Podiatric physicians use foot orthotics daily to treat a myriad of lower extremity conditions. Yet while the foot orthotics industry has been growing each year, researchers in the field of biomechanics have begun to challenge previous clinical studies showing that foot orthotics really work. At the same time, third party insurance payors have started questioning the value of foot orthotic therapy on the grounds that this treatment intervention is “experimental” and still without verification of the overall benefit. Podiatric physicians may have a false sense of confidence that the medical literature can verify the positive treatment outcomes achieved with custom foot orthotics. Further scrutiny will reveal that strong scientific evidence supporting the benefits of foot orthotic therapy is hard to come by. In their comprehensive review of the medical literature of all studies conducted on custom functional foot orthoses (FOs), Landorf and Keenan concluded: “(While) these papers have included results which are generally quite supportive of FOs, some have found either inconclusive or negative results. Much of the research to date could be improved upon and from this perspective, it is clear that further randomized controlled trials assessing outcomes for specific clinical conditions are necessary.”1 Out of all the pathologies that podiatric physicians can prescribe foot orthoses for, the one condition which has the most compelling scientific evidence justifying orthotic use is the one least likely to be considered for orthotic treatment. This condition is chronic instability of the ankle. Chronic ankle instability can be due to mechanical or functional causes. Immediately after an acute ankle sprain, ligamentous injury can cause mechanical instability of the ankle, which one can detect with manual testing, stress radiography or via magnetic resonance imaging (MRI). At the same time, most patients after moderate ankle injury will also show evidence of functional instability, which is basically a loss of neuromuscular control over the ankle and subtalar joints. While there is significant scientific evidence that foot orthotics can address both forms of ankle instability, the primary focus of this article will be on the effects of these devices to treat functional instability.
A Closer Look At Postural Control
In 2003, I described essential components of neuromuscular control of the ankle.2 These elements include proprioception, muscle reaction time, muscular strength and postural control. Of all these physiologic functions, the one finding that clinicians commonly see in individuals after acute ankle injury is a loss of postural control. Postural control is defined as the ability of an individual to maintain his or her center of mass over a single supporting foot. One can measure this in the clinical setting by having the patient perform a modified Rhomberg test. The patient stands on one foot with the arms crossed over the chest and with the eyes closed. A healthy patient should be able to maintain a single leg stance in this situation for at least 10 seconds. In the research setting, one can measure postural control with special devices that track deviations of center of pressure. Accordingly, one can measure total body sway in terms of velocity and magnitude. This measurement technique is known as stabilometry. In the practice setting, podiatric physicians can measure postural sway with a Matscan® (Tekscan). With the Matscan, one can track migrations in the center of pressure while a patient stands on one foot. Most authorities agree that measuring postural control will give the best estimate of loss or restoration of functional control of the ankle.3,4 At the same time, loss of postural stability may be the single best predictor of risk for future ankle injury.5,6 The studies cited in this article all measured postural control to determine the potential benefits of foot orthotic therapy. Why postural control is compromised after acute ankle injury is not well understood. This appears to be due to dysfunction of both the afferent (sensory) and efferent (motor) segments of neuromuscular control over the ankle. On the afferent side, disruption of ankle ligament mechanoreceptors will cause a loss of proprioception or joint position sense. In addition, it has been shown that patients after acute inversion ankle sprain have abnormal nerve conduction velocities in both the peroneal and tibial nerves. Hematoma and mild compartment syndrome after ankle trauma can also compromise sensory nerve function. The efferent loop of motor nerves and muscular activation of the lower leg muscles have been extensively studied with conflicting results. Many studies have focused on peroneal muscle reaction time, which may or may not be delayed in people with symptoms of functional instability of the ankle. Physicians and therapists have traditionally cited peroneal muscle weakness as a cause of ankle instability but recent research has shown a surprising contradiction. Muscle weakness is not always present in patients with unstable ankles and when researchers have found such weakness, it is always in the ankle invertors, not the evertors.7,8 How could this be? I will come back to this point after discussing how the mechanism of foot orthoses can improve ankle instability. Ligament mechanoreceptors and sensory nerves around the ankle are part of what is termed the “somatosensory system” of afferent feedback for balance and postural control. The two remaining elements of this system are the muscle spindles of the lower leg and the mechanoreceptors on the plantar surface of the foot. It has recently been discovered that the plantar surface of the foot plays a critical role in providing sensory input to the central nervous system for balance and postural control. There are three distinct types of mechanoreceptors on the sole of the foot and they include Merkel cell complexes (responding to pressure), Meissner’s corpuscles and Pacinian corpuscles (both responding to pressure). Neuroscientists have discovered that the plantar surface of the foot will detect increases in pressure and vibration, which signal “total body sway” to the central nervous system. This, in turn, activates muscles in the lower leg to correct body alignment over the fixed foot. Accordingly, adjustments in postural control can be directed by sensors located on the plantar surface of the foot. Foot orthotics have the potential to enhance sensory feedback for improvements of balance and postural control. In addition, these devices could address the mechanical components of ankle instability by reducing strain around the soft tissue structures of the ankle and enhancing muscular strength for stability.
