Prosthetic liners and sockets play an important role in amputee patient rehabilitation. They serve as the linkage between the residual limb and the mechanical components of a prosthetic leg, providing the support necessary to regain mobility after amputation. Although much has been done to improve the mechanical knee and ankle units of prostheses, little progress has been made with regard to improving the thermal conditions experienced by the residual limb that result from the liner and the socket.1
The liner and socket are in close contact with the residual limb. Amputee patients often comment that current commercially available prosthetic liners and sockets insulate their residual limbs. Heat is trapped inside the double-layered liner (fabric and elastomer) and the socket for a prolonged period of time, causing the temperature of the residual limb to increase.2–4 The normal cooling mechanisms of the body are compromised in an amputee patient, making it difficult for their body to effectively regulate the unnatural thermal environment around the residual limb.3,4
Elevated temperature around the residual limb typically causes increased sweating of the residual limb, or hyperhidrosis.2,4 Increased temperatures and humidity levels, coupled with friction and atypical loading of the tissues, can lead to a variety of dermatologic conditions.4–8 Hagberg and Brånemark9 reported that 72% of amputee patients were troubled by sweating of their residual limb when wearing their prosthesis during the summer months. Furthermore, 62% of that same group reported that their prosthesis has caused sores, chafing, or skin irritation.9 These skin problems cause unnecessary physical and psychological burdens on the amputee patient, hindering their rehabilitation process.7
To understand why existing materials used in prostheses trap heat around the residual limb, the thermal properties of the materials must be assessed. This is an aspect that is not reported in current product specifications. Thermal conductivity is one way to assess how well a material conducts heat. It is the rate of heat transfer through a unit thickness of a material per unit area per unit temperature difference, reported in W/m K.10 The value of thermal conductivity depends on the material in question, as well as the temperature of the material.3 Materials having thermal conductivities less than 1.0 W/m K are insulators.10Table 1 lists the thermal conductivities of common materials used to construct prosthetic liners and sockets.11 These values are approximate, and actual values vary depending on the unique formulation used by each prosthetic manufacturer.
In a prominent study on the thermal conductivity of prosthetic materials, Klute et al.3 measured the thermal conductivity of selected commercially available prosthetic liners and sockets. A temperature differential was created across a sample by sandwiching it between a plate heated to 40°C and a second plate at 30°C.3 Fourier’s law for steady-state conduction through a specimen of constant area was used to calculate thermal conductivity, where q is the heat flux across the sample, L is the sample thickness, A represents the area of the sample, and ΔT is the temperature differential across the thickness of the sample (Equation 1).3 Klute et al. acknowledged the limitations to their study, namely, the influence of thermal grease on the samples’ thermal conductivities. Thermal grease is not used in a clinical setting where an amputee patient is wearing the liner and socket. Furthermore, Klute et al. recognized that the effective thermal conductivity while a patient is wearing the prosthesis possibly changes with the applied load on the prosthesis because of the fluctuating volume of trapped air observed while the prosthesis is in use.
Equation 1. Thermal conductivity
From patient accounts and literature reports of problems with prostheses, it is evident that the skin temperature of the residual limb typically is elevated compared with the rest of the body when wearing the liner and socket.1,6 It is difficult to say exactly how much greater the temperature is because of the variation that exists between patients, the activity dependence of skin temperature, and the location on the residual limb.6 For example, Peery et al.1,8 observed an average skin temperature increase of 2.9°C from donning to walking for one patient, whereas only a 0.9°C increase was observed in a second patient. It was also noted that, overall, the coolest points on the residual limb were located in the proximal anterior region while donning. The warmest skin temperatures observed across all subjects were in the posterior aspect of the residual limb after walking for 15 minutes.1,8
The current study focused on quantifying the thermal barrier posed by materials currently used in prosthetic liners and sockets by measuring thermal conductivity. It was hypothesized that materials used in commercially available prosthetic liners and sockets have poor thermal conductivities and thereby insulate the residual limb. More specifically, it was hypothesized that all prosthetic elastomers, fabrics, liners, and socket materials tested would be classified as insulators, having thermal conductivities significantly lower than 1.0 W/m K.
