Characterization of Prosthetic Liner Products for People with Transtibial Amputation

INTRODUCTION

Use of elastomeric or “gel” liners in prosthetics practice has become increasingly common, with recent data suggesting that practitioners select gel liners for about 85% of their prosthetic patients.¹ To accommodate the clinical needs of a broad range of prosthetic limb users, liner manufacturers have created an array of different liner products. Although elastomeric liners are derived primarily from the same base materials (i.e., urethane, silicone, and thermoplastic elastomer (TPE]), differences in a liner’s chemical composition, shape, geometry, and manufacturing determine a liner’s overall performance. Presence of fabric backing, antibacterial additives, or features such as sealing sleeves, distal pads, or reinforcement can additionally affect mechanical performance of a liner. Given the number of variables that contribute to a liner’s design, it is not surprising that a large number of liner products are commercially available – a recent study identified more than 70 liner products on the market.²

Despite the availability of so many different liner products, a recent survey indicated that a typical practitioner choses among only two or three products when selecting liners for his or her prosthetic patients.² An explanation for such limited selection among many available options may be a lack of comparable, objective information about the similarities and differences among liner products.³ Without information about how a new liner compares to the two or three familiar products, practitioners are hesitant to change. The same study found that only 33% of practitioners surveyed obtained information from scientific journals, while the majority sought information directly from manufacturers (94%) or clinical magazines (65%). These results suggest there is a need to distinguish liners and provide practitioners with practical information to help them identify products that best match individual patient needs.

The purpose of this research study was to characterize mechanical properties of commonly used prosthetic liner products and to compare findings with previous liners characterized in the literature. We also evaluated the influence of design variables fabric backing and liner thickness on testing results. Selecting a liner for an individual patient may require the practitioner optimize the most preferred property or identify a liner that exhibits a balance among several preferred properties.

METHODS

We compared commonly used, commercially available prosthetic liners across six material property tests. Compressive elasticity and shear elasticity tests were conducted to characterize each liner’s ability to distribute ambulatory loads during stance phase and couple the socket to the residual limb. Tensile elasticity tests were conducted to measure each liner’s resistance to axial stretching during the swing phase of gait. Coefficient of friction (CoF) tests were conducted to determine the degree to which each liner would adhere to a residual limb. Finally, thermal conductivity and volumetric elasticity tests were conducted to assess each liner’s accommodation to limb temperature increases and volume fluctuations, respectively. Development and validation of the liner material property tests used in this study are described in detail elsewhere.⁴ Brief description are included below.

Instrumentation

The compressive elasticity, shear elasticity, tensile elasticity, and volumetric elasticity tests were conducted with a material testing machine (MTM) (5944, Instron, Norwood, Massachusetts) and custom test fixtures. Full scale range of the MTM’s load cell was 1–500 N with a mean load measurement error of ±0.25% and a displacement error of ±0.005 mm.

CoF tests were conducted using a planar friction tester (PFT) (Advanced Friction Tester, Hanatek, East Sussex, United Kingdom). CoF test parameters were determined primarily from an ASTM testing standard⁵; no custom fixtures were required. However, the PFT specimen sled was covered with a soft leather (Cream Cow, WBC Industries, Westfield, New Jersey), which served as a skin surrogate and provided a physiologically meaningful contact surface. As configured, the PFT measured CoF with a repeatability of ±0.008.

Thermal conductivity was measured with a guarded heat flow meter (HFM) (Unitherm model 2022, Anter Corp, Pittsburgh, Pennsylvania). Similar to the CoF test, thermal conductivity test parameters were primarily determined by an ASTM test standard⁶ and no custom fixtures were required. Repeatability of the HFM was ±0.001 W/m∙K.

Materials

Prosthetic liner manufacturers were invited to donate samples for material property testing. Liner products sold most often were requested, but any liners donated by manufacturers were included in the study. Additional liners were purchased as needed to ensure that liner products used most often by prosthetists, as reported in a recent survey,² were tested.

Specimens were excised from each donated liner based on its thickness profile (Figure 1). Liners with uniform thickness profiles had specimens excised from a single area of the liner, while tapered liners had specimens excised from proximal and distal areas so as to characterize the range of materials present in the liner. If a liner presented different anterior and posterior thickness profiles, test sections were extracted from the anterior region, since soft tissues in the anterior region are closer to bone; thus, this region was expected to be of greater clinical interest. Specimen thicknesses were measured with a custom test instrument⁷ used in prior studies to assess prosthetic sock thickness (resolution 0.04 mm).^8,9 All liner material tests were performed in a temperature controlled room with ambient conditions of 21 ± 1 deg C. Each specimen was thermally preconditioned for at least 24 hours before testing was initiated.

