Therapeutic Effect of Botulinum Toxin A for the Treatment of Plantar Fasciitis.

Plantar fasciitis represents the most frequent cause of chronic heel pain, usually occurs in patients 40 years or older, overweight, sedentary or with intense physical activity.

The plantar fascia function is to prevent foot collapse by its anatomical orientation and by its tensile forces; It originates at the base of the calcaneus and extends distally to the phalanges. The plantar fascia stretching prevents the displacement of the calcaneus and the metatarsals and maintains the medial longitudinal arch. Simulates a cable attached to the calcaneus and metatarsophalangeal joints. The windlass mechanism described by Hicks, for the action of the plantar fascia is usually explained when a dorsiflexion of the fingers occurs, this leads to an effective shortening of the length of the plantar fascia causing an elevation of the arch. The extension of the fingers increases the arch of tension with the metatarsophalangeal joint as axis or anchor point. The shortening of the plantar fascia resulting from the dorsiflexion of the hallux is the essence of the windlass mechanism. When a fasciotomy is performed, this mechanism is lost, decreasing the stability of the arch and this does not allow a phase of stable terminal stay.

Historically the development of plantar fasciitis is attributed to biomechanical defects such as hyperpronation, this contributes to excesive mobility of the foot, which increases the stress applied to the musculofascial structures and soft tissue through an elongation of plantar fascia. There are other studies that have shown that one of the main factors for the appearance of this disease is the mechanical overload and it has been reported that the tension necessary for the rupture of the windlass mechanism ranges from 1.4 to 3.4 of the body weight of the subject.

There is a great variety of therapies reported for the treatment of this pathology, intralesional application of steroids, platelet-rich plasma, intralesional botulinum toxin A, treatments such as extracorporeal shock waves, all of which are assisted by stretching excercises of the gastrocnemius and sole muscles or stretching of the plantar fascia.

The clinical use of botulinum toxin A has expanded beyond the original indications by its effects on cholinergic neurons. The increased interest in the potential role to treat conditions of chronic pain is partially based on the effects of the toxin on the modulation of the release of substance P, on the calcitonin-related gene peptide and glutamate. On the other hand, the toxin has shown its effect on the inhibition of inflamatory pain and on the release of neurotransmitters from primary sensory neurons in a rat model. It also inhibits peripheral sensitization, which leads to an indirect decrease in central sensitization. It is unclear whether the treatment of chronic plantar fasciitis with botulinum toxin A works by causing muscle paralysis or by analgesic anti-inflamatory effects or by both mechanisms. A combined effect, induction of paresia of the muscles originating in the medial calcaneus process and direct analgesia due to analgesic anti-inflamatory properties.