June 2010

Postural control deficits: the role of foot orthoses

The mechanism by which foot orthoses improve postural control remains uncertain, but research suggests that it may stem from improvements in sensory feedback or changes in center of pressure that in turn affect joint moments.

By Douglas Richie, DPM

The role of foot orthoses in the treatment of lower extremity pathologies and functional disorders has been extensively studied over the past 20 years.1-11 Hawke et al reviewed randomized controlled trials utilizing foot orthoses to treat various painful conditions of the human foot and concluded that the best studies have shown favorable outcomes in the treatment of painful pes cavus, idiopathic juvenile rheumatoid arthritis, plantar fasciitis and hallux valgus.12 Often overlooked in literature reviews of foot orthotic treatment effects, however, is the role of these devices in improving postural control and in treating patient populations with chronic ankle instability or at increased risk of traumatic falls. Richie published a literature review and analysis of this subject in 2007.13 Since then, even more research has provided insight into the role of foot orthotics in balance control.

Clinical relevance of postural control

The term balance describes a person’s ability to remain upright in stance.14 Postural control is the ability to maintain the body’s center of gravity (CoG) within the borders of support.15 Postural control, therefore, is a strategy to maintain balance. Balance occurs during static stance and dynamic gait. Postural control is typically measured during quiet static stance and can be evaluated during either single- or double-limb support.

Loss of postural control has been consistently observed in patients with chronic ankle instability16-21 and after an acute ankle sprain.22-24 However, Mckeon and Hertel have demonstrated that studies using traditional force plate testing have consistently linked poor postural control only to an acute ankle sprain rather than subjects with chronic ankle instability.25 Elderly patients at risk for falling have also shown consistent deficits in postural control.26,27 Thus, efforts to improve postural control may potentially benefit diverse patient populations with different apparent needs and activity levels.

Studies of foot orthoses

At least a dozen studies have been published documenting the effects of foot orthoses on postural control.28-42 These studies focused on various populations, including healthy subjects and patients with acute and chronic ankle sprains. In these investigations, the types of orthoses being studied varied significantly, whether custom fabricated or pre-fabricated, posted or non-posted. However, several trends and consistent findings can be found.

Subjects who sustained an acute ankle sprain demonstrated poor postural control compared to healthy controls in three of the studies.28,29,32 In two of these studies, intervention with custom molded, posted foot orthoses significantly improved postural control in the ankle sprain subjects.28,29 In another study of patients with an ankle sprain, several styles of both custom and pre-fabricated foot orthoses failed to improve postural control.32

In the same three studies, foot orthotic intervention failed to improve postural control in healthy subjects28,29 or the uninjured limbs of subjects with unilateral ankle sprain.32 However, another three studies found that  when asymptomatic subjects with forefoot varus or abnormal hindfoot pronation were fitted with custom foot orthoses, significant improvement in postural control occurred.35-37 In these three studies, significant improvement in postural control with orthoses was not seen initially, but only after a period of acclimatization (two to six weeks). Improvements in postural control using custom foot orthoses occurred over a six week period in a study of asymptomatic subjects with rearfoot malalignment.37

Attempts have been made to improve postural control by altering the texture of the foot orthosis. One study38 of middle- aged female subjects failed to show any improvements in postural control with textured pre-fabricated foot orthoses, at baseline or after four weeks of wear. Another study39 of younger subjects did find significant improvement in postural control with textured insoles in double-limb stance, but not in single limb support.

An interesting study was recently conducted comparing the effects of foot orthoses to dental appliances in postural control.40 Postural sway has been previously shown to be affected by alterations in occlusion and function of the temporomandibular joint and these problems can be corrected with specialized dental appliances fitted by an orthodontist.  A unique study of patients who were fitted with these dental appliances for malocclusion, and who were also fitted for custom functional foot orthoses was conducted by Sloss et al.40 This study showed benefit from both the dental appliances and foot orthoses in reducing postural sway in double leg stance.   Foot orthoses alone showed the best reduction of postural sway compared to dental appliances or combination of these appliances and foot orthoses.

