Terminal devices

1. What are the advantages and disadvantages of a passive TD?
2. What is a three-jaw chuck pinch?
*3. What is the difference between a voluntary-opening and voluntary-closing terminal device?
4. What are externally powered TD?
*5. What are the two types of prosthetic wrists?

Answers:
1. Lighter, but with no functional mechanism and no grasp, intended for cosmetics only. Flexible passive TD may absorb shock.
2. Grip with thumb, index, and middle fingers, which is provided by a prosthetic hand.
3. VO is most common and practical type, powered by proximal muscles to open the TD against springs. VC is most physiological, but is heavier and less durable.
4. Controlled by switches or myoelectric signals and powered with energy from external batteries.
5. Friction and locking. Friction permits pronation and supination of TD and hold it in selected position via friction. A locking wrist permits manual rotation then locks the TD in the selected position, which is an advantage in that it prevents inadvertent rotation of the TD when a heavy object is grasped.