Targeted Reinnervation: A Functioning Neural-Machine Interface for
Improved Myoelectric Prosthesis Control
Kristina Dare O’Shaughnessy, MD, Todd A. Kuiken, MD, PhD, Gregory A.
Dumanian, MD
INTRODUCTION : Traditional upper extremity myoelectric prostheses require uncoordinated single motion operation of the elbow, wrist, and terminal device that must be performed sequentially. Amputees find these systems slow, difficult to operate, and unnatural.
Targeted reinnervation, an innovative surgical technique, involves nerve transfers of the residual amputated peripheral nerves to otherwise functionless muscle segments on the limb or chest, thereby creating new myoneurosomes which act as biologic amplifiers for the signals still contained within the peripheral nerve. These unique concepts permit the simultaneous control of multiple limb functions resulting in additional degrees of freedom.
METHODS : A prospectively analyzed cohort of five patients with a mean follow-up of 24 months (range 6-48 months) and an average age of 47 years (range 25-56 years) underwent targeted reinnervation in a single operation using two to four peripheral arm nerves anastomosed to arm or chest muscle segments. Patients with transhumeral amputations underwent median and radial nerve transfers to the medial head of the biceps and brachialis, respectively. Patients with shoulder disarticulations underwent median, radial, ulnar, and musculocutaneous nerve transfers to the two heads of the pectoralis major, to the serratus anterior, and to the pectoralis minor muscle.
In addition to these motor nerve transfers, one of these 5 patients received end-to-side sensory nerve transfers using the supraclavicular and intercostal brachial nerves to the median and ulnar nerves in order to bring the feeling of hand sensibility to the chest skin.
RESULTS : Four of five patients had successful nerve transfers based on both visualization and tactile feedback from the contracting muscle. Successful nerve transfers were also validated by electromyography. Outcome measures include a standardized box-andblocks test, clothes-pin moving test, and a validated assessment of the patient's performance with Activities of Daily Living (ADLs).
Performance in all three categories improved significantly in these four patients when compared to preoperative results. Furthermore, these patients expressed preference for the new myoelectric prosthesis due to speed, ease of operation, and the ability to perform multiple extremity functions simultaneously. Both patients with shoulder disarticulations have gained hand sensibility on their chest skin. With new unconventional devices, a feedback has been developed, allowing the patients to know how hard they are closing the terminal device of their prosthesis by graded stimulation of the chest skin.

CONCLUSION : Targeted muscle reinnervation to create new myoneurosomes is a novel development for improved myoelectric prostheses control.
The ability to use muscles as biological amplifiers of residual peripheral arm nerves that would otherwise remain functionless has offered amputees simultaneous control of multiple limb functions.
Through surgically created myoneurosomes, the preexisting neural inputs to these residual nerves function identically to control the prostheses as they did to control the native arm. In addition, targeted sensory reinnervation completes the neural interface loop, thereby allowing afferent sensory feedback from the prosthesis back to the part of the brain that interprets hand sensation.
Figure 1.
A. An intact native myoneurosome and large residual peripheral arm nerve with end neuroma formation.
B. The small motor nerve to the native myoneurosome is cut 1 cm from its entry point into the muscle and the peripheral nerve neuroma is excised back to healthy fascicles.
C. The large peripheral nerve is secured to the muscle segment, covering the original motor nerve at its entry site, thereby creating a new myoneurosome.

Figure 2.
Shoulder Disarticulation Patient #1: Schematic diagram illustrating targeted reinnervation and the creation of four new myoneurosomes.
The sternal head of the pectoralis major is divided into an upper and lower segment for transfer of the median and radial nerves, respectively. The musculocutaneous nerve is transferred to the clavicular head, while the ulnar nerve is transferred to the pectoralis minor muscle.