Effect of sock on biomechanical responses of foot during walking
Introduction
Foot lesions are common and costly complications in people with diabetes (Litzelman et al., 1997). Diabetic patients may not be able to sense the painful stimulus due to neuropathy. Diabetics suffering from neuropathy can develop minor cuts, scrapes, blisters, or pressure sores that they may not be aware of due to their insensitive feet. If these minor injuries are left untreated, complications may result and lead to ulceration and possibly even amputation. Inappropriate footwear has been identified as a major cause of ulceration in diabetes (Apelqvist et al., 1990). An European study reported that inappropriate footwear contributed to 21% of all ulcer formation (MacFarlane and Jeffcoate, 1997), showing that proper footwear is an important element for protecting the diabetic feet. Various preventive and therapeutic modalities, including shoes and insoles, have been developed for diabetics. A number of studies have been carried out to investigate the role of footwear in preventing foot lesions (Litzelman et al., 1997, Maciejewshi et al., 2004, Mueller et al., 1997, Viswanathan et al., 2004). It has been shown that appropriate footwear helps to redistribute plantar pressure and reduce peak plantar pressure and shear, which are suggested as major causes of calluses, sores and ulcers.
Further research about foot care showed that appropriate shoes and insoles are not enough and attention must also be paid to socks (Mayfield et al., 1998, Phillips et al., 2000, Scheffler, 2001). It has been a common clinical practice that people with diabetes are advised not to wear shoes without socks. Hosiery helps to remove perspiration from the skin, regulate foot temperature, provide pressure relief, and protect the skin from abrasion. However, except a few research to investigate the effect of padded sock on plantar pressure (Scheffler, 2001, Veves et al., 1989), and to measure the static coefficients of friction between skin and socks (Sanders et al., 1998), the effect of sock wearing on foot biomechanical response has not been studied in terms of its frictional properties.
It is estimated that an individual takes about 8000–10,000 steps a day. During walking, foot presses and rubs against footgear and if the situation persists, blisters and calluses will result. These foot problems may likely ignite foot ulceration in diabetic patients with foot neuropathy. It has been reported that repetitive pressure and shear on the foot are correlated with the development of blisters and further ulcers (Phillips et al., 2000). Therefore, reducing both pressure and shear is crucial in prevention of ulceration. Various pressure measurement methods have been developed to measure the static and dynamic plantar pressure (Cavanagh et al., 1992). Unlike the plantar pressure that can be measured directly by commercially available pressure sensors, it is difficult to measure the shear on the foot. Several experimental techniques for shear measurement have been reported (Akhlaghi and Pepper, 1996, Lord et al., 1992). However, crucial factors that influence the rubbing process when wearing shoes remain unknown.
Socks are able to change the frictional properties between the foot–shoe interface. Abrasion of the foot skin can be avoided by reducing the shear between the contact interfaces with the use of socks made from textile fibers of low frictional coefficients (Delporte, 1997). In this paper, we aimed at studying to study the mechanical effect of sock with different frictional properties on foot during the stance phases of gait. Since direct measurement of friction and shearing in situ is difficult, we turned to the computational method. Finite element models have been employed to investigate the biomechanical effects of soft tissue stiffening in the diabetic feet (Thomas et al., 2004, Gefen, 2003, Jacob and Patil, 1999). Several finite element analyses (Chen et al., 2003, Cheung and Zhang, 2005) have modelled the foot–insole interface to investigate the effect of insole cushioning on the plantar pressure distribution.
Currently, no attempt has been made to model the foot–sock–insole interface using a 3-D FE model and the biomechanical behaviour of the foot during rubbing and slippage within the shoe has not yet been analysed. The objective of this study was to build a foot, sock and insole contact model to investigate the effect of sock wearing on the plantar pressure and shear during the stance phases of gait.
Section snippets
Methods
Foot consists of bones, cartilages, ligaments, soft tissue and skin. Since the purpose of this study was to investigate the frictions and slippages between the foot, sock and insole, the stress distribution in bones was not the subject of research. For the sake of simplification, all the bones and cartilages were combined skeleton with the five rays of toes combined as a whole and all connecting ligamentous structures neglected (Fig. 1(a)). The foot skeleton was encapsulated by a volume of
Results
Fig. 2(a) and (b) shows the plantar pressure distributions of foot-flat phase and push-off phase of the barefoot simulation. During the foot stance phase transition from foot-flat to push-off, the concentration of plantar pressure moved from the heel region to the forefoot region. The predicted plantar pressure patterns agreed qualitatively with the measurement shown in Fig. 2(c) and (d). However, the measured peak pressures 0.20 MPa at the heel and 0.23 MPa at the forefoot were lower than the
Discussions
The predicted plantar pressure distributions of barefoot were in general comparable to the F-scan measurement although the predicted pressures were higher than the measured values. There may be several causes for the discrepancy. One was the resolution of the F-scan sensors, which reported an average pressure for an area of 25 mm2. As the FE analysis provided solution of nodal contact pressure per element’s surface area, the F-scan measured peak plantar pressure was expected to be lower than the
Conclusions
Through this study, the dynamic 3-D distributions of the plantar pressure and shear force were investigated by developing biomechanical models and computational simulations. It has been demonstrated that high shear force on foot sole due to high friction is a direct factor causing skin abrasion, friction blisters and ulcers. Wearing sock of low friction allowing proper sliding in footwear was able to reduce the shear force significantly. The shear force could be further reduced when wearing the
Acknowledgement
We would like to thank The Hong Kong Polytechnic University for funding this research through the projects A188 and G-YD31.
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