To measure the thermal conductivity of commercially available prosthetic materials, a custom-built heat flux meter was used (Figure 1). This machine was built to comply with ASTM Standard C518-10.12 A temperature gradient was established across the sample by heating the upper plate to approximately 27°C and cooling the lower plate to approximately 10°C. The upper plate nested on top of the lower plate during testing. This nesting feature helped minimize heat loss to the surrounding environment, which could impact thermal conductivity calculations. During testing, heat moved from the upper plate, through the sample, to the lower plate.
Custom heat flux sensors (International Thermal Instrument Company Inc, Del Mar, CA, USA) were built into the upper and lower plates. These sensors measured the heat flux and temperature on both sides of the sample. Temperatures of the upper and lower blocks were measured by two fixed thermistors, one in the upper block and the second in the lower block (Omega Engineering, Stamford, CT, USA). Although these block temperatures were not used in any calculations, they were used to ensure that there were not any major temperature fluctuations in the machine.
The custom heat flux meter was calibrated using materials with similar physical characteristics to the prosthetic samples. Instrumental error was calculated by error propagation, using the manufacturer-provided error values for the thermocouples and heat flux sensors. Sylgard 528 (Dow Corning, Midland, MI, USA) was used as a standard of calibration for the elastomer and liner samples. Typical thermal conductivity for poly(dimethylsiloxane), such as the elastomer used in calibration, was 0.15 W/m K, varying slightly from the value reported in Table 1 because of its specific composition.13 The rigidity of the socket sample was approximated by using borosilicate glass (McMaster-Carr, Aurora, OH, USA) and thermally conductive polycarbonate (McMaster-Carr, Aurora, OH, USA). To minimize heat loss, sample thickness was restricted to less than 6 mm.
Specimens were required to be 1-in-diameter circles to completely cover the heat flux sensors for proper testing. A 1-in-diameter punch was used to cut out the elastomer, fabric, and liner samples. Five samples of each material were prepared for testing. Socket materials were machined to the specified dimensions. Sample thickness ranged between approximately 1 and 6 mm, depending on the material available. Three elastomers were tested: two different silicone blends (samples A and B) and one polyurethane blend (sample C). Nine fabrics of varying weaves and thread composition were tested (samples D–L). Five variations of liners, all with an outer fabric layer, were tested. Four of those liners (samples M–P) contained a silicone elastomer layer, whereas one liner (sample Q) contained a polyurethane elastomer layer. The elastomers and liners were from three separate companies: Otto Bock, Ossur, and Ohio Willow Wood. A total of six different socket materials were tested. One of those samples was a thermoplastic (sample U), whereas the remaining five materials were thermosetting polymers including epoxies (samples R and S), polyethylene terephthalate (sample T), and polyethylene copolymers (samples V and W). Each specimen had a uniform thickness across its entire surface. To avoid excess heat loss, samples were inspected for overhanging or uneven edges before testing.
During testing, the temperatures and heat flux on both sides of the sample were collected continuously at 1 Hz using Tracer DAQ (Measurement Computing, Norton, MA, USA). Each sample was tested for at least 1 hour to reach a steady state, defined as reading fluctuations neither greater than 100 m°C nor 100 μV over a period of 15 minutes. Some samples required a longer testing period to meet the steady-state criteria. Final temperature and flux measurements were taken once each sample reached a steady state. The sample thickness (L) was measured using a digital indicator (Mitutoyo America, Aurora, IL, USA) at the end of testing, and then it was used along with the average heat flux (Qavg) and temperature difference (ΔT) across the sample to calculate the thermal conductivity (k) using Equation 2. The average and standard deviation of thermal conductivity were calculated for each material using the five thermal conductivity values of the samples.
