Nothing Like the Human Hand

Boiven, M.D., C.R.C.P


There is nothing like a hand- nothing that looks like a hand or operates like one. The obviousness of this fact often blurs our vision in the field of prosthetics, since we are so impressed by the refinement of the normal hand that we tend to capitulate a priori and tell ourselves, "We will never reproduce it." Therefore, we still accept that rather crude tool which is the conventional prosthetic hook.

In 1967 we are still prescribing, and many amputees are using, the conventional hook system because no one has yet built an artificial hand that performs as well as, let alone significantly better than, a hook. But the hook is not an artificial hand: it is a tool, a curved pair of pliers providing pinch and suspension. Its functional resemblance to any of the normal prehension patterns is really only to that of laterolateral prehension (see Prehension Patterns, pattern 6, on page 18), and this is not the most practical pattern. As far as cosmesis is concerned, the hook can only be accepted as a tool, and it is therefore hard to see the need for a more cosmetic socket. Whether the hook terminal device be conventionally, pneumatically, or electrically powered makes no difference whatsoever, since only the driving force is different.

North American mechanical hands are cosmetically acceptable, but provide one function almost exclusively. This function is a palmar-tip prehension between thumb, index finger, and middle finger (pattern 3). It is not a true three-jawed-chuck prehension type (pattern 4), as we used to call it, since in that type of pinch the tips of the three jaws join at the same point by relative shortening of the middle finger and by abduction and lateral rotation of the metacarpophalangeal joint of the index digit.

Research Efforts Uncoordinated

Recent advances in biomedical engineering are certainly promising, but to this observer the research efforts seem scattered and lacking in coordination and integration. Some research centers appear to concentrate on control systems almost regardless of terminal devices and sockets, some specialize in terminal prehension devices without regard for controls, some elaborate external sources of power without enough assessment of the terminal device and control system, and no one pays due regard to cosmesis. Each center apparently tries to qualify itself as a "system controller," a "terminal devicer," or an "external power sourcer," but each of them is supposedly an "acceptable cosmesis provider."

One of the reasons for this confusing situation may be that we are trying to provide a substitute for normal hand functions without sufficient consideration of the actual physiological functions of the hand, according to the kinesiological data presently available.

Prosthetists and biomedical engineers are making new designs for artificial hands, hopefully under medical guidance. However, I believe there is a dearth of specialized medical guidance to the first group, while engineers may tend to be too technical and sometimes not practical enough.

Patterns

The task of providing medical guidance to the prosthetic research group is difficult because of the physician's heavy clinical load and also because of our still incomplete knowledge of normal hand function. A better understanding of the biomechanics of the hand should be a prerequisite to a better design for an artificial hand, and more systematic research in that field should be conducted first. We should aim at duplicating the normal mechanics of the hand as closely as control sites and power sources permit.

It does not seem possible that we will ever produce an artificial hand capable of reproducing true manipulative patterns. However, prehen-sive patterns could be investigated with a reasonable chance of success if we first studied the different joint positions and sequences of motion in the normal hand for different prehension patterns, then made a thorough survey of the most commonly used patterns in activities of daily living and common professional duties, and finally tried to simplify or integrate these patterns into a reproducible, practical device.

Proper hand positioning has been overlooked too long, and passive friction wrist units that only permit pronation-supination do not provide sufficient positioning for many useful activities. Incorporation of wrist flexion and extension would also be advantageous. Rigid forearm sockets provide stability, but at the same time block pronation and supination movement; whereas semirigid sockets for longer stumps would permit reasonably useful forearm rotation.

Soft Covering Desirable

Artificial hands today are made either in the same size or just a little smaller or larger than the amputated hand, and are then covered with a thin glove. A much smaller size would permit the provision of a skin and soft subcutaneous tissue-like material that would preserve the glove, give a smoother feeling on touch, provide smoother contours, and improve grasp by a better molding of the fingers over the objects to be grasped.

Nature has made the flexor digitorum profundus the most active finger flexor, flexing the distal interphalangeal, the proximal inter-phalangeal, and the metacarpophalangeal joints, and yet we keep making hands with rigid interphalangeal joints and allow motion at the metacarpophalangeal joint only.

Nature has also provided hyperextension at the distal interphalangeal joint for a better palmar prehension (pattern 2) and we keep hampering this type of prehension with rigid, semiflexed distal interphalangeal joints.

In extension the fingers normally abduct or separate from each other, thus positioning the hand for spherical grasp (pattern 10), while in flexion they adduct and approximate. This characteristic is not given enough consideration in prosthetics design, although it is recognized that a larger and wider opening is necessary to grasp bigger objects.

The human hand has a normal transverse arch that could be reproduced rather easily in artificial hands just by respecting anatomy and physiology-that is, by providing some mobility to the fourth and fifth rays of the hand. This provision would certainly add to cosmesis and to function.

Conclusion

These simple observations are only a few examples of well-known anatomical and physiological principles that could be incorporated rather easily into prosthesis making, considering our advanced state of technology and our large variety of materials.

The use of external sources of power has permitted the fitting of many amputees who would otherwise have been helpless, and control systems have been developed to enable the patients to use these sources. Nevertheless, we still need better actuators and better terminal devices, and it is to be hoped that in the near future we will be able to produce artificial hands that will provide much better function and look much more alive and natural. Let us be concerned for a while with a more cosmetic and functional terminal device and worry later about sources of power and controls.

Boiven, M.D. C.R.C.P is associated with the Rehabilitation Institute of Montreal in Montreal, Quebec, Canada