Morton’s Neurectomy

Christopher P. Chiodo

Abstract

The plantar interdigital neuroma or Morton’s neuroma is caused by a thickening of the interdigital nerve as it crosses under the intermetatarsal ligament just distal to the division of the nerve traveling to the two adjacent toes. The third web space nerve is the most commonly affected due to its relative immobility as the junctional terminal nerve between the medial and lateral plantar nerves. Patients present with pain in the ball of the foot, often radiating to the third and fourth toes, and numbness. A cortisone injection can be used therapeutically and diagnostically to confirm the diagnosis. Failure of nonoperative modalities is treated with surgical neurectomy, usually through a dorsal third web space approach, as outlined in this chapter.

Keywords: interdigital, Morton’s neuroma, neurectomy

15.1 Indications

• Mild intermittent nerve pain is not necessarily an indication for surgery and may respond to nonoperative measures or may be tolerable to the patient.

• Surgery is indicated in patients who have at least 3 months of daily or regular pain that is refractory to nonoperative measures and interferes with activities of normal living.

• It is recommended that all patients undergo a cortisone injection into the web space before surgery given this will help confirm the diagnosis (if the pain is temporarily relieved), and may resolve the swelling and cure the neuroma.

• Finally, patients may occasionally present with numbness but no pain. Neurectomy is contraindicated in these instances, as it will not improve sensation.

15.1.1 Pathology

• In 1876, Morton described painful thickening of the interdigital nerves at the level of the metatarsal heads. Such “in situ” neuromas are histologically characterized by concentric perineural fibrosis.

• The vast majority of these lesions occur between the third and fourth metatarsals. In those cases in which symptoms are attributed to a second interdigital neuroma, the alternative diagnosis of second metatarsophalangeal (MTP) synovitis and instability should be carefully considered.

• The exact etiology of a Morton neuroma is not entirely understood and may be multifactorial.

• Potential intrinsic factors include irritation of the nerve as it crosses under the intermetatarsal ligament, bursa formation, ischemia, tethering of the nerve, and the fact that the third interdigital nerve is thicker than the other interdigital nerves.¹

• Potential extrinsic factors include high-heeled shoes, shoes with a narrow toe box, and proximal nerve compression.

15.1.2 Clinical Evaluation

• A Morton neuroma may occur in any of the lesser web spaces, with pain radiating to the two adjacent toes supplied by the nerve.

• As mentioned previously, the third web space is the most commonly affected, followed by the second and fourth web spaces, respectively.

• A small number of patients may have more than one neuroma, although the additional neuromas may not contribute to the patients’ symptoms.

• Patients with interdigital nerve pain are typically tender with palpation of the soft tissues between the metatarsal heads and distal metatarsals. With a primary neuroma, there is often a painful “click” with palpation of the intermetatarsal space while simultaneously compressing the metatarsal heads (i.e., Mulder’s sign).

• Additionally, pain is reproduced by squeezing the forefoot medially–laterally, rather than plantar-dorsal as is seen with MTP pathology. MTP synovitis and instability is characterized by pain and tenderness located more medial, at the base of the toe.

• The second MTP joint will also be unstable and painful with dorsal translation of the toe (a positive “drawer” test).

15.1.3 Radiographic Evaluation

• Preoperative diagnostic studies should include weight-bearing radiographs of the foot. These are important to rule out other potential sources of pathology, including pain from a long second metatarsal (e.g., second MTP synovitis and instability), stress fracture, or Freiberg’s disease.

• Advanced imaging, specifically magnetic resonance imaging (MRI), is not routinely necessary, but may be helpful to confirm the presence of a neuroma or multiple neuromas in atypical presentations. It may also help to rule out other potential diagnoses such as MTP capsulitis, plantar plate injuries, stress fracture, or Freiberg’s infraction.

• Ultrasound evaluation can give real-time imaging of the plantar foot structures and help identify a neuroma, the presence of multiple neuromas, or the presence of plantar plate MTP pathology or synovitis.

• Finally, the treating physician should have a low threshold to use diagnostic lidocaine injections. These injections are invaluable when it comes to confirming or ruling out the diagnosis of neurogenic pain.

15.1.4 Nonoperative Options

• Pain due to a Morton neuroma may often respond to accommodative orthotics and a cortisone injection, as well as nonsteroidal anti-inflammatory medications and oral nerve stabilizing agents (e.g., gabapentin or pregabalin).

• More than one cortisone injection should be used with caution, however, given that this can lead to plantar fat-pad atrophy or MTP instability.

15.1.5 Contraindications

• Active infection.

• Vascular insufficiency.

• Uncontrolled complex regional pain syndrome.

• Lack of a clear diagnosis.

15.2 Goals of Surgical Procedure

The primary goal of a neurectomy is pain relief. Neurogenic pain can be disabling, especially in the foot where cyclic loading can lead to thousands of painful stimuli each day. Neurectomy alleviates the majority of symptoms in most patients. Nevertheless, patients must be counseled about the fact that there will be permanent postoperative numbness in the foot. Additionally, they should be aware that any peripheral nerve transection can result in a recurrent stump neuroma, albeit one that is less painful or asymptomatic.

15.3 Advantages of Surgical Procedure

The main advantage of neurectomy is long-term pain relief. The interdigital nerves generally travel in the second plantar layer of the foot. The advantages of performing this procedure through a dorsal approach are that a plantar scar is avoided and that patients can bear weight in the immediate postoperative period. While a plantar approach has been described for the treatment of primary neurectomy,²^,³ it is more useful for a recurrent neuroma in which more proximal exposure is necessary.

15.4 Key Principles

• Adequate exposure is essential.

• An appropriate level of proximal transection is critical for success. Ideally, the distal aspect of the residual proximal nerve segment should be well proximal to the weight-bearing portion of the forefoot.

• All nerves must be handled with delicacy to avoid scarring and iatrogenic damage. This can theoretically lead to pain in the residual nerve ending.

15.5 Preoperative Preparation and Patient Positioning

The patient is positioned supine with the operative extremity elevated on blankets or a foam wedge. If necessary, a bump is placed under the ipsilateral hip so that the toes are oriented toward the ceiling. The use of a calf or ankle tourniquet facilitates visualization, as does the use of loupe magnification. Regional anesthesia with sedation is preferred, specifically either an ankle or popliteal fossa block.

15.6 Operative Technique

• A 3to 4-cm longitudinal incision is made on the dorsal aspect of the foot centered between the metatarsal heads and extending to the distal web space (Fig. 15.1).

• The underlying subcutaneous adipose and dorsal fascia of the foot are sharply divided. The latter structure is thin and membranous and should not be confused with the more robust and plantar transverse intermetatarsal ligament.

• A smooth lamina spreader is then placed between the metatarsal necks and used to gently distract the metatarsals. This tensions the intermetatarsal ligament and thereby facilitates its identification and exposure (Fig. 15.2).

• The intermetatarsal ligament is then sharply divided after its proximal and distal edges are identified. Passing a Freer elevator deep to the ligament will minimize inadvertent damage to the nerve or artery.