The influence of passive wrist joints on the functionality of prosthetic hands

Background: The addition of a passive wrist to a single degree of freedom prosthesis has an effect on its functionality. The amount of impact is undetermined. Objectives: To measure the grasping function of a commercial single degree of freedom hands with and without two forms of passive wrist flexor. Study Design: Form-board and timed tasks. Method: Repeated measures with a single subject using a validated assessment tool. The test measured the function of one conventional, single axis, powered hand controlled by the same myocontroller format. It was used in conjunction with a passive three position wrist flexor, a wrist with compliance in the flex/extend and radial/ulnar deviation and compared with a hand without these axes. Results: The overall functional score of the hand alone was 80 out of 100. The use of a wrist flexion unit resulted in a higher score (83) and the compliant wrist achieved a score of 79. The addition of a wrist allowed improved performance in Power, Lateral and Tips grips for both wrist designs, in addition the Extension grip was improved with the compliant wrist. Conclusion: Wrist flexion had a positive impact on the functional score. It enabled some tasks to be performed quicker and with less difficulty. Clinical relevance The majority of prosthetic hands have pro/supination yet there is little evidence about what impact this, or any other motion at the wrist, has on the overall functional capabilities of a prosthetic hand (i.e. grip). A commercial myoelectric prosthetic hand when used with and without two different wrist designs can potentially improve grip performance.