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PATHOLOGY PERSPECTIVES Morton's neuroma turns out not to be a true neuroma (nerve tumor) at all. A more correct term is perineural fibrosis: a thickened perineurium, or nerve sheath. numbered 1–5 from the medial to the lateral side. The range of motion at the tarsal-metatarsal joints is very limited: the bones can move just enough to allow the arches of the foot to flatten and rebound with each step. However, mobility of these joints increases toward the lateral aspect: the 4th and 5th metatarsals move more freely than their medial partners. Nerve supply for the distal part of the foot is provided by the common digital nerves. These nerves are the terminal endings of the lateral and medial plantar digital nerves, which originate as part of the sciatic nerve. This is significant, because any restriction or limitation of free movement from the buttocks downward can irritate nerve fibers that eventually reach the bottom of the foot. The common digital nerves converge at the metatarsal heads, or "balls" of the feet. This location is especially vulnerable to compressive forces: the nerve structures are thick where they converge and their location is superficial to both the metatarsal heads and the transverse intermetatarsal ligaments. In other words, this bulkiest part of the nerve with its connective tissue sheath is in a position to be squeezed and irritated from all sides when pressure is translated across the bottom of the foot and toward the toes, as during the "toe-off" phase of walking. CONTRIBUTING FACTORS TO MORTON'S NEUROMA When branches of the common digital nerves converge and are irritated by compressive forces, the structure's connective tissue sheath tends to thicken. This protective response adds to compression on the nerve and exacerbates symptoms of nerve irritation. Morton's neuroma turns out not to be a true neuroma (nerve tumor) at all. A more correct term is perineural fibrosis: a thickened perineurium, or nerve sheath. 112 massage & bodywork may/june 2009 TIGHT FASCIA AND MUSCLES Several issues can contribute to nerve irritation in the ball of the foot. For example, the nerve might be embedded in thick fascia all the way down the leg. If that fascia is tight and restrictive, it inhibits the ability of the nerve to function well and increases the risk of entrapment or stretching of the nerve. Additionally, muscle tightness in the hamstrings or plantar flexors can pull on or compress the medial and lateral plantar nerves that eventually become the common digital nerves. People who spend a lot of their day in high heels put particular pressure at the metatarsal heads, just where the nerves are compressed under the intermetatarsal ligaments. This is exacerbated by squatting with the feet in plantar flexion: this is a position often adopted by people with tight hamstrings and calf muscles when they must squat. OTHER FOOT PROBLEMS Compensation patterns in the feet can also contribute to compression of the common digital nerves. Plantar fasciitis is a condition involving damage to the plantar fascia of the foot, making it painful to walk through a normal range of foot motion. For these people, spreading the arches of the foot is irritating, so they tend to walk on the balls of their feet and thus pressure on the common digital nerves under the metatarsal heads. Pes planus (flat feet) or pes cavus (jammed arches) likewise create compensation patterns that alter how force is transferred through the bottom of the feet. With little or no rebounding action in the arches of the feet, the nerves have no protection from constant compression. Any condition that alters how weight is distributed through the foot can contribute, including plantar warts, bunions, and even temporary blisters.