Targeted Muscle Reinnervation (TMR) & Regenerative Peripheral Nerve Interface (RPNI)

When a peripheral nerve is transected (as in amputation or nerve injury) and the regenerating axons have no distal target to reinnervate, they sprout into a disorganised tangle - a neuroma - which forms a hypersensitive free nerve ending that can cause debilitating residual-limb (stump) pain and neuropathic symptoms. Amputees may additionally experience phantom limb pain. Together these are a major cause of pain, poor prosthesis use and disability after amputation, and no traditional technique has given consistent relief.

In TMR, the cut major (mixed) nerve is coapted (transferred) to a small, expendable motor nerve that supplies a nearby muscle (which is thereby reinnervated). This gives the regenerating axons a target to reinnervate ('somewhere to go and something to do'), so they do not form a neuroma. Originally developed to create additional myoelectric control sites for advanced prostheses, TMR was then found to prevent and treat neuroma and phantom limb pain as well. It is performed by (or with) a surgeon experienced in nerve transfers, commonly at the time of amputation or as a secondary procedure.

In RPNI, the transected nerve end is implanted into an autologous free skeletal muscle graft (a small, devascularised piece of muscle that revascularises and is reinnervated by the implanted axons). The graft provides the axons with a target - preventing neuroma formation - and acts as a biological signal transducer that amplifies neural signals for prosthetic control. Because it uses a small free muscle graft rather than a nerve transfer, RPNI is technically simpler in some respects and can be used for multiple individual nerves/fascicles.