Development of Standardized Material Testing Protocols for Prosthetic Liners

Abstract

A set of protocols was created to characterize prosthetic liners across six clinically relevant material properties. Properties included compressive elasticity, shear elasticity, tensile elasticity, volumetric elasticity, coefficient of friction (CoF), and thermal conductivity. Eighteen prosthetic liners representing the diverse range of commercial products were evaluated to create test procedures that maximized repeatability, minimized error, and provided clinically meaningful results. Shear and tensile elasticity test designs were augmented with finite element analysis (FEA) to optimize specimen geometries. Results showed that because of the wide range of available liner products, the compressive elasticity and tensile elasticity tests required two test maxima; samples were tested until they met either a strain-based or a stress-based maximum, whichever was reached first. The shear and tensile elasticity tests required that no cyclic conditioning be conducted because of limited endurance of the mounting adhesive with some liner materials. The coefficient of friction test was based on dynamic coefficient of friction, as it proved to be a more reliable measurement than static coefficient of friction. The volumetric elasticity test required that air be released beneath samples in the test chamber before testing. The thermal conductivity test best reflected the clinical environment when thermal grease was omitted and when liner samples were placed under pressure consistent with load bearing conditions. The developed procedures provide a standardized approach for evaluating liner products in the prosthetics industry. Test results can be used to improve clinical selection of liners for individual patients and guide development of new liner products.

Fig. 1

Instruments used for testing: (a) uniaxial material testing machine, (b) planar friction tester, and (c) guarded heat flow meter

Fig. 2

Stress concentrations in shear elasticity testing. Stress concentrations at the upper and lower specimen edges were predicted to be the greatest source of error. Height was optimized first, because adjustments changed the proportional area of influence within the 2D profile. Width was optimized second, because the area of influence was consistent in the extruded profile.

Fig. 3

Tensile elasticity and volumetric elasticity test configurations: (a) tensile fixture with adhesive fixation. Left-side specimen mounts were fixed in place, while right-side specimen mounts were attached to a linear slide rail, allowing the specimen to thin while under tension. (b) bulk test fixture with air pocket. Air pockets were removed by center-punching with a large knitting needle, the eye of the needle created a gap that provided a low resistance path to outside of the well.

Fig. 4

Compressive elasticity–parameter assessment: (a) the most compliant liner achieved 102 kPa at 60% strain, while the stiffest liner achieved 200 kPa at 47% strain. Averages for 18 liners specimens evaluating (b) strain rate and (c) specimen diameter.

Fig. 5

Compressive elasticity–effects of lubricants. Examples from two lubricants tested on the softest liner material (TPE): outlast synthetic oil (left) HAAS automation oil (right). Upper plots show overlayed stress–strain curves at nine load cycles and lower plots show displacement over time.

Fig. 6

Shear elasticity–error assessment: (a) the influence of stress concentrations was minimized as length-to-thickness ratio was increased to 20, (b) load measurement error for the chosen specimen size decreased as linear modulus increased, and (c) experimental data showing an adhesive failure that began to peel at 15% strain and delaminated at 55% strain

Fig. 7

Tensile elasticity–error assessment: (a) adhesively mounted specimens minimized measurement error and increased the achievable maximum strain, (b) load measurement error decreased relative to specimen length, and (c) decreased with increased specimen stiffness at a length of 200 mm

Fig. 8

Coefficient of friction. Single liner model variability for (a) static and (b) dynamic friction. (c) mean and standard deviations of three liner models when measuring dynamic friction.

Fig. 9

Volumetric elasticity–error assessment. Venting specimens significantly reduced the impact of air.

Fig. 10

Thermal conductivity–error assessment. The absence of thermal grease resulted in a consistent change in thermal conductivity.

Fig. 11

Materials comparison. Ashby plot of the materials contained in a human prosthesis system.