Biomechanics of Plantar Fasciitis
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Composition, Origin and Insertion of the Plantar Aponeurosis
The plantar aponeurosis is a ligamentous structure that supports the plantar arch of the foot. It originates on the medial calcaneal tubercle and inserts on the deep, transverse ligaments of the metatarsal heads. The plantar aponeurosis is triangular and consists of three parts: medial, central and lateral.
Functions of the Plantar Aponeurosis
The Windlass Mechanism
The Windlass mechanism describes how the plantar aponeurosis elevates the medial longitudinal arch. To begin, the foot can be described as a triangular arch, bordered by the metatarsals, calcaneus, and metatarsal joints. The way the plantar aponeurosis supports the medial longitudinal arch is analogous to how a rope or cable becomes taut. The plantar aponeurosis acts like a cable tethered to the metatarsophalangeal joints and calcaneus at either end. As the propulsive phase of gait occurs, dorsiflexion causes the plantar aponeurosis to tighten around the heads of the metatarsals. This creates tension in the plantar aponeurosis and decreases the distance between the calcaneus and the metatarsals; which elevates the medial longitudinal arch and prevents it from collapsing.
The Plantar Aponeurosis and Collagen
The plantar aponeurosis is composed of type I collagen. This type of collagen is characterized by elasticity and tensile strength. Collagen fibers are stable and provide great tensile strength because of their microscopic structure; the triple helix. Additionally, the subunits of type I collagen fibers (fibrils) can uniquely withstand a large amount of stretch and tension. Type I collagen fibers are oriented in a parallel fashion with respect to each other. This means that collagen fibers are effective in transmitting and distributing forces that are also longitudinal or parallel to the fiber direction. However, forces that are compressive or perpendicular in nature cannot be transmitted as effectively and result in degradation of the collagen fibers.
What is Plantar Fasciitis?
Simply put, plantar fasciitis is an overuse injury. Accumulation of micro-damage leads to the degradation of the collagen fibers that make up the origin point of the plantar aponeurosis. This prevalent condition is the most common cause of heel pain. It is important to note that despite the presence of the suffix “itis”, this condition lacks inflammatory mechanisms and agents. 
Collagen Degradation and Plantar Fasciitis
Forces that are perpendicular to the fiber direction of collagen or compressive in nature are most likely to cause micro-tearing of the collagenous structure. Thus, the forces that cause degradation to the plantar aponeurosis are body weight and vertical ground reaction forces.  The magnitude of the forces experienced by the plantar aponeurosis are also important to consider.  Having a high BMI can increase body weight forces experienced by plantar aponeurosis. This increases the rate at which these compressive forces cause micro-damage to the collagenous structure.  Additionally, as the duration of the forces applied to the plantar aponeurosis increases, the rate of collagen degradation also increases.  This is particularly relevant to activities like prolonged standing or walking.
The Gait Cycle and Plantar Fasciitis
Movement at the ankle like pronation and supination increase plantar fascial tension. This tension is beneficial because it regulates the movement of the talus and prevents it from moving too far anteriorly during pronation or too posteriorly during supination. Pronation and supination are important movements in the gait cycle. Thus, tight control of the plantar aponeurosis allows for regulated pronation and supination which in turn results in systematic and efficient gait.  However, prolonged gait creates excessive tension on the origin point of the plantar aponeurosis (medial calcaneus). The first instance in the gait cycle where plantar fascial tension is highest is the transition between heel strike and weight acceptance; this transition is where maximum ankle pronation occurs.  The second instance is during the transition between midstance and toe-off; maximum ankle supination occurs during this transition. Excessive plantar fascial tension results in the degradation of the collagenous fibers along the origin point of the plantar aponeurosis. This manifests in symptoms like heel pain.
Risk Factors and Underlying Biomechanics
Overpronation: Overpronation results in weakness of the posterior tibialis. This proves to be problematic because tibialis posterior normally functions to reduce tension applied to the plantar aponeurosis by eccentrically contracting during weight acceptance. Weakness in tibialis posterior will result in an increase in tensile forces applied to the plantar aponeurosis which leads to collagen degradation.
High-Arched Feet: A high arch decreases the distance between the two attachment points of the plantar aponeurosis. Meaning, that the plantar aponeurosis is continuously in a shortened state.This increases the tension along the plantar aponeurosis.
Leg length discrepancy: this results in uneven distribution and transmission of ground reaction forces to the feet. Compensatory mechanisms like excessive hip and knee flexion, and excessive hip circumduction all increase the stress and tensile forces on the plantar aponeurosis. Plantar fasciitis commonly affects the longer limb because greater forces are transmitted to the foot on the longer limb.
Footwear: The sole and heel of old shoes can begin to wear down with continual use.  This decreases the ability of the shoe to absorb and dissipate ground reaction forces. Thus, the foot and the plantar aponeurosis will experience more stress, and compressive forces as most of the ground reaction forces are being transmitted directly to the plantar fascia, rather than the shoe.
Treatments and Underlying Biomechanics
Night splints: Plantar fasciitis pain and symptoms are most severe in the morning because overnight, the foot is in a sustained plantarflexed position. This leads to plantar fascial shortening. Night splints hold the ankle in a dorsiflexed position. This elongates the plantar aponeurosis and releases tension on the structure.
Gastrocnemius and soleus muscles stretching: Stretching focused on these muscles is effective because it increases Achilles’ tendon flexibility which in turn reduces the tensile forces applied to the plantar aponeurosis.
Orthotics: A shoe insert controls biomechanical risk factors of plantar fasciitis like overpronation of the feet and leg length discrepancy. These orthotics should be semi-rigid, ¾ to full length and have longitudinal arch support. For orthotics to successfully treat plantar fasciitis they need to control overpronation and the motion of the first metatarsal head. 
Supportive shoes: Shoes should provide stability and motion control by regulating the amount of pronation in the feet. The shoe should be constructed to have a wide toe-box and allow for forefoot flexibility. Forefoot flexibility enables dorsiflexion by allowing the midfoot and rearfoot to move over the forefoot with ease.  The midsole of the stability shoe should be made from supportive material like ethylene vinyl acetate. This material provides cushioning and counteracts compressive forces. 
Plantar Fasciotomy: A small incision is made at the origin point of the plantar aponeurosis.  Typically, this incision is made on the medial band of the plantar aponeurosis. The purpose of this operation is to release a contracted plantar aponeurosis. During the healing process, the place of the incision will grow in length, effectively elongating the plantar aponeurosis.
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