The PIP Driver is intended for finger amputations distal to the proximal interphalangeal joint, where sufficient length and flexion mobility of the residual middle phalanx remains. A proximal frame surrounds the proximal phalanx with a second frame surrounding the middle phalanx. Flexion and extension between these two frames are captured by a linkage joint that transmits flexion force and movement to a prosthetic distal interphalangeal joint, moving a prosthetic distal phalanx.

The MCP Driver is intended for finger amputations approximating the PIP
joint. Flexion of the metacarpophalangeal (MCP) joints is transmitted to distal prosthetic interphalangeal joints through a metal linkage system (Figure 1). Current manufacturing constraints are such that the optimal amputation length for such systems is just proximal to the PIP joint as this provides an optimal functional lever arm for the body segment driving the system, while leaving enough length distal to the amputation to fit the necessary length of the prostheses. Given its robust stainless steel construction, the MCP Driver is currently the most heavy-duty prosthetic solution at this amputation level.

Similar in some respects to the MCP Driver, the Thumb Driver is intended for amputations just distal to the MCP joint of the thumb. Driven primarily by movement at the carpometacarpal joint, and secondarily at the MCP joint of thumb, the Thumb Driver provides active flexion of a prosthetic interphalangeal (IP) joint and dynamic opposition against the remaining digits of the hand.

The X-Finger system is intended for finger amputations distal to the metacarpophalangeal joint. It is anchored at the wrist joint with a linkage mechanism spanning the dorsum of the hand. As with the MCP Driver, flexion of the remnant fingers drives polycentric linkage mechanisms to flex the prosthetic fingertip. Variants for amputations both proximal and distal to the DIP joint are commercially available (Figure 2). The design of the X-finger is not as robust as that of the MCP Driver and it appears better suited to lighter duty applications.

The Partial M-Finger is also intended for finger amputation distal to the MCP. Similar in some respects to the MCP Driver, the M-Finger uses a cable system rather than a rigid linkage system, that is mounted on the dorsal surface of the hand. The mounting acts as the anchor, and metacarpophalangeal flexion creates cable tension that causes the partial finger element to flex volarly (Figure 3). Internal springs extend the interphalangeal joint in the absence of cable tension.

M-Fingers use the movement of wrist flexion as the prime mover. As the user flexes the wrist, cables, which are anchored on a frame mounted proximal to the dorsal aspect of the forearm, are pulled. The frame acts as the anchor, and the action of wrist flexion acts to close the fingers about an object. The available force and excursion are both limited, making this type of prosthesis better suited for lighter duty applications (Figure 4).

The Minnesota split-hand device is an example of a prosthesis that uses wrist flexion and extension to activate a hinged, split hand. The hand is split at its base, making the thumb a stationary component, while the top section of the hand is activated with wrist flexion. Force and excursion are moderate to good with this type of device. It is appropriate for use in an individual with a congenital limb deficiency at the transcarpal level or a traumatic partial hand amputation (Figures 5 and 6).

Another option is the use of traditional body-powered terminal devices and a figure-of-8 harness for a partial hand amputee (Figure 7). This option can support carrying heavier loads as the durable construction of the terminal device can transmit considerable force through the broad surface area of the
harness. At this distal amputation level, the harness also allows the anatomic motions of wrist flexion and extension with full pronation and supination, which allows the user many options to position and then activate the prosthesis in space.

Artifacts and drawings have documented historical attempts at producing functional upper limb prosthetic devices. As early as the 16th century and continuing through the US Civil War, hook-like shapes were often fashioned as useful prosthetic implements for stabilizing, pulling, and carrying objects⁵ (Figures 8 and 9). Patent filings in the 19th century document the use of body-powered control through a harness, such as William Selpho’s 1857 patent for a body-powered prosthetic arm^6,7 (Figure 10).

The primary body-powered system still currently in use can be attributed to Dorrance’s 1912 patent⁸ of the split hook. The original hook design (Figure 11) was refined into multiple models and shapes and is currently sold by the Fillauer Corporation.⁹ Both the Selpho and Dorrance designs demonstrate the use of a harness to both suspend and activate a cable that is pulled to open a terminal device.

Building on these original concepts, methods to efficiently transmit force through a harnessing system have been developed and refined. To better understand these concepts, it is useful to review force transmission and goals of efficiently harnessing a body-powered system.

The goal when setting up body-powered prostheses is to create a transmission of force in the most efficient manner possible. To achieve this,
the following criteria must exist. (1) The prosthesis must be securely suspended or anchored to the individual’s body. (2) There must be a harness and cabling system designed to transmit body movements to efficiently activate a terminal device. (3) There must be an available power source in the form of movement generated by an intact body segment that can activate a body-powered component. (4) The body movement must be able to travel a sufficient distance to complete the action with adequate force to achieve the desired outcome.

The figure-of-8 harness system has several key component parts (Figure 12).

The body movements available to activate a body-powered component are dependent on the level of amputation. As a general rule, the more distal the amputation, the more options remain available for body activation. At the transradial level, the two key movements in body-powered activation are glenohumeral flexion of the ipsilateral shoulder and biscapular abduction. In the former, as the upper arm translates forward with the harness anchored to the contralateral shoulder, the cable is pulled and the terminal device opens. Relaxing this movement allows the rubber bands on the voluntary-opening terminal device to close. This movement generates excellent force and is a key prime mover in body-powered activation (Figure 15).

With biscapular abduction, the user moves both scapulae in opposing directions. This widens the back and thereby creates tension on the cable that opens the terminal device. This movement is very useful when operating close to the body and when the user desires to open the terminal device while keeping it in a stationary position (Figure 16).

The movements of glenohumeral flexion and biscapular abduction can be used discreetly or simultaneously in combination. The activation movement chosen depends largely on the location in space at which the user would like to activate the terminal device.

The power the body must generate to activate a terminal device is referred to as the activation force needed for activation. The distance the intact segment of the body must travel is known as the activation excursion needed for activation. These concepts are illustrated in the two previously described examples of body activation. Glenohumeral flexion produces both substantial force and excursion. The ipsilateral shoulder muscles generate excellent force and can travel a substantial distance.
In contrast, biscapular abduction generates good force but more limited excursion because the two scapulae can travel only for a short distance when generating the movement.¹¹