Peripheral Machine Interfaces
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Current Prosthetics The present Functional Prosthesis can be divided into two categories: Body-powered prostheses and myoelectric prostheses. Body-powered Prostheses: Body-powered prostheses, sometimes called conventional prostheses, are the most durable prostheses and have higher sensory feedback than myoelectric prostheses. However, body-powered prostheses are less cosmetically pleasing than a myoelectric unit, and they require more limb movement as a whole. A body-powered prosthesis is powered and controlled by gross body movements. These movements, usually of the shoulder, upper arm, or chest are captured by a harness system which is attached to a cable that is connected to a terminal device (prosthetic hook or hand). For a patient to be able to control a body-powered prosthesis he or she must possess at least one or more of the following gross body movements: Glenohumeral flexion Scapular abduction or adduction Shoulder depression and elevation Chest expansion Also necessary are: Sufficient residual limb length Sufficient musculature Sufficient range of motion Myoelectric Prostheses: Prostheses that are operated by myoelectricity may give more proximal function and increased cosmetic appeal, but they are heavy and expensive. They have less sensory feedback and require more maintenance than body-powered prostheses. Myoelectric control uses the electrical signals generated by muscle contraction (electromyogram-EMG) of the residual limb with recorded activity from peripheral trunk nerves (electroneurogram-ENG) to control input for a prosthesis controller. They function by transmitting electrical activity that the surface electrodes on the residual limb muscles detect to the electric motor. Two types of myoelectric units exist: The 2-site/2-function device and the 1-site/2-funtion device. The 2-site/2-function device has separate electrodes for flexion and extension. The 1-site/2-function device has one electrode for both flexion and extension. The patient uses muscle contractions of different strengths to differentiate between flexion and extension. For example, a strong contraction opens the device, and a weak contraction closes it.
 
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BME 240
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