Modelado de la estructura y propiedades del tejido óseo trabecular

Relative motion between the myoelectric socket and the residual limb is clearly key to the production of motion artifacts, but is relatively common within standard prosthetic sockets at virtually all levels of limb absence (71, 116). Small displacements between the skin and the inner surface of the socket often occur with most socket types, and anecdotal evidence suggests that some prosthesis users prefer a slighter looser fit, particularly if the skin is sensitive or fragile. Cotton or woolly socks can be worn over the residual limb, cushioning the effects of loads incurred during prosthesis usage. However, excessive movement, or ‘pistoning’, can lead to uncomfortable skin abrasions and a lack of proprioceptive feedback (114). The development of roll-on sockets, such as the ICEROSS (see chapter 2), has been made to improve suspension and reduce these movements (131).

Motion between the socket and the residual limb in upper limb prostheses usually involves smaller loads than would be recorded in lower limb prostheses, since upper limb

186 prosthesis usage is not subject to the relatively large ground reaction forces associated with gait (110, 115). However, even though the loads on the prosthesis will be smaller, the range of upper limb movements that may occur during normal daily living are extensive, depending on the individual involved and their own daily requirements, activities and occupation (see chapter 2). If the prosthesis is light, and / or the suspension is effective, then motion between the residual limb and the socket should be minimised (23, 104).

Depending on the prosthesis type, either the self-suspending socket or the harness should maintain the upper limb prosthesis in the correct anatomical position during prosthesis motion (104). As most transradial prostheses are light cosmetic types, they will normally only exert small loads on the interface between the residual limb and the socket (102). Therefore, motion between the socket and the residual limb is unlikely to be significant for wearers of cosmetic prostheses. In addition, even if slight movements between the residual limb and the socket do occur, these should have little impact on the effectiveness of the cosmetic prosthesis. The small forces generated within the socket mean that skin abrasions are unlikely, and even if the residual limb is sensitive (often following a trauma-related amputation, see chapter 2) the use of socks to cushion the interface will not hinder prosthesis usage (116). Cosmetic prostheses remain the most popular choice for the upper limb prosthesis wearer, despite their obvious limitations (6, 14).

Similarly, as previously noted in chapter 2, the socket in body-powered prostheses plays no functional role in prosthesis activation or functional usage. The harness will absorb most of the forces generated during prosthesis usage, as well as ensuring that the prosthesis components remain in the correct relative anatomical positions (256).

For myoelectric prosthesis users, the problem with relative movement between the socket and the residual limb becomes more significant, primarily because of the reasons, such as electrode motion and lift, discussed in chapter 2. However, anecdotal evidence suggests that most sockets employed in clinical use for myoelectric prostheses are very similar to those employed for cosmetic prostheses and body-powered prostheses. Traditionally, it is the length of the residual limb and the composition of the remaining tissues within the residual limb that determine the transradial socket type for each prosthesis user (28, 37). Socket tightness may be increased to accommodate the heavier components fitted to a myoelectric prosthesis in an attempt to reduce the slippage that may occur between the residual limb and the socket at

187 their interface. However, the prosthesis user may not accept a tighter fit and may wish to revert to a standard, looser fitting, or wish to wear socks, particularly if the residual limb is unable to tolerate greater loads e.g. because of tissue sensitivity.

6.3.1 The effect of load variations on socket movement during prosthesis usage

Two main factors will influence the effective load acting on the socket during daily living activities:

1. The weight of the prosthesis itself, particularly the prosthetic hand;

2. The effective load being moved or carried by the prosthesis during the activity.

The weight of the hand will be particularly significant, since this will act at the distal end of the prosthesis, and will therefore have a larger lever effect on the socket. Myoelectric hands are significantly heavier than cosmetic hands, even though the socket types are similar (118, 119). Numerous surveys have highlighted prosthesis users’ wishes for lighter hands and prosthesis components, and prosthesis manufacturers have tried to accommodate these wishes into current designs (12, 14, 15, 51, 224). However, the need for greater functional and technical capability has also contributed to the necessity for a relatively heavy myoelectric hand (118, 119).

The length of the residual limb will also affect the load acting on the socket (4). Shorter residual limbs will be more susceptible to high loads due to the extended lever effect. Soft, fleshy residual limbs will also be prone to movement between the residual limb and the socket, since these will inherently provide a looser interface with the socket (42, 125). Anecdotal evidence suggest that these factors may have inhibited the prescription of myoelectric prostheses to those potential users who do not have either long or relatively firm residual limbs.

Loads moving the myoelectric prosthesis socket with respect to the skin can potentially affect the functional capabilities of the differential electrodes. This part of the study evaluates the motion artifacts that may occur in commonly prescribed socket types currently used in clinical prescription in the UK for transradial prostheses and discusses the potential effect of these artifacts on prosthesis functionality. Standard sockets and componentry (listed below) were incorporated within a bespoke modular prosthesis that

188 enabled sockets to be interchanged between users, thereby maintaining procedural consistency. Each prosthesis user undertook a series of movements representing common activities of daily living, using various loads to simulate either an item being carried or a heavier terminal device being worn. The signal generated at the interface of the electrode and the residual limbs were recorded using the ‘Myoboy’ prosthetic myoelectric assessment system.

The following methodology describes in detail the above procedure, the equipment used and the processes involved to complete this investigation.

6.4 Methodology

Prior to the commencement of this part of the study, ethical approval was sought and provided by the relevant National COREC ethical committee following the submission of the requisite protocol and other relevant material plus the relevant local ethical approval. Following this, five transradial prosthesis users with experience of using myoelectric control were recruited for the study, from the University of Salford’s professional patient database. The selection and recruitment criteria are described in section 4.4, chapter 4. The selection procedure is outlined in section 4.4, chapter 4. Relevant ethical approval documents, together with a patient consent form, can be viewed in Appendix B-Ethical approval and related documents. Associated documents with this investigation also included a patient information sheet, patient consent form and an investigation protocol adapted from those available within Appendix A-Questionnaires and Questionnaire development, with alterations provided detailing the differences in the analysis regime.

This investigation evaluated specific factors that could potentially produce motion between the socket and the residual limb. These factors are:

1) Prosthesis loading: the weight of the prosthesis, plus any other load that is carried or lifted by the prosthesis;

2) Prosthesis/ upper limb movement: the approximate movement of the limb and prosthesis during the activity.

For these factors to be evaluated accurately and consistently, a prosthesis that allowed the interchange of different sockets within a standard arrangement was required. In addition,

189 this prosthesis needed to be adaptable in terms of its length and be able to permit the addition of small loads where appropriate. As stated previously (see chapter 3), most upper limb prostheses are exoskeletal, with the socket laminated within the prosthetic forearm for transradial prostheses. This arrangement does not allow socket interchange or length alteration once the prosthesis is manufactured. Consequently, a standard exoskeletal prosthesis was not suitable for the requirements of this study.

Unlike upper limb prostheses, the vast majority of lower limb prostheses are endoskeletal. Endoskeletal prostheses permit the type of adaptations and alterations as listed above, although there are few upper limb endoskeletal prostheses currently provided and these are usually light cosmetic types primarily developed for more proximal levels of limb absence (see chapter 1). A suitable prosthesis for this study would therefore need to be endoskeletal, but suitably robust to permit the attachment of a relatively heavy myoelectric hand. The production of a bespoke prosthesis arrangement was described in chapter 5 and this prosthesis would again be employed within this part of the study.

Each socket fitting was adjusted to provide optimum comfort for the prosthesis user. In addition, the gain or amplification setting was adjusted to meet the specific requirements of each subject.