A prosthetic wrist is an integral component of any upper limb prosthesis, whether for transradial amputees or for transhumeral amputees. Prosthetic wrists help in the attachment of the terminal device to the forearm of the prosthetic arm and also allow pre-positioning of the terminal device according to the needs of the amputee, which is especially required for transhumeral amputees or amputees with very short transradial residual limbs who have less than 50% of residual limb length and therefore, active supination and pronation is absent. As compared to the anatomical wrists, the range of motion exhibited by prosthetic wrists is limited and is accomplished mechanically through different joint mechanisms, varying with the type of wrist configuration being used.
Like any other upper limb component, prosthetic wrists are also of different types – passive, mechanical wrists and active, electronic (also called as myoelectric) wrists, used with bionic arms or hybrid prosthetic systems.
The passive, mechanical wrists are used along with body powered terminal devices that are operated through the use of the amputee’s residual limb motions, harnessed through control cables either single (transradial) or double (transhumeral). Amputees using these passive wrists have to manually position and adjust the terminal device as well as the wrist, using their contra lateral limb or their surroundings. Also, these wrists are accompanied with either friction levers or locking mechanisms to limit wrist movement after locking.
The active, electronic wrists usually consist of electric motors and electronic or pneumatic actuators that provide external power through different power sources to operate the prosthetic wrists and the terminal devices. The most common method of operating powered wrists is through electromyograhic signals picked up by electrodes placed over residual limb muscles. A battery is used as the power source in these electronic wrists. These myoelectric wrists can either be single units, that can be used with different terminal devices or they can be incorporated with the arm/forearm part or the terminal device of the prosthetic arm. Like most common bionic components, these electrodes are placed on agonist-antagonist residual muscles and the amputee is asked to perform phantom motions to produce EMG signals that will be picked up by the electrodes placed on the limb.
There are different types of myoelectric wrists, based on the type of function or motion that they provide. They can provide the amputee different degrees of freedom (1, 2 and/or 3) in different planes and configurations. The more degrees of freedom a wrist will perform, the more actuators will be required to control these motions, usually leading to an increase in the weight, complexity, length and cost of the wrist displaying multiple degrees of freedom. This poses a problem with upper limb amputees, especially those with longer transradial limbs as there is a limitation in the space available for fitment of the wrist, leading to further constraints in motion. Moreover, ongoing researches are still attempting to investigate better ways to achieve the simultaneous control of wrists exhibiting multiple degrees of freedom.
The primary function of any powered wrist is to imitate the functioning and working of the anatomic wrist and fulfill the needs and requirements of the amputee with respect to performing ADLs using their terminal devices. These wrists also reduce the forces that are applied on the residual lime during arm movements. Another major benefit obtained with the use of myoelectric wrists is that they increase patient comfort and minimize the occurrence of overuse injuries that occur due to increased compensatory movements that occur at the torso, shoulder and elbow joints. This is mostly due to the pre-positioning benefits offered by the myoelectric wrists.
The current myoelectric wrists work on the sequential control mechanics whereby 2 control sources are used; one that is used to select the motion and the other to control the motion.
Future Directions
There is a need to carry out and document more research in the area of myoelectric wrists. Current powered wrists need to be modified and made light-weight to reduce the torque that decreases the lifting capability of the prosthetic arm for higher level amputees. A major constraint while using the active wrist systems is the lack of control sites for offering simultaneous control. Moreover, powered wrists can only perform single degree of freedom currently that is supination-pronation. With respect to control mechanisms, each powered wrist joint requires an additional source for proportional control motion.
