Overview:

The majority of myoelectric terminal devices utilize cylindrical and tip prehension in a single geometrical configuration. This position is reasonable as it combines the two dominant prehensions used by nondisabled people. It does not account, however, for lateral prehension, which in nondisabled people accounts for 33 percent of all pick up prehension. Often overlooked, lateral prehension is especially critical for non-manual labor. As the population of America ages, more and more patients that do not typically do manual labor receive transradial amputations. These patients often need to perform tasks that require lateral prehension, such as turning a key, or more importantly holding a book or a piece of paper.

The principle novel concept of the Dual Mode Prosthetic Terminal Device (DMP-TD) is that it incorporates both tip/cylindrical and lateral prehension. This feature allows the user to continue to use the terminal device for manual labor and other tasks that require tip/cylindrical prehension, but also gives the user the ability to switch modes such that the prosthetic device may then be used for lateral prehension. This is a proof of concept design, and as such not concerned with power consumption, weight, maximum exertable torque, or cosmetic design. Rather, it focuses on creating a fluid transition between the two modes, and the ability to reasonably mimic the motion of a nondisabled hand in those two prehensions.

In our initial design input for the system came from electromyogram (EMG) activity measured by surface electrodes. After signal processing, these would be fed into the Handyboard, which would process them and send signals, amplified in a post-processing circuit, to one or more of three servo motors that control prehension. The EMG circuit was unfortunately never implemented given the time frame of the project because of problems filtering 60 Hz noise.

The prehension portion of the TD consists of five bodies: one palmer box (H1), which contains the servomotors, one finger body (H2), and two thumb bodies (T1+T2, T3). Rubber bands control contraction of H2, the servomotors control flexion, and a tendon system composed of pulleys and wires translates the mechanical motion from the palmer box to the fingers.

The hand operates in two different modes: In the first mode the thumb is pronated as far as it will go, and remains stationary during both extension and contraction. The H2, arched in a cylindrical position, touches the thumb at the tip, creating a surface contact between the two parts. Extension affects H2, which extends. In the second mode of operation, the thumb is slightly supinated and rotated superiorly such that it is in contact with the side of H2, again creating surface contact. A way to visualize this position is to think of a human hand in the position it would be placed to hold a key, to turn it in a lock. When the TD is extended, H2 moves in the same manner as it did in the first mode. However, in this mode the thumb rotates supinely from the carpel joint, near the wrist.