FIGURE 1.

Test specimen locations. Test specimens were removed from anterior regions of the liner when possible. For uniform gel thickness profiles, only a narrow acceptable source region (blue) was available for certain liner products (e.g., WillowWood Hybrid Select), because the mid-anterior region had a different fabric, while distal and posterior regions had different gel thickness (beige).

Compressive Elasticity

Compressive elasticity characterizes a liner’s ability to distribute concentrated stresses and to secure the liner (and by association, the socket and prosthesis) to the residual limb. Distributing compressive stresses over the residual limb, particularly in sensitive regions with bony prominences, is desirable since concentrated stresses are associated with adverse limb health issues like discomfort, pain, and skin breakdown.¹⁰ Good limb-socket coupling facilitates a sense of stability to the prosthesis user, thus it is also desirable. However, distributing concentrated stresses and coupling to the residual limb require opposing characteristics. A soft liner (low compressive elasticity) will facilitate comfort by better distributing peak pressures, while a stiff liner (high compressive elasticity) will deform less and create a more stable limb-socket coupling.

Three circular specimens (19 mm [D]) were evaluated per liner test section. Specimens were lubricated (Outlast, C&C Synthetics, Mandeville, Louisiana) and then installed into the MTM. Presence of the lubricant had minimal effect on compressive elasticity measurements.⁴ Specimens were cyclically preconditioned for 15 minutes and then re-lubricated before the test. For the measurement of compressive elasticity, specimens were compressed to 60% strain or 250 kPa engineering stress, whichever induced greater stress, under the condition that an upper limit of 500 kPa was not exceeded. Tests were repeated until specimens produced a consistent stress-strain response. To make the data more comparable to interface stress studies reported in the scientific literature,^10–16 the measured engineering stresses were converted to true stresses under the assumption of material incompressibility. The reported compressive elasticity value for each liner was the average of the three specimen stiffnesses.

Shear Elasticity

Shear elasticity characterizes each liner’s ability to propagate and distribute shear stresses. Importantly, shear stresses are thought to be a leading cause of mechanically induced skin breakdown.^17,18 Unwanted shear can also slow or exacerbate wound healing following an adverse event.^19,20 Thus, consideration of a liner’s shear elasticity is of particular importance when selecting liners for people with delicate skin.

One rectangular specimen (127 mm [L] × 32 mm [W]) was excised per liner section to conduct the shear elasticity test. Specimens were bonded to a custom test fixture and stretched to 60% strain. FabTech 90 adhesive (FabTech Systems, Everett, Washington) was used to bond the majority of liner specimens to the test fixture. Loctite 409 (Henkel Adhesives, Rocky Hill, Connecticut) showed significantly better bonding with several TPE and silicone liner products, and was used for those samples. Specimens were cyclically sheared until a consistent stress-strain response was achieved (about three cycles). The repeatable response was recorded as the property measurement. Specimens were not cyclically pre-conditioned for 15 minutes, like in the compressive elasticity test, because the selected adhesives could not bond all liner products for that entire period of testing.

Tensile Elasticity

Tensile elasticity characterizes each liner’s ability to resist axial stretching. Axial stretching in an elastomeric liner can cause pistoning (i.e., axial movement) of the prosthetic socket relative to the user’s residual limb during ambulation. Increased tensile elasticity may encourage a closer coupling between the limb and liner, and thus reduce pistoning. However, increased tensile elasticity may also decrease a liner's ability to conform to the shape of a highly contoured limb or to a limb with bony prominences. In these cases a liner with high tensile elasticity may be uncomfortable to the prosthesis user.

The tensile elasticity test evaluated one rectangular specimen (210 mm [L] × 32 mm [W]) per liner section. Specimens were bonded to a custom test fixture and then stretched to 60% strain or 500 kPa engineering stress, whichever was achieved first. Similar to the shear elasticity test, specimens were not cyclically conditioned, but instead repeatedly stretched until a consistent stress-strain response was produced. Like compression test specimens, tensile test specimens changed in cross-sectional area during testing, so measured engineering stress was converted to true stress under an assumption of incompressibility. Three tests were conducted, and the mean reported.

Coefficient of Friction

CoF characterizes each liner’s ability to adhere to a residual limb. Adherence of a liner to a residual limb is clinically relevant because skin health depends, in part, on the shear stress (tangential stress with no slip) and frictional stress (tangential stress with slip) applied to the residual limb. A liner with a high CoF will enhance mechanical coupling between the prosthesis and limb, and may provide the user with a sense of security. However, a liner with high CoF will also increase shear stresses applied to the limb. Increased shear stresses may be detrimental to skin tissues.^21,22 CoF also affects the shear load that a liner is capable of transmitting before slipping relative to the skin.

