The primary aim of this study was to investigate the effect of total contact standard diabetic insoles on balance in people with diabetes and neuropathy. We found that the offloading standard diabetic total contact insole with arch fill and smooth, cushioned top cover, (similar to that commonly used to reduce foot ulcer risk in people with diabetic peripheral neuropathy) reduced static balance (increased postural sway velocity) in participants with diabetes and neuropathy. However insole type had no effect on stepping reaction time.
The findings might suggest that insole design could artificially alter (dampen or enhance) any remaining somatosensory awareness, that contributes to the maintenance of postural stability, in people with diabetes and neuropathy. However variations in insole design may not influence the reaction time of any rapid voluntary corrective balance response. Stepping reaction time is in part dependent on the planning, transmission and response of the descending motor command and confounded by a number of contributing variables including visual acuity, level of motivation and concentration, amount of practice, decision making ability and cognitive function. Moreover reaction time has been found by others to be independent of body sway [
19]. Thus it is unsurprising to find that stepping reaction time was not a sensitive measure of the afferent feedback provided by insoles in this study.
The second aim of the study was to systematically manipulate each design component common to insoles for people with diabetes to investigate which component altered balance in people with diabetes and neuropathy. Specifically three insole design components were manipulated; 1. The total contact insole with arch fill was made flat; 2. The resilient, fast return top cover material was swapped for a material of low resilience, a slow return memory material 3. The smooth top surface was covered with a textured material.
The effect of the insole profile on standing balance
Static balance was significantly worse when participants wore the standard diabetic and low resilient memory insoles compared to the flat and no insole conditions. For example total COP path length increased by a mean of 14 % (absolute mean 92 mm) when the standard diabetic insole was compared to the no insole condition. The percent difference in static balance was even greater in the medial/lateral direction when the standard diabetic insole was worn; increasing by a mean of 17 % (absolute mean 48 mm). The standard diabetic and flat insoles were constructed from the same materials. The only known difference between the two devices was that the standard diabetic insole had the moulded arch fill and heel cup, whilst the other was flat. It would therefore appear that a moulded arch fill and heel cup impaired the static balance of participants with diabetes and neuropathy.
Only one other study investigated the effect of arch supports on standing balance. Gross and colleagues compared the ability of older people without neuropathy, but with a history of falls, to stand in tandem stance whilst wearing a semi ridged custom made foot orthosis [
20]. The authors reported that stance time increased significantly when participants wore the foot orthosis, incorporating an arch support. The discrepancy in findings between studies may be explained either by differences in the insole design or participant characteristics. Both studies used a full length insole design incorporating an arch fill and heel cup, therefore appear that differences in static findings may not relate to the arch fill but instead be attributed differences in insole firmness or participant group. We found that standing balance in people with diabetes was not influenced by insole firmness, and so postulate that people with diabetes and neuropathy display a disease specific response to an arch fill different to those seen in the older population.
Any disease specific alteration in balance stability could be a mechanical or sensory phenomenon. Cutaneous sensation from the plantar mechanoreceptors provides the central nervous system with critical stability information about the proximity of the centre of mass to the base of supports limits and the potential for impending loss of balance [
21]. Plantar pressure sensation appears to play an important role [
21]. It is reasonable to suppose that the introduction of an arch fill alters the plantar pressure pattern. People with diabetes may detect the proximity of the centre of mass to the limits of the base of support through inversely proportional gross changes in pressure ratios between forefoot and rearfoot. Filling the arch could provide additional confusing sensory midfoot information that makes detection of these proportional changes in regional forefoot and rearfoot pressures more difficult for people with sensory deficit to decipher.
Participants lacking cutaneous perception could be more reliant on foot and ankle joint mechanical stability to retain postural stability. Without arch fill participants maybe more predisposed toward standing with midtarsal, subtalar and ankle joints at end range of motion. We speculate that in this mechanically close packed, more stable position, there is the increased potential to make use of the high load compression and tension mechanoreceptors within the joints that relate information about body schematics. The addition of the arch fill and heel lift could shift these joints out of their close packed position and reduce the loads generated with joints as well as the ability to use high load joint receptors to give proprioceptive information.
The effect of covering materials on standing balance
Insoles for people with diabetes and neuropathy are often covered with soft thick foam materials designed to cushion and protect the foot from the high loads associated with foot ulcer development. Clinically to meet that objective two types of foam covering materials are routinely selected: 1. Traditionally used open cell foams such as poron®; These are highly resilient materials with excellent, compression set resistance. 2. The more recently developed slow return memory foams such at Memory V9 (Algeos Ltd); This set of materials display low resilience and excellent dampening properties, enabling them to mould to the foot when loaded. Resilience is defined as the amount of energy returned during unloading as a percentage of the amount of energy absorbed during loading [
22]. The lower the resilience the greater the dampening or shock attenuation capacity of the material [
22]. For this reason memory foams are becoming an increasingly popular insole material choice for people with diabetes. We found that neither the traditional open foam nor the newer slow return memory foams had any effect of standing balance when compared to the no insole condition.
