Lower limb amputees suffer complete lack of sensory feedback of current available prostheses, which excludes the central nervous system from the correct sensory-motor integration. It causes serious problems as: falls due to unexpected perturbations, asymmetric walking, low mobility, higher power consumption, feeling the prosthesis as a foreign body, high cognitive burden, and phantom limb pain occurrence.

Investigators will provide amputees with a prosthesis restoring sensory feedback and will assess its benefits on the users.

The clinical investigation described here aims at evaluating the benefit of a lower limb prosthesis providing sensory feedback (bidirectional prosthesis), in selected transfemoral amputees. The device is constituted by i) Rheo Knee XC, Pro Flex foot and customized socket/liner structure from OSSUR. Rheo Knee embedded already an encoder coupled with a Bluetooth unit, which is used to make communication with devices external to the knee itself; ii) nerve stimulating system constituted by implantable intraneural electrodes from IMTEK, iii) an external neurostimulator from AXONIC, and iv) sensorized sole from SensArs Neuroprosthetics to apply under the prosthetic foot and driving the stimulating system. The readout of the sensorized insole are transmitted via Bluetooth, together with the encoder readout, to an external controller, which transduces it in parameters of stimulation. These parameters are sent to the stimulator, which injects current into the intraneural electrodes. As final result, the subject perceives sensory feedback from the prosthesis when using it.

Investigators call SENSY the sensory feedback restoration system. The subjects will use the prosthesis with and without sensory feedback to execute walking tasks. The average change of kinematics (including falls), metabolic cost, embodiment, and cognitive burden (EEG) when intraneural stimulation is provided will be compared to the case in which there is no sensory feedback. Standard clinical tests (as PEQ and CB&MS) will be executed with and without sensory feedback. Also, when the subjects will report attacks of phantom pain, they will undergo a 10-minutes-stimulation session. Only stimulation without prosthesis is used in this intervention.

Finally, imaging of the cortical activity will be executed through functional magnetic resonance imaging (fMRI). This procedure will be only observational since no prosthesis or sensory feedback will be used by the subjects.

Inclusion Criteria:

• Uni-lateral transfemoral amputation above the knee level
• Other treatments for phantom limb pain should have been tried with poor results
• The subject should experience phantom limb pain at a level of 6 or higher measured on on a visual analog scale (VAS) ranging from 0-10
• Phantom limb pain should be experienced at least once a week
• The subject should be in a chronic and stable phase, and the stump should have healed
• The subject should otherwise be healthy and able to carry out the experiment
• If pain medication is used it will be acceptable that the person continues to use the medication

Exclusion Criteria:

• Cognitive impairment
• Pregnancy
• Prior or current psychological diseases such as borderline, schizophrenia, depression or maniodepression
• Acquired brain injury with residual impairment
• Prior neurological or musculoskeletal diseases
• History of or active substance abuse disorder
• Excessive sensitivity to electrical stimulation with surface electrodes
• Persons with fear for electrical stimulation, pain cannot participate
• Persons that are hypersensitive to electrical stimulation and experience the stimulation as unpleasant cannot participate
• Since the protocol includes MRI scanning of the brain, persons that may feel claustrophobic cannot participate