Upper limb Prosthetics

Designed to replicate the cosmetic size, shape and appearance of the missing limb, these can also act as basic holding or securing tools

They are the most commonly prescribed upper limb prostheses

Uses a biomechanical harness: either a:

• Full appendage - used to suspend and operate the terminal device in conjunction with a ‘cup’ socket, or a:

• Simpler harness such as a P-loop (transradial only), which will only be used to operate the terminal device and will be used with a self suspending socket

• Advantages: Good feedback, good control

• Disadvantages: Poor cosmesis, dated design, comfort of harness

May be either active (with prehension) or passive (no prehension)

• Passive e.g. foam hands, knife appliance
• Active e.g. split hooks
• May be interchangeable when used in conjunction with single knob wrist rotary

Advantages over body powered devices:

• More powerful, greater grip strength and opening
• No need for harness for control
• Can potentially use limb in more positions around the body
• High-tec appeal
• Open to improvements in technology

Disadvantages:

• Heavy, costly, not as durable, harder to control, usually slower, less proprioception (feedback)

Myoelectric Signals

• Created when muscles contract, these provide means of operating electrically powered prostheses

• They are picked up and amplified via electrodes located within the inner wall of the socket

Myoelectric Control System

Types of Myoelectric Control

• Threshold: 15 microvolts at skin’s surface; any signal above this = same response from hand

• Proportional: 15 microvolts required for action again, but stronger signal=equivalent proportional response from the hand or component

• Site/State: no. of electrodes used= no. of sites no. of functions = no. of states

• Most hands are single degree of freedom hands, offering one grip type only

Multi-Functional Hands

• 14 grip patterns
• passive wrist options including low profile
• Full hand grip, individual motors in each finger and thumb

• 7 grip patterns
• Independently controlled thumb option
• Thumb, forefinger and middle finger grip
• Axon electric wrist option

The Clinical Reality

Real-world Usage:

Chadwell A, Kenney L, Granat MH, Thies S, Head J, Galpin A, Baker R, Kulkarni J. Upper limb activity in myoelectric prosthesis users is biased towards the intact limb and appears unrelated to goal-directed task performance. Scientific reports. 2018 Jul 23; 8(1):11084.

Actual Control

Head, J.S., Howard, D., Hutchins, S.W., Kenney, L., Heath, G.H. and Aksenov, A.Y., 2016. The use of an adjustable electrode housing unit to compare electrode alignment and contact variation with myoelectric prosthesis functionality: A pilot study. Prosthetics and orthotics international, 40(1), pp.123-128.

• The human body acts like a giant antenna, and attracts electricity from sources such as overhead power cables, the mains supply and electrical appliances

• As a result, it constantly carries a ‘common mode voltage’ many times larger than a myoelectric signal

• If the electrode is displaced in relation to the skin’s surface, then a movement artefact, very similar in size and strength to the myoelectric signal, will be produced

Filtering the signal

• The common mode voltage is filtered using a differential amplifier.

• As the contracting signal is discharged along the length of the muscle, the difference at each myode is registered as a positive and a negative signal and the difference is amplified

• If the electrode moves or lifts, the common mode voltage may become active, or a small electrical signal may be caused by the electrode and the skin acting as charged capacitors

• Cross talk occurs when a muscle other than the target muscle affects the signal acquisition

• Often occurs in congenital cases, where standard muscle boundaries are less well defined

Subcutaneous Fat & Surface Impedance

• Subcutaneous fat will increase the chance of cross talk and dissipate the signal, reducing its strength at the surface

• Surface impedance caused by skin particles, hairs and other barriers will also affect signal size and clarity

Kuiken, T.A., Lowery, M.M. and Stoykov, N.S., 2003. The effect of subcutaneous fat on myoelectric signal amplitude and cross-talk. Prosthetics and orthotics international, 27(1), pp.48-54.

Potential Solutions: Adjustable Electrode Housing, with De-coupling from the Socket

Use of an adjustable housing with de-coupling has produced an 83% reduction in signal production above prosthesis ‘activation’ thresholds during simple arm movements

Pattern Recognition

Englehart, K. and Hudgins, B., 2003. A robust, real-time control scheme for multifunction myoelectric control. IEEE transactions on biomedical engineering, 50(7), pp.848-854.

Targeted Muscle Re-ennervation

Kuiken, T.A., Li, G., Lock, B.A., Lipschutz, R.D., Miller, L.A., Stubblefield, K.A. and Englehart, K.B., 2009. Targeted muscle reinnervation for real-time myoelectric control of multifunction artificial arms. Jama, 301(6), pp.619-628.

Kuiken, T.A., Dumanian, G.A., Lipschutz, R.D., Miller, L.A. and Stubblefield, K.A., 2004. The use of targeted muscle reinnervation for improved myoelectric prosthesis control in a bilateral shoulder disarticulation amputee. Prosthetics and orthotics international, 28(3), pp.245-253.