In analyzing their results, the authors of these studies propose several mechanisms by which foot orthoses may improve postural control: Improving alignment of the subtalar or ankle joints,28,29,33,35,36 reducing strain on ligaments or muscles around the ankle joint,28,30 and improving sensory feedback from the plantar surface of the foot.29,38-40 None of these investigations actually measured improvements of foot alignment, changes in joint moments or improvements in sensory function of the foot, so these proposals are purely speculative. However, one can analyze previous research on the kinematics of postural control and the effect of foot orthoses on some of the key kinematic variables.

Kinematics of Postural Control

Figure 1. The ankle strategy for postural control.

In upright stance, balance and postural control of the body involves four key segments: the head, the trunk, the legs, and the feet. In healthy subjects, measurements of postural control in single leg stance reveal that the head, trunk and leg function as one unit, with rotations and corrections occurring at the ankle. Horak and Nashner describe this mechanism as an “ankle strategy” where muscular contractions in the leg will reposition the body over the supportive foot to maintain balance (Figure 1).43 The same authors also described a less efficient “hip strategy” of postural control, in which the trunk is rotated over the leg.  These investigators determined that the ankle strategy required input from the somatosensory system in the feet and legs, while the the hip strategy relied more on vestibular input.44

Mergner used the term “inverted pendulum” to describe postural corrections utilizing an ankle strategy where the head, trunk and leg function as one unit and rotate over the fixed foot (Figure 2).45 These researchers assumed that frontal, transverse and sagittal plane corrections would all occur at the ankle joint, although most clinicians recognize that inversion and eversion would have to occur primarily at the subtalar joint.

Orthotic mechanisms

Figure 2. The “inverted pendulum” utilizing an ankle strategy for postural control.

Foot orthoses are thought to exert their influence primarily on the subtalar joint of the foot.5-9 Several studies of orthotic improvements in postural control led researchers to conclude that their findings were due to a positive effect of the device on subtalar joint position.28,29 Yet, no verification of change of foot alignment was provided in these papers.  We know that studies of foot orthotic treatment effects show very minimal change of skeletal alignment.8,9 On the other hand, we also know that ligament mechanoreceptors are compromised when a joint is positioned at end range of motion.46 Therefore, even subtle changes of alignment achieved by foot orthoses may affect joint position such that mechanoreceptor function is improved, which in turn improves proprioception and postural control.42

Lim et al studied 25 subjects with unilateral functional ankle instability and measured joint position sense and kinesthesia ( perception of joint movement) comparing the symptomatic vs asymptomatic side.47 No significant difference was found between both sides leading the researchers to conclude that proprioceptive deficits may not be always present in patients with functional ankle instability. However, the authors also point out that there could be equally poor deficits in joint position sense and kinesthesia in both ankles of patients with unilateral functional ankle instability, and thus, no side-to-side differences were measured.  Also, the authors note that their measurements of joint positioning were only detecting changes on the efferent end of the proprioception pathway. They recognize that measurement of the afferent side, particularly the muscle spindles, would be more helpful in determining proprioceptive deficits in patients with functional ankle instability. How foot orthoses affect the muscle spindles remains speculative. However, studies of joint moments, particularly internal joint moments (the forces on muscles, tendons and ligaments) have shown measurable changes with foot orthoses.

Foot orthoses have their most significant effects on the lower extremity by changing joint moments.7-9 By reducing strain or load on the supportive ligaments, tendons and muscles of the lower leg—all vital components of the somatosensory system—foot orthoses may facilitate proprioception and postural control mechanisms. For example, Munn et al determined that patients with chronic ankle instability had eccentric strength deficits in the ankle invertor muscles.48 Several studies of foot orthoses have demonstrated an effect of reducing internal ankle joint inversion moment.7,49

Figure 3. Excursion of center of pressure in single-leg stance can be reduced with a custom orthotic device.

Reduction of eccentric load on the medial ankle musculature with foot orthoses may address one component of chronic ankle instability.

Recently, the retinacula ( extensor retinaculum, peroneal retinaculum, flexor retinaculum) of the ankle joint have been studied to evaluate their role in proprioception of the ankle ankle joint. Stecco and co-workers conducted histologic studies and found evidence of sensory nerve fibers and corpuscles, i.e. mechanoreceptors, within the ankle retinacula.50 In further studies of patients with an ankle sprain, these researchers found that the ankle retinacula underwent MRI changes of thickening and altered signal intensity-similar to what is seen in plantar fasciitis.  The authors propose that the ankle retinacula are important sensors for proprioception of the ankle joint which respond to tension of the muscles and tendons to which they are attached, as well as direct stretching when the ankle is moved in various directions.

