Evaluation of Myoelectric Implantable Recording Array (MIRA) in Participants With Transradial Amputation

A well-documented challenge with current myoelectric devices is the common use of only two surface EMG electrodes to record control signals, typically one on each of the extensor and flexor muscle compartments in the residual limb. With this electrode configuration, just a single motion can be controlled at any given time. Changing to new motions or grasp patterns typically requires a secondary switching method. This sequential control enables pre-programmed functional grasps, and can be selected by patterns of co-contraction of antagonist muscle groups. However, this approach has limited success due to the slow and non-intuitive nature of the required muscular activations. For example, contraction of flexor muscles may be used to close the hand, but in order to rotate the wrist, a co-contraction of the flexors and extensors (to switch control modes) followed by flexor contraction is required. This non-intuitive approach ignores the normal function of forearm muscles and has a further disadvantage that simultaneous control of hand aperture and wrist rotation is impossible. Adding additional grasping patterns can make the problem worse and often requires the user to step through the different pre-programmed patterns by making multiple co-contractions until the correct movement is selected. Surface electrode-based systems also rely on strong muscle contractions, which are inefficient and contribute to awkward usability and ultimate rejection of the prosthesis. Other complications that are associated with surface electrodes are signal changes due to environmental conditions such as sweating, daily changing of electrode locations due to skin-related issues, and susceptibility to crosstalk and movement artifacts. In contrast, implantable electrodes are protected from environmental conditions, remain fixed in place, can record small signals from gently contracting muscles, and can record many different signals to potentially allow for more complex and simultaneous control of multiple joints.

Recent research with myoelectric control has focused on developing new control paradigms, such as simultaneous multi-degree of freedom control and proportional velocity control. Often, surface EMG recordings from an antagonistic pair of muscles are used to control a single degree of freedom. Implantable electrodes have shown promise as an alternative method in preliminary human subject studies, and recent research has demonstrated that up to three degrees-of-freedom have been simultaneously controlled in a virtual environment using intramuscular electrodes, as well as proportional velocity control. The Implantable Myoelectric Sensor (IMES®) System utilizes eight implanted electrodes (six actively needed for control, two as back-ups) to control three degrees-of-freedom. Another study using the same EMG leads and electrode configuration as MIRA, implanted percutaneously, enabled simultaneous control of up to 6 degrees of freedom (thumb flexion, index flexion, middle flexion, thumb adduction and abduction, wrist flexion and extension, and wrist pronation and supination) of an advanced prosthesis. As myoelectric prostheses move away from preprogrammed movements and pattern recognition, a minimum of two electrodes implanted in separate muscle targets appears to be necessary to control a single degree of freedom.