The CoF test evaluated one rectangular liner specimen (210 mm [L] × 70 mm [W]) per liner section. Each specimen was tested with three leather specimens (63 mm [L] × 63 mm [W]) since there was some variability since they were a natural leather. Specific to our evaluations, leather samples were installed into the instrument’s test sled using the provided magnetic specimen clamps. The leather samples were then pulled across the liner until three consistent measurements were achieved. Each liner’s CoF was characterized as the mean dynamic CoF across the nine pulls (three pulls for each of the three leather test surfaces). Dynamic CoF, rather than static CoF, was measured because it was found to be a more reliable indicator of a prosthetic liner’s adherence performance.⁴

Thermal Conductivity

Thermal conductivity characterizes each liner’s ability to transfer heat away from the residual limb. Overheating and excessive sweating are two of the most commonly reported issues affecting active prosthesis users, and many users would prefer liners that do not so strongly insulate their residual limb.¹⁷ Conversely, another study²³ reported that 15% of lower-limb prosthesis users experienced issues with their residual limb being too cold. A liner with high thermal conductivity will move more heat away from a warm limb to the cooler external environment. A liner with low thermal conductivity will help to retain heat (i.e., insulate the residual limb), and may be suitable for lower activity users or those who use their prosthesis in a colder environment.

The thermal conductivity test evaluated one circular specimen (51 mm [D]) per liner section. Specimens were placed into the instrument and compressed with a pneumatic piston pressurized to 69 kPa. Specimen thickness was measured with calipers (Model 2416, Mitutoyo, Aurora, Illinois) immediately after installation in the testing jig and again after 15 minutes to ensure that creep was not impacting the test results. Once conditioned, two thermal set points were measured with a contact temperature range of 10–40 deg C. Each liner’s reported thermal conductivity was determined as the mean of these two measurements.

Volumetric Elasticity

Volumetric elasticity characterizes each liner’s stiffness when it is not allowed to expand laterally (flow) under compression. A recent survey found that volume management was the most common problem experienced by persons with lower-limb amputation.²⁴ A liner with a low volumetric elasticity is more capable of adjusting to residual limb volume changes, while a liner with high volumetric elasticity is less capable of accommodating volume changes.

The volumetric elasticity test evaluated three circular specimens (10 mm [D]) per liner section. Before installation into the test fixture, specimens were lubricated and then center-punched with a large knitting needle. This puncture provided a path for trapped air to escape during installation and demonstrably improved measurement results.⁴ Specimens were repeatedly compressed to 500 kPa until a consistent stress-strain response was achieved. The volumetric elasticity reported for each liner was the mean tangent modulus in the linear region of the stress-strain curve (generally between 200 and 500 kPa). The mean volumetric elasticity of the three specimens was reported.

Analysis

Elastomeric liners in the prosthetics industry are typically classified according to their base polymer. There are in general three types of polymers^25–30 – polyurethanes, silicones, and thermoplastic elastomers. Watts (2014) found that for practitioners selecting a gel liner, 92% of them made their selection based on the elastomer polymer.³¹ We grouped the liners so that we could compare test results among the three groups (polyurethanes, silicones, thermoplastics) and by comparing results with prior testing reported in the literature characterize the history of material changes for the three material groups. We also compared individual liners that were of common elastomer but different fabric backing, and we compared liners that were of common elastomer and fabric backing but of different thickness. Results for these latter two comparisons were expressed as a percentage change of the specimen with the higher magnitude material property relative to the specimen with the lower magnitude material property.

RESULTS

Twenty-three commercial elastomer-fabric liners and one room-temperature-curing silicone used for fabricating custom liners (no fabric backing) (Renew Silicone 10, Smooth-On Inc., Macungie, Pennsylvania) were tested using the described methods. The room-temperature-curing silicone material was rated by the manufacturer to have a hardness of Shore 10A; thus, it served a reference by which to evaluate other liner materials. Of the 23 commercial liner products,⁵ were made from a polyurethane base material,⁸ from silicone, and 10 from a TPE. Results from material tests were compared across material groups (i.e., polyurethane, TPE, and silicone) as discussed below.

Compressive, Shear, and Tensile Elasticity

Most of the liners demonstrated a viscoelastic material response as evidenced by an initial, non-linear toe region followed by a linear region in the stress-strain curve (left sides of Figures 2–4).⁴ To create metrics that would facilitate comparison across the greatest number of liner products, compressive, shear, and tensile elasticity were quantified with a single value (tangent modulus) derived from the linear portion of the stress-strain curve. The linear portion of the curve was in a comparable strain range across all liner products for each type of test. The stress or strain ranges used to quantify tangent modulus for each test were determined by the investigators, as described below.

FIGURE 2.

a,b. Compressive elasticity test results. a: Typical stress-strain curve. The strain region in which the compressive elasticity was taken is indicated with a dashed line. b: By material group. Identification numbers are liner codes listed in Table 1. P= proximal. D= distal.

FIGURE 4.

a–c. Tensile elasticity test results. a: Typical stress-strain curve. The strain region in which the tensile elasticity was taken is indicated with a dashed line. b: By group. Identification numbers are liner codes listed in Table 1. P= proximal. D= distal. c: Materials with tensile elasticities below 400 kPa are shown. All of these materials achieved less than 250 kPa true stress at 60% strain, thus their tensile elasticities were defined using Definition 1.

The tangent modulus for the compressive elasticity test was defined between 10–40% strain. Polyurethane liners had a mean stiffness of 300 ± 40 kPa, silicone liners 310 ± 100 kPa, and TPE liners 140 ± 30 kPa. Results from individual liner models are listed in Table 1 and illustrated in Figure 2. In general, polyurethane and silicone liners had comparable compressive stiffness, while TPE liners had lower stiffness.

TABLE 1.

Material Testing Results.

Mat.	Manufact.	Product	Liner Code	Section Thickness (mm)	Compress. Elasticity (kPa)	Shear Elasticity (kPa)	Tensile Elasticity (kPa)	Volumetric Elasticity (kPa)	Poisson Ratio (mm/mm)	Coeff. of Friction (N/N)	Thermal Conduct. (W/m·K)
Polyurethane	Otto Bock	6Y512(NF)	1	2.79 (prox) 4.45 (dist)	290 (prox) 296 (dist)	59.3 (prox) n/a* (dist)	157 (prox) n/a* (dist)	92,600	0.4995	0.51	0.164
	Otto Bock	6Y512	2	3.59 (prox) 5.15 (dist)	347 (prox) 336 (dist)	65.2 (prox) n/a* (dist)	n/a**	128,000	0.4996	0.44	0.145
	Otto Bock	6Y520(NF)	3	4.56	252	59.3	124	173,000	0.4998	0.67	0.170
	Otto Bock	6Y520	4	5.26	318	62.2	216	191,000	0.4997	0.44	0.143
Silicone	Medi	Relax 3C	5	3.12 (prox) 5.96 (dist)	458 (prox) 359 (dist)	104.6 (prox) 79.6 (dist)	353 (prox) 268 (dist)	78,300	0.4991	2.65	0.158
	Össur	Iceross® Comfort	6	5.88	224	41.5	158	147,000	0.4997	2.82	0.171
	Össur	Iceross® Dermo	7	6.19	156	51.2	154	8,360	0.4969	2.18	0.151
	Össur	Iceoross® Original	8	2.11 (prox) 3.81 (dist)	354 (prox) 375 (dist)	84.3 (prox) 84.2 (dist)	232 (prox) 244 (dist)	92,400	0.4993	3.07	0.175
	Össur	Iceoross® Sport	9	2.92 (prox) 3.62 (dist)	263 (prox) 228 (dist)	56.7 (prox) 56.8 (dist)	194 (prox) 173 (dist)	8,785	0.4953	1.95	0.150
	Össur	Iceoross® Synergy	10	3.04	370	76.0	241	8,740	0.4929	1.79	0.122
	Otto Bock	6Y75	11	3.15	384	68.5	286	82,500	0.4992	2.23	0.183
	Prosthetic Design	SealMate	12	2.99 (prox) 6.34 (dist)	186 (prox) 187 (dist)	56.1 (prox) 64.4 (dist)	170 (prox) 199 (dist)	113,500	0.4997	1.95	0.160
	Renew	Shore 10A Silicone	13	5.70	307	81.0	237	208,000	0.4998	2.95	0.181
	WillowWood	Alpha® Si-Se	14	4.36	349	53.6	3,450	111,000	0.4995	1.36	0.146
TPE	Alps	EasyLiner	15	4.00	141	15.2	124	94,700	0.4998	0.81	0.124
	Alps	Extreme	16	3.90	111	16.0	1,510	73,000	0.4997	>3.0	0.144
	Alps	General Purpose	17	3.75	96	16.0	225	77,000	0.4998	>3.0	0.126
	Alps	Winter's Gel	18	5.24	96	21.4	1,560	101,000	0.4998	>3.0	0.130
	Freedom Innovations	Inception	19	5.87	144	39.3	1,554	126,000	0.4998	1.52	0.139
	Otto Bock	6Y92	20	4.49 (prox) 6.57 (dist)	121 (prox) 116 (dist)	24.9 (prox) 22.8 (dist)	136 (prox) 110 (dist)	123,000	0.4998	2.80	0.128
	WillowWood	Alpha® Cl-Mx	21	6.84	134	26.2	145	151,000	0.4999	2.78	0.124
	WillowWood	Alpha® Cl-Or	22	6.33	157	26.1	294	143,000	0.4998	2.79	0.141
	WillowWood	Alpha® Cl-Sp	23	6.39	144	28.0	309	125,000	0.4998	2.75	0.138
	WillowWood	Alpha® Hy-Se	24	6.14	196	22.2	2,280	99,500	0.4997	>3.0	0.130

