Upper Limb Orthotics Flashcards
What are the 2 main reasons for upper limb prosthesis
Congenital absence
Trauma
What 6 factors will affect the outcomes of a prosthetic upper limb replacement
Comfort Function Appearance/cosmesis Weight Power Control and ease of use
What devices allow for good function in transradial prostheses
Terminal devices
What are the 3 main categories of supplying power to artificial hands/arms
Body power
External power
Hybrid power
(also zero-powered/passive devices)
How do body powered mechanisms work
Cables attached to the terminal device are linked to a harness anchored around the contralateral axilla. Power is generated by either glenohumeral flexion or biscapular abduction
Benefits of body powered mechanism
Good feedback
Simple design
Lightweight
Cheaper than externally supplied methods
How do externally powered mechanisms work
Energy is derived from either battery or pneumatic/electro-hydraulic power
How to passive/zero power devices work
Static devices where the position of the parts is manually adjusted
What is a servomechanism
A device used for the automatic control of a mechanism or source of power/energy. Uses error-sensing negative feedback to correct the action of a mechanism
Give examples of closed loop systems and open loop systems
Closed loop: