The main similarity of all Foot Orthotics is the principle of their structure. The core is comprised of a plastic material more commonly referred to as the shell. External support structures on the underside of the shell material are utilized for balance and support inside the footwear; these are commonly referred to as extrinsic posts. Soft padding is placed on the underside and the topside to absorb compression forces when walking and running, this includes the top covers, underlay materials and arch fills.
Dependent on the reason for the prescription of Foot Orthotics, all of the above mentioned materials can be utilized in many combinations to achieve three broad categories of Foot Orthotics: