A Closer Look At Foot Orthoses For Chronic Ankle Instability

Author(s): 
Douglas Richie, Jr., DPM, FACFAS, FAAPSM

In light of an increasing body of research on the effect of orthoses on chronic ankle instability and postural sway, this author discusses essential biomechanical considerations and recommends orthotic prescriptions and modifications to treat unstable ankles.

Although clinicians use foot orthoses to treat a myriad of clinical conditions, they rarely employ these devices to treat chronic ankle instability. When they do use foot orthoses to treat chronic ankle instability, prescription strategies and the design of foot orthotic devices vary significantly among practitioners. This may reflect a lack of understanding about this enigmatic condition, which poses a challenge for even the most skilled podiatric physician.

   Understanding the complexity and nuances of chronic ankle instability will provide the practitioner greater insight to understand how to implement foot orthotic therapy effectively in combination with other interventions, which can result in very favorable patient outcomes.

   The ankle sprain is the most common injury in sport and accounts for more time lost from participation than any other injury.1 Almost half the patients who suffer a significant ligamentous injury to the ankle continue to suffer the long-term, serious disability known as chronic ankle instability.2 This suggests that we do not treat ankle sprains properly as our treatment protocols are failing at least 50 percent of the time.

   One reason for this high failure rate is a prevailing standard of practice to return an athlete to sport too soon after an ankle sprain before adequate ligament healing has occurred. While basic science has taught us that ruptured ligaments require at least six months to repair and restore strength adequately, high school and collegiate athletes routinely return to play within weeks of this injury.3 Even in the non-athletic patient who has suffered an ankle sprain, practitioners fail to protect the injured ligaments beyond a few weeks, allowing unprotected ambulation, which dooms the patient to incomplete healing.

   When ligaments fail to heal after an ankle sprain, there are long-term, if not permanent, consequences. These impairments include permanent pain, swelling and recurrent ankle sprains resulting in up to 72 percent of people having an inability to return to previous activity.4 There is a known link between chronic ankle instability and the development of osteoarthritis of the ankle.5 Researchers have reported serious quality of life consequences of chronic ankle instability such as decreased participation in exercise and withdrawing from occupational activity.6

A Closer Look At The Pathophysiology Of Chronic Ankle Instability

Unhealed ligaments do not entirely account for all of the contributing factors of chronic ankle instability. Traditionally, the literature has attributed chronic ankle instability to two potential causes: mechanical instability and functional instability.7 Functional instability may be caused by specific insufficiencies in proprioception, neuromuscular control, postural control or strength. Mechanical instability may be caused by factors that alter the mechanics of one or more joints within the ankle complex. Potential mechanical insufficiencies include pathologic laxity, impaired arthrokinematics, synovial inflammation and impingement, and degenerative changes.8 Patients with chronic ankle instability may have only mechanical instability, functional instability or a combination of these entities.

   While clinicians can understand mechanical instability from a physiologic standpoint, the notion of functional instability, particularly neuromuscular control of the ankle, is more complicated. At the same time, restoration of neuromuscular control is the mainstay of treatment for chronic ankle instability. As we will see, foot orthotic therapy can play a significant role in this treatment strategy.

   Postural control describes the ability of humans to remain upright, keeping their center of mass within the borders of the support of the feet. Researchers have identified a loss of postural control as a causative factor of chronic ankle instability and repetitive sprains.9 Postural control requires an intact neuromuscular control mechanism, which foot orthoses can affect in several ways.

   The visual, vestibular and somatosensory systems provide the sensory feedback to maintain upright balance and postural control. Of most interest to the podiatric physician, the somatosensory system provides feedback from the ligament mechanoreceptors in the ankle joint, the muscle spindle system of the lower leg and the cutaneous sensors on the soles of the feet. An ankle sprain disrupts the ligament mechanoreceptors but the other two levels of input from the leg muscles and the plantar surface of the foot are able to compensate adequately and provide essential proprioceptive input.9 Therefore, this challenges the notion that an ankle sprain causes loss of proprioception. While balance and postural control are compromised after an ankle sprain, the problem seems to lie beyond the sensory mechanisms.

