Plantar Digital ‘Neuroma’/Mortons neuroma
Common problem – 9.3% of foot complaints in one clinic . Also called - intermetatarsal neuroma; interdigital neuroma; Morton’s toe; neuritis; metatarsalgia; ‘pinched nerve’; metatarsalgia. Originally described by Durlacher in 1845 and Morton in 1846. Middle aged women most commonly affected.
‘Neuroma’ is a misnomer – histological studies have revealed fibrosis and fibronoid degeneration of the soft tissue around the nerve, and demyelination and endoneural fibrosis of the nerve. These findings are consistent with other entrapment neuropathies.
The cause is thought to be due to trauma on the nerve or in the region of the nerve (possibly compressive forces against deep transverse metatarsal ligament). Tight fitting footwear is probably a significant contributing factor. Obesity also appears to be a risk factor.
Root et al (1977) & Carrier et al (1975) hypothesise that the shearing stresses associated with abnormal pronation of the foot during propulsion causes the ’neuroma’. In a pronated foot the 3rd metatarsal head is assumed to move closer to the 4th compression/impingement. May be due to adverse neural tension in an excessively pronated foot during propulsion – nerve is placed under tension during MPJ dorsiflexion and pronation of rearfoot (as 3rd interspace is supplied by two nerves, the neural structures here may not be as mobile and adapt to the neural tension). A stretch is also placed on the nerve during prolonged stance phase pronation. However, Kilmartin et al (1994) were not able to demonstrate any changes in the symptoms with the sue of felt padding to pronate or supinate the foot.
The neurovascular bundle in third interspace runs medially to the lumbricle muscle, whereas in the second interspace it runs more plantar with respect to the muscle – this may play a role in the 3rd interspace being the more common site.
Intermetatarsal bursa are in close proximity to nerve, so a bursitis may cause irritation of nerve . 40% of patients seen at one clinic that presented what was a diagnosis of Morton’s interdigital neuroma either had diagnosed rheumatoid arthritis at the time or later developed the disease
Appears to be more common in females in 4th to 6th decade (? Role of footwear)
Most common between the 3rd and 4th metatarsal heads (2/3rds here; second interspace is next most common site)
Three stages of pain:
• Initial Stage – focal distant pain (not always sharp) usually around 3rd and 4th metatarsal head on weightbearing; often described as standing on a lump or walking on a wrinkle in stocking.
• Second stage – sharp shooting pain radiating into the toes on weightbearing
• Later stages – excruciating pain present most of the time – very distinct sharp pain may radiate into the digits and/or across the dorsum of the foot
Pain can initially be relieved by taking off the shoe and massaging the area; pain is worse on dorsiflexion; patient may describe a feeling of numbness, but sensory deficit is uncommon; toes adjacent to affected area may splay on weightbearing, especially if neuroma is large.
Key feature – Mulder’s Click:
Pressing finger in between the 3rd and 4th web space and squeeze laterally and compress the foot around the metatarsal heads, a positive diagnosis is present if an audible click is heard in the early stages and symptoms produced in the later stages. Anatomically, Mulder’s sign or click is due to a plantar subluxation of the digital nerve
Diagnostic testing – x-ray (no findings, but helps rule out other causes); ultrasound (useful if experienced); MRI – may not be very reliable as neuromas were found in 19/57 (33%) of those with no clinical evidence of it .
Capsulitis; bursitis; metatarsal overload syndromes; rheumatoid arthritis; other inflammatory arthritis; synovitis; stress fracture; flexor tendonitis; vasospastic conditions; neuritis associated with proximal nerve impingement (eg tarsal tunnel syndrome).
Metatarsal padding to offload and splay metatarsals – can be combined with an anti-pronatory strapping and toe crest padding. Wider footwear with lower heels are key to initial management. Orthoses to limit forefoot hypermobility if rearfoot excessively pronated are widely recommended with little evidence to support their use. Butler (1991) suggests performing gliding motions of the third and fourth metatarsals while having the patient sit in the slump test position with ankle dorsiflexed (to increase tension in the neural pathway). Corticosteroid/local anaesthetic injections (literature reports variable results)
Chemical neurolysis with 4% alcohol sclerosing injection (82% success rate for recalcitrant neuromas reported – Dockery (1999). Surgical removal through a plantar or dorsal approach.
Bennett et al (1995) evaluated a staged treatment protocol that consisted of:
Stage one – patient education, footwear modifications (wider), metatarsal head relief with padding just proximal to inflamed area (43% improved)
Stage two (after 3 months) – steroid and local anaesthetic injection (50% improved)
Stage three (after 3 months) - surgical incision (100% improved)