Phase 2: The subject will return to one of the three centers for Targeted Muscle Reinnervation (TMR) surgery. The surgery is an outpatient procedure that will be performed under general anesthesia, consistent with current standard of care. The surgery will involve the transfer of the median nerve to the flexor digitorum superficialis (DFS) muscle and the ulnar nerve will be transferred to the flexor carpi ulnaris (FCU) muscle. Both transfers can be done with a single incision. Alternative target muscles (e.g. the brachioradialis muscle) can be used if trauma interferes with use of the DFS and/or FCU. Some post-operative pain is expected and the subject will be given a prescription for pain medication if needed. Two weeks after the surgery the subject will be asked to start doing exercises to help with muscle recovery. They will return to using the prosthesis from Phase 1 with either conventional or pattern recognition control for 6 months after surgery. Socket adjustments will be made if needed. If they return to using pattern recognition control, their usage and performance data will be tracked during this 6 month period. The subject may be provided with a laptop computer and asked to perform a series of virtual tests. They will receive additional training on how to complete these tests. If the subject returns to using conventional control, they will not be asked to complete the additional tests, nor will the usage or performance data be tracked. Six months after surgery, when reinnervation of the target muscles has been completed, the subject will return to using the prosthesis with pattern recognition control. They will be asked to return to one of the three centers if socket adjustments or a replacement socket is needed. The subject will receive 1-3 days of pattern recognition control training, only this time special attention will be paid to utilizing EMG from target muscles for improved function. Preliminary testing will be done to ensure that they can adequately use the prosthesis. The subject will then take the prosthesis home for a third 8-week home-use trial. The subject will be asked to log daily use of the device, note any issues, and follow up weekly with a therapist. After this home-use trial, the same 1-3 days of comprehensive outcomes testing will be completed and usage data from their prostheses will be downloaded.