Chapter 2. Prehension

Appendix D - Prosthetic and Robotic Hands 407
hooks, so that hook opens when the user activates it, and it closes by
use of a spring. Much of their simplicity is due to the fact that they
have one degree of freedom. Studies have been done to determine the
useful acceptance of prostheses. Millstein et al. (1986) studied 314
patients and found the advantages of using the cable-operated hook to
include that they are good for manual skills, not easily damaged,
designed for rugged conditions, and easy to clean. However, the hook
lacked cosmetic appeal, and there was difficulty in stabilizing some
objects due to the hook’s shape and sometimes insufficient gripping
force.
Table D. 1, displaying a list of commercial hooks currently
available, evaluates their functionality in opposition space
terminology. Hooks are categorized as having a side opposition
because the opposition is occurring transverse to the palm. Hooks are
structured so that at the tips, a finer side opposition is available for fine
precision grasping between tip one (VF1) and tip two (VF2). The
hook becomes bowed more proximally, providing a wider opening for
dealing with larger objects in a grasp between one (VF1) and two
(VF2). In general, hooks do not have the surface area necessary for
palm opposition, and their ability to pick up larger and irregularly
shaped objects is limited. Using both fingers together, it is possible to
create a virtual finger three (VF3) useful for lifting and carrying
objects, such as suitcases, plastic bags, etc. Recall from Chapter 2 that
since this virtual finger is opposing gravity, and not another virtual
finger, it is designated a VF3.
Of these devices, the Contourhook (Hosmer Dorrance) is a body-
powered device having a unique design that provides two neoprene
covered curved fingers that can apply side opposition very much like
the human thumb in opposition to the side of the index finger.
Another unique device is the Utah Split hook (Mauch), which uses an
improved design over the standard split hook for gripping shapes and
surfaces. This device has wider gripping surfaces than normal, and a
special interlocking knife grip. The outside surface of the tips is ure-
thane-rubber-coated for better friction for grasping paper. Cable
excursion remains constant over the full range of movement, due to
the way the cable housing is attached directly to the terminal device.
Side opposition is used between the two fingers, as in the Hosmer
Dorrance devices. However, the Utah hooks interlocking knife grip
allows objects to be held in a side opposition between the base of the
two hooks, using the distal end of one of the hooks as a VF3 in
opposition to the upward forces developed in cutting with the knife
(Sears et al., 1989).