By Koen Andre Horstink, Lucas Henricus Vincentius van derWoude, and Juha Markus Hijmans
Diabetes mellitus (DM) is a major problem worldwide, especially among elderly. Diabetic patients are prone to develop neuropathy at their feet and ankles. The incidence of diabetic peripheral neuropathy (DPN) among newly diagnosed diabetic patients is approximately 30%. This increases to approximately 50% in patients diagnosed with DM for more than 20 years. DPN is characterized by functional loss of cutaneous receptors and proprioceptive sensation, also referred to as somatosensory loss. Loss of somatosensory information indicates reduced perception, which starts most often in feet and lower legs of DM patients, and subsequently, may lead to foot ulcers. High plantar foot pressure and shear stress in combination with high blood sugar levels increases risk of developing diabetic foot ulcers (DFU). One in 4 diabetic patients develops DFU, which can have serious consequences and may lead to an amputation or even death. Consequently, prevention of DFU through reduction of peak plantar pressure and shear stress during standing and walking is important in patients with DPN. Older people with DM experience higher fall risks compared to healthy older people. When accompanied by DPN fall risk increases even further. The central nervous system uses somatosensory information for maintaining balance in static and dynamic situations, such as bipedal balance control, which is evidently compromised in patients with DPN. Therefore, patients with DPN are at higher risk of falling compared to healthy older adults. Other studies in healthy people found similar balance reductions as patients with DPN, eg, when the feet of healthy people were placed in an ice bath. This temporarily decreases somatosensory information from the feet. Besides reduced balance in patients with DPN, offloading devices such as insoles or diabetic footwear influence pressure distribution, afferent somatosensory information, and perception in patients with DPN, which may also negatively influence balance control in patients with DPN.
Offloading devices are commonly used to prevent DFU occurrence or recurrence. There are many different devices used for plantar offloading and most of them aim at altering plantar pressure through foot positioning, roll-off characteristics, cushioning, and increasing foot surface support. Offloading devices have different sole characteristics than regular footwear (eg, thickness, stiffness, rocker position, base of support), which can have other positive or negative side effects. For example, shoes that consist of thicker midsoles and smaller base of support lead to decreased control of the center of pressure (CoP), and thus, decreased postural stability. In addition, decreased CoP control results from decreased reactivity characteristics of the shoes or insoles on the foot surface. Research in healthy older people showed that postural stability decreases when standing on materials with low resiliency. Besides CoP control, shear stress can also have effects on balance control. Shear stresses are related to development of DFU and some offloading devices implement shear stress reducing techniques in their designs, eg, shear stress reducing insoles. These insoles permit lateral motion in the device to reduce shear stress at the skin and deeper tissues. However, side-to-side motion might cause patients with DPN to become unsteady. On the contrary, other insole designs might be beneficial for balance control, since textured insoles have some beneficial effects in increasing somatosensory information and perception in patients with DPN, and subsequently increase postural stability.
All in all, the exact relation between plantar offloading in diabetic patients and balance control in diabetic patients is not clear, although both received much attention separately. It is important to examine this relation, since most patients with DPN are already at higher risk of falling. The aim of this systematic review is to investigate the effects of plantar offloading devices used for prevention of DFU on static and dynamic balance control in patients with DPN. It is expected that offloading techniques negatively influence static and dynamic balance control, and consequently, can lead to higher fall risks in patients with DPN.
PubMed and Embase were systematically searched using relevant search terms. After title selection, abstract selection, and full-text selection only five articles could be included for further analysis. Two articles included static balance measurements, two articles included dynamic balance measurements, and one article included both. [Further details on the methodology can be viewed at the URL on the bottom of page 45.]
Results suggested that static balance control is reduced when rocker bottom shoes and different insole configurations are used, however, toe-only rockers showed less evidence for reduced static balance control. There was no evidence for reduced dynamic balance control in combination with offloading devices. However, these results should be interpreted with care, since the number of studies was very small—5—and the quality of the studies was moderate. [Detailed results can be viewed at the URL on the bottom of page 45.]
This systematic review aimed to study the effects of plantar offloading devices used for prevention of DFU on static and dynamic balance control. Included papers were limited in number and quality. Only five papers met all inclusion criteria and these were reviewed in detail. Three articles described offloading shoes, one article described the effects of different types of insoles, and the last article included several offloading devices. Three studies found significant reductions of static or dynamic balance control in combination with offloading devices. One study found no significant effects of rigid and flexible shoes on balance control. Interestingly, of the three studies that included dynamic balance outcome measurements, two studies found no effects of offloading devices on dynamic balance control and one study found small positive effects of different footwear types on balance control. Perceived balance was included in one article as an additional balance outcome measurement, and no effects of insoles on perceived balance were found.
