Individual grip type functionality assessment scores

It was noted in chapter 2; section 2.2.1, that the human hand affords six different grip types. It was also noted, in chapter 4; section 4.4.2, that the SHAP quantifies these as part of its assessment scoring process.

As well as providing overall functionality scores for each user, the SHAP produced index of functionality scores for each of the grip types. The mean values for all these scores, for each test condition, are presented below in figure 5.12:

177 Legend

Std = assessed alignment position by an experienced Prosthetist within standard housing Bespoke-1 = assessed alignment position within bespoke electrode housing

Bespoke-2 = alignment rotated 250 ‘nose down’, with respect to Bespoke 1, within the bespoke electrode housing

Bespoke-3 = alignment rotated 250 ‘nose up’, with respect to Bespoke 1, within the bespoke electrode housing

Figure 5.12: Mean hand grip type functionality scores from each SHAP assessment with respective bespoke alignment positions (1, 2, 3) using the bespoke housing unit, and the standard socket housing (Std).

Grip type

Functionality index score/position

Mean SD

Std. Bespoke 1 Bespoke 2 Bespoke 3

Spherical 23 48 49 41 40 12.0 Power 21 43 38 30 33 9.6 Tip 23 53 55 45 44 14.7 Tripod 21 42 61 29 38 17.5 Lateral 20 61 65 48 49 20.3 Extension 37 63 68 45 53 15.1 SD 6.4 8.9 11.2 8.2

Table 5.4: Functionality index scores for each grip type, also showing overall mean scores for each grip type and appropriate Standard Deviations (SD) between housing positions, and between grip types.

0 10 20 30 40 50 60 70 80

Spherical Power Tip Tripod Lateral Extension

std bespoke 1 bespoke 2 bespoke 3

178 The ‘power’ grip afforded the smallest mean functionality index score (score = 33), although variations between scores for the ‘power’ grip using different electrode positions were also the smallest (SD = 9.6). The ‘extension’ grip had the highest mean functionality index score (score = 53), followed by the ‘lateral’ grip (score = 49), although the ‘lateral’ grip also appeared to be the one most susceptible to change with regard to electrode position (SD = 20.3). The standard housing produced the lowest variation between grip scores (SD = 6.4) but this reduced substantially to only SD = 1.3, when the ‘extension’ grip score was removed from the calculation. These results show that different electrode housing and alignment positions will have varying levels of influence on the functionality of different grip types. A further study is required to investigate whether electrodes positioned within the standard housing reduce the available functionality of specific grip types.

The standard electrode housings rely on the Prosthetist carefully contouring the appropriate electrode site area onto the positive plaster model, but do not provide the finite levels of contact control that are provided by the bespoke housing unit tested in this study. Plastic or felt washers and elastic bands may subsequently be used on the outer surface of electrodes to enhance contact security in conjunction with standard housings, but this was not tested in this study. Additionally, these methods do little to ensure secure contact across the surface of the electrode and could actually disrupt this, if for example the electrode is able to pivot around the elastic band.

The variations demonstrate that a housing arrangement which enables alteration to electrode alignment would be of benefit to Prosthetists who are attempting to achieve maximum signal acquisition for the prosthesis user. Electrodes aligned in the bespoke 3 test condition (i.e. with the distal end of the electrode in a ‘nose up’ position with respect to the standard position) produced a significantly larger reduction in prosthesis functionality than electrodes aligned in the relative ‘nose down’ position. There were significant functionality score variations between the test conditions where the electrodes were rotated away from the assessed standard alignment position (i.e. bespoke 1 and bespoke 3), and also between bespoke 2 and bespoke 3, even though the contact security could be adjusted using the bespoke housing mechanism. For one subject, user ‘A’, electrodes in the bespoke 3 position proved unusable. This highlights the fact that even when connect security is enhanced, alignment of the electrode will still affect the functionality of the prosthesis, within what could be considered a reasonable practical alignment range.

179 It is therefore important to recognise these factors when determining electrode alignment, particularly as they can lead to a significant change in the resultant functionality of the prosthesis. It is also worth noting that these alignment positions may be altered if the transfers of alignment marks from the original negative cast taken by the Prosthetist are not accurate, or if the dummy housing slips during the manufacturing process.

The lack of electrode adjustment in the myoelectric control system contrasts with the evident adjustability of other types of prosthesis control. For example, in lower limb prostheses, prosthesis control source is usually reliant on the biomechanical relationship between the body’s weight line and the ground reaction forces. By providing the correct alignment between the prosthesis components, the Prosthetist is able to provide the platform for effective control for the prosthesis user. For this reason, alignment devices and components within the prosthesis allow finite levels of adjustment, enabling suitable settings to be included prior to the delivery of the prosthesis, and at later dates should there be changes to the user’s anatomy or requirements.

5.10 Discussion

The scores for the functionality index scores (figures 5.11 and 5.12) clearly indicate that the use of an independent, adjustable housing unit that offers increased contact security as perceived by the prosthesis user, can significantly improve myoelectric prosthesis functionality.

