Redesign and validation of a handheld tribometer to determine the coefficient of friction between the prosthesis and the residual limb of people with a transfemoral amputation

A new design of a handheld tribometer that allows measurements on different anatomical regions is proposed. More specifically, within this research, the device will be used to measure several body regions of the residual limb of transfemoral amputees. The device assesses the normal load through a load beam cell and the friction force through the motor torque. The device was validated using a gauge repeatability and reproducibility study and a 2-variance test. For the first validation test, non-biological materials were used and a sensitivity analysis of the different degrees of freedom was conducted (i.e. operators, parts and materials). Including all operators, parts and replicates, the part-to-part variability was 89.14% with the ANOVA method. After elimination of the most variable observers, the part-to-part variability could be enhanced to 97.94%. For the second validation, measurements of polypropylene against the skin were used. The variance ratio between the Coefficient of Friction (COF) with the presence and the absence of sweat was statistically significant for all anatomical regions where measurements were taken (0.012 ≤ p − value ≤ 0.045). More specifically, the COF variances were larger in the presence than in the absence of sweat. Consequently, the new handheld tribometer can be used for in vivo measurements of the COF with any tribological pair where the skin is involved. However, previous training is recommended for the person conducting the tests, together with taking extra measurements to discard atypical values (i.e. values increasing the variance) resulting from the intrinsic variability due to the portability of the device.