Paton et al showed reduced static balance control of patients with DPN while wearing standard diabetic insoles and insoles with low resilient memory (relatively soft material). Balance was worse compared to no insole and the insole with removed arch fill. These results suggest that the molded arch fill and heel cup induce postural instability in patients with DPN. Softness of insoles might compromise stability in patients with DPN since CoP control of healthy people is dependent on the rigidity of the material on which someone is standing. Several studies already found better CoP control in soft shoes compared to rigid shoes, suggesting the same results for soft insoles compared to rigid insoles. In addition, insoles with molded arch fills have a larger contact area with the foot. One possible explanation for the difference in balance might be that increased contact areas leads to decreased pressure, which decreases the somatosensory feedback, and consequently, decrease balance. On the other hand, research in healthy older adults shows that arch support insoles enhance standing balance and are beneficial for fall prevention, indicating a difference between patients with DPN and healthy older adults. Paton et al also found that added textured covers decrease negative effects of insoles on static balance. However, similar to the results from the removed arch fills, in healthy people it was found that textured insoles had no significant effect on static balance in eyes open and eyes closed conditions. Reduced somatosensory information in patients with DPN might explain these differences with healthy people, because healthy older adults without neuropathy already have sufficient somatosensory information available for balance control. Textured insoles and insoles with removed arch fill seem to stimulate somatosensory information, resulting in increased balance performance in patients with DPN. For that reason, patients with DPN may benefit more from stimulation of somatosensory information through the use of textured insoles or insoles with removed arch fill compared to healthy people.
Regarding dynamic balance, Paton et al found no differences in dynamic balance using different insoles. Other research found no negative effects of insoles or small positive effects of different types of insoles on dynamic balance. A reason for the difference between static and dynamic balance might be the sensory input during dynamic activities. Dynamical situations provide more somatosensory feedback from muscles, joints, ligaments, and other structures. This increase in somatosensory feedback results in reduced reliance on somatosensory information from the feet and an increased reliance on somatosensory information detected in other structures during dynamic balance control. This theory is supported by research on modelling human locomotion, where control of leg and foot joints requires afferent feedback from all lower leg structures. It is also important to note that an increase in somatosensory information is not dependent on walking speed, since walking stability is the same for slow, normal and fast walking speeds. Thus, increased availability of information from other structures might be used as compensation for the loss of peripheral somatosensory information to prevent reductions in dynamic balance, independent of walking speed.
Dynamic balance is characterized by the ability to maintain a stable gait. This can be done by keeping the CoP within the base of support during dynamical activities such as walking. Measuring dynamic balance is challenging, since it consists of walking, turning, dual tasks, stepping over or on an object, and resisting perturbations. In the study of Paton et al, the step reaction time test was used to evaluate dynamic balance. However, it can be argued that the step reaction time test does not reflect the whole concept of dynamic balance. Tests for dynamic stability should include multiple aspects to obtain overall dynamic balance results, since stepping is only one aspect of dynamic balance.
Albright et al found evidence that rocker bottom shoes decreased postural stability during perturbed stance in diabetic patients. In contrast, Ghomian et al [both 2016 and 2019] found no or little effects of toe-only rocker shoes, different toe-only rocker settings, and rigid or flexible shoes on postural stability respectively. Outcome differences between these studies can possibly be explained by the characteristics of the shoes. Rocker bottom shoes have reduced base of support. Base of support is an important factor in CoP control, and therefore, influences balance control in static situations. In addition, stiffer soles are associated with faster and more reactive CoP control. In healthy populations, evidence was found for decreased postural stability with increased softness of midsole, decreased postural stability with thicker midsoles, increased postural stability with high-collared shoes, and increased postural stability with increased base of support. Diabetic patients often wear rocker shoes for ulcer prevention. Rocker shoes are characterized by thick and stiffer soles, apex positions around 55 – 60%, and, with that reduced base of support. Toe-only rockers, which are rocker shoes without a heel rocker, have a larger base of support compared to rocker shoes with a heel rocker. For that reason, rocker bottom shoes evidently have more negative effects on postural stability in patients with DPN compared with toe-only rockers and regular shoes with different rigidities.
