Parameters for Quantification

Following is a review of orthotic design those characteristics related to the ability of a device to control foot function were identified and defined in mathematical terms.

The potential advantage of a material that can reduce shock is clear, if that were the primary intent here a flat piece of material with physical properties demonstrating the highest elastic modulus would suffice. It is the intent here to quantify those physical characteristics of an orthotic that alter foot position during function. It is accepted that a device designed to control position of foot function may benefit from the addition of shock absorption characteristics.

The ability of a material to resist deformation must be considered in any measurement or quantifying process. When the material used for construction does not compress during function the physical support is equal to a non-weight bearing measurement. If the material compresses then physical support characteristics of the device would be reduced accordingly during stance phase of gait. Any quantifying process must consider the compressibility of the material used for construction. Although it is generally accepted that foot control takes place primarily during stance, or weight bearing, phase of gait. It is not as well defined as to which portions of Stance Phase are of primary concern.

Biomechanical examination of the lower extremity may reveal as much as 20 degrees of varus positioning of the rear foot at heal contact. This suggests a strong need to begin foot control early during stance phase of gait. . In fact both medial and lateral wedging of the most proximal cupola of the orthotic has and is currently attempted. Several authors contend that the spherical design of the heel of the foot and the cupola of the orthotic obviate any benefit of extrinsic posting during early heal contact.

The next functional phase is that portion of time between heel contact and mid-stance when the foot is resting fully on the highest part of the devise. As the plantar surface of the foot becomes less spherical past mid heal on the orthotic, wedging applied after this point would have a much greater effect on foot function. Many of the inventions described placement of the varus wedging beginning in the middle and others at the end of the orthotic cupola. They all described a gradual increase in the slope of the wedge extending to a point in the middle of the arch near or at the Talo-Navicular Joint (TNJ). Contoured plate orthotics generate additional varus wedging to a point near the TNJ, at this point the plate is bent plantar grade so as to maintain the proximal part of the plate in the more varus position.

Clearly from middle to late heal contact through all of midstance those physical characteristics of an orthotic device associated with foot function are most important. Although important elements of foot function occur during propulsion most authors agree foot position at the end of mid-stance sets the stage for what will happen in propulsion. Some inventions reviewed added small amounts of mass beyond the metatarsals, many added cushion to the metatarsal head region. Contoured plate design terminated proximal to the metatarsals in all patents reviewed. Other than the transverse metatarsal arch reported in most designs, placed proximal to the metatarsal heads, few researchers attempted to alter foot function during propulsion. Shock absorption was the only frequent physical embodiment utilized during propulsive phase of gait.

When viewed clinically, or even through the "eyes" of computer assisted technology, a predominant varus angulation is observed in a plate contoured to the plantar surface of a human foot. This varus angulation begins where the plate cups the heel, increases distally to an area corresponding with the TNJ, and then decreases to the metatarsal head. Points on the superior surface of the plate and the horizontal plane create this angle.

Regardless of the actual angle all plates progressively increase from 0 where the plate joins the cupola to a maximum angulation at a point in the center of the arch more specifically the TNJ of the foot, and then decreases to 0 again before the metatarsal heads. In fact measurement of this varus angulation at any given point from heal to metatarsal area is a function of the varus angulation measured at the TNJ or apex of the orthotic. Therefore, measurement of the varus angulation at the apex establishes and defines the varus angulation of the entire plate from heel to metatarsal area.

A Logical section for measurement of this angle, created by the superior surface of the contoured plate and the horizontal plane, is that portion of the plate measuring the greatest angulation more specifically, a part of the plate that consistently corresponds with the TNJ. A frontal plane section through the apex region of an orthotic can be identified and the superior surface of the plate can be related to the horizontal plane in terms of degrees.

Efforts to plot, or attempts to represent this angular relationship between the superior surface and the horizontal plane at the specified frontal section created immediate difficulty. Whereas the proximal portion of the plate is cupola in design, the portion of the plate corresponding with the apex is hyperbolic, having a portion of the curve that is more planer laterally and more inclined medially. The point to be the apex of the varus angle located on the horizontal plane was directly under the lowest lateral part of the curve. This point was approximately 25 % of the orthotic width from the most lateral part of the curve. A point on the most medial superior surface was identified and used to finish the angular measurement. It was quickly apparent that if the orthotic was narrowed significantly the angle changed dramatically. Three years of effort to relate orthotic function to this described angular measurement failed to produce a consistent statistical relationship.

When the most medial and vertical part of the hyperbolic curve is used the angular measurement is recorded much higher than if the central 50 percent of the curve is used. Basically when that part of the contoured plate and finished orthotic known as the medial phalange is used as part of the angular measurement, the measurement is 10 to 20 degrees higher.

The patent literature revealed considerable controversy concerning that part of orthotic design referred to as the medial flange. Some patents designed two options of their invention one with and one with out a medial flange. One current orthotic fabrication technique eliminated the medial flange in the finished orthotics by using a fill process on the positive mould before the contoured plate was formed. Points on the curve representing the medial flange were then eliminated from the angular measurement.