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Mar 11

Written by: Larry Huppin, DPM
3/11/2010 7:51 AM

Casting Criteria for Optimum Clinical Outcomes

When casting for functional foot orthoses, in what position should the foot be held in order to achieve optimum clinical outcomes. McPoil et al looked at this question in 1989 and found that foam box semi-weightbearing casting resulted in a forefoot-to-hindfoot angle that was significantly smaller than the angle measured using either of two nonweightbearing methods.(2) It is suggested by McPoil that the difference is due to the inability of the midtarsal joint to lock using the semi-weightbearing method.

Davis et al did a similar study in 2002 where they compared methods for taking a negative cast of the foot.(3) Non-weightbearing plaster was compared to semi-weightbearing foam. Their conclusions were that NWB casting had the highest level of agreement with the clinically measured forefoot-to-rearfoot relationship while SWB foam casting had the lowest level of agreement.

The conclusion of the study was that plaster casting is recommended as the most reliable and valid method in situations where the forefoot-to-rearfoot relationship is of importance.

If semi-weight bearing casting methods risk putting the foot in a position of increased forefoot varus, how will this affect orthoses? Roukis and Scherer found that prevention of first ray plantarflexion resulted in decreased first MPJ dorsiflexion (hallux limitus).  A subsequent study by Scherer, et. al looked at the effect in stance of a polypropylene orthosis made from a NWB negative cast taken with the first ray plantarflexed on patients with a functional hallux limitus. Results of this study were that mean dorsiflexion at the first MPJ increased 90%.

In a 2000 study, Harradine found that increasing eversion of the heel decreased available dorsiflexion at the first MPJ.  According to Root, et. al. “ground reaction against the first metatarsal head will force the first ray into a dorsiflexed position when the foot is everted by pronation”,  and based on the study by Roukis, et. al it can be hypothesized that the decrease in hallux dorsiflexion is secondardy to the dorsiflexion of the first ray.

These studies indicate that first ray dorsiflexion leads to decreased hallux dorsiflexion, resulting in functional hallux limitus. Casting the foot with the first ray dorsiflexed will result in an orthotic device that holds the first ray dorsiflexed. This may contribute to increased first MPJ symptoms as a result of joint jamming.

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