Morton Neuroma

Etiology

The exact etiological cause of Morton neuroma is not known, but four major hypotheses have been described. The chronic trauma theory, the most widely accepted hypothesis, states that the mechanical effects of walking cause chronic micro-traumas to the intermetatarsal plantar digital nerves, which become compressed between two metatarsal heads and the metatarsophalangeal joints.[4] 

The entrapment theory, one of the earliest proposed theories, states that interdigital neuromas occur due to compression of the interdigital nerve against the anterior end of the deep transverse metatarsal ligament and plantar soft tissue structures. The intermetatarsal bursa theory states that bursitis in the intermetatarsal region causes compression and inflammation with subsequent fibrosis of the affected common plantar digital nerve. Interestingly, bursae are close to the neurovascular bundle in the second and third intermetatarsal spaces where Morton neuromas are most common, but Morton neuromas are rare in the fourth intermetatarsal space where the bursa rarely ever contacts the neurovascular bundle.[5] The fourth theory is the ischemic theory, based on histopathological findings of the common plantar digital artery exhibiting degenerative changes prior to the fibrous thickening of the nerve.[6] Common causes include narrow toe-box footwear, hyperextension of the toes in high-heeled shoes, deviation of the toes, inflammation of the intermetatarsal bursa, thickening of the transverse metatarsal ligament, forefoot trauma, high-impact sporting activities, metatarsophalangeal joint pathology, and lipoma.[7][8]

Epidemiology

Morton neuroma is common in middle-aged women, and the incidence is at least five times more common in females when compared to men.[9] The exact incidence is unknown. Rarely are both feet affected. Finding two neuromas on the same foot is common.[10]

Pathophysiology

Morton neuroma is common in the third interspace because it is narrower compared to other spaces. As the common digital nerve to the third space receives branches from both medial and lateral plantar nerves, it has increased thickness and is prone to compression and trauma. Trauma from a crush injury, penetrating injury, thickened transverse metatarsal ligament, enlarged bursa in the interspace, and repetitive trauma from running have all been linked to the development of Morton neuroma. The compression and repetitive trauma to the nerve results in vascular changes, endoneurial edema, and excessive bursal thickening leading to perineural fibrosis.[11]

Histopathology

Grossly there is a fusiform swelling near the bifurcation of the plantar interdigital nerve with thickening of adjacent tenosynovial tissues. Microscopically, the common plantar digital artery displays disruption of the arterial wall, thrombosis, and incomplete recanalization, which are findings that support the ischemic theory.[12] In addition, there is fibrosis surrounding and within the nerves, Schwann cell and fibroblast propagation, and damage to myelinated nerve fibers.

History and Physical

The most common symptoms are plantar pain between metatarsal heads that are aggravated by walking and wearing tight-fitting, high-heeled shoes and relieved by resting and removing shoes. Patients describe the pain as burning, stabbing, or tingling with electric sensations. Some patients describe the sensation as walking on a stone or marble. Numbness between the toes is present in less than half of the patients. With prolonged walking, the pain can radiate to the hindfoot or leg, causing cramps.

Evaluation

Diagnosis is usually based on history and clinical examination. Palpation in the affected space may reproduce the symptoms. Compression of the forefoot in the mediolateral direction while palpating the affected space often results in a significant crunching or clicking feeling, commonly known as the "Mulder's click." Some surgeons inject the affected webbed space with a diagnostic injection of lidocaine. A maximum of 1 ml to 2 ml of the anesthetic agent should be injected. Plain, weight-bearing radiographs should be taken to rule out any bony masses, deformities, subluxation, dislocation, or arthritis. Additionally, gapping of the distal intermetatarsal space and/or divergence of adjacent digits may be noted, and this is known as Sullivan's sign.[13] 

A radio-opaque foreign body can also be visualized. An ultrasound scan performed by an experienced radiologist can be a useful diagnostic aid. A steroid injection under ultrasound guidance can be given at the same time. MRI can be obtained based on the clinical scenario, especially to rule out other pathologies.[8][14] MRI imaging demonstrates a dumbell-shaped soft tissue lesion within the intermetatarsal space. The T1 signal is often low, the T2 signal is frequently low or intermediate, and enhancement is variable. The sonographic evaluation also similarly demonstrates a noncompressible dumbell-shaped soft tissue lesion with hypoechogenicity within the intermetatarsal space. A "Mulder's click" can potentially be elicited on palpation with the probe.