In general, the number of studies conducted on dynamic balance in combination with offloading footwear is very small. In this systematic review, only one study [Grewal] on dynamic balance in combination with offloading footwear was included and the study provided little to no evidence for negative nor positive effects on dynamic balance. It was found that different types of offloading footwear showed longer double support phase and less center of mass (CoM) sway during walking, which are not clear indications for impaired balance. However, the design of the study was a three-group comparison and all participants wore their own prescribed footwear. Participants wore different types of footwear with varying mechanical characteristics (offloading footwear, offloading sandals, and removable cast walkers). As a result, effects of different footwear on balance could not be determined. For that reason, balance outcomes of the study were difficult to interpret and difficult to compare with other studies. Najafi et al [JAPMA 2013] investigated gait unsteadiness in patients with DPN during barefoot walking and walking with regular shoes. Results revealed gait unsteadiness in both conditions compared to healthy controls. It was also found that the addition of footwear improved gait steadiness in patients with DPN, indicating beneficial effects of footwear on dynamic balance.
Besides textured insoles, vibrating insoles also stimulate proprioception through vibrations under the surface of the foot and they showed some beneficial effects in patients with DPN. Hijmans et al [JRRD 2008] tested vibrating insoles on healthy participants and patients with DPN. In healthy participants no significant effects were found. In contrast, patients with DPN were found to significantly improve in balance during an attention-demanding task. Other research showed that vibrating insoles are also beneficial for healthy older adults, since they enhance somatosensory information and improve balance in dynamic and static situations. Vibrating insoles might be used in combination with offloading devices that lead to decreased balance, in such a way that patients have the offloading benefits of the offloading devices without compromising balance. In other recent research prototype footwear and insoles were developed to optimize gait in older adults. The footwear consisted of a firm rubber sole (25mm thick underheel and 18mm under the forefoot), a high collar to support the ankle, a firm heel counter, a 10-degree bevel in the heel, and a slip resisting outsole. The insole was textured with 4mm ethyl vinyl acetate with dome-shape projections. Results revealed better balance in healthy older adults. These findings are interesting in the context of this review; however, it is not clear if the prototype footwear and insoles were also beneficial for offloading purposes. Both the vibrating insoles and the prototype devices have potential and future research is needed to evaluate their effects in patients with DPN.
As discussed earlier, increased rigidity of shoes is associated with better CoP control. Research on healthy participants suggests the same results for insoles, since evidence supports that rigid insoles had potential beneficial effects compared to soft insoles. Rigid insoles show less postural sway and CoP velocity. In addition, when visual feedback was removed, postural sway in the soft insole increased whereas the rigid condition showed better postural stability without visual feedback, indicating decreased fall risk. It is suggested that in patients with DPN rigid insoles have similar effect, while textured insoles and vibrating insoles also showed beneficial effects on balance in patients with DPN. It could be interesting for future research to combine one of these two with rigid insoles.
Limitations included: only static balance and dynamic balance were considered; only offloading devices for ulcer prevention were considered (offloading devices used for ulcer healing, such as cast walkers, are mechanically different from offloading devices used for prevention.); only articles in English and Dutch were included; and measurement outcomes of dynamic balance control varied. [View full limitations and suggestions for future research at the URL listed on page 45.]
Only five studies on the effects of offloading footwear on balance were found. The included studies showed negative effects of offloading devices on static balance control. Standard diabetic insoles, insoles with low resilient memory cover, rocker bottom shoes, and negative heel shoes showed negative effects on static balance. There was no evidence for negative effects on dynamic balance control. However, these results are based on a small body of evidence that investigated short-term effects of footwear and insoles on balance. Thus, conclusions regarding change in balance control as a result of wearing offloading devices used for prevention of DFU should be interpreted with care.
More research is necessary to gain insight into the benefits and side effects of offloading devices. Future research should focus on comparing different offloading devices or different settings of offloading devices using a within-subject design. Furthermore, studies should include measurement outcomes aimed at static balance control, dynamic balance control, and perceived stability on the short and long term.
Koen Andre Horstink is with the Center for Human Movement Science, at the University of Groningen in The Netherlands.
Lucas Henricus Vincentius van der Woude is with the Center for Rehabilitation, University of Groningen in The Netherlands; and the School of Sport, Exercise & Health, Peter Harrison Centre for Disability Sport at Loughborough University in the UK.
Juha Markus Hijmans is with the Department of Rehabilitation Medicine at the University of Groningen in The Netherlands.
This is an extended excerpt of an article with the same title that originally appeared in the journal BMC Sports Science, Medicine and Rehabilitation; (2021) 13:31. doi: 10.1186/s13102-020-00230-8. Use is per the Creative Commons License CC BY 4.0. Some summarization and editing has occurred and references have been removed for brevity. Find the original article at https://bmcsportsscimedrehabil.biomedcentral.com/articles/10.1186/s13102-020-00230-8