The hand is an amazing organ of versatility and dexterity. Not only can it be used to grasp everyday objects such as balls and pens, but it also can perform delicate maneuvers such as those performed by the skilled hands of a surgeon. The basic functions of the hand can be grouped into grasping activities (latching onto objects) or non-grasping activities (touching, feeling, tapping, etc.). Imagine how limited your life would be without the use of your hands. Though they are not essential to sustain your life, they very well may be the most important organs that allow you to enjoy your life. Thinking about movement of your own hands, there is an immense range of motions that can be performed. A hand can move not only up-down (y-axis), left-right (x-axis), and forward-backward (z-axis), but can also rotate about these three axes. In addition, each finger can flex and extend itself. The thumb in particular has a wide variety of movements. The opposition of the thumb to the other finger is what allows for grasping of objects. This diagram demonstrates some of the many motions that are involved with everyday activity:

Anatomy The hand owes its structure to bones, its mobility to joints, its force generation to muscles, and its control to nerves. The following will briefly discuss each of these aspects. The basic hand bones consist of the carpals, metacarpals, and phalanges. The carpal bones form the wrist and consist of eight bones arranged in two rows of four. The metacarpals form the palm area of the hand and consist of five long bones that radiate from the carpal bones. Finally, the phalanges form the fingers of the hand. There is a proximal phalanx, a middle phalanx, and a distal phalanx for each finger. The hand bones are demonstrated in the following diagram.

There are a total of 15 joints (including the wrist) in each hand. These joints are named logically according to their position. Thus the metacarpal-phalangeal (MP) joint is found between the metacarpal and phalanx, the proximal interphalangeal (PIP) joint is found between the proximal and middle phalanxes, and the distal interphalangeal (DIP) joint is found between the middle and distal phalanxes. Each joint is composed of ligaments, cartilage, and a synovial capsule. The synovial capsule provides for lubrication of the joint spaces, to allow for almost frictionless motion of the bones (Valero-Cuevas). The human hand has about 40 muscles that control it, which are classified into those outside of the hand (extrinsic muscles) and those that are within the hand (intrinsic muscles). Though the extrinsic muscles are located in the forearm, they transmit their force through tendons attached to the hand. The median and ulnar nerves are the major nerves of the hand, which run the length of the arm in order to transmit electrical impulses to and from the brain to create movement and sensation. The median nerve is mainly responsible for muscles associated with wrist and finger flexion while the ulnar nerve is responsible for the rest of the muscles (all of the intrinsic muscles) in the hand. The importance of the median nerve can be appreciated by its medical nickname, the $1,000,000 nerve (because of its importance in thumb opposition, severing this nerve during surgery could result in a $1,000,000 lawsuit!). The median and ulnar nerve also provide cutaneous sensation to the hand, which allows one to adjust one's grip in response to tactile feedback from the hand (Mathers). Biomechanics: The subject of hand biomechanics is a well-studied one. There are whole books dedicated to the topic. The basic task of an engineer is to conceptualize a machine into its fundamental components. A bioengineer dealing with the hand must figure out a way to break down the hand into constituents that can be characterized mathematically. The ultimate goal is to one day approximate all of the motor functions of the human hand. One may simplify things by thinking of the hand as 15 joints, each of which is restricted to its own type of movement. These types of movement can be referred to as degrees of freedom (DOF). The nine interphalangeal joints can be described as only having one DOF: flexion-extension. The five metacarpophalangeal joints, however, have two DOFs: flexion-extension and abduction-adduction (i.e. spreading fingers apart). The thumb is a little bit more complicated, with the base of the thumb having an extra two DOFs when compared to the other fingers. And finally, the wrist itself has six DOFs. Using these fifteen DOFs, one can approximate all of the possible movements of the human hand (Bebis).

Bottom Line The interaction among the nerves, muscles, tendons, ligaments, and bones is what makes possible all of the hand movements. The great challenge in engineering a bioprosthetic hand not only lies in simulating the complex interplay among bone, muscles, and specialized ligaments, but also in emulating the look and feel of the natural human hand. References: (1) Mathers, LH, Chase RA, Dolph J., et al. Clinical Anatomy Principles. Mosby 1996. (2) Valero-Cuevas, Francisco J., Hentz VR., “Anatomy and Physiology of the Human Hand” (http://www-cdr.stanford.edu/Touch/workshop/private/drafts/valero.html) (3) Pons JL, Ceres R, Pfeiffer F. “Multifingered dextrous robotics hand design and control: a review” Robotica 1999: 17; 661-674. (4) Bebis G., Harris F., Erol A., Yi B., "Development of a Nationally Competitive Program in Computer Vision Technologies for Effective Human-Computer Interaction in Virtual Environments", Space Grant/EPSCoR Annual Meeting, University of Nevada Reno, Nov. 7, 2002.