^*Liner has an anatomic thickness profile, which made it impossible to collect distal specimens of uniform thickness

^**Fabric backing is present, but was not bonded to the elastomer above mid-patellar-tendon. Property is best approximated by liner no.12

The tangent modulus for the shear elasticity test was defined from 20 – 60% strain. Polyurethane liners had a mean shear stiffness of 63 ± 3 kPa, silicone liners 69 ± 17 kPa, and TPE liners 21 ± 5 kPa (Table 1, Figure 3); thus, polyurethane and silicone liners had comparable shear stiffness, while TPE liners had a lower stiffness.

FIGURE 3.

a,b. Shear elasticity test results. a: Typical stress-strain curve. The strain region in which the shear elasticity was taken is indicated with a dashed line. b: By group. Identification numbers are liner codes listed in Table 1. P= proximal. D= distal.

For tensile elasticity, because some products experienced high strains at low stress while others experienced low strains at high stress, it was not possible to create one single definition for tangent modulus. Therefore, we defined the tangent modulus for each product one of two different ways. For liners that achieved less than 250 kPa true stress at 60% strain, the tangent modulus was defined between 10–40% strain (Definition 1). For liners that achieved 250 kPa before reaching 60% strain, the tangent modulus was defined between 100–250 kPa (Definition 2) (Figure 4a). Results showed that tangent moduli for tensile elasticity averaged 170 ± 50 kPa for polyurethane liners, 210 ± 70 kPa for silicone liners, and 1,460 ± 1,790 kPa for TPE liners (Table 1, Figure 4b); thus, on average polyurethane and silicone liners had comparable tensile stiffness, while TPEs had a higher tensile stiffness. However, we noted that for liners that achieved less than 250 kPa true stress at 60% strain (thus, tangent modulus defined between 10–40% strain (Definition1)), stiffnesses were comparable among polymer groups, averaging 166 ± 47 kPa for polyurethane liners, 224 ± 57 kPa for silicone liners, and 192 ± 84 kPa for TPE liners (Figure 4c).

Coefficient of Friction

Static and dynamic CoF could not be measured for all liners, because several liner products had static CoFs that approached the limit of the load cell and risked catastrophic failure of the instrument. Data from other liner products showed that a static CoF of 4.0–6.0 typically resulted in a dynamic CoF of 2.7–2.8. Therefore, a test was stopped if a liner product had a static CoF greater than 7.0. The liner was then characterized as having a dynamic CoF of >3.0. Because of this limitation, ranges of dynamic CoF are reported for each polymer group. Dynamic CoF ranged from 0.4 to 0.7 for polyurethane, 1.4 to 3.1 for silicone, and 0.8 to greater than 3.0 for TPE (Table 1, Figure 5). Thus polyurethane liners demonstrated relatively low CoFs, while silicone and TPE liners had higher CoFs and wider ranges of CoF.

FIGURE 5.

Coefficient of friction test results. Identification numbers are liner codes listed in Table 1.

Thermal Conductivity

Thermal conductivities were comparable among the three types of materials (Table 1, Figure 6). Polyurethane liner results averaged 0.16 ± 0.02 W/m∙K, silicone liner averaged 0.16 ± 0.02 W/m∙K, and TPE liners averaged 0.13 ± 0.01 W/m∙K.

FIGURE 6.

Thermal conductivity test results. Identification numbers are liner codes listed in Table 1.

Volumetric Elasticity

For the volumetric elasticity test the incompressible nature of elastomers meant that the linear region occurred at very low strains (1–5%). These strains corresponded to a stress range of 200–500 kPa (Table 1, Figure 7). Tangent moduli in the linear regions averaged 146,000 ± 44,000 kPa for polyurethane liners, 96,000 ± 58,000 kPa for silicone liners, and 114,000 ± 29,000 kPa for TPE liners. Thus all materials had high bulk moduli thus all were essentially incompressible. In clinical application, in terms of the impact of volumetric elasticity, there is likely little difference in performance among liners from the three different groups.

