Effects of Different Types of Socks on Plantar Pressures，In-shoe Temperature and Humidity in Diabetic Patients

YICK K L (易潔倫)，CHEUNG N C (張念澄)，LEUNG K Y (梁潔恩)，NG S P (吳新培)，YIP J (葉曉雲)，LO W T (廬慧婷)，YU Annie (余芷慧)*

1 Institute of Textiles and Clothing，The Hong Kong Polytechnic University，Hong Kong，China

2 Hong Kong Community College，The Hong Kong Polytechnic University，Hong Kong，China

Introduction

According to the International Diabetes Federation，there were more than 371 million diabetics worldwide in 2012［1］.By the year 2025，the total number of individuals with diabetes is projected to reach 300 million.People who suffer from diabetes may experience diabetic foot infections，foot pain，ulcerations，and even amputations.The literature has reported that patients with foot ulcers localized on the plantar surface of their feet experience a 15 times higher overall risk of amputation compared to non-diabetes［2］.Due to the loss of the protective sensation that is associated with neuropathy，diabetics are unable to respond to pressure，cold and heat，minor cuts and injuries，or deep pain.As diabetes mellitus also enables infections by microorganisms，injuries could quickly become infected and lead to serious health conditions.In many cases，diabetic foot ulcers can lead to necrosis and eventually，the amputation of the foot or even the whole lower leg［3-4］.It is estimated that 15%of the diabetic population will develop a foot ulcer during their lifetime and 2% -3% may develop a foot ulcer annually［5］.With the rapid increase in the prevalence of diabetes over recent years，the prevention and treatment of ulcerations and their related costs impose challenges and a large economic burden on healthcare systems.

Foot ulcers mainly develop as a result of sustained high pressure on a particular area of the foot，frequently over bony prominences，such as the plantar surface of the toes and forefoot［6］.The increased pressure， repetitive mechanical trauma beneath the foot，poor vascular supply，and nerve damages in the diabetic foot may eventually result in cell and tissue death［4-5］.Foot ulcerations also occur at the site of irritation due to poorly fitted footwear.Slipping of the foot inside an oversized shoe may cause excessive shearing stress at the interface and results in soft tissue breakdown［3］.The peak pressure exerted onto a diabetic's foot when walking may reach 1 000 kPa，which is double the peak pressure of a normal foot［7］.Over the past decades，much of the work that aimed to reduce stresses on feet had concentrated on the design and fabrications of footwear that were routinely used in clinical practice for the prevention and management of diabetic foot ulcerations.Materials with good shock-absorbing properties are frequently adopted.Compared with shoes and insoles，socks are much simpler to apply，and offer the first line of defence to the foot.As indicated by Li et al.［8］，the pressure induced from socks could deform the skin and soft tissue which may result in pressure discomfort and even pain sensations to the wearer.The designing and engineering of socks play a key role in maintaining suitable pressure and contact between the socks and feet.Although cotton has good water absorption performance，it retains water.This not only affects the humidity inside the shoe microenvironment，but is also associated with the increase of temperature and friction coefficients of the socks.Higher temperature and moisture content in footwear will also encourage the growth and different types of bacteria on the feet，which will further result in sores and wound infection for patients with diabetes.The current study therefore aims to examine the pressure-relieving properties of different types of materials for socks as compared with normal daily socks.As it is also crucial for diabetic patients to maintain a dry and cool foot-skin microclimate environment，the designing of different sock types in relation to foot skin temperature and humidity will also be investigated.

1 Experimental

In this study，a total 5 types of socks，including 3 types of diabetic socks and 2 types of daily/sports socks of various structures，are examined.It is worth noting that the fabrication of the 3 types of diabetic socks is generally complicated，comprising 4 or more different knitting structures for the leg，heel，plantar，toes，etc.As shown in Fig.1，vertical inlays are used in the leg region for graduated compression，cushioning and preventing foot and leg fatigue during prolonged periods of standing and walking.A full terry structure is frequently adopted in the foot region which provides a soft cushioned heel and toe to enhance comfort for wearers.It is believed that the changes in plantar foot pressure are closely associated with the fabrication on the underfoot region of socks;thus a summary of knitting structures for the underfoot region in the 5 sock samples is presented in Table 1.

Fig.1 A typical diabetic sock and its knitted structure at the calf and underfoot

Table 1 A summary of the knitting structures (underfoot)of the socks

In this study，a total of 4 subjects (including 2 subjects with diabetes)were invited for the wear trials.The NOVEL Pedar-x system is used to record plantar pressure data in this study，whilst the temperature and humidity of the microenvironment for the sock are recorded by placing a sensor(Thermocrons HC，OnSolution)inside the sock (see Fig.2).The Pedar-x system provides bipedal plantar pressure and force measurements by using thin (thickness of 2 mm)and flexible pressure sensors (including 99 sensors for each foot).As shown in Fig.3，the measured plantar pressure obtained from each foot is divided into 7 anatomical regions for analysis，including the whole foot (WF)，forefoot (FF)，rear foot(RF)，midfoot (MF)，first metatarsal head (1stMTH)，second and third metatarsal heads (2nd＆ 3rdMTHs)，and fourth and fifth metatarsal heads (4th＆ 5thMTHs)，respectively.A series of activities were designed for each subject，including indoor sitting and walking，and outdoor walking.The changes in the mean and peak plantar pressures for different activities with different sock types and barefoot(BF) were recorded respectively.The temperature and humidity loggers were inserted in the arch region so that changes in foot skin temperature and humidity were recorded every minute.In order to ensure accuracy，the two sensors were calibrated beforehand and the differences in the temperature and humidity recorded under the same conditions were within 0.1 ℃and 1%，respectively.

