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Mar 18

Written by: Larry Huppin, DPM
3/18/2010 10:06 AM

Casting Rules Part 3 of 4

In parts 1 and 2 in this series we discussed the following casting rules:

Today, we will look at the importance of capturing the posterior heel and balancing the forefoot to the rearfoot.

By definition, a functional orthosis is one in which the forefoot is balanced to the perpendicular axis of the rearfoot in the frontal plane when the foot is held non-weightbearing in subtalar neutral with the midtarsal joint locked. Without this balancing, the orthosis is providing only a simple arch support. Kogler’s study demonstrated effectively the importance of frontal plane valgus wedging of the forefoot in order to decrease strain on the plantar fascia.

Valgus wedging is provided by a functional foot orthosis by capturing a valgus forefoot position in the negative cast and then balancing (supporting) the valgus position of the forefoot. If the valgus in the forefoot is not supported (balanced to the rearfoot) then the orthosis will not be optimized to decrease strain on the plantar fascia. As far back as the late-1950s, Manter, whose work influenced Root, suggested that supination of the midtarsal joint leads to destabilization of the rearfoot. This would suggest that a functional orthosis that stabilizes the midtarsal joint would in turn stabilize the rearfoot and assist in treating pathologies associated with excessive rearfoot inversion. Roukis, et. al. in a 1996 study proposed that “the decrease of first metatarsophalangeal joint dorsiflexion that results from dorsiflexion of the first ray is the predominant factor behind the development of hallux abducto valgus and hallux rigidus deformities” and that one of the possible deforming forces that lead to this is a “flexible forefoot valgus or everted forefoot-to-rearfoot deformity” For a functional foot orthosis to mitigate this deforming forces, it must support (balance) the frontal plane everted position of the forefoot.

For a fabrication laboratory to be able to balance an orthosis in the frontal plane, the negative cast must capture the posterior heel as well as the plantar aspect of the foot.

Casting Rule #4: To allow frontal plane correction of the orthosis (balancing the forefoot-to-rearfoot) the posterior heel must be captured in the negative cast.

Rules of Negative Casting Index
Negative Casting Instructional Video
Evaluation of the Negative Cast Video

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