Foot orthotics are typically designed based on an image of the plantar surface of a patient's foot. Standard foot orthotics attempt to treat foot or arch pain by providing cushioning, stability or support, sometimes attempting to adjust or stabilize movement about the subtalar joint. Prior to about 1950, there was little to no standardization in the methods used to treat mechanically-induced foot pain. A standardized approach to the design of foot orthoses was introduced in 1954, when Merton L. Root, DPM, revolutionized the field with the theory of the Subtalar Neutral Position (STNP) (Lee, 2001, “An Historical Appraisal and Discussion of Root Model as a Clinical System of Approach in the Present Context of Theoretical Uncertainty,” Clinics in Podiatric Medicine and Surgery, 18(4)).
The subtalar joint is the joint between the talus and calcaneus bones. Subtalar neutral is where the subtalar joint is neither pronated nor supinated and its importance was based on observations of what Root subjectively deemed to be “normal” feet. According to Root's theory, correction of a foot to a “normal” position involves placing of only the subtalar joint into a “neutral” position, the so-called subtalar neutral position or STNP. Root's theory involved only correction of the subtalar joint and did not involve manipulation or correction to other bones or joints in the foot. Further corrections, postings and wedges were then added to an orthotic after the subtalar joint was placed in neutral position to compensate for any perceived abnormalities. In the present application, the subtalar joint is not a controlling element; rather it is an adaptive joint governed by the position of the joints distal to it. The subtalar joint functions in synergy with other foot structures to allow the foot to adapt to an infinitely variable topography, within physiologic tolerances.
There are two basic types of custom orthoses made today; accommodative and functional. An accommodative orthosis is typically made from a soft or flexible material that cushions and “accommodates” any deformity of the foot. This cushioning also results in some dissipation of the forces required for efficient gait that ordinarily would be transmitted up the kinetic chain. In addition to force dissipation, accommodative orthosis made of EVA and similar soft materials are unable to control foot mechanics. A functional orthosis is one that controls joint movements and/or foot position. Because they are rigid, clinicians utilize these orthoses to hold the foot in a position they deem therapeutic. This is problematic because the foot must be allowed to continually adapt to the ground in order to operate efficiently. For the manufacture of both types of orthoses, the plantar surface of the patient's foot is captured and its mirror image is produced on the surface of the orthotic device that contacts the patient's foot. Materials used to make orthotic devices designed in accord with Root's theory are typically both strong and rigid, to support the patient's weight in a durable manner, as the foot cannot bear the weight itself. Such orthotics abnormally maintain the foot's arch throughout gait, with the orthosis supporting the body's weight and compressing the soft tissue between the bones and the orthosis. An ideal configuration for an orthotic device that is beyond the capability of current functional or accommodative orthoses, would adjust the bones of the foot to create an internal load-bearing structure that is self supporting, bears weight on the calcaneus and metatarsal heads, and enables the foot to adapt to uneven topography.
In practice today, most functional orthotic devices are designed to establish STNP and maintain it from heel strike to the beginning of toe-off. Capturing and maintaining STNP is too simplistic an objective to apply to the complicated kinematics of the foot with its 33 joints, 28 bones, supporting ligaments, tendons, and other structures. Under Root's SNTP theory, and other models of foot function such as Rotational Equilibrium Theory, Sagittal Plane Theory, and the Tissue Stress Model, little, if any, consideration is given to correcting the underlying pathologic changes to foot structure and function. While functional and accommodative orthotics may temporarily decrease foot pain due to restricting pathologic range of motion and in cushioning the foot, they necessarily cause pathologic gait, and this approach will inevitably cause pain in other joints in the foot, leg, pelvis and/or back as they compensate for this abnormal motion. There remains a need for improved methods and systems for designing foot orthotics, and for improved foot orthotics to correct and/or restore the ideal alignment and/or positioning of foot structures.