Morton's neuroma is a benign fibrotic thickening of a plantar interdigital nerve that is a response to irritation. Also known as Morton's metatarsalgia, Morton's entrapment, interdigital neuroma, intermetatarsal neuroma, and interdigital nerve compression syndrome, it is not a true neuroma, as the condition is a degenerative process rather than a neoplastic process. The mean age at presentation is 55 years. It is 4-15 times more common in women than in men.
Morton’s neuroma is a benign fibrotic thickening of a plantar interdigital nerve that is a response to irritation. Typical symptoms of Morton’s neuroma include pain in the forefoot, most commonly felt in the third inter-metatarsophalangeal space. Pain is exacerbated by walking, or particular footwear, and relieved by removal of footwear and massaging the toes. First-line management includes avoiding shoes with thin soles, high heels, or a constricting toe box. If symptoms persist for more than 3 months specialist treatments may be considered including corticosteroid injections, radiofrequency ablation or surgery.
Morton neuroma was first described by Thomas Morton in 1876 as “a peculiar and painful affection of the fourth metatarsophalangeal articulation”. However, subsequent studies have shown that the third and second spaces are the more common sites for a neuroma.
The exact pathogenesis of Morton neuroma remains controversial. It is generally accepted that the development of Morton neuroma may be induced by repetitive compression of the plantar nerve against the deep transverse intermetatarsal ligament, with subsequent perineural fibrosis.
Typical symptoms of Morton’s neuroma include:
The recommendation that investigations are generally not necessary for Morton’s neuroma as it is essentially a clinical diagnosis is based on expert opinion in a clinical consensus statement. (Thomas et al, 2009)
Ultrasound scan and MRI are highly accurate in detecting and localizing Morton’s with high sensitivity and specificity.
The possible clinical differential diagnoses that can cause metatarsalgia include intermetatarsal bursitis, stress fractures, necrosis of the sesamoid bones, synovitis of the metatarsophalangeal joint, infection and true neoplasm.
Freiberg’s disease (also known as Freiberg’s infraction – osteochondritis of the metatarsal head) – causes metatarsalgia, most commonly in the area of the second metatarsophalangeal joint area.
Injection of local anaesthetic and steroid under ultrasound guidance can help if simple treatments have not worked. Repeated corticosteroid injections into the tight web spaces of the foot should be performed cautiously since it may lead to plantar fat pad atrophy and eventually avascular necrosis.
Another nonoperative treatment that has been shown to have a high success rate in patients with Morton’s neuromas is alcohol neurolysis under ultrasound guidance.
Recently, radiofrequency (RF) ablation has been used as a less invasive method for the treatment of Morton’s neuroma before considering surgical resection. RF utilizes the electricity generated from a radiofrequency wave to disrupt soft tissue molecules, resulting in frictional heating. When the temperature rises to a certain point, normally higher than 70 degrees Celsius, instant tissue coagulation and cell death occur resulting in the destruction of neural tissue. In one retrospective study, 83% of patients with Morton’s neuromas who underwent RF expressed complete relief of symptoms after one month. Additionally, in a longer term study, 87% of patients who underwent RF had good results with a 70% reduction in the number of patients progressing to surgery.
Pulsed radiofrequency (PRF) ablation is another type of radiofrequency applied intermittently at lower temperatures as compared to the continuous high heat seen in RF. A higher voltage is used in PRF that produces a brief rise in temperature followed by elimination of heat at timed intervals. The mean tip temperature remains below the neurodestructive range, which preserves the structural integrity of the nerve; but it still has ablative effects due to the temperature spikes and/or electrical field. PRF may minimize the risk of nerve damage while still providing good clinical outcomes; however, there is limited data supporting this.
By combining PRF with RF, a shorter duration of RF can be utilized thus decreasing nerve and tissue damage, while still achieving pain relief through the combined ablative effects.
Surgical options include Neurectomy of the involved interdigital nerve and neuroma or decompression of the interdigital nerve by deep transverse metatarsal ligament release, with or without neurolysis.
Surgical neurectomy has been established as a viable treatment option with good outcomes for many patients with Morton’s neuromas. In a long term follow-up study performed by Coughlin et al., 85% of the patients who underwent surgical excision were satisfied with the results from the surgery and 65% were pain-free 6 years later. Another study demonstrated that 82% of patients reported excellent or good postoperative results, but 71% still had restrictions with footwear.