Key Insights On Modifying Orthoses For Specific Conditions

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Guest Clinical Editor: Ronald Valmassy, DPM

Q: What modifications would you use for lateral instability in a patient who functions with the subtalar joint maximally pronated yet is still inverted in stance and in gait?

A: In a patient with lateral ankle instability, Dr. Kirby says the biomechanical problem that causes the ankle instability could be one of three mechanical factors. He says there may be some structural abnormality in the foot or lower extremity that is causing excessive STJ supination moments. Alternately, there may be relative weakness within the peroneal muscles, which actively resist supination or there may be some structural defect within the lateral ankle ligaments, which are the anatomical structures that passively resist supination when STJ supination occurs suddenly during weightbearing activities.

When it comes to patients with lateral ankle instability, Dr. Kirby suggests designing the foot orthosis to increase the subtalar joint pronation moments in order to counterbalance the subtalar joint supination moments that result in inversion ankle sprains. If the patient with lateral ankle instability has an inverted calcaneus but is maximally pronated at the STJ, he says one should design the foot orthosis to increase the STJ pronation moments even though the foot is already maximally pronated.

One can do this by ordering a vertical heel balancing position with a 16-mm heel cup, a 2- to 3-mm lateral heel skive, a flat rearfoot post and a valgus forefoot extension, according to Dr. Kirby.

“These orthosis design modifications will shift the ground reaction force acting on the patient’s plantar foot more laterally so there will be an increase in subtalar joint pronation moments and less likelihood that there will be some sudden increase in inversion moments that will cause another inversion ankle sprain for the patient,” explains Dr. Kirby.

In addition to the specially designed foot orthosis, he says one may use high top shoes/boots or modify the shoes with a lateral sole flare to give further supination stability to the patient.
A patient with a partially compensated rearfoot varus (i.e. maximally pronated at 3 degrees inverted) is only a diagnostic challenge, according to Dr. Blake. He says such a patient is maximally pronated yet stands inverted. “When the medical world or non-medical world gives this patient an orthotic that forces him or her to a vertical position, the patient cannot tolerate them,” notes Dr. Blake.

Dr. Blake notes one must tell the lab to pour the cast in the patient’s maximally pronated position (i.e. 3 degrees inverted). Then one should use 27- to 30-mm heel cups (sort of a modified UCBL device). He says such patients are severe pronators in function but they are severely supinated to the ground. One should treat the pronation more or the supination more, advocates Dr. Blake. He says physicians can normally achieve this with a 0-degree rearfoot post and a Denton modification to stabilize the lateral column (arch fill under the lateral arch without valgus wedging).

Dr. Blake adds that propulsive phase supination control may be necessary with 1/8- to 3/16-inch sub 4/5 valgus support and with maximal forefoot valgus support if it exists in the foot.

Dr. Blake is the Past President of the American Academy of Podiatric Sports Medicine. He practices in San Francisco.

Dr. Kirby is an Adjunct Associate Professor in the Department of Biomechanics at the California School of Podiatric Medicine at Samuel Merritt College. He is the Director of Clinical Biomechanics at Precision Intricast Inc.

Dr. Valmassy is a Past Professor and Past Chairman of the Department of Podiatric Biomechanics at the California College of Podiatric Medicine. He is a staff podiatrist at the Center for Sports Medicine at St. Francis Memorial Hospital in San Francisco.

Dr. Wernick is Professor and Chairman of the Department of Orthopedic Sciences at the New York College Of Podiatric Medicine (NYCPM). He is also a Diplomate of the American Board of Podiatric Orthopedics and is the Medical Director of Eneslow Comfort Shoes and Langer, Inc.

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