Another important parameter of the orthosis prescription is orthosis shape and positive cast work. Cast correction is the term used to “balance the positive cast to neutralize the compensation motion of forefoot valgus or varus.” Once a positive is made, either electronically or in plaster, from the negative impression, it contains the forefoot deformity. In a forefoot varus the positive will appear to have an everted heel when placed on a level surface and the opposite for valgus. The cast correction technique is the creation of a platform at the fore part of the positive cast that makes the heel perpendicular. This new shape of the positive and subsequent orthosis is intended to prevent the consequences of midtarsal joint compensation for the forefoot to rearfoot deformity.

This essential part of orthosis construction has drifted tremendously in the past decade. Some orthotic laboratories have completely or partially discarded the cast correction or balancing technique for either reducing cost or out of ignorance of the purpose of the technique. Eliminating this step produces a foot orthosis that is a similar shape as the pathologic shape of the foot in stance and doing little to prevent the consequential compensation of an abnormal forefoot to rearfoot position. The resulting device, without balancing, is little more than an arch support from the 1950s and probably has the same effect as a drug store prefabricated device. The practitioner is urged to investigate whether a cast correction technique is actually performed by the orthotic laboratory producing their device.

The orthotic prescription must include heel cup depth, orthosis width, cast fill , medial skive , and positive cast inversion. Examples of how each relate to some pathologies can be described but obviously not how they relate to all foot pathology.

Heel cup depth, from most orthotic labs, includes shallow (10 mm) standard (14 mm) deep (18 mm) and extra deep. The primary concept to remember when choosing a heel cup depth is the deeper the heel cup, the greater the surface area of plastic and the greater the control of the rearfoot. If the calcaneus is everted, a deep heel cup will provide greater control. The only reason to use a standard or shallow heel cup in the presence of an everted calcaneus is to accommodate the patient’s athletic shoe selection, or because the pathology originates distal to the midtarsal joint. A rigid ski boot or hockey skate is so stable that heel cup depth is of little consequence. An attempt to treat posterior tibial tendinitis with an orthosis with a shallow heel cup is an effort in futility.

Orthosis width generally refers only to the width of the distal edge of the orthoses and the resulting breadth of the arch area. Width determines the stability of the orthotic in the athletic shoe during and after midstance and control over the first ray. The longest horizontal support against frontal plane motion of the orthosis in the shoe is the distal edge. The wider the orthoses, the less likely it will tilt with pronation at midstance. When treating pathology that involves excessive midtarsal joint motion, like plantar fasciitis and functional hallux limitus, a wider front edge withstands the deforming forces that are present in a dysfunctional foot. An orthosis raises the base of the first metatarsal to increase hallux dorsiflexion in functional hallux limitus. If the orthosis is narrow, it cannot create a force to hold the base of the first metatarsal up. A wide front edge is rarely an athletic shoe problem, with the exception of extreme styles like soccer cleats. Insisting on choosing orthosis width appropriate for the patients’ pathology rather than allowing the orthotic lab to default to narrow so that the CFO fits in any shoe is essential.

Cast fill was originally introduced by Dr. Merton Root [5] as a technique intended to blend the forefoot correction into the arch of the positive. An orthotic lab should offer several cast fills to address the need of a specific pathology. An orthosis made from a positive cast with minimum fill will conform close to the arch of the foot. Minimum fill offers the most control over arch collapse and is essential for symptoms produced by cavus feet and hard to control pronated feet.

Standard fill lowers the arch slightly and makes the orthosis less “tight” against the foot in stance. This is useful when there are secondary issues with the foot, like limitations of motion secondary to osteoarthritis or intense sport activities both of which require a more gentle control of the foot. Maximum fill for equinus, muscle spasm, or tarsal coalition is a strategy that allows for minimum control in situations where the least control can produce enough symptom reduction without creating other problems. Again, allowing the laboratory to select the arch fill without knowing the condition of the patients’ foot could produce a clinical failure or a very uncomfortable orthoses .

It is critical that the practitioners control how much cast fill is added to the positive cast. Adding excessive cast fill is a common lab error practice since it produces a more forgiving CFO with less potential to cause arch irritation. Although somewhat less likely to cause arch irritation, an orthosis made from a positive cast with excessive fill will result in an orthosis with inadequate control, since the corrective forces that an orthotic device creates are ameliorated. Prescribing a minimum fill orthoses can be confirmed by matching it to the arch of the foot closely when the foot is held in casting position before dispensing.

The medial skive technique was probably one of the most significant and effective developments in orthosis design. This contribution to the custom functional Root-type design, developed by Kevin Kirby, D.P.M. [6] allowed for the manipulation of ground reactive force to provide better control of the rearfoot. Treating athletes with flexible flatfoot, plantar fasciitis with an everted heel, or PT dysfunction without this modification usually produces a less than optimal result. Most pathologies that include an everted calcaneus in stance are treated more successfully with this technique, which produces a rise in the medial side of the heel cup by 2, 4, or 6 mm. Clinicians who are introduced to this modification frequently discover significantly improved clinical outcomes when they add this modification to the prescription of patients with pathology related to an everted calcaneus. This modification is not effective with a shallow heel cup; it requires a deep or at least standard depth. Most labs don’t charge for this additional modification.