The plantar fascia runs from the medial tubercle of the calcaneus to the transverse ligaments of the metatarsal heads of the foot. The fascia has medial, central, and lateral parts, underneath which lie the abductor hallucis, flexor digitorum brevis, and flexor digiti minimi muscles, respectively. It holds down muscles and tendons in the concave surface of the sole and digits, facilitates excursion of the tendons, prevents excessive compression of digital vessels and nerves, and possibly aids in venous return (1). The origin of this fibrous aponeurosis is rich in sensory innervation and has fibrocartilage with longitudinal collagen fibers that resist tension. This fibrocartilage is also metabolically active in forming cartilage. Overuse of this structure can lead to a condition known as plantar fasciitis. Because fascia has little elastic properties, repetitive stretching can cause microtears at its origin.
Although originally perceived to be an inflammatory condition, histological findings are consistent with a degenerative process. Tissue analysis reveals a thickened fascia (up to 15 mm) as well as fibrocyte necrosis, microtears, chondroid metaplasia, angiofibroblastic proliferation and type I collagen fibers. This myxoid degeneration, which occurs in chronic conditions, replaces the normal cellular matrix and is mechanically inefficient. During the night, as the foot rests in the equinus position, the plantar fascia contracts. Thus, the first step out of bed in the morning abruptly stretches the fascia and causes irritation and pain.
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