Test condition 2: Best practice method-separated from socket

To accurately assess the influence of the socket motion on prosthesis response and functionality, it was necessary to provide a comparable, separate method of electrode attachment which effectively removed the influence of relative motion between the electrode and the skin due to it being fixed into the socket walls. To achieve this, the electrode would have to be directly secured to the skin, yet still be able to operate a myoelectric prosthesis via connecting leads in the same way a standard housing arrangement would achieve this. In addition, the electrodes and the prosthesis used for both methods of assessment would have to be identical for an accurate relative prosthesis functionality assessment to be completed.

The most suitable method established to provide electrode detachment from the socket but still maintain the electrode on the residual limb over the contact sites was to secure the electrode surface to the skin of the residual limb in line with and directly underneath the position where the electrode would normally sit within the socket aperture in the standard arrangement. The electrode would not be able to be secured within the socket housing, since this clearly would leave it to vulnerable to the same movements as previously described.

132 Therefore, the semi-rigid attachments, used to secure the electrode within the standard housings, would not be necessary and would in fact obstruct the fitting of the electrode within the socket apertures.

Although current clinically available electrodes are manufactured by numerous prosthetic companies, the main structure of each is very similar. The signal acquisition and amplification electronics are located on a miniature circuit board, which in turn is located within a plastic housing.

The three electrode contacts which acquire the signal protrude from the housing by approximately 2mm. Since they must be maintained in close contact with the skin, any adherent material securing the electrodes to the skin must not cover these contacts. As these are the only parts of the electrode that normally contact the skin, adaptations had to be made to the electrode in order to successfully adhere it to the skin and prevent motion.

Two options were available which potentially enabled the electrode to be secured to the skin of the residual limb within the socket aperture whilst keeping the electrode contacts interfaced with the skin:

a) Building up the area of the electrode interfacing with the residual limb around the actual contact sites with ‘pelite’ polyethylene foam, thereby creating a flush surface between the contact faces and the ‘pelite’. Adherent material, such as strong double sided tape, could then be applied to the foam areas only, thereby securing the electrode to the skin. Adhesive was unsuitable, because it would have been difficult to remove and may have damaged the skin.

b) Attaching plastic tabs, or extensions, to each end of the electrode; these could be adhered to the skin and would again be flush with the signal-acquiring electrode contacts.

An initial trial of each of the above methods was undertaken on the author’s forearm using a bespoke socket design over suitable attachment points that corresponded closely to those used to acquire myoelectric signals. Consequently, a plaster cast was taken around the proximal forearm segment, and also around the elbow joint, in a fashion similar to the

133 standard ‘hybrid supracondylar’ cast used in standard UK clinical practice (see chapter 2). From this cast, a supracondylar socket was manufactured, and apertures were created over the signal positions as deduced during the myoelectric assessment procedure.

Two electrodes were positioned within the socket and were subsequently connected to the myoelectric prehensor and control system. The connections and the components used were identical to those that would be employed within the prosthesis user investigation. An image of the author’s socket is shown in figure 4.2 (below):

Figure 4.2: Bespoke socket on author’s forearm (author’s own image)

The author conducted two trials, lifting and releasing objects for a period of 5 minutes, with his natural hand whilst wearing the bespoke forearm on this side, using electrode attachment methods (a) and (b). It was found that the electrode maintained a secure fit to the skin of the forearm when fitted using option (b).

134 Figure 4.3: Electrode attached to author’s forearm using option (b).

During trials of option (a) the electrode was dislodged three times during the trial procedure and therefore the control was not deemed to be effective. This option was therefore discarded. Option (b) was the chosen method of attachment, and is illustrated in figure 4.3. This was shown to retain the electrode onto the skin particularly effectively.

The electrode with the extension tabs was secured onto the skin within the apertures that had been created by the dummy housings during the lamination process; the adherent material found to be the most successful at adhering the tabs to the skin was extra-strong double–sided tape.

4.4.1.3 Test Condition 3: The use of an external assistance device to improve contact