The Science of Morton's Neuroma
Morton's neuroma, more accurately termed intermetatarsal neuroma, represents a localized thickening of the common digital nerve as it passes beneath the deep transverse metatarsal ligament. Despite its name, this condition is not a true neuroma but rather perineural fibrosis - a reactive thickening of the nerve sheath and surrounding connective tissue. The condition most commonly affects the third intermetatarsal space (between the third and fourth metatarsals) due to unique anatomical factors. In this location, the medial and lateral plantar nerves converge, creating a larger nerve bundle that is more susceptible to compression. The nerve becomes trapped between the metatarsal heads above, the deep transverse metatarsal ligament below, and compressed laterally by adjacent metatarsals during toe-off. Repetitive mechanical irritation leads to chronic inflammation of the nerve sheath, followed by progressive fibrosis and thickening. This creates a pathological cycle where the enlarged nerve becomes increasingly susceptible to further compression. The resultant ischemia and mechanical deformation of nerve fibers produces the characteristic neuropathic pain symptoms. Contributing factors include biomechanical abnormalities that increase forefoot loading, such as excessive pronation, first ray insufficiency, or metatarsal length variants. Intrinsic factors like hammertoe deformities or claw toes can further compress the intermetatarsal spaces. The condition is significantly more common in women (8-10:1 ratio), largely attributed to restrictive footwear with narrow toe boxes and elevated heels that force the metatarsals together and increase forefoot loading.
Contributing Factors
Normal forefoot mechanics distribute weight-bearing forces relatively evenly across all five metatarsal heads during push-off. The intermetatarsal nerves lie in the relatively protected spaces between adjacent metatarsals, with adequate clearance during normal gait patterns.
When biomechanical dysfunction occurs, several factors can increase nerve compression. Excessive forefoot loading, whether from first ray insufficiency, flexible flatfoot, or gastrocnemius tightness, increases the forces transmitted through the lesser metatarsals. This creates a more forceful "squeeze" of the intermetatarsal spaces during propulsion.
High-heeled shoes create a cascade of biomechanical changes: the elevated heel shifts body weight forward, increasing forefoot loading by up to 75%. Narrow toe boxes force the metatarsals into a more compressed position, reducing the available space for neural structures. The combination creates maximum compression precisely where the nerve is most vulnerable.
Forefoot width also plays a critical role. As the foot widens with weight-bearing, the intermetatarsal spaces normally expand slightly to accommodate nerve movement. Restrictive footwear prevents this natural accommodation, maintaining compression throughout the stance phase of gait.
The plantar fascia's role in maintaining arch stability affects metatarsal mechanics. When the plantar fascia is tight or the windlass mechanism is inefficient, compensatory overloading of the forefoot can occur. This explains why Morton's neuroma often coexists with other forefoot pathologies like metatarsalgia or plantar fasciitis.