Treatment / Management

Non-operative Treatment

Wearing a wide, soft-soled, laced shoe with a low heel can effectively relieve pressure on the nerve. Some surgeons recommend a firm-soled shoe instead. Soft metatarsal support with a metatarsal pad, or neuroma pad, placed just proximal to the metatarsal heads, can help to spread the metatarsal heads and relieve mechanical pressure on the neuroma. In the presence of synovitis, instability, or deformity of the toe, a Budin splint or canopy toe strapping can decrease secondary neuralgia. Anti-inflammatory medications, tricyclic antidepressants such as amitriptyline, and anti-seizure medications such as gabapentin can be administered to lessen the severity of related nerve symptoms. Blind or ultrasound-guided steroid injections can occasionally help, but their effect is rarely long-lasting. Atrophy of the subcutaneous fat and plantar fat pad, discoloration of the skin, and disruption of the joint capsule adjacent to the injected site causing deformity of the toe, are some of the reported side effects. Radiofrequency ablation, cryotherapy, and alcohol nerve injections have been proposed as less invasive and more conservative methods of treating neuromas.[1][15][16](A1)

Surgical Treatment

If non-operative management fails, then surgery is indicated to treat recalcitrant cases. The neuroma is excised using a dorsal or plantar approach. The dorsal approach is better tolerated by patients as the plantar scar can be painful. A 3 cm to 4 cm incision is made just proximal to the involved webspace in the midline to prevent injury to the dorsal cutaneous nerves. The incision is deepened to the transverse metatarsal ligament, which is transected. The common digital nerve is identified in the proximal portion of the wound and is traced distally to its bifurcation, and any soft tissue adhesions, if present, are released around the nerve. At this point, nerve decompression or neurolysis has been performed, and some surgeons will end the dissection here without any further intervention to the nerve itself. Previous reports in the literature noted satisfactory results with decompression alone.[17] (B2)

To intervene on the nerve, the common digital nerve is cut proximal to the metatarsal heads and then traced distally past the bifurcation, where both branches are transected. Some surgeons suture the cut ends of the nerve to the side of the metatarsal or one of the intrinsic muscles to prevent the formation of a painful stump neuroma. As little plantar fat as possible should be removed. A postoperative shoe is worn till the stitches are removed at about 7 to 14 days, and a compressive wrap is used for 2 to 6 weeks.

The plantar incision is mainly reserved for recurrent neuromas or when the patient has a proximal focal tender trigger point for neuralgia. It decreases the rate of missed neuroma, and it does not require an incision of the transverse metatarsal ligament. The plantar approach permits a more direct exposure of the nerve and allows its more proximal resection. The artery and vein can be better visualized and preserved. The main disadvantages are painful plantar scars and plantar keratosis in about 5% of cases.[18]

Differential Diagnosis

• Metatarsal stress fracture
• Hammertoe
• Rheumatoid or osteoarthritis
• Malignancy
• Ganglion cyst

Prognosis

Conservative Approaches 

Conservative approaches to treatment have shown varying degrees of effectiveness.[19][20] Change of footwear, activity modification, and pharmacotherapy are often used to minimize pain and lessen symptoms. Employing multiple strategies has shown to be effective.[21]  A systematic review and meta-analysis of non-surgical approaches were performed, comparing both non-invasive (manipulation/mobilization, wider footwear and metatarsal padding, extracorporeal shockwave therapy, and varus/valgus wedge) and invasive (corticosteroid injections, sclerosing injections, radiofrequency ablation, cryo-neurolysis, and botox injection) methods. The analysis found corticosteroid injections and manipulation/mobilization to have the strongest evidence for pain reduction.[22]

Surgical Cases 

Various papers have investigated success rates after surgery.[23][24][25][26] Kasparek et al. evaluated 81 patients and noted excellent subjective results in 45% of patients and good results in 32% of patients. Approximately 8% of patients had noted a poor result.[23] The question of whether to perform neurolysis versus a neurectomy is debatable, but one study noted that neurolysis yields satisfactory results overall, but in the presence of a pseudo-tumor or considerably thickened nerve, a neurectomy may be a more viable option.[27] 

A systematic review and meta-analysis of 35 articles, including 2998 patients with Morton neuromas, compared injection therapy (both alcohol and non–alcohol), neurolysis, and neurectomy. The analysis found that 43% of patients noted complete pain relief after injection therapy, 68% were pain-free after neurolysis, and 74% were pain-free after neurectomy.[28]

Complications

• Chronic pain (chronic regional pain syndrome)
• Recurrence of the deformity due to inadequate excision or converting a Morton neuroma into a true neuroma
• Complications related to surgery (infection, pain, bleeding)
• Complications associated with corticosteroid injections (skin/fat pad atrophy, skin discoloration)