FIGURE 7.

a,b. Volumetric elasticity test results. a: Typical stress-strain curve. The strain region in which the volumetric elasticity was taken is indicated with a dashed line. b: By group. Identification numbers are liner codes listed in Table 1.

We calculated a Poisson ratio to confirm that the materials were functionally incompressible and to offer insight into any differences in liner products’ tendencies to flow under compressive stresses. The Poisson ratio (PR) was calculated from the volumetric elasticity (VE) and compressive elasticity (CE) results using an equation from the generalized form of Hooke’s law and the definition of the Poisson ratio,

$$\text{PR}=(3\times \text{VE}-\text{CE})/(6\times \text{VE})$$

Results showed that liners tested in the present study had Poisson ratios between 0.493 and 0.4999.

Effects of Fabric Backing Material

Among the materials tested, there were three sets of liner products made from a common elastomer but from different fabric backings (Set A: products #16 and #17 in Table 1; Set B: products #3 and #4; Set C: products #21, #22, and #23), allowing us to test the influence of different fabric backing materials on liner properties. In general, shear elasticity, CoF, thermal conductivity, and volumetric elasticity were relatively unaffected by different fabric backing materials (Table 2). Tensile elasticity, however, demonstrated strong sensitivity (74–571% differences) for all three groups except for liner material #21 and #22 (set C) (5% difference). Compressive elasticity results showed that set B’s compressive elasticity was more sensitive to fabric backing (>20% difference) than Set A’s or C’s (<20% differences). CoF results in Set B (54% difference in dynamic CoF between product #3 and #4) reflected a reduced surface roughness (bumpiness) of the elastomer for material #3 compared with that for #4, an alteration from a difference in manufacturing, not a difference in fabric backing.

TABLE 2.

Sensitivity to Fabric Backing Material

Group	Liners Compared	Base Polymer	Fabric Backings Compared	Percent Difference (absolute value)
				Comp.	Shear	Tens.	Vol.	CoF	Therm.
A	#16, #17	TPE	Brown v. Tan	15%	0%	571%	5%	0%	14%
B	#3, #4	Poly.	Non v. Unknown	26%	5%	74%	0%	54%	19%
C	#21, #22	TPE	Max v. Original	17%	0%	103%	6%	0%	14%
C	#22, #23	TPE	Original v. Spirit	10%	7%	5%	14%	0%	2%
C	#23, #21	TPE	Spirit v. Max	7%	7%	113%	21%	0%	12%

During tensile testing, the fabric backing limited strain to less than 60% at 250 kPa for some products, most of which were TPE polymers. For these products (indicated with bold text in Figure 4), the tensile tangent modulus was defined based on a 500 kPa stress threshold rather than the 250 kPa stress threshold. The fabric backing stiffened and contributed to liner tensile elasticity at high strains for those products, and thus was primarily responsible for the heightened tangent modulus.

Effects of Liner Thickness

Two specimens from the same base material but with different thicknesses were tested from six liner products (#2,8,9,11,12,20). The difference in thickness between specimens was due only to the thickness of the elastomer. Liner #5 was not included because it did not have a consistent fabric backing thickness; it was composed of two layers of fabric at the distal end and a single layer of the same fabric at the proximal end.

Comparison of compressive, shear, and tensile elasticities demonstrated relatively low sensitivity to thickness (all differences <25%) (Table 3). The percent differences listed in Table 3 were calculated: ((higher value – lower value)/lower value). CoF, volumetric elasticity, and thermal conductivity were not tested for both samples since they were expected to be unaffected by changes in liner thickness.

TABLE 3.

Sensitivity to Liner Thickness

				Percent Difference (absolute value)
Liner Product	Base Polymer	Low thickness sample (mm)	High thickness sample (mm)	Comp.	Shear	Tens.
#8	Silicone	2.11	3.81	6%	0%	5%
#9	Silicone	2.92	3.62	15%	0%	12%
#1	Poly.	2.78	4.45	2%	N/A	N/A
#2	Poly.	3.59	5.15	3%	N/A	N/A
#20	TPE	4.49	6.57	4%	9%	24%
#12	Silicone	2.99	6.34	1%	15%	17%

N/A – Thickness differences were anatomically based and thus small in size, thus it was only possible to extract thick/thin specimen sets for compressive elasticity testing

DISCUSSION

In the discussion below, we place results from the present study within a historical perspective in an effort to highlight changes made to elastomeric liners by the prosthetics industry over the past two decades and to highlight future research needs. Clinical implications of the material property results are discussed.