   There is mounting evidence that the disruption of neuromuscular control in chronic ankle instability is due to the “feed-forward” side of the system in which central processing, nerve activation and muscular contraction are not working properly.10 Instead of focusing on therapies to improve “proprioception” in patients with chronic ankle instability, new protocols now employ balance training, gait training, jump and landing training as well as muscular strengthening, which starts at the hip and works down to the feet.

   Donovan and Hertel have proposed four specific symptom domains of chronic ankle instability: reduced range of motion, reduced strength, impaired proprioception and neuromuscular control, and altered gait patterns.11 They describe a systematic method of treating chronic ankle instability deficiencies to enable patients to return to full function. Foot orthotic therapy can provide an adjunct to treatment of chronic ankle instability in all four of the symptom domains that these experts describe.

How Can Foot Orthoses Treat Chronic Ankle Instability?

In 2006, I wrote an article on this same subject for Podiatry Today.12 At that time, seven studies had been published in the medical literature describing the effects of foot orthoses on postural control in both healthy patients and those with chronic ankle instability.13-19 Since that time, we have gained significantly more insight and verification of those previous studies.

   While initial studies of foot orthoses and postural control focused on people with chronic ankle instability, more recent research has studied patient populations at risk for catastrophic falls. The findings continue to be quite remarkable given the simple nature of this treatment intervention.

   Recently, Gross and coworkers studied a group of older adults with a history of falls and tested the use of custom foot orthotics on several static and dynamic tests for balance.20 The authors noted improvements in balance immediately and after two weeks of use of the custom orthotic devices. Note that these orthoses were custom fabricated and posted according to the specific biomechanical findings of each patient.

   Another randomized controlled clinical trial tested a custom foot orthosis prescribed to 94 elderly women with osteoporosis.21 The women who wore foot orthoses showed improvements in dynamic balance and foot pain. The researchers speculated that the foot orthoses optimized sensory information from the soles of the feet to improve proprioception and postural control.

   Researchers have made attempts to study the effects of shoe inserts with textured surfaces or vibrating sources designed to enhance sensory input and improve balance. Rome and I described these studies in a previous article published in Podiatry Today.22 Currently, there is a lack of consistent evidence that textured or “enhanced” orthoses can improve sensory feedback to improve balance. Most of these studies were on healthy patients or elderly patients at risk for falling.

   More recently, McKeon and colleagues studied the effects of textured insoles on patients with chronic ankle instability.23 The authors found a significant negative effect on postural control when patients wore a textured insole in comparison to a sham device. The researchers concluded that the textured surface of the insole caused a distortion or alteration feedback rather than enhanced sensory input from the sole of the foot.

How Should We Design Foot Orthoses To Treat Chronic Ankle Instability?

When prescribing foot orthoses to treat patients with chronic ankle instability, it is important to study the gait pattern of each and every patient. Besides learning about foot type and alignment of the feet during various phases of gait, the practitioner should look for evidence of lost range of motion, muscle weakness and balance deficits, all of which can become apparent during simple walking activity.

   Patients with chronic ankle instability have demonstrated an alteration of walking mechanics and an inability to distribute pressure normally across the plantar surface of the foot.24 Studies have shown that patients with chronic ankle instability place greater pressure on the lateral aspect of their foot while walking.25 Schmidt and colleagues replicated this finding in runners with chronic ankle instability.26 In this study, patients also demonstrated a reduced loading or slowing down of the loading response from the rearfoot to the midfoot. These findings may indicate a weakness of the peroneal muscles to counteract the ankle invertors during gait and a possible abnormality in structural alignment, which directs loading to the lateral surface of the foot.