What Does The Literature Indicate?
There are currently seven published studies in the medical literature documenting the positive effects of foot orthotics on postural control in healthy people and in people with chronic ankle instability. Orteza, et. al., were the first to study the effects of foot orthotics on patients after an acute ankle sprain and in healthy control patients.9 Comparing molded, custom foot orthotics made of Aquaplast® to flat inserts, the authors gauged the effects on balance control and reduction of pain in people running after an ankle sprain. The molded orthotics improved balance control only among the injured patients. Furthermore, the molded orthotics significantly reduced pain with jogging in the injured people while the flat inserts had no effects.9 Guskiewicz and Perrin used a more traditional semi-rigid polypropylene custom foot orthosis to treat a group of injured patients after ankle sprain.10 Again, this study showed that custom foot orthotics significantly improved balance and postural control among the injured patients in comparison to healthy people. Ochsendorf, et. al., showed some fascinating effects of custom foot orthotics on healthy people who had been subjected to muscular fatigue.11 In this study, fatigue of the lower leg muscles caused a significant compromise in postural control but the use of custom foot orthotics completely removed the negative effects of fatigue. Hertel, et. al., published two studies measuring the effects of various types of prefabricated foot orthoses as well as a direct mold Aquaplast custom (non-posted) orthotic on healthy patients as well as those who had suffered an acute ankle sprain.12,13 The first study documented increased postural sway on the injured side but there was no benefit from any of the prefabricated foot orthoses to improve the condition over a period of four weeks. The second study of healthy people showed that medial posting of prefabricated devices significantly reduced medial-lateral sway in comparison to lateral (valgus) posting. The authors concluded that medial posting of foot orthotics rather than lateral posting was the best way to achieve improvements of postural control. They speculated that the medial posting reduced pronation.12,13 This is the first suggestion that reducing foot pronation vial medial posting would help someone with lateral ankle instability. This suggestion runs contrary to the accepted notion that inversion instability is the dominant mechanical problem with ankle instability conditions. It is interesting to note that none of the previously cited studies verified the foot type or structural alignment of the subjects. Furthermore, while most of the researchers speculated that improvements in postural control were correlated with improved foot alignment, no study produced data to confirm the achievement of this treatment effect. The most recent studies of foot orthoses and postural control have focused on people with significant foot pronation or forefoot varus. In both studies, a foot type that compensates with rearfoot pronation was quite responsive to foot orthotic therapy in terms of postural control improvements. However, the researchers did not see these improvements until at least four to six weeks after initiating orthotic therapy. Rome and Brown studied 50 subjects with pronated feet as determined by the Foot Posture Index.14 They utilized prefabricated foot orthoses with 5-degree rearfoot varus posts in the experimental group and compared this group to the untreated group with the same foot alignment abnormality. They found no differences in postural control at baseline between the two groups. However, after wearing the foot orthoses for four weeks, the experimental group showed significant improvements of postural control in medial-lateral sway. The authors emphasized this was the first study on healthy patients (i.e. those with no ankle instability) with a specific foot deformity (pronated hindfoot) that showed the positive effects of orthotics to improve balance and postural control. Cox, et. al., have recently studied the effects of orthotics on postural control in patients with forefoot varus.15 In a previous study, Cox, et. al., had shown that patients with forefoot varus greater than 7 degrees had compromised postural control in single leg stance, particularly with anterior-posterior sway. In the more recent study, Cox, et. al., showed that patients with greater than 7 degrees forefoot varus would improve significantly by wearing custom polypropylene foot orthoses. However, as in the previous study by Rome, the treatment benefits were not immediate.14 The patients in Cox’s study did not see improvements in postural sway until they had worn orthotics for six weeks.15 This suggests that foot orthotics may play a role in the “learning phase” of functional rehabilitation and that patients may not realize the immediate benefits until the central nervous system has adapted to the intervention.