Subjects with chronic ankle instability have been shown to have laterally deviated center of pressure.51 The center of pressure under the feet is thought to accurately reflect the center of gravity of the subject.52 Studies of laterally wedged insoles demonstrated that the center of pressure would shift laterally in healthy subjects and subjects with chronic ankle instability, although the results were sometimes divergent and reversed.53,54 When the center of pressure is shifted either medial or lateral to the subtalar joint axis, the moment arm lengthens accordingly, and the magnitude of joint moment will increase for a given amount of applied force.55 Medial wedging of insoles will shift the center of pressure medially, while lateral wedging will shift the center of pressure laterally.56 Therefore, in patients

Figure 4. The Star Excursion Balance Test (SEBT).

with poor postural control, determining the location of the center of pressure may help guide the clinician to post or wedge orthoses to affect the lever arm and torque created by the tendons and ligaments of the ankle joint complex (Figure 3).

In a clinical setting, it would be ideal to have instruments available to determine the location of the center of pressure under the feet so that foot orthoses could be designed or posted accordingly to enhance postural control. However, clinicians can also evaluate foot type and perform simple tests of balance and postural control to develop the proper orthotic prescription.

The Star Excursion Balance Test is a reliable dynamic test of single leg balance that can be easily implemented in the clinical setting.57 A subject is evaluated in single-leg stance while reaching as far as possible with the other leg in eight different directions (Figure 4). Cote and colleagues demonstrated that subjects with pronated feet could reach farther in an anterior and anterior-medial direction than subjects with a neutral foot, while supinators reached farther in the posterior and posterior-lateral direction.58 They suggested that these differences may be related to shifts in center of pressure specific to foot type.

Figure 5. Patients with pronated (top image) and supinated feet (bottom image) will have tendency for medial vs lateral deviation of center of pressure in single leg stance.

Evaluating foot type will not always predict location of center of pressure or improvement of postural control with posting of the foot orthosis. Theoretically, subjects with pronated feet will have a center of pressure deviated medially, while supinated feet will have a center of pressure deviated laterally (Figure 5). However, studies of displacement of center of pressure in pronators and supinators in single-leg stance do not consistently verify this notion. Tsai and colleagues found that pronators had greater excursion and displacement of center of pressure in an anterior-posterior direction than the neutral group while supinators had greater excursion in a medial-lateral direction than the neutral group.59 Hertel et reported that cavus feet had greater excursion of CoP than rectus feet and speculated that lack of medial arch contact in high arched feet would limit the ability of the body to sway medially over the supportive foot.60

These studies suggest that assessment of static and dynamic balance should be done on an individual basis before posting or wedging of foot orthoses is attempted. Neutral, non-posted orthoses have already demonstrated improvements in postural control in groups of subjects with various foot types. Additional wedging and posting can enhance postural control if the location of center of pressure can be determined or if performance with static and dynamic testing validates improvement from the standard orthotic prescription.

Summary

Studies of the effects of foot orthoses on postural control in healthy subjects and patients with chronic ankle instability have provided fascinating insight into the potential benefits of these devices to prevent traumatic falls and recurrent ankle sprains. The mechanisms by which foot orthoses improve postural control have not been clearly elucidated, but several possibilities appear plausible. Foot orthoses may improve sensory feedback to the foot, and may improve position of the ankle and subtalar joint to enhance propriception from ligament and retinacula mechanoreceptors. More likely, foot orthoses can shift the center of pressure and change joint moments to facilitate the ankle strategy for postural control during standing and walking.

Douglas Richie, DPM, is an associate clinical professor in the department of applied biomechanics at the California School of Podiatric Medicine in Oakland, CA.

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One Response to Postural control deficits: the role of foot orthoses

  1. The role that postural distortions play in the development of chronic musculoskeletal pain has become increasingly more apparent in recent years. This article discusses some of the studies and their importance in understanding the link between foot mechanics and musculoskeletal health.

    Professor Rothbart

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