   Postural control is compromised in both supinated and pronated foot types. A study has shown that supinated individuals had greater speed and maximal displacement in the medial-lateral direction of center of pressure than neutral individuals.27 In the same study, pronated individuals had greater maximum displacement in the anterior-posterior direction than neutral individuals. They also had a shorter single limb stance duration. The researchers concluded that individuals with pronated feet and supinated feet have poorer postural control than individuals with neutral feet, but perhaps through different mechanisms.

   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.

   A medial arch flange will enhance stability for the patient with chronic ankle instability in almost every case. This may be a surprise to many practitioners. The use of medial flange has traditionally reduced pronation and encouraged supination of the foot. This would seem contraindicated for the patient with chronic ankle instability. However, the opposite is true. A lateral flange on a foot orthosis will resist abduction and may force the foot to invert, which will exacerbate the symptoms of lateral foot and ankle instability. Conversely, a medial arch flange will act as a blocker to foot inversion and increase the total contact surface area of the orthosis.

   When implementing foot orthotic therapy for the patient with chronic ankle instability, I recommend starting with an intrinsically balanced orthotic shell, which has no rearfoot posting. In most cases, the patient will report improved confidence in gait, balance and comfort. Unless one observes significant alignment or balance issues, further modification of this device is not necessary.

   In the event the patient is not improving, one can apply further posting or enhancement of the device in the office setting. Foot orthoses have demonstrated an ability to decrease strain on soft tissue structures around the ankle and hindfoot.28 With wedging or posting of these devices, changes in moment or torque at the ankle and subtalar joints can reduce strain of the ligaments that resist these forces. The posting material in the office setting can include various thickness of felt, Korex or ethyl vinyl acetate (EVA) foam. In all cases, apply the posting to the anterior edge (forefoot posting) of the device. Only a forefoot post can invert or evert the entire orthosis. If a patient with a mobile, pronated flatfoot deformity has a significantly pronated gait, apply a forefoot varus post. Similarly, if the patient still ambulates inverted in the hindfoot, I suggest application of a forefoot valgus post, starting with 1/8-inch Korex wedging.

   Wedging or posting of orthoses for patients with chronic ankle instability will not predictably improve postural control. Kakihana and colleagues showed that lateral wedging will shift the center of pressure laterally in both healthy people and patients with chronic ankle instability, but there may be diverse and sometimes reversed effects.32,33 Therefore, while many patients with chronic ankle instability have a laterally deviated center of pressure in stance, application of wedging laterally will only further increase this deviation.

   A novel and unscientific method of fine-tuning a foot orthosis for improvement of balance is to use the single leg stance test for stability. Ask the patient to stand on both orthoses, placed on the floor outside the shoe. The patient then stands on one foot at a time, balancing for at least 20 seconds while the practitioner observes for body sway and compensation. If body sway predominates to the lateral side, place a forefoot valgus or lateral wedge under the orthosis. This usually occurs in 1/8-inch increments with Korex material until the patient feels more secure or the practitioner observes reduced sway. Similarly, place a varus or medial wedge under the orthosis when sway is predominately medial. Ask the patient to give feedback regarding any improvement of stability with the additional posting.

In Conclusion

The net effect of these adjustments is to achieve a “blocking” effect of the orthosis to reduce body sway. In other words, the patient will feel an improved base of support to “push against” when his or her body sways out of alignment during difficult balancing tasks. Often, input from the patient regarding improvements of balance with orthotic adjustment will supersede any intuitive decision making by the clinician.

   Dr. Richie is an Adjunct Associate Professor in the Department of Applied Biomechanics at the California School of Podiatric Medicine at Samuel Merritt University. He is a Past President of the American Academy of Podiatric Sports Medicine, and a Fellow of the American College of Foot and Ankle Surgeons. Dr. Richie is in private practice in Seal Beach, Calif.

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   Editor’s note: For related articles, see “Chronic Ankle Instability: Can Orthotics Help?” in the October 2006 issue of Podiatry Today, “Can Orthoses And Insoles Have An Impact On Postural Stability?” in the October 2010 issue or “Can Foot Orthoses Help Improve Postural Control?” in the May 2007 issue.

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