Equation 2. Thermal conductivity calculation
Elastomer thermal conductivities ranged from 0.145 to 0.155 W/m K (Table 2). For reference, the thermal conductivity of Sylgard 528 was calculated as 0.124 W/m K . The nine different fabric samples had lower conductivity values (0.045–0.074 W/m K) when compared with the other materials tested. Fabric sample H had the poorest thermal conductivity of all prosthetic materials tested (0.045 W/m K). Liners exhibited a similar range of thermal conductivities (0.116–0.143 W/m K) when compared with the elastomers tested. The prosthetic materials that conducted heat the best were the socket materials, having a range of thermal conductivities from 0.133 to 0.189 W/m K. The prosthetic material with the greatest thermal conductivity was sample U (0.189 W/m K), the thermoplastic socket material. It is important to note that although the socket materials had the greatest thermal conductivity of the materials tested, all materials tested had measured thermal conductivities of substantially less than 1 W/m K (Figure 2), indicating that they all are insulators.
The results presented above add quantitative data that support anecdotal patient reports of liners and sockets trapping heat around their residual limbs. All materials tested had low thermal conductivities comparable with other common insulators such as skin (0.37 W/m K ), wood (0.17 W/m K ), soft rubber (0.13 W/m K ), and air (0.026 W/m K ).9 Fabrics had the poorest thermal conductivity of the materials tested. This was expected because of the numerous air pockets present in the weave of the fabric. The measured conductivity values for elastomers and liners were comparable. This similarity was anticipated because the liners are composed primarily of an elastomer, with a thin fabric layer on one side. Overall, the fabric’s thermal conductivity is quite low, but it is partially penetrated by the elastomer in a liner, reducing the number of air pockets. Because of this penetration, the influence of the fabric layer on thermal conductivity was minimal and the liners’ thermal conductivities were comparable to the elastomers’.
The thickness recorded and used in thermal conductivity calculations is the thickness during testing, with the heat flux meter closed and the upper plate resting on top of the sample. This is of insignificance when looking at incompressible socket specimens; however, this could introduce a small amount of error in the conductivity calculation of the more compressible elastomer, fabric, and liner specimens. Compression of the specimens was not restricted. Another possible limitation of this study is that the interface resistance between the sample and the heat flux sensor was not accounted for. When looking at thinner samples, this could result in some error when calculating the thermal conductivity. The thicker the sample, the less influence the interface resistance has on the calculated conductivity because the bulk behavior of the material dominates. To limit this influence, optimum sample thickness was defined as 2–6 mm. Unfortunately, most fabric samples were only commercially available in thicknesses less than 2 mm.
Klute et al. reported liner thermal conductivity values ranging from 0.085 to 0.266 W/m K and socket thermal conductivity values of 0.148 and 0.150 W/m K, values similar to those presented above. Although methodological differences exist between the two studies, namely, the use of thermal grease in the Klute et al. study, the ranges of thermal conductivities can provide valuable information regarding how well the groups of materials conduct heat. Although thermal grease ensures close contact of the specimen and the sensors, the decision was made to test samples in this study in their natural state, without thermal grease. This more closely represents the in vivo environment and also avoids shorting out circuitry in the custom heat flux sensors.
The reported thermal conductivities allowed for a quantitative baseline to be established that correlates with anecdotal patient reports of the prosthetic materials insulating their residual limbs. The poor thermal conductivities of all prosthetic materials tested indicate that there is much room for improvement regarding the thermal properties of prosthetic materials. Amputee patient quality of life could improve significantly by focusing future efforts on creating new materials and designs for prosthetic liners and sockets that allow for a more natural thermal environment to be established around the residual limb.
All of the prosthetic materials tested had poor thermal conductivities. These results concurred with those presented by Klute et al.3 regarding select liner and socket material conductivity. Thus, these materials are insulators, making them less than ideal for use in prostheses that surround the residual limb for many hours each day. In the future, it would be beneficial to take this into consideration when improving prosthetic design. Using materials that could eliminate the abnormal thermal environment around the residual limb could improve the comfort, safety, and overall quality of life for amputee patients.
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Keywords:© 2014 by the American Academy of Orthotists and Prosthetists.
prosthetics; amputees; thermal conductivity; materials testing; polymers; biomechanics