Pressure relief and load redistribution by custom-made insoles in diabetic patients with neuropathy and foot deformity
Introduction
Custom-made insoles (CMIs) are routinely used in clinical foot care settings for diabetic patients with neuropathy and foot deformity. The success of such insoles and the associated footwear is primarily evaluated based on whether the patient remains free of ulceration while wearing the footwear and insoles. Elevated barefoot plantar pressure has long been associated with plantar ulceration (Boulton et al., 1983; Veves et al., 1992). Therefore, one of the major goals of any footwear intervention must be to protect the foot at sites that are at risk for plantar ulceration or re-ulceration by reducing pressure to a level below some (currently unknown) threshold for ulceration.
CMIs are prescribed primarily, if not exclusively, to protect the plantar surface of the foot. However, little quantitative data is available on if and how CMIs achieve their action. Among the putative methods that have been discussed is the redistribution of load to adjacent foot regions through accommodative moulding––the “total contact” concept––and the incorporation of additional “reliefs” or, conversely, elevated contact surfaces such as medial arch supports and metatarsal pads, which go beyond merely providing a mirror image of the plantar contour (Bowker et al., 1993; Cavanagh et al., 2001; Janisse, 1995).
Several authors have reported successful relief of dynamic plantar pressures and load at the metatarsal heads (MTH), a plantar region where ulcers commonly occur (Albert and Rinoie, 1994; Brown et al., 1996; Lord and Hosein, 1994; Novick et al., 1993; Postema et al., 1998). However, others have found no significant off-loading effects of moulded insole interventions (Ashry et al., 1997; Hewitt et al., 1993; Uccioli et al., 1997). The discrepant results from these various studies are likely a function of different approaches to insole manufacture, subject selection, and experimental procedure.
The lack of a standard and comprehensive method of pressure and load analysis in these studies has prevented a better understanding of the mechanisms by which insole modifications act to relieve pressure and redistribute load under the foot. One of the methods by which this relief can be achieved is through comparison of the load distribution patterns from a CMI and some controlled condition, such as a flat insole, worn sequentially both on the same foot and in the same shoe, from which load transfer analysis can be performed. Therefore, the purpose of the present study was to compare the mechanical behaviour at the foot–insole interface of CMIs and flat insoles in diabetic patients with neuropathy and foot deformity and to use this comparison to define a calculation method for assessing load redistribution.
Section snippets
Subjects
Twenty diabetic patients with neuropathy and foot deformity (13 men, 7 women), recruited from a specialist diabetic foot clinic, participated in the study after giving informed consent. The mean (SD) age, height, and weight of the subjects were 64.4 (11.2) years, 1.73 (0.10) m, and 99.5 (15.7) kg, respectively. Neuropathy was confirmed by a loss of protective sensation on the plantar surface of the foot, as determined by the inability to feel the 10-g Semmes–Weinstein monofilament on the hallux
Results
The mean (SD) walking speed with both flat insoles and CMIs was 0.83 (0.31) m/s. The mean absolute intra-subject difference in walking speed between the two insole conditions was 1.6% (range 0–4.7%).
Discussion
This study has shown that, on average, this type of CMI, intended for at-risk neuropathic feet, can reduce PP and FTI at MTH1 by 16% and 8%, respectively, compared with a thick, flat, over-the-counter cushioned insole. It remains to be demonstrated whether or not such alterations are sufficient to reduce tissue loading at this site below the threshold for injury. Neither is the relative importance of reductions in PP or FTI for prevention of tissue injury established. These results show a lower
Conclusions
This study provides a perspective on alterations in the loading of the feet in a group of patients with diabetic neuropathy and foot deformity who were using CMIs. On average, the CMIs significantly reduced PPs and FTIs in MTH1 (RoI) when compared with flat insoles, but their mechanical effects were much larger in more proximal regions of the foot, which are less at risk for plantar ulceration. In particular, dramatic pressure reductions were achieved in the heel as a result of load
Acknowledgements
The authors thank Mary Becker, RN, for her contributions in subject recruitment and screening, data collection and data analysis. This work was supported by NIH grant HD037433 and by ConvaTec, Inc. Jan Ulbrecht and Peter Cavanagh are principals in DIApedia LLC (State College, PA, USA), a company that performs research and development in the area of diabetic foot disease.
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