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The Art of Prescribing Foot Orthotics

This article discusses the importance of considering foot flexibility when prescribing orthotics.  To ensure maximum therapeutic value from the orthotics, extra care and attention to the degree of compensation of the foot is vital.

 

It is not sufficient to simply measure a subtalar position or a forefoot angle in an attempt to quantify a deformity as to the number of degrees.  For example, one patient could have a 5 degree rearfoot varus deformity.  The foot is hypermobile of compensates fully.  This patient requires full rearfoot control.  Whereas another patient with the same 5 degree rearfoot varus has a tightly structured foot, small ranges of motion.  This patient will only compensate partially or not at all and require little if any control.  The previous example illustrates the importance of flexibility of the foot.  Or, if you will, is the foot uncompensated, partially compensated, or fully compensated?

 

Let’s take a closer look at compensation.  A foot which has an uncompensated subtalar varus deformity will show signs of callus formation or skin thickening on the entire lateral border of the heel and sole with a small discrete callus (usually not symptomatic) under the head of the 5th metatarsal bone.  That same foot partially compensated, will show less callus on the lateral border and less of a sub-5th metatarsal callus, but the callus might be symptomatic due to a now partially unstable 5th metatarsal bone.  We might even see the start of a sub-4th metatarsal callus. The fully compensated deformity would now move up and out of the way of the reactive force of the ground, thus eliminating any likelihood of any callus formation under the 5th metatarsal head. However, the 4th metatarsal head, which is the next most stable segment, might show evidence of shearing callus. 

 

The foot can be totally compensated and consequently unstable.  In this case the 1st and 5th segments would be hypermobile thereby exposing the 2nd and 3rd metatarsal heads to trauma resulting in a sore, diffuse callus under these metatarsal heads.  It should be apparent that the lesion distribution pattern must be considered as a valuable diagnostic determinant bearing heavily upon the type and degree of control of foot function that should be considered therapeutically.

rearfoot varus

The 1st metatarsal segment also varies greatly in its range of motion.  For example, a rigid plantarflexed 1st ray type forefoot valgus will often display a thick painful callus under the 1st metatarsal head.  This deformity causes a supinatory compensation to take place at the subtalar joint and may be a factor in ankle spraining.  The orthotic prescription in this case would call for a full forefoot valgus post and a zero or no rearfoot post, since any degree of rearfoot varus posting would tend of increase the force that that peroneus longus exerts in plantarflexing the 1st ray, thus making the condition worse.  However, if the 1st ray was semi-rigid, that is, partially movable in a dorsal direction, all of these findings would be reduced and the orthotic prescription should similarly be altered.  Finally, if the plantarflexed 1st ray was flexible, the subtalar joint would be in a position to pronate fully instead of supinating, assuming that its range of motion was restricted.  Thus the orthotic prescription would be designed to fully control the subtalar joint pronation.  As you can see this is opposite to the prescription used for the rigid plantarflexed 1st ray.

Hallux deformities

Observation of the position of the hallux and range of motion in the 1st metatarsophalangeal joint is a tremendous diagnostic tool since the most common cause of hallux abducto valgus is subtalar joint pronation at propulsion in a foot which has an adducted metatarsal pattern.  On the other hand, a foot that has a straight (rectus) metatarsal pattern, pronation at propulsion may result in hallux limitus leading to hallux rigidus.  Patients demonstrating these conditions may have complaints and lesion distribution patterns consistent with fully compensated pronatory deformities.  If they have ample ranges of motion, they should receive full orthotic control. On the other hand, if they have insufficient ranges of motion such as in the congenital equinus deformity, these patients will not be able to tolerate full orthotic control and, if subjected to it, may develop other symptoms or extreme foot discomfort.

 

The previous hypothetic conditions serve to illustrate the importance of recognition and evaluation of all factors affecting foot function.  Proper foot care depends on an accurate, coherent appreciation of all factors that influence lower extremity iomechanics and how they interact.

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