Similar to Covey et al. (2000),³² Emrich and Slater,³³ Sanders et al.,³⁴ and Klute et al.,³⁵ we chose to test elastomeric liners that were popular clinically at the time of the study. We used results from a recent survey of prosthetists to select most of our liner products.2 Though most of the product names differed between prior and present studies, liners in each of the referenced studies were made from one of three base polymers: polyurethane, silicone, or TPE (Appendix 1).

Compressive, Shear, and Tensile Elasticity

Covey et al. (2000) measured liner products’ combined compressive and volumetric response by selectively constraining the lateral expansion (flow) under compressive stresses.³² This test method is challenging to compare directly with the current test methods since there are two independent responses combined into one. However, consistent with Covey et al.’s results from 2000, we found that silicone and polyurethane liners were relatively stiff, while TPE liners had the softest compressive elasticity. It is important to note that the liner product Covey labeled “Silicone B” was ALPS EasyLiner, which is a TPE.

Direct comparison of compressive, shear, and tensile test results in the present study with those from Sanders (2004) is possible, since curve fit coefficients were provided in Sanders’ publication.³⁴ We performed the same modulus calculations as in the present study using data from the prior study. Results showed lower compressive and shear stiffness for polyurethane liners in the present study compared with the 2004 investigation (i.e., polyurethane liners are generally now more stretchy) though we note that only one polyurethane liner was tested previously (Figure 8a–c). TPE liner tensile stiffnesses were higher in the present study than in 2004, while TPE compressive stiffness and shear stiffness results were comparable. We note in both studies that compressive and shear stiffnesses of silicone liners tended to be more similar to those of polyurethane than to TPE.

FIGURE 8.

a–c. Compressive, shear, and tensile elasticities – comparison of present study and prior study results. Present study results are shown using light color bars, and prior study results³⁴ using dark color bars. Each horizontal line represents one liner.

Coefficient of Friction

Our dynamic CoF results are not consistent with those from Emrich et al. (1998), who also tested prosthetic liner frictional characteristics.³³ Emrich found similar dynamic CoFs when comparing polyurethane and silicone (range of 0.76–1.01 for polyurethane liners, and 0.60–1.06 for silicone liners). In the present study, we found that polyurethanes had lower dynamic CoFs than silicone (range of 0.4–0.7 for polyurethane liners and 1.2–>3.0 for silicone liners). Thus, unlike 17 years ago when the need for a very sticky liner would result in the choice of polyurethane over silicone, presently the choice would be silicone over polyurethane.

We note that, in general, CoFs of contemporary liners were relatively high compared to those from Emrich’s tests. Of the 24 products tested in the present study, only five had CoFs less than 1.1.

Thermal Conductivity

Our thermal conductivity results are comparable to those from Klute et al. (2007) and Webber et al. (2014) who measured thermal conductivities without applying compression to the liner material.^35,36 Klute reported thermal conductivities ranging from 0.11–0.27 W/m·K, while results from the present study ranged from 0.12–0.19 W/m·K. Since values from the present study did not exceed the range of those reported in the prior study, compression during thermal conductivity testing did not appear to alter results appreciably. The slightly higher thermal conductivities measured by Klute et al. may be due to inclusion of thermal grease in their experimental protocol. Use of thermal grease has been shown to slightly increase thermal conductivity in prosthetic liners.⁴ To put these thermal measurements in perspective, elastomeric liners have a thermal conductivity higher than sheep wool (0.04 W/m·K) and comparable to leather (0.14 W/m·K). Tightly fitting elastomeric liners move heat almost exclusively via conduction, and their low thermal conductivities dominate any capability for cooling a warm residual limb in a room temperature environment.³⁷ While it would be challenging to increase the conductivity of the base elastomers, some groups have explored the use of air channels within sockets³⁶ and the use of vacuum to remove perspiration.³⁸

Volumetric Elasticity

Unlike the above material characteristics, volumetric elasticity (bulk modulus) was not reported in any scientific literature testing prosthetic liners. However, the findings here help to put in perspective the term “flow,” used clinically in the prosthetics industry to describe a liner’s ability to deform away from areas of high pressure to areas of low pressure.^25–30 All materials tested here were essentially incompressible (i.e., a Poisson ratio of 0.493–0.4999), which means that their ability to flow reflected their compressive stiffness. Compressive stiffness can be thought of clinically as characterizing a liner’s ability to flow, thus allowing a quantitative metric (compressive stiffness) to be used to describe this quality.

