caution is advised in interpreting this as confirming an interdigital neuroma because this injection may also relieve pain from local pathologic conditions, such as degeneration of the joint capsule or plantar plate. Younger and Claridge ⁶⁰ 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. Schon performed a recent cadaveric study comparing injection of 1 mL of fluid versus 2 mL into the third web space and found increased extravasation of fluid with the 2-mL dose. Hembree et al²⁶ found that injection of 1 mL of fluid was sufficient to effectively envelope the nerve but not selective enough to be used diagnostically because, even with a lower dose, there was still a high rate of extravasation into adjacent web spaces.

Administration of cortisone can be useful in about one third of patients, but cortisone can cause deterioration of the joint capsule or possibly the plantar fat. Only infrequently should more than one hydrocortisone injection be made into the web space. In some patients, medial or lateral deviation of the adjacent MTP joint has developed after multiple steroid injections, presumably from damage to a collateral ligament (Fig. 12-10). Decreasing the volume effect of the injection can help diminish the risk of damage to the MTP joint capsule, ligament, or 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.