Robbins et al suggested that thick, soft soled footwear intended to provide cushioning to the plantar surface of the foot may reduce dynamic balance through a decrease in foot position awareness in older people [
23,
24]. Conversely Lord and colleagues concluded that soft and hard soled shoes made no difference to balance when compared to barefoot in a sample of 42 older women [
25]. Likewise, we found that insole firmness had no statistically significant effect on standing balance in people with diabetes and neuropathy. That is, there was no statistically significant difference in standing balance when the flat (high resilient, fast release 6 mm poron cover) and no insole conditions were compared. Furthermore we concluded from the comparison diabetic (high resilient, fast return, 6 mm poron cover) and memory (low resilient, slow release, 6 mm Memory V9 cover) insole conditions, that material resilience has no statistically significant effect on standing balance in people with diabetes and neuropathy. Similarly, Van Geffen and colleagues compared the body sway of 30 people with diabetes and neuropathy standing with feet 2.5 cm together whilst wearing no insoles, 8 mm soft 15 Shore A insoles and 8 mm firm 30 Shore A value insoles [
10]. This group measured Route Mean Square of the anterior posterior centre of mass velocity, and likewise found no difference in sway values between each insole tested [
10].
There may be a number of possibilities to explain why variations in insole firmness and resilience had no apparent effect on standing balance in people with diabetes and neuropathy. First the 9 mm thickness of the insole condition used in this study may not be sufficient to cause a balance perturbation. Second, people with neuropathy, may not be as reliant on cutaneous sensory perception to maintain postural stability as those without neuropathy. In compensation for their sensory deficits, people with neuropathy may select to employ an alternate balance strategy. Thus important information about foot position awareness generated by changes in insole material is either overridden or goes unnoticed.
The effect of insole texture on standing balance
When the smooth topped low resilient memory insole with arch fill was covered with a textured material static balance improved by a statistically significant amount, to a similar level to that found when participants wore the flat insoles and no insoles without arch fill. We recorded a mean reduction in total COP path length of 8 % (absolute mean 62 mm) when the low resilient memory cover was replaced by the textured cover.
This study is novel in its inclusion of people with severe diabetic neuropathy. No other study has investigated the effects of textured insoles on standing balance in people with moderate and severe neuropathy. The lack of work in this area has been rationalised by the assumption that people who are seemingly unable to perceive texture would be unlikely to respond to its affect. Our study suggests that this assumption is unfounded and texture can affect postural stability even in people with severe neuropathy.
Further exploratory subgroup analysis was undertaken to investigate if the differences in the insole effect on postural stability were dependant on neuropathy severity. Participants were split into two groups; moderate and severe neuropathy. Severe neuropathy was defined as participants with a VPT threshold of more than 40 volts. The moderate neuropathic group was had a VPT threshold of between 25 and 40 volts. The findings for the moderate and severe neuropathic groups are comparable to each other and similar to that of the whole group.
The central nervous system continuously adjusts the relative contribution of the different sensory inputs (somatosensory, visual, vestibular) according to differing environmental constraints [
26]. This capacity of the central nervous system enables the body to adapt to changing conditions (e.g. light verses dark) to stay upright [
26]. The introduction of the textured surface (in the absence of visual cues) may have heightened the participant’s reliance on and responsiveness to plantar sensory inputs.
Several studies have demonstrated the relationship between plantar surface sensitivity and postural control. However Palluel and colleagues found that wearing spike insoles did not enhance cutaneous sensation, yet did improve postural stability in elderly people [
27]. They concluded that the absence of a correlation between plantar surface sensitivity and postural control suggested that the spikes stimulated other deep receptors. We believe that this concept may provide an explanation to the counter intuitive finding of this study, suggesting that insole texture may alter the standing balance of people with neuropathy.
There are four types of mechanoreceptors in the sole of the foot. Steady indentation pressure is sensed by the merkel disks (SAI). Rapid indentation is sensed by the Ruffini corpuscles (SAII) resulting in the sensation of skin stretch. Vibration and texture are sensed by the Pacinian corpuscles (FAII). While Meissner corpuscles (FAI) sense low frequency vibrations, resulting in the sensation of gentle fluttering [
28]. Our findings may be explained if we suppose that the textured material created focal points of skin stretch or indentation pressure to generate increased SAII or SAI afferent firing. Hatton and colleagues examined the effects of textured insoles in 50 older adults’ quiet standing with eyes closed and open. They found that whilst textured pyramids (with the potential to cause focal points of skin stretch) reduced ML sway with eyes closed, the concave dimples (with less potential to cause skin stretch) had no effect [
29].
SAI receptors are the most sensitive to maintained indentation [
30]. They are mostly are located on the borders of the sole and represent just 14.4 % of the mechanoreceptors of the foot soles in young adults [
28]. The moulded profile of the textured insole used in our study could have increased surface contact with the boarders of the sole. Thus quiet standing on the textured surface could have provided the steady indentation stimulus at the key location required for afferent firing of these mechanoreceptors.
Such a response by SAI and SAII afferents if present would not correlate well to neuropathy severity as tested by light touch or vibration thresholds [
31]. The SAII afferents, are least sensitive to the light touch and vibration stimulation [
32] and the SAI afferents are focused in the plantar boarders, a location not included in our neuropathy testing.
When considering the clinical relevance of these results there are a number of important limitations integral to the design of this pilot study that must be taken into account. First, changes in postural sway with the different insoles conditions in quiet standing individuals may not indicate that the insole components will have a clinical impact on every day balance activities. Second, this study focused on eyes closed conditions with feet close together, work is needed to explore whether the insole components have a similar effect on balance control under eyes open conditions. Third, the results of the study only reflect the immediate effect of the insoles tested on standing balance; the longer term effect of the insoles conditions on balance was not tested. Clinicians involved with the care of the diabetic foot should continue with current best practice; to provide those people with diabetes at increased risk of foot ulcer with an offloading insole. However, clinicians should carefully weigh up the offloading effectiveness of insoles against the possible impact of a particular insole design on balance. Especially when instability is cited by an individual as the underlying cause for insole non-adherence and an appropriate alternative offloading insole design could be provided.