Research findings

If there is a biomechanical basis for patient improvement, one of many possible kinematic or kinetic parameters should be altered by foot orthoses. Increased magnitude of the pronation angle and increased pronation velocity have been postulated as risk factors for lower extremity injury. A number of investigations have shown the potential of an orthosis with an external medial post to decrease the magnitude of pronation.¹ Not unexpectedly, a decrease in tibial internal rotation has also been shown with medially posted orthoses.² However, Johanson et al¹ observed that a non-posted orthotic shell reduced the maximum pronation angle as much as either a forefoot or a rearfoot post, as well as a combination of a forefoot and rearfoot post.

However, attempts to reduce the velocity of pronation through foot orthoses have proved less successful.³ Pronation velocity may be influenced more than the magnitude of the motion by the eversion moment that results from the point of application of the ground reaction force. Investigations showing reduced motion often find no change in pronation velocity. With a restriction to normal pronation, Perry and Lafortune⁴ found no change in impact loading during walking. However, during running, the same pronation restriction produced an increase in impact loading. This suggests that the influence of an orthosis differs between walking and running and should be considered at the time of prescription.

Research contradictions

It appears that for every study showing a positive change in a biomechanical parameter produced by foot orthoses, another study can be cited showing no change. Some of these discrepancies could be due to methodological differences. These include the measurement system, marker placement, skin movement artefact, variable subject/patient groups, lack of statistical power, individual subject response, and the type of orthotic intervention. Reinschmidt et al⁵ showed substantial errors between skin markers and intracortical pins in the frontal and transverse planes (63% and 70% respectively). Advances in measurement technique should resolve some of the contradictions; however, a recent study using intracortical pins⁶ showed that orthotic effects were subject specific and non-systematic across conditions.

Orthotic behaviour is generally assessed using some measure of rearfoot motion to describe the subtalar joint action. Unfortunately there is no direct method to do this. Subtalar and talocrural joint motion can only be inferred from the measures that biomechanists often use. Part of the problem may be that the wrong parameters have been measured or that the changes made by the orthoses are too subtle for the measurement system to detect.

In many studies, the subjects are not patients and therefore may not respond to the orthotic intervention as a patient may. The unimpaired subjects may attempt to override any of the “unnecessary” effect of the orthosis that would force them into a less efficient locomotory pattern. Nawoczenski et al⁷ observed that different foot structures showed different amounts of frontal and transverse plane motion. Both of these factors could contribute to a range of responses in individual subjects, evident by the observation of internal tibial rotation changing from −80% to +60% with the use of an orthosis.⁷

Further, the foot motion observed may not be dictated primarily by foot structure. Movement patterns of the foot may be driven by (and the effects of orthosis found in) the proximal joints. Bellchamber and van den Bogert⁸ calculated a proximal to distal energy flow between the tibia and foot among all subjects during walking and some subjects during running. From the observed direction of energy flow, the authors suggested that the use of foot orthoses may be ineffective in controlling tibial rotation. This conclusion, however, is countered by studies cited above.⁷

Finally, the variation in patients' response to foot orthoses may be largely influenced by the methods used in fitting. Foot orthoses are typically fitted on the basis of a static clinical examination of various measurements of lower extremity alignment. The assumption is that the position of the foot and ankle in the static position reflects the motion of the foot and ankle during ambulation—that is, an increased static pronation angle will produce an increased maximum pronation angle during ambulation. However, much evidence has shown a rather poor relation between static measures and dynamic lower extremity motion.⁹ Hamill et al⁹ showed that various static clinical measures of lower extremity alignment are limited in predicting dynamic lower extremity function. On the basis of similar results, Hunt et al¹⁰ questioned the appropriateness of using such measures in prescribing and fitting foot orthoses. If the static measures do not accurately reflect the dynamic motions of the foot and ankle, then designing an orthosis on the basis of these static measures may not provide adequate correction for the dysfunction. Mueller¹¹ suggested that orthoses should not be prescribed on the basis of specific foot alignment measures, but rather on the patient's symptoms.

Summary

This article is not intended as a comprehensive review of the literature. Rather, its purpose is to bring to the reader's attention several of the key issues involved with foot orthoses and their prescription. In spite of the rather equivocal findings from the numerous investigations involving the efficacy of foot orthoses, their success in reducing pain and symptoms cannot be denied. However, the mechanism by which this is accomplished is certainly open to question. The determination of the mechanism may involve the procedures used to evaluate orthoses biomechanically.