Accurate and Repeatable Identification of Mechanical Innocuous and Pain Thresholds Using a New Mechatronic Testbed

The restoration of mechanical somatosensory functions in amputees using instrumented prosthetic hands requires realistic feedback, grounded on the accurate knowledge of the human physiological mechanical innocuous and painful thresholds. Multiple technologies are commercially available to identify these thresholds. However, they have low repeatability and accuracy due to intrinsic limitations in the delivery of the stimulation force, which is either discrete or continuous but non-automated, as a result of a manual application of the stimulus. To overcome these issues, a new mechatronic testbed has been developed, that consists of a high-resolution positioning system and an automated mechanical stimulator. The testbed can deliver continuous stimulation forces with a positioning and force resolution of 0.5 $μ m}$ and 1 mN respectively, and includes a small circular stimulation tip $(0.8 mm})$. The developed testbed was employed to determine the mechanical innocuous and painful thresholds on the hand and the forearm of 12 healthy volunteers. No statistically significant difference was observed when comparing the results from the two stimulated anatomical sites. Furthermore, a statistically significant difference was found between innocuous sensation and pain on both sites, highlighting the importance of investigating accurately the range of force to appropriately discriminate thresholds. A direct comparison of our results on innocuous thresholds with literature data highlighted the difference between our continuously increasing force delivery and the discrete stimulations pursued with standard devices. Our painful thresholds were found to be consistent with literature results also confirming the dependency of the values from the dimension of the stimulation surface.