The variation in the results suggests that the most effective electrode site for signal acquisition is not easy to recognise, even for experienced Prosthetists. Simply positioning the electrode in line with the line of progression of the residual limb is not always the best way to achieve optimal signal results. The variations also suggest that a housing arrangement that enables electrode alignment alteration would benefit Prosthetists who are trying to achieve maximal signal acquisition for the prosthesis user. Significantly higher functionality index scores may be achieved using electrodes housed in specific alignment positions (i.e. as assessed or ‘nose down’) in the bespoke housing unit with respect to the standard housing unit.

The inclusion of modular systems, particularly in lower limb prosthetics, has greatly enhanced the adjustment available to Prosthetists when trying to ensure the natural gait,

180 comfort and function for prosthesis users. Even if original alignment or other discrepancies are present within a prosthesis, these may be altered and refined to provide suitable usability for the recipient. The availability of positional adjustments post-fitting for myoelectric prosthesis electrodes would appear to be much more limited. The implications of prosthesis user function and usage are no less relevant, as the effective control of a myoelectric prosthesis will ultimately determine its level of usage and potential rejection. Attempts to improve contact security through the use of roll-on sockets and ‘snap fit’ electrodes will meet the needs of some prosthesis users, but if the residual limb is not suitable for the application of a roll-on socket, or indeed if the user does not have the capacity or the willingness to wear one, then this will not be suitable. Many prosthesis users are children, who will have changing residual limb sizes and shapes-at present, meeting their exact needs may be challenging, even for the most experienced Prosthetists.

Having some adjustment available within an electrode housing system will provide the capacity for contact security to be enhanced within a relatively large socket. In addition, those users who have more proximal limb absence will benefit from local electrode adjustability, as current socket designs for these levels do not provide the snug fit that is achievable at the transradial level.

Experienced Prosthetists will be more able to recognise suitable electrode positions, but the relatively small number of myoelectric prosthesis users means that this experience will be limited even amongst those Prosthetists that are more specialised within upper limb prosthesis rehabilitation. More recently, the role of the upper limb Prosthetist has changed within the United Kingdom, from a more specialised role to one that is carried out in

conjunction with lower limb prosthetics and even orthotics (see Appendix E- The changing education of Prosthetists). This role change has further reduced the levels of experience that many of those dealing with upper limb prosthesis users, including myoelectric users.

Obtaining functional electrode positions may not be easy for those Prosthetists that will only be presented with a very limited number of myoelectric prostheses during their careers.

5.11 Limitations and potential errors

Although the bespoke housing unit was able to be moulded to each socket, the exact fit and contour was sometimes difficult to achieve. This could potentially affect the results, as the electrode may sit more securely on the socket wall in one of the positions with respect to

181 other positions if variations in contouring occur. This could therefore have influenced, albeit to a small degree, the functionality index scores that were achieved.

Heating and remoulding the bespoke housing unit also influenced the shape of the unit round the screw-thread fastenings and the semi-rigid rod locators. This again could alter the contact and security of the electrode in any of the positions within the housing unit. In addition, the standard housing system had to be assessed first, because it had to be removed and an aperture in the socket created to allow for the fitting of the bespoke housing unit. Despite the user subject having time to practice prior to each assessment, this could have influenced the results negatively with regard to the standard housing unit, albeit not to the degree that was seen within the data.

Despite these potential limitations, the significant variance in the results suggests that fitting a bespoke electrode housing unit can produce a significantly positive improvement to prosthesis functionality. In addition, variances within electrode alignment can influence prosthesis functionality when designated by Prosthetists who are not experienced with the assessment and fitting of myoelectric prostheses.

5.11 Chapter summary

The use of an adjustable housing unit which provided the facility to provide alignment and contact security variations demonstrated significant variances in prosthesis functionality compared to the commonly accepted clinical standard. The number of alignment variations was limited to three in this study, and the unit itself was fixed which restricted the capacity of the system to provide more clinical fitting. Nevertheless, despite these limitations, the unit was able to illustrate the changes in prospective functionality that may be recorded when even relatively small alignment alterations and contact security arrangement are provided.

As the clinical profile of the upper limb Prosthetist changes, and the capabilities of upper limb devices improve along with their costs, it is vital that every effort is made to ensure that adjustments are available for upper limb myoelectric prostheses that provide effective levels of electrode contact if the prosthesis user is to acquire the maximum benefit from their device.

182 Chapter 6:

An analysis of motion artefacts produced from the electrode / residual limb interface during movements associated with daily living activities

6.1 Introduction

In chapter 2, the process of myoelectric signal acquisition using differential electrodes was described and, more specifically, how motion artefacts could occur if the electrode’s surface contacts moved or lifted with respect to the skin. Chapter 2 also highlighted how the production of motion artifacts that can mimic the myoelectric signal and impede genuine activation signals could potentially disrupt myoelectric hand control and activation.

In chapter 3, a correlation between relative socket and electrode tightness and disrupted control of the myoelectric hand, including false and unwanted activation of the myoelectric hand, was established. However, no specific link between the production of motion artifacts and control disruptions was possible, as other factors could also lead to activation disruption of the myoelectric hand. These include mechanical and technical failures of the hand, low battery power, electrical interference, and the users’ inability to produce and regulate the myoelectric signals.

Chapter 3 also indicated that myoelectric prosthesis usage patterns resembled those