Can Orthoses And Insoles Have An Impact On Postural Stability?
In recent years, there has been a growing interest in the neurophysiological effects that orthoses can have on the foot. Accordingly, these authors review the literature and offer their perspectives on the ability of orthoses and insoles to enhance postural stability.
Podiatrists frequently prescribe foot orthoses to treat and prevent overuse running injuries, as well as enhance comfort, cushioning, realignment, redistribution of forces and stability for chronic conditions such as diabetes and rheumatoid arthritis. However, the exact biomechanical mechanism of the action of orthoses remains unknown. Researchers have attempted to understand the biomechanical and anatomical factors associated with the prescription of foot orthoses. Only recently has there been an interest into the neurophysiological factors.
There is evidence that neurophysiological factors can affect lower limb biomechanical alignment. This evidence suggests that disrupting the function of intrinsic foot muscles, via induced fatigue, results in excessive foot pronation. The evidence also suggests that impact forces during daily physical activity produce a reaction in the muscles to minimize soft tissue vibration.1,2 Orthoses can augment the medial longitudinal arch, thus altering the muscular work required to maintain foot posture. Foot orthoses also may act to dissipate and absorb this impact, subsequently affecting levels of fatigue, the intensity of muscular work, performance and comfort.2
In addition, when one places orthoses under the midfoot and forefoot, they may provide biofeedback by enhancing afferent feedback from cutaneous receptors on the plantar surface of the feet and reducing eversion due to contraction of the inverting muscles.3 Furthermore, anatomical factors can affect lower limb biomechanical alignment. Research has shown that induced excessive foot pronation significantly increases internal tibial rotation, internal hip rotation and anterior pelvic tilt.4 Correcting anatomical malalignment at the foot with orthotic devices may indirectly realign the lower limb more superiorly.
Nigg postulated that by inserting a corrective interface between the plantar aspect of the foot and sole of a shoe, orthoses can counteract abnormal biomechanics, support natural joint motion and alter muscle activation patterns.2 The foot orthotics filter information generated by the ground reaction force and this information transfers to the central nervous system, which elicits a subject specific dynamic response.
The central nervous system controls upright posture and integrates visual, vestibular and somatosensory information. The somatosensory system is a diverse sensory system, which comprises the receptors and processing centers to produce the sensory modalities such as touch, temperature and pain. This initiates appropriate motor and muscle responses at the trunk and lower limbs.
Current evidence suggests that orthotic devices may affect lower limb muscle activity while patients are walking and participating in physical activities. Three recent literature reviews have been published on the subject.4-6 Recently, there has been exponential growth in the use of orthoses and insoles to help enhance postural stability or balance. Although it is essential to understand the muscle activity that includes strength, range of motion and muscle tone, let us take a closer look at orthoses, footwear and insoles that may facilitate improved postural stability.
Researchers have explored the effects of balance-facilitating, noise-enhanced, textured and functional orthoses on postural stability. The relevance of these studies centers around postulations that improved balance performance with such interventions may be due to alterations in sensory information at the plantar surface of the foot.
Balance-facilitating foot orthoses. Maki and colleagues designed a balance facilitating foot insole, which is comprised of a compliant raised ridge located around the posterior perimeter of the rearfoot.7 The authors suggest this ridge will indent the skin and stimulate mechanoreceptors around the periphery of the plantar surface of the foot when an individual starts to lose his or her balance. This research group also investigated the long-term effects of the orthosis on balance performance in young, healthy adults, older adults with a mild, age-related loss of foot sensation and those with Parkinson’s disease.7-9 The authors found that the insole significantly increased postural stability in all of these groups.
A Closer Look At Noise-Enhanced Foot Orthoses
Noise-enhanced foot orthoses. A number of recent studies have reported on enhancing postural stability via noise-enhanced sensors in older adults, patients with diabetic neuropathy, stroke patients, and those with Parkinson’s disease.10-11 Sub-sensory noise is based upon the concept of stochastic resonance, which is when the addition of noise to a system changes the system’s behavior in some fashion.
Priplata and colleagues assessed the use of vibrating insoles in 15 patients with diabetic neuropathy and 15 patients with stroke, and compared the results to the use of vibrating insoles in 12 healthy older adults from another study.11 Using identical methodology, the application of noise resulted in a statistically significant reduction in each of eight sway parameters in the subjects with diabetic neuropathy, the subjects with stroke and the elderly subjects. The authors also reported that higher levels of baseline postural sway in sensory impaired individuals were correlated with greater improvements in balance control with output noise.
What The Research Reveals About Textured Orthoses And Footwear
Textured foot orthoses and footwear. Various studies have investigated the use of textured orthoses, in the format of simple insoles or sandals, as a means of improving postural stability in young, middle-aged and older people.12-14 The researchers included foot orthoses cut from commercially available textured sheeting and footwear products such as sandals.
Wilson and colleagues concluded that the insoles showed neither a therapeutic benefit nor detrimental effects on postural stability in middle-aged females.12
Palluel and co-workers explored the effect of wearing “spike insoles” within sandals.13 The term spike insoles refers to a commercially available design of footwear for pool activities, which is comprised of an insole covered with semi-rigid plastic spikes of 3 mm in diameter. In comparison to a control condition, spike insoles caused significant reductions in sway amplitude and velocity in young and older participants. These findings in healthy adults may support the potential for textured orthoses to have even greater effects on postural stability in people who have poor balance or problems with sensory systems.
