Function of Orthotics
To explain how orthotics function, it is important to understand the mechanics of walking. With each step, the vertical axis of the heel ideally should land slightly inverted to the ground, with an inclination of only a few degrees toward the outside of the heel. From there, the foot begins to pronate (flatten) and then comes off the ground at the toes (resuppinates). So, during each step, weight shifts from laterally to medially and back to laterally. A properly designed orthotic controls how the foot strikes the ground, absorbs shock, and reduces stress in the foot.
This coordinated motion is a complex process in which many things can go wrong. If a structural problem is present, the foot can collapse under the body's weight. Over time, stress on the feet can result in deformities. Running exerts much greater force on the feet than walking and can lead to more severe injuries, such as sprained ankles, shin splints, and fractures.
One of the foot's main functions is to absorb shock as the body's weight shifts with each step. It does this through a complex process in which the arch of the foot flattens slightly. This absorbs and distributes the weight throughout the entire foot. There are two major problems that can occur in this mechanism.
The first problem occurs when the arch does not flatten at all. This typically occurs in a person with a high arch, called a cavus foot. Because the arch does not flatten, it absorbs shock poorly. Instead of spreading the weight throughout the entire foot, it falls only on the heel and the base of the toes. This increases stress on the foot, especially the heel. Furthermore, because the weight is not absorbed well in the foot, it radiates up the leg to other joints. Over time, this can cause pain in the knees, hips, and lower back.
To correct this condition, an orthotic is used to adjust and even out the contact between the foot and the ground. This allows the entire foot to support the weight of the body. Also, extra cushioning can be built into the orthotic so that some of the force does not reach the foot.
A different problem results when the arch flattens too much. This is known as a planus or flatfoot. In this condition, the weight distribution on the foot is too far on the medial side. A flatfoot is unstable and cannot maintain a proper arch. Over time, the weight of the body on an unstable foot can cause the bones of the foot to become misaligned. This can lead to the development bunions, hammertoes and other foot deformities, as well as knee and low back pain.
To address this problem, an orthotic with an increased arch can be prescribed to distribute the weight laterally. Depending on shape of the foot, the heel of the orthotic can be slanted to shift the weight more toward the center of the heel.