Pathology
The spring and propulsion of the plantar fascia work like a nautical device called a Spanish Windlass (see Figure 1). The plantar fascia functions to maintain the longitudinal arch when the foot is in a normal weight-bearing position. In the windlass mechanism, tension on the "cable" (in this case, the plantar fascia) is increased as the second segment (the phalanges) is brought into extension. Therefore, during the end of the push-off phase of gait there is a greater degree of tension generated in the foot to help propel the body forward (see Figure 2).

Normal weight-bearing activities also create a natural degree of increased tension on the plantar fascia. As pressure is placed directly downward on the longitudinal arch, tension will naturally be increased along the plantar fascia. The increased tension will pull on each end of the plantar fascia. Due to the windlass arrangement in the foot, the tension will be greatest when the toes are in hyperextension.

The calcaneal attachment site for the plantar fascia is much smaller than the distal attachment spread across the metatarsal heads. Therefore, when tensile loads within the plantar fascia are spread to each end, the calcaneal attachment site bears a greater pulling force. Because the plantar fascia pulls on the periosteum at the attachment site, this site becomes the central problem in plantar fasciitis. Since the periosteum is one of the most pain-sensitive tissues in the body, this condition causes significant foot pain.

As mentioned earlier, plantar fasciitis has traditionally been characterized as a chronic inflammatory problem, although there is controversy about just how much inflammation is actually present. It is likely that a primary part of the problem involves collagen degeneration in the fibers of the plantar fascia more so than an inflammatory reaction in the tissue.

When a tendon or connective tissue like the plantar fascia inserts into a bone, it doesn’t stop right at the bone. It has fibrous continuity with the bony matrix. Therefore, excessive tensile stress on that site may also affect the bone. This often occurs in plantar fasciitis. As a result of the tensile stress placed on the bony attachment site, an exostosis (bone spur) may develop.

Plantar fasciitis can occur from a number of causes. One of the most common causes is biomechanical dysfunction in the foot. While improper footwear may contribute, overpronation is frequently cited as the primary biomechanical dysfunction. Pronation is a diagonal movement of the foot that includes the combined motions of abduction, eversion, and dorsiflexion. Pronation happens during the normal gait, but if the person rolls over too far on the medial side of the foot this is considered overpronation. When the individual overpronates, the plantar fascia must take on a greater role in absorbing shock in the lower extremity. The increased tensile stress on the plantar fascia will often lead to fiber breakdown with resultant stress on the calcaneal attachment site.

Overpronation often accompanies a flat foot (pes planus), the presence of which is a strong indicator that plantar fasciitis may occur. However, a pes cavus (high arch) foot is also likely to contribute to the problem.

References