Complex upper extremity prosthetics

The upper extremity consists of three parts; the shoulder, the arm and the hand. The arm, or brachium, is technically only the region between the shoulder and elbow. It consists of a single long bone called the humerus. The forearm is the region between the elbow and the wrist. It is formed by the radius on the lateral side and the ulna on the medial side. The hand consists of three parts (the wrist, palm, and five fingers) and 27 bones. This list already shows the complexity of upper extremity prosthetics. The function of the hand grip is above all else still a big problem.

As with the lower extremities the upper extremities are differentiated by the height of the amputation, that is; finger-, hand-, forearm-, elbow-, or upper arm exarticulation.

Additionally, the prosthetics are classified according to their technical capability. Cosmetic prostheses are used purely for aesthetic reasons, be that due to the amputation of a single finger, a hand or the whole arm. With hydraulically driven hand- or arm prostheses it is the functional capability and not the aesthetic which is important.

Progress thanks to Myoelectronics

Much progress has been made in the last number of years in the area of myoelectonic prostheses. Surface electrodes embedded in the prosthesis socket make contact with the skin and detect and amplify muscle action potentials from voluntarily contracting muscle in the residual limb. The amplified electrical signal turns on an electric motor to provide a function, eg opening , closing and rotating of an electric arm or hand. Patients must possess a minimum microvolt threshold (i.e., minimum strength of microvolt signals emitting from the remaining musculature of the arm) and pass a control test to be considered a candidate.

Combining various concepts

The hybrid prostheses combine many concepts with one another, allowing for example the bending movement of the elbow by means of a body powered prosthesis and the gripping function of the hand by myolelectronics. Every technical forms of exarticualtions prostheses are very difficult, like for example the hip exarticulation, as the holding points are missing. In this case the strapping of the prosthesis is one of the most important tasks.

Author: pg - 09/2007, translation: PmcC - MyH