Changes in Liners over the Last 15–20 Years

The comparisons discussed above indicate that liner properties have changed in the last 15–20 years. For example, changes to polyurethane liners that allow them to stretch more (i.e., increased compression and shear elasticity) and be less adherent may simply reflect the clinical desire for a less stiff liner. More than 50% of the people in our survey stated that “cushion” was a property they looked for in the liner (5th-highest priority). It is noted that Covey and Sanders each tested only one polyurethane liner, since it was the only commonly-used polyurethane liner at the time. Manufacturers may have reduced CoF simultaneously with increasing compression and shear elasticity to limit the potential for mechanical failure. However, this interpretation is conjecture and rigorous mechanical testing of liner durability would be needed to evaluate this interpretation. The tensile stiffness of some TPE liners have been increased, perhaps by using stiffer fabric backings, a result that may reflect in part an effort to make them more durable and less prone to failure. A wide range of properties for silicone liners have been maintained from 2004 to the present study, the only polymer group that did not show substantial differences.

Of the properties tested, tensile elasticity was most strongly affected by the nature of the fabric backing material (Table 2). Results from the present study demonstrate that liner manufacturers have the flexibility to adjust a liner’s overall tensile stiffness according to the properties of the fabric backing. Some manufacturers claim “no stretch” fabrics or describe a “stabilizing matrix,” both of which are desirable clinically because they limit axial stretch and reduce pistoning. Excessive pistoning is considered clinically unfavorable because it may induce shear stresses that contribute to abrasions or skin breakdown.²² Results from the present study showed that tensile stiffness could be increased by more than 200% by changing to a stiff fabric backing. Sanders et al. (2004) did not demonstrate a dependence of tensile elasticity on fabric backing,³⁴ presumably because either liners at the time did not have stiff fabric backings or the maximum applied tensile stresses (80 kPa) were too low for an effect to be observed.

Unlike the choice of fabric backing stiffness, liner thickness was not a strongly influential variable affecting mechanical properties. The slight changes we did see (Table 3) may reflect changes to the ratio of elastomer to fabric backing. In other words, the thickness of the elastomer differed between the thinner and thicker liner samples but the thickness of the fabric backing was kept the same.

Clinical Implications

Changes in mechanical properties of prosthetic liner products over time demonstrate the importance of practitioners keeping up to date on new liners as they enter the market. Information that practitioners learn about prosthetic liners during their clinical education or early in their clinical careers is likely to change as materials science, manufacturing methods, and clinical practices evolve. As such, practitioners may be challenged to stay up-to-date on the relative performance of the broad range of commercially available prosthetic liners. Manufacturers would advance practitioners’ knowledge by conducting standardized material property tests⁴ and disseminating results upon introduction of a new product. Professional societies supporting such testing as standard practice in the field would also help practitioners to have a complete understanding of available products. We created a web-based resource, termed the Prosthetic Liner Assistant (PLA) [www.LinerAssist.org],³⁹ to present data collected in this study to practitioners. The PLA is an interactive database (Appendix 2) that provides relative material property information across the different tests reported here. Results in the PLA are presented on a relative scale so that practitioners can assess new liners relative to those products with which they are familiar, rather than according to the quantitative results (i.e., specific mechanical property values) reported in this study. Clinical descriptions of materials properties listed in the PLA are also included to help practitioners understand the influence of each on a liner's performance. As an example, the PLA description for compressive elasticity is shown in Figure 9. Extension of this clinical resource to include new liners (or other existing liners) would facilitate practitioners’ ability to stay informed and select confidently among the many products available on the market.

FIGURE 9.

Example of a PLA material property description – compressive elasticity. The PLA converts material properties like compressive elasticity to a normalized scale of 1–5 to provide practitioners a quantitative metric to build a practical knowledgebase. A description of the measure and its clinical importance is included.

Future Studies

Liner samples tested in the present study were new. They had not been subjected to extended wear and cleaning practices liners typically undergo in the field. Regular use reduced the thickness of a related material, prosthetics socks, by about 25% within a month but had minimal impact on material properties9. It is unclear to what degree clinical use affects elastomeric liner material property results. A needed next step in prosthetics liner research is to quantify mechanical properties of used liners, and determine if the trends reported here are maintained. It would also be helpful to conduct all tests at close to body temperature rather than room temperature.

CONCLUSION

Material properties of prosthetic liners meet the clinical needs of contemporary patients and clinical practices. Polyurethane and TPE liner properties have been changed over the past 16 years – polyurethane is softer and less sticky, TPEs have higher tensile stiffness. In today’s liners, fabric backings contribute primarily to tensile elasticity (and thus pistoning) and to a lesser extent compressive elasticity. All materials tested were essentially incompressible so their ability to flow reflected their compressive stiffness. The high bulk moduli and low thermal conductivities of all liners tested indicated that future efforts should focus on compressible materials and thermally conductive materials for people with limb loss using prosthetic limbs.