The effect of alignment and other electrode positional variations

Achieving a secure and intimate fit between the electrode and the residual limb within the socket of a myoelectric prosthesis is generally regarded as essential for uninterrupted prosthesis usage and control (184, 194). The results from the pilot study in chapter 4 also suggested that fixed electrode housings do not always deliver optimum contact. However, the security of this interface is not the only factor requiring consideration with regard to signal acquisition. The myoelectric signal will pass along the length of the muscle fibres and, for the electrode to acquire it evenly across all of the contacts, the electrode will in theory have to be positioned parallel to the muscle fibres when the electrode is housed within the prosthetic socket (31, 39, 40, 169).

157 Anecdotal evidence suggest that common clinical practice is to follow the natural alignment of the residual limb and place the electrodes parallel to this alignment above the site of maximum signal strength. However, the remains of the muscle tissue, and how it presents upon palpation of the residual limb, may differ between prosthesis users, particularly at the transradial level of limb absence which has multiple wrist flexors and extensors (see tables 2.1 & 2.2, chapter 2). Factors such as cause of limb absence, nature of the injury (if the cause was traumatic) plus any unique techniques employed during surgery will all contribute to differences in the layout of the muscle tissue and importantly its alignment with respect to the residual limb.

Palpation of the residual limb may offer some information with regard to the optimum alignment position of the electrode. However, the presence of other soft tissues may make precise muscle location more difficult. In addition, within the transradial residual limb there are a number of different muscles which contribute to the overall muscle mass, and these muscles may cross over each other, making an initial electrode position selection more difficult (see tables 2.1 & 2.2, chapter 2).

The transmission of the myoelectric signal is also dependent on the alignment of the muscle fibres with respect to the electrode contacts (40). Each of the 3 contacts must be aligned along the long axis of the same muscle fibres in order for the same signal to be acquired by each contact. If not, then the contacts will potentially acquire different signals from various muscle fibres, causing disruption to the clarity of the signal and potentially failing to initialise a response from the prehensor (39, 40, 157-160, 174, 175).

In addition, keeping the electrode along the long axis of the bulk of the muscle fibres will maximise the signal strength, since the signal acquired will be the summative voltage of all the fibres that are in acquired via the surface electrode (169). Misaligning the electrodes will also increase the chance of cross-talk, where muscle fibres from muscles other than the target muscle may affect the signal acquisition of the electrode and hence the operation of the prehensor (39, 40, 157-160, 174, 175).

The problem for the Prosthetist is that the alignment of the muscle fibres is not necessarily clearly defined on the surface of the residual limb. Remnants of multiple muscles may contribute to the myoelectric signal that is acquired from the surface of the skin at the

158 transradial level. Re-attachment of these muscles following amputation may result in the fibres becoming aligned in a non-specific pattern, meaning that there may be no discernible ‘norm’ with regard to electrode alignment (95, 96, 100).

For prosthesis users who have a congenital limb absence, the muscle structure within the residual limb will often vary due to the unique genetic causes and subsequent characteristics of the limb malformation (99). For these reasons, it is difficult for the Prosthetist to be sure that the electrode is actually aligned in an optimum position with regard to signal acquisition, even if they have experience of fitting myoelectric prostheses. For those Prosthetists who have limited myoelectric prosthesis fitting experience, the task is even more challenging.

Surface electrodes have a large myoelectric signal pick-up area (171, 210). Essentially, the cumulative effect of many fibres both close to the electrode and relatively far from it will be recorded (171, 210). This is both advantageous and disadvantageous; many fibres will produce a larger cumulative effect and hence a stronger signal, but the disparate sources will potentially provide fluctuating signal strengths and cross-talk, which will interfere with the signal acquisition process (39, 40, 157-160, 174, 175).

Each muscle fibre will contribute to the overall myoelectric signal at the skin’s surface by a value equivalent to 1/rn, where ‘r’ is the distance between the electrode and the muscle fibre and ‘n’ is an arbitrary constant disputed in value by various authors (158, 201). Therefore, a larger electrode-to-fibre distance will provide a lower contribution to the summative myoelectric signal, thereby highlighting the need for the electrode to be placed over the largest number of target fibres to acquire the maximum summative signal.

Muscle force during isometric contraction is often regarded as being proportional to the resultant myoelectric signal strength, but variations have been reported (158, 165, 183, 250, 251). This may be due to the changes in recruitment of slow twitch and fast twitch fibres, which may alter the signal size as contraction progresses (39, 250). This could potentially result in prosthesis users attempting to apply greater levels of contractile force but not producing proportionally larger myoelectric signal sizes and thereby not receiving appropriate prosthesis feedback from their actions.

