A Wrist-Powered Prosthesis for the Partial Hand

Z. WEDDERBURN, R. R. CALDWELL, E. R. SANDERSONAND M. OLIVE*Fredericton, New Brunswick


Partial hand amputees are more likely to reject their prostheses than any other upper-limb amputee. Rejection stems from lack of tactile sensation, poor appearance, lack of ventilation in the prosthesis, and limited function. A new prosthesis (Fig. 1 ) is based on a design used for several years in Sweden and subsequently modified by the authors.

Components

The prosthesis consists of two sections, thumb and hand, hinged to a forearm cuff. The hand section includes a socket for amputation-limb control of the prosthesis which is covered with a cosmetic glove. Component specifications (Fig. 2 ) are:

  1. Finger base plate made from 2024T aluminum alloy, 65 mm long, 170 mm wide, and 14.5 mm high, with a 6 deg. taper making the distal margin slightly smaller than the proximal margin. The top has four holes 14 mm apart to accommodate the stainless steel rods which form the fingers. The holes are 4 mm in diameter, 11 mm deep. Perpendicular to the holes are four 3 mm tap holes for stainless steel set screws to secure the rods in place.
  2. Four stainless steel rods for the fingers.
  3. One stainless steel rod for the thumb.
  4. A single-axis hinge (Fig. 3 ) made of 2024T aluminum alloy. The rods or ends of the hinge are approximately 30 mm long, 9 mm wide, and 4 mm thick, and have a radius on them to conform to the wrist area. The yoke of the hinge is 20 mm in diameter and 9 mm thick. A 4 mm wide slot is in the middle of the yoke to accommodate the other rod. Two pieces of brass shim stock 0.003 mm thick lie between the inner yoke and the outer rod to minimize friction. The pivot pin is a 6 mm stainless steel pin, newly designed.
  5. One laminated socket.
  6. One Otto Bock inner glove.
  7. One cosmetic glove, Centri or other make.
  8. One " D" ring.
  9. Two pieces of Velcro for closure.

Fitting and Fabrication

The prosthetist takes a plaster mold of the amputation limb in the usual manner, then fills and modifies the model so that the distal end of the amputation limb can be accommodated into the socket eventually. The finger base plate should lie transverse to the long axis of the forearm, secured to the plaster model by plaster, Otto Bock soft putty, or other adhesive. The prosthetist then pulls a polyvinylalcohol bag over the model and cuts it at the line where the finger plate joins the model so that the finger plate can be incorporated in the lamination. Holes in the finger plate are protected and lamination is performed.

After breaking the plaster out of the socket, the prosthetist makes an opening on the medial side of the socket large enough to allow entry of the amputation limb into the socket. Weight is added to the distal end of the socket to permit checking comfort and fit. The socket is then divided at the articulating point of the wrist, and the hinge aligned parallel to the long axis of the socket unit in both sections of the lamination. The lamination is then removed from the distal portion of the finger plate and the finger rods are installed, cut to fit an Otto Bock inner glove, and shaped in a flexed posture. The thumb of the inner glove is removed.

Next, the prosthetist fills the fingers of the inner glove with a mixture of polyurethane foam, at 1:1 ratio foam/hardener, Silastic "E" silicone rubber, or bathtub caulking, and inserts the finger rods. The thumb rod is aligned to oppose the middle and index fingers; the thumb of the glove is filled with the same material and the rod inserted. When the material has cured, the prosthesis is filled with plaster and a mandrel inserted.

The fingers, thumb, and hinge are protected so that resin cannot clog or damage them during final lamination. Strips of glass fiber or carbon fiber are used to reinforce the thumb rod and hinge. Two layers of stockinette are pulled over the prosthesis and the final lamination is done. The prosthesis is prepared for final fitting.

Stretching the cosmetic glove is necessary so that adequate opening range may be achieved. The technique developed in Sweden requires lubricating the inside of a Centri glove of appropriate size, then warming it gently with a heat gun and pulling in onto the prosthesis. The glove must be pulled to the tip of the fingers. The most difficult step of the procedure involves preparing a series of wooden blocks ranging in thickness from 35 to 50 mm, in 5 mm graduations, for a small child's glove, and from 35 to 70 mm for a 6 1/4 or 7 1/4 glove. The prosthetist also needs a deep pot of rapidly boiling water and a deep pot of ice water. With the smallest block placed between the thumb and fingertips, plunge the hand into the boiling water for 30 seconds, avoiding wetting the interior of the prosthesis. Immediately plunge the hand into ice water for 30 seconds. Replace the block with the next thicker one and repeat the immersions into hot and cold water, until the maximum width of 50 or 70 mm is obtained. The glove may tear if the procedure is not followed exactly.

Prosthetic Function

The single-axis joint linking the hand and fingers to the forearm shell is aligned parallel to the anatomical wrist joint, thus allowing freedom of flexion and extension of the prosthetic hand (Fig. 4 ). The thumb, aligned in opposition to the middle and index fingers, is stabilized to the forearm shell, thus providing pinch force proportional to the amount of force applied by the wearer.

Because of the small surface area of the amputation limb which is covered by the prosthesis, the user has superb awareness of the environment surrounding the prosthesis. The small contact area also minimizes interference with normal heat loss; thus, the prosthesis does not feel hot. The cosmetic glove improves the appearance of the prosthesis greatly, and should enhance user compliance. The prosthesis opens far enough to grip the same range of articles possible with an electric hand of similar size. Grip force is perceived by pressure directly on the skin surface.

Acknowledgements

The prosthesis is based on a design from the Department of Clinical Neurophysiology Regionsjukhusset, 71085 Orebro, Sweden. The assistance and cooperation of Dr. Rolf Sorbye, head of that department, is acknowledged. Prosthetics research at the Bio Engineering Institute is funded in part by the Canadian Imperial Bank of Commerce, the War Amputations of Canada, and Health and Welfare Canada.

*Prosthetics Research Centre, University of New Brunswick Bio-Engineering Institute, 180 Woodbridge Street, Fredericton, New Brunswick, Canada E313 4R3