For a foot orthotic to be biomechanically correct it must be a custom device constructed individually from a positive mold. The patient's feet are cast in a subtalar neutral impression (negative cast). Then a filler (plaster of paris) is poured into the impression creating a positive mold. This positive mold duplicates the exact shape of the individual foot in its own biomechanically correct position. Materials are then fitted on to this mold that take into consideration the weight, activity level, and pathology of the patient. Orthotics for a 90 pound patient compared to a 200 pound patient, or an athlete compared to a sedentary individual require different specific durometer (firmness) values.
But the most important factor that distinguishes a biomechanical orthotic from an accommodative orthotic is what is known as subtalar neutrality. Subtalar neutrality can only be achieved by a practitioner holding the talar joint in a non-weightbearing neutral position (patient sitting or prone), while the orthotic is cast.2,3
So what is subtalar neutrality and why is it essential for the biomechanical correction of excessive pronation or excessive supination? Briefly, the subtalar joint (STJ) is the articulation between the inferior surface of the talus and the superior surface of the calcaneus.
Fig. 1 goes here
The normal range of motion for the calcaneus beneath the talus is 10 degrees inversion and 20 degrees eversion. Excessive pronation (Fig. 1C) is usually over six degrees of calcaneal eversion, and excessive supination is usually over twelve degrees of calcaneal inversion. During the stance phase of gait (heel strike to heel rise) the STJ should normally pronate only during the first 25 percent of stance in order to provide shock absorption and foot adaptation to the ground. A normal foot after the first 25 percent of stance begins to move towards supination which is necessary for the foot stability required for push off. Subtalar neutrality is the point in gait just before the end of the stance phase where the STJ is neither pronating or supinating. It is at this point in gait where the foot and leg should be in the proper relationship for effective and efficient locomotion.1
A foot that reaches the subtalar neutral position at the correct moment will not demonstrate excessive pronation or supination. Excessive pronation means that the STJ is pronating beyond the normal 25 percent at mid stance. Excessive pronation of the STJ is a compensation for the abnormally functioning foot or leg. In order for the foot to function normally, it must function from the subtalar joint neutral position, i.e., as close as possible to the uncompensated position because "motion at the subtalar joint affects the position and function of the entire foot and each of its major joints."1
A major role of a functional foot orthosis is to enable the foot to move as normally as possible throughout the stance phase of walking.2 The orthotic must limit the pronation so that the foot will be in a position to resupinate at the proper moment (subtalar neutral position). At the subtalar neutral phase of gait the orthotic must maintain the subtalar and midtarsal joints ready for propulsion. During the propulsion phase the orthotic has less effect as the weight is distal to the device.2
Weightbearing casted accommodative orthoses in which subtalar neutrality is not accurately established allows the foot to abnormally compensate and, therefore, cannot be considered a biomechanical orthotic.
It is a fact that abnormal pronation of the foot is directly or indirectly related to a myriad of functional insults to the human structure. Dr. Roots states in his classic text on the foot that pronation is the most frequent compensation that occurs due to bony and soft tissue irregularities of the foot and leg and "any condition which prevents normal pronation of the subtalar joint results in pathological shock. This shock is transmitted up the leg, into the pelvis, and on to the lumbar spine."3
Doctors interested in achieving the most functional biomechanical status for their patients should be aware that if a patient requires a foot orthotic that it must be cast in a non- weightbearing subtalar neutral position, and be custom made from a positive mold; the construction of the orthotic should take into consideration the weight, the pathology, and the activity of the patient.
References
- American Physical Rehabilitation Network: When the Feet Hit the Ground Everything Changes. P.O. Box 8864, Toledo, Ohio 43623-0864.
- Burns MJ: Non-weightbeaing cast impressions for the construction of orthotic devices. JAPA, 67(11): 1977.
- Root ML, Orien WP, Weed JH: Normal and Abnormal Function of the Foot: Clinical Biomechanics, Vol 2. Clinical Biomechanics, Los Angeles, 1977.
- Hunt GC: Physical Therapy of Foot and Ankle. New York, Churchill Livingstone, 1988.
Ken Silverman, D.P.M.
New Rochelle, New York