159 Although research on the subject area is limited, there is some evidence to suggest that electrode alignment plays a significant role in the variations in acquired signal strength, although whether this role is as significant as electrode pressure or contact is disputed (166, 167, 201). Current electrode housings within the prosthetic socket also secure the electrode in a fixed alignment position (249). Therefore, if the electrode is significantly misaligned, the prosthesis may require a completely new socket, and sometimes even a new limb entirely, due to the exoskeletal construction used in most upper limb prostheses (1, 4, 23). In addition, the prosthesis user may not realise that misalignment is the reason for problematic prosthesis usage and may actually mistakenly abandon myoelectric limb wearing believing that there is simply not enough signal available or that there is a fault with the prosthetic system or componentry. Myoelectric prosthesis rejection is commonly cited, with functional problems often quoted as a reason for lack of usage (6).

However, it should be remembered that the effect of misalignment, especially when minimal, is not clearly understood in terms of its effect on resultant prosthesis functionality. In addition, any apparent variations in the amount of electrode alignment between Prosthetists for a given prosthesis user are again not well documented. Any new design of electrode housing device would therefore need to offer the capability of applying viable variations in electrode alignment and should be able to afford prostheses usage and data collation from these different positions.

An intimate prosthetic socket fit is designed to limit movement between the residual limb and the prosthesis for a number of reasons; as discussed in chapter 2. However, variances between what could be perceived as a ‘good fit’ by both the Prosthetist and patient are also relayed frequently, if anecdotally. Factors such as tissue stiffness, shape and volume match, cause of limb absence and the user’s own personal preferences will all contribute to small but nonetheless significant changes to the socket fit (4, 42, 102, 128, 139). These become particularly significant when considering the nature of myoelectric control and the importance of the electrode-skin interface.

There can be little doubt that movements, even minute ones, can occur between the skin covering soft tissues and a solid interface such as the prosthetic socket (42). It is, however, less clear as to what extent these movements occur during ADLs and more specifically what impact these have on differential electrode contact security and signal

160 acquisition as well as prosthetic functionality. Variances between the perceived tightness of myoelectric sockets and electrode contacts and their relationship with prosthesis response and control were highlighted in chapter 3. In addition, chapter 4 provided a comparative assessment between various basic electrode-to- skin attachment methods and their association with resultant prosthesis functionality. However, the ability to enhance electrode to skin contact using finite alterations within a feasible system that can be implemented within a commercial myoelectric prosthesis is still unavailable at present.

Maintaining contact between the skin and the residual limb during prosthesis operation is widely reported as being essential in achieving effective signal acquisition (39, 40, 160). The production of motion artifacts however is not the only problem caused by poor contact. Lack of prehensor activation (as described in chapter 3), and relative motion between the electrode and the muscle belly away from its original position, both reduce the capability of the prehensor and potentially the functionality of the prosthesis. Not only would the signal from the target muscle group be reduced if the electrode position moved with respect to the residual limb, other factors such as cross-talk could further diminish the clarity of the signal (39, 40, 157-160, 174, 175).

According to anecdotal evidence, current practice employed to improve the security of electrodes such as the SEA200 (figures 5.2 & 5.3) involves the application of elastic bands wrapped around the outside of the electrode surface and outer socket wall. The tension within the elastic should impart reasonable force onto the electrode to help to maintain immovable contact. However, the flexibility within both the elastic and the semi-rigid electrode locators will by necessity allow some degree of motion to occur. In addition, the tension created within the socket will be related to the tightness of the socket tightness. However, in chapter 3 it was noted that socket tightness did not have the same effect on prehensor or hand control as electrode tightness. Therefore, it should not be assumed that the provision of a tight socket without direct electrode contact improvement is sufficient to improve prehensor control; and as a result, prosthesis functionality.

The fit of the socket and the electrodes to the residual limb requires a great deal of skill if suitable secure electrode-to-skin contact is to be achieved (222). Even then, as the muscles contract inside the socket, there is a tendency for the shape of the residuum to alter

161 within a fixed environment, thereby potentially altering the contact scenario between the electrode and the skin (4).

The results of the pilot study described in chapter 4 showed that the use of a simple attachment unit incorporated into the design of the prosthetic socket enhanced myoelectric signal acquisition and produce improved functionality when compared to the standard electrode housing arrangement. However, there were obvious limitations to both the electrode housing arrangement used and also to the prosthetic socket, which were as follows:

1) The housing was fixed within the socket walls and, as per the standard housing arrangement, in a position that relied upon the skill and experience of the Prosthetist.

2) There was no means to change the alignment of the electrode with respect to the underlying muscle fibres of the residual limb.

3) The single plunger in the centre of the housing unit did not impart contact security onto the most important areas of the electrode (i.e. the contacts positioned at either end of the electrode), and did not provide the means to maintain similar security for both contact areas. 4) All three prostheses were constructed to suit the requirements of the three users employed within the pilot study. Although every effort was made to ensure consistency in the design and manufacturing of each prosthesis, minor differences in length and materials employed may have led to a slight lack of consistency between the results obtained from each user.

It was therefore thought prudent to design a further study which took account of the limitations highlighted in the pilot study reported in chapter 4.