Applying The Research Findings To Clinical Practice
The studies cited above have some common findings and also have some shortcomings that require further research. None of the studies utilized custom functional foot orthoses fabricated from plaster casts of the feet using a “Root style” neutral suspension technique. When custom orthotics were used, they were either directly molded to the foot of the subject or they were fabricated from molds of the feet using foam boxes. Whether a true Root style functional foot orthosis would produce the same or better result remains speculative. In most cases, a custom molded orthotic improved balance better than a prefabricated device. However, some studies showed impressive improvements in balance when one combined a prefabricated device with medial posting. In the few studies in which researchers applied lateral posting, the results were negative in terms of an improvement in balance. Almost all of the researchers attributed their positive results with foot orthotics improving postural control to the fact that these devices optimized positioning of the foot. However, no study verified that improvements of alignment had occurred. What is apparent and somewhat surprising from all this research is the fact that efforts to reduce pronation of the foot are more successful in improving ankle instability than strategies that prevent supination or inversion. Furthermore, researchers have shown that patients with chronic ankle instability tend to have weakness in the invertor muscle group of the lower leg, not the evertors. Lateral body sway over a fixed foot in single leg stance causes closed kinetic chain pronation of the rearfoot. A body “falling” laterally would need to fire the medial leg flexors (i.e. tibialis posterior, flexor digitorum longus and flexor hallucis longus) to pull the tibia medially over the fixed foot. If this does not occur, then further closed chain pronation of the rearfoot would use up all available range of motion of the subtalar joint and further lateral sway would force the foot and ankle into sudden inversion. Pronated feet have used up all their available range of motion in the subtalar joint so lateral body sway will immediately force the ankle into inversion. Pronated feet will generally have their subtalar joints functioning at end range of motion. Joints at extremes of range of motion have been shown to have compromised proprioceptive input. This is why nearly all patients with advanced stage adult-acquired flatfoot will demonstrate significant balance deficits with clinical testing. Accordingly, in terms of the mechanical effects of foot orthotics to improve chronic instability of the ankle, researchers have found the best results using medially-posted orthotics, particularly in pronated feet. These are not the only foot types that suffer lateral ankle sprains. In fact, we have traditionally attributed this disorder to the cavus or inverted foot type. Thus far, the medical literature has not verified that cavus feet are more vulnerable to lateral ankle sprains than pes planus foot types. All of the researchers cited in these articles speculated that foot orthoses may enhance balance and proprioception by stimulating the sensors on the plantar surface of the foot. A molded foot orthosis would conceivably increase the surface area of contact between the foot and the ground, and thus increase the receptive field for sensory input to the central nervous system. Studies have shown that thin foam mats actually improve postural control by increasing the surface area of contact under the foot. Other studies have shown that irregular firm surfaces will also improve postural control, presumably by enhancing tactile input to the bottom of the foot. The dilemma is that we do not know what the ideal orthotic material is to fully maximize sensory input on the plantar surface of the foot. Some of the best results of the aforementioned research were achieved with rigid plastic orthoses. In most cases, custom molding of the devices worked better than prefabricated inserts.
The research on the positive effects of foot orthotics on chronic ankle instability have led many authorities to recommend their use for any patient with this condition or for any patient who has suffered a significant acute ankle sprain. It appears that custom foot orthoses can significantly improve balance and proprioception in the lower extremity, and can be a valuable adjunct to a functional rehabilitation program after an ankle sprain. Podiatric physicians should follow their traditional biomechanical prescription approach when treating ankle instability. Custom semi-rigid or rigid devices closely molded to the shape of the foot in a neutral position would be recommended. While one can intrinsically balance the positive casts to perpendicular, additional posting is not recommended. If and when one applies posting to orthotics in the treatment of ankle instability, mild medial posting may be of more benefit than lateral posting. Finally, foot orthoses should only be one part of a comprehensive functional rehabilitation program, which is vital to the overall recovery of a patient with chronic instability of the ankle. Dr. Richie is an Adjunct Associate Clinical Professor in the Department of Applied Biomechanics at the California School of Podiatric Medicine at Samuel Merritt College. He is in private practice in Seal Beach, California. He can be reached at email@example.com
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