Functional Thumb Prosthesis

Loss of the thumb at or proximal to the interphalangeal (IP) joint typically from a traumatic injury results in major impairment of hand function as the thumb is essential for many important functions of the hand including pinching and grasping. The current treatment for the traumatic loss of a thumb is surgical removal of the big toe or second toe and a transplant to the location of the thumb. Extensive physical therapy is required for the patient to gain control of the transplanted toe. This technique, however, is not always successful and may impair a person’s balance during walking and running because of the loss of the big toe.

 

A mechanical thumb prosthesis that recreates a functional IP joint has an advantage to the toe-to-thumb transfer in that it does not sacrifice the toe and does not have the surgical risks of transplant failure. A functional thumb IP joint requires the user to have voluntary control over the flexion and extension of the joint to allow for grasping and releasing. For the most typical functions of the hand including pinching and grasping, thumb IP joint motion is coupled with index finger proximal interphalangeal (PIP) and distal interphalangeal (DIP) joint motion in that when the index finger flexes for grasping the thumb also flexes and when the index finger extends for release the thumb also extends for release.

 

The design is a mechanical thumb prosthesis that takes advantage of the coupling of the action of the index finger to control the motion of the thumb prosthesis. The main proposal to achieve this goal is a solely mechanical system. A solely mechanical design can employ a compact pulley system that couples the index finger action with the thumb prosthetic action of the person wearing the prosthesis. The design has a cord along the dorsal surface of the index finger that runs to a pulley system at the base of the prosthetic thumb. In order to control the behavior of the thumb prosthesis, the pulley system is designed such that as the index finger flexes, the cord along the dorsal side of the index finger pulls the cord running along the palmar side of the thumb prosthesis causing the thumb to flex. Passive extension of the thumb prosthesis is achieved with an elastic band running along the dorsum of the prosthesis  and occurs with extension of the index finger. This coupled system enables the thumb prosthesis to flex when the coupled index finger flexes and extend when the coupled index finger extends, restoring pinching and grasping of the hand. The concept for this design is a prosthesis that is worn like a glove over the index finger and the base of the hand with the thumb prosthesis including the IP joint and pulley system incorporated into the glove prosthesis. The fingertip of the index finger will be free in order to allow the user to have normal sensation at the tip of the finger.

 

The video shows a prototype of the thumb prosthesis as it pinches to tear foil and grasps to pull away the piece of paper. To improve upon the design, the addition of a surface with greater friction at the tip will aid in reducing slip during pinching and grasping. A lower profile for the pulley system and a thinner glove will help in adoption of the prosthesis. The prosthesis in the video has an adapter in the glove to allow the prosthesis to be worn and tested by an individual with an intact thumb. The next step will be to test the use of the prosthesis on a person with a loss of the thumb proximal to the interphalangeal joint. The other goals include using a stronger cable system with less stretch to allow for finer control of the prosthesis and for greater force to be applied when pinching.