Fig.2 The Pelay-x in-shoe pressure measurement system(left)and temperature and humidity sensor (right)

Fig.3 Foot anatomical regions for examination of plantar pressure

2 Results and Discussion

2.1 Plantar pressures

In reviewing the peak plantar pressure distributions obtained from the diabetic patients，exceptionally high pressure was obtained at the RF，when the subjects were walking in BF(Fig.4).The results could be explained by the high friction forces between the foot skin and the insole leading to extremely high shearing forces and plantar pressure［8］.The results are also highly consistent with the discussions in previous studies，in which an increase in the coefficient of friction will result in increasing the shear force［9］.When the foot is wearing the sock，the high friction between the foot skin and the insole can be dramatically reduced，hence reducing the potential for the formation of various foot lesions［10］.In this study，regardless of the type of sock and its different knitting structure，thickness and airspace ratio，both diabetic and daily/sports socks can effectively reduce high pressure on the RF.The peak plantar pressure at RF reduced from 632.5 kPa to around 200 kPa when socks are worn.The results reveal that high magnitudes of plantar pressure on the RF are re-distributed to the other foot regions，like the metatarsal heads and/or FF region by wearing socks.The overall plantar pressures can be considerably reduced to the 300 kPa range，which is effectively for the prevention of foot ulcerations.

Fig.4 Peak plantar pressure when standing with and without socks

Amongst the 5 sock types，sample DS3 has the best pressure relieving performance at the WF，FF，RF，MF，and 1stMTH.The magnitude of the peak plantar pressure was reduced by 52% at the WF，70% at the RF，29% at the MF，and 36% at the 1stMTH.Even though the long terry loops at the FF and toe portions of the diabetic sock could probably enhance the subjective comfort perception of the wearer in terms of cushioning， their effects on alleviating and/or redistributing excessive plantar pressures are somewhat similar to the single jersey structure of the daily sock and/or short terry structure of the sports sock.In designing socks for diabetic patients，apart from thick cushioned soles and seamless toes，the surface frictional properties of the fabrication materials which allow the foot to slide on the insole and reduce the shearing force，should also be taken into consideration.It is also noted that when friction is too low，the foot will move easily in the shoe.Such excessive movement not only gives rise to a feeling of underfoot insecurity，but also leads to excessive pressure and rubbing between the top and upper parts of the foot and the shoe［11］.

2.2 Foot skin temperature and humidity

The thermal comfort properties of the 5 sock samples were measured in terms of air permeability，thermal conductivity，moisture regain，water vapour transmission rate (WVTR)，etc.By using Kawabata Standard Evaluation System (KES)(Kato Tech Co.，Ltd.，Japan)Thermo Labo Ⅱ，the warm/cool feeling evaluation value (Qmax)of the sock samples was also measured.As shown in Table 2，the results indicated that Sample DS2 has the highest air permeability value (344.9 cm/s)，but the lowest value in thermal conductivity (0.003 2 W/m·℃)，moisture regain (3.76%)，WVTR (2.77 g/(m2·h)and Qmax(0.038 W/cm2)，amongst the 5 types of socks studied.Its superior breathability characteristics，warmth keeping and low moisture absorption performance could be explained by its X-static silver contents that enable a dry sock-foot interfacial surface and thus，enhance the overall comfort of the wearer as well as protect against bacteria and fungi.In view of the wear trials，the foot skin temperature and humidity obtained from DS2 with dense inlay structures of underfoot padding were consistently higher than those of the other sock samples (see Fig.5).Due to its low thermal conductivity and poor water vapour transmission，heat and sweat cannot be dissipated，thus offering a warm sock-foot contact interface to wearers.The results also indicate that the thick and long terry stitches of the diabetic sock prevent transpiration of sweat and in-shoe humidity，which results in minimal changes in the in-shoe humidity during various activities.It is helpful to maintain a stable thermal microenvironment for the foot，particularly in the winter time.Foot skin temperature and humidity obtained from daily socks(with 80% cotton)and sports socks (with 100% cotton)considerably increased when walking and/or carrying out daily activities.

Table 2 Thermal comfort properties of sock samples

3 Conclusions

Fig.5 Skin temperature and humidity measured in different sock samples

The wearing of socks can effectively alleviate peak plantar pressure in the RF region by reducing high shearing and friction stresses between the foot skin and the insole and/or shoe.Both full long-terry and single jersey fabrics can substantially reduce plantar pressure.Nevertheless，to control plantar pressure so that ulcerations can be prevented for diabetic patients，material of suitable frictional properties which allow the foot to slide on the insole and reduce shearing forces should be considered during the designing and engineering of socks.Sock fabrication and structure have major impacts on maintaining and controlling foot skin temperature and humidity.The long terry stitches of the diabetic sock not only provide soft and thick cushioning，but also prevent transpiration of sweat and in-shoe humidity， thus maintaining a stable thermal microenvironment for the foot.With regards to sock materials，since high humidity may be associated with increase in temperature and friction coefficients of socks，they must be able to wick moisture away from the foot，leaving the skin dry and reducing sticking.

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