A Closer Look At Foot Orthoses For Chronic Ankle Instability
Hertel and coworkers have shown that people with cavus feet have greater excursion of center of pressure or reduced postural control in comparison to people with rectus feet.28 These authors speculated that patients with cavus feet have no “anatomic block” medially, meaning that there is less contact area of the medial side of the foot on the supportive surface. They also proposed that cavus feet have less surface area for plantar cutaneous afferent feedback.
This is not to suggest that all patients with chronic ankle instability have a varus instability of their ankle or hindfoot. It is interesting to note that several studies of foot orthoses that showed improvements of postural control were on patients with pronated feet. In fact, the most common mechanism that researchers propose to explain the benefits of foot orthoses for chronic ankle instability is “reduced pronation” of the foot.29 In a previous article, I explained how this mechanism is plausible.12 It is based upon the notion that improved positioning of a body segment to allow greater range of motion will actually improve postural control and prevent injury.
Simply stated, the more range of motion available in the ankle and subtalar joints, the more opportunity for the foot to stay on the ground when the upper body is perturbed or thrust out of alignment. Therefore, if we can design foot orthoses to restore or improve range of motion rather than restrict range of motion, the devices will be more effective in preventing an ankle sprain.
Foot orthoses prescribed for the patient with chronic ankle instability should do the following:
• enhance sensory input from the plantar surface of the foot;
• reduce strain or load on the soft tissue supportive structures around the ankle;
• reduce compensation;
• improve range of motion; and
• provide a stable base of support for “blocking” or resistance of body sway.
A Guide To Orthotic Prescription Recommendations
To enhance sensory input from the plantar surface of the foot, the evidence supports the use of a custom device versus a prefabricated device based upon the improvement of contact surface area.30 For further enhancement of capture of the native shape of the foot, one should perform impression casting with the patient off-weightbearing. Over ground, weightbearing scanners will produce an orthotic equivalent to a prefabricated device in my opinion.
The choice of orthotic shell material should favor semi-rigid to rigid materials. This will ensure better leverage of support of the foot to “block” sway of the body. Semi-rigid versus “soft” orthotic materials will maintain their shape and contour over repetitive loads to continue to provide total surface contact to the foot.
To position the foot optimally, the impression cast must capture forefoot to rearfoot deformity and the lab must intrinsically balance the deformity to prevent rearfoot compensation. This is particularly important for the patient with a forefoot valgus deformity. Balancing of forefoot valgus reduces compensatory rearfoot inversion, which can exacerbate chronic ankle instability. The orthotic laboratory should use minimal arch fill in the cast correction to enhance the contour and contact of the device to the plantar surface of the foot.
One should avoid rearfoot posts in most cases. These posts will almost always tend to invert the hindfoot and ankle. The goal of foot orthotic therapy in chronic ankle instability is to allow range of motion rather than restrict motion, which occurs with rearfoot posting. One can add forefoot posting after dispensing the intrinsically balanced devices. The practitioner can add more posting when he or she observes the patient in gait and before achieving optimal alignment.
Full-length top covers with medium thickness foam are preferred. These will allow more contact surface area to the sulcus and toe sections of the foot for enhanced sensory feedback. Also, full-length extensions allow for application of sulcus wedging, which one can add later later to enhance control.