A healthy individual is said to have a bilateral temperature difference of no larger than 1°C between the upper extremities and core regions of the body.1,2 In general, people with amputations have a skin temperature asymmetry exceeding 1°C, indicative of a potential pathophysiology. The mechanism of this temperature asymmetry after limb amputation is unknown.24,25
We verified our method to measure skin temperature asymmetry by confirming previous findings of larger than 1°C difference in people with unilateral amputations wearing conventional socket prostheses. We found the residual limb on average 1.49°C (SD = 0.84) colder than the contralateral, which is in agreement with previous work reporting an average difference of 1.1°C.6
Using the same thermal imagining technology, we observed a shift towards bilateral temperature symmetry in users of neuromusculoskeletal prostheses. Temperature asymmetry in these participants was less than half of those without the neuromusculoskeletal interface investigated in here and in work by others.6 More importantly, removing the neuromusculoskeletal prosthesis exacerbated the asymmetry to values similar to the participants using conventional prostheses, thus indicating causality. Interestingly, the changes in temperature after donning and doffing of the neuromusculoskeletal prostheses were not immediate. In preliminary experiments, we attempted to capture such changes by taking thermal pictures every five minutes during 30 minutes after removing or wearing the prosthesis but found no measurable change. We therefore decided to utilize spans of six hours.
The neuromusculoskeletal interface allows for more reliable, responsive, and precise prosthetic control than conventional non-invasive technologies.26 People with neuromusculoskeletal prostheses have shown improved grasping function, particularly under uncertainty, thanks to the provision of somatosensation elicited via direct neural stimulation,27 which lacks in conventional prostheses. The difference in control and somatosensation between neuromusculoskeletal and conventional prostheses results in a higher degree of agency and ownership,14 both conclusive to prosthetic embodiment.28 In addition, the superior comfort provided by direct skeletal attachment via osseointegration, as opposed to socket suspension, allows for patients to wear their prosthesis all day and every day.10,11,29 Overall, living with such an integrated human-machine interface has shown to have positive social and psychological effects in the users, who see these prostheses as part of the bodies.14 This embodiment of the prosthesis could be a potential reason for the reduction in temperature asymmetry, as subconscious changes in the body representation have been thought to cause an optimization of autonomic efforts to supply blood to parts of the body no longer present. Restoration of the body representation by an embodied prosthetic limb might reverse said optimization restoring in turn the temperature in the residual limb. We therefore propose skin temperature asymmetry as an objective biomarker for artificial limb integration.
Despite the improvements reported by neuromusculoskeletal prostheses, this technology is still far from equating a biological limb. At present, they provide limited control and sensory feedback,30,31 which might explain why there is still certain level of temperature asymmetry even in users of thes more integrated prostheses. Further improvements to this, or any other technology for artificial limb replacement, could be potentially evaluated by the symmetry of skin temperature.