What About The Impact Of Functional Foot Orthoses For Postural Control?
Functional foot orthoses. A number of studies have reported altering sensory feedback signals from the plantar surface of the foot with custom-made orthoses.15-22
Cobb and co-workers reported that after six weeks of foot orthoses intervention, postural stability improved in participants with more than 7 degrees of forefoot varus.15 Mattacola and co-workers evaluated the effect of custom-fitted foot orthoses on postural sway over a six-week acclimation period.16 The authors concluded that the application of foot orthoses appears to improve postural control in people with rearfoot conditions.
Rome and Brown reported a reduction in postural sway after four weeks of wearing prefabricated functional orthotics with a 4-degree rearfoot wedge.17 The study focused on 50 asymptomatic patients with diagnosed excessive foot pronation. The study authors concluded the use of orthotic devices may have improved postural control by stabilizing the rearfoot and maintaining balance as a result. By the same argument, the benefits of limiting excessive foot pronation may contribute to effective control of internal rotation of the tibia and thereby reduce counter-rotatory motion at the knee and lower leg, and maintain balance.
Hertel and colleagues found that medially posted rearfoot orthotics enhanced postural control by significantly decreasing the frontal plane center of pressure length and velocity in healthy participants in comparison with footwear only.18
Orteza and colleagues, as well as Guskiewicz and Perrin, found that molded subtalar neutral orthotics improved balance and caused a decrease in pain by reducing the magnitude of postural sway in various balance tasks in participants diagnosed with acute lateral ankle sprains.19,20
Percy and Menz demonstrated that foot orthoses had no significant beneficial or detrimental effect on postural stability in asymptomatic professional soccer players.21 Stude and Brink concluded that wearing custom-made foot orthoses for six weeks has a positive influence on balance ability and proprioception in experienced golfers.22
What Are The Clinical And Research Implications?
This body of research has provided new insights into the potential benefits of foot orthoses and insoles, insights not previously recognized by most clinicians. Traditionally, foot orthotic therapy has always focused on improvement of function based upon correction of alignment of specific segments of the lower extremity. Indeed, foot orthotic therapy has produced reliable treatment outcomes for a myriad of musculoskeletal pathologies yet the mechanism of this therapy may not be what we always thought.
Today, we must evaluate foot orthoses, insoles and footwear in terms of their potential effects on neuromuscular function of the lower extremity. Unfortunately, we do not have enough information to make reliable recommendations about prescription criteria for foot orthoses and their expected effects on neuromotor control. However, we do know certain aspects of foot orthoses therapy that may be beneficial.
Custom molded orthoses will probably enhance recovery from acute ankle sprains and will also improve postural control in patients with chronic ankle instability. In general, devices that are custom molded tend to work better than prefabricated devices but this has not been conclusively proven.
It is tempting to assume that the positive effects of foot orthoses on improving balance and postural control in patients with ankle sprains could also translate into potential benefits for other populations at risk for catastrophic falls. Elderly patients and patients with diabetic sensorimotor neuropathy not only have an increased risk of falls but also have poor postural control, which may improve with foot orthotic therapy. No studies have been published showing that foot orthoses will actually reduce the frequency of falls in these populations yet the link between postural control and falls would lead one to have some confidence in this treatment option.
Traditional podiatric foot orthoses do not use textured topcovers or have noise-enhancing properties such as those used in previously cited research. However, one could certainly modify the standard orthotic prescription based upon a desired effect of improving sensory feedback from the foot.
Custom contouring of the shape of the device to increase contact with the field of sensory receptors on the plantar surface of the foot would be recommended while also utilizing more rigid materials in the footplate construction of the orthotic. Adding medial and lateral flanges may increase the surface area of contact while also providing enhanced pressure on the perimeter of the foot to provide sensory feedback.
Finally, obtaining patient feedback about the effects of footwear, orthotics or insoles may provide as much valuable information as any laboratory measurement. Most patients can immediately perceive the treatment effects of foot orthotics on balance and stability. Adding more posting or contouring to an orthotic device in the clinic, and then testing the device on the patient in double and single leg stance as well as dynamic gait should allow the opportunity for the patient to tell the practitioner that the device is improving stability.
The evidence to support the notion that foot orthoses and footwear can have a beneficial impact on the neurophysiological system is promising. However, there is much more work to be undertaken in this area and the profession must still determine the optimal design to promote neurofacilitation and subsequently enhance postural stability.
Clinicians should consider undertaking not only a musculoskeletal assessment but balance testing as well. Balance assessment and evaluation can occur with limited equipment and clinicians should work closely with other healthcare professionals, especially in the high-risk groups such as patients with diabetes and those older adults with a history of falls.
Dr. Rome is a Professor in Podiatry at the Centre for Health and Rehabilitation Institute and School of Podiatry at AUT University in Auckland, New Zealand.
Dr. Richie is an Associate Clinical Professor in the Department of Applied Biomechanics at the California School of Podiatric Medicine at Samuel Merritt University. He is a Fellow of the American College of Foot and Ankle Surgeons, and the American Academy of Podiatric Sports Medicine.
Ms. Hatton is a Research Fellow at the Centre for Health, Sport and Rehabilitation Sciences Research at the University of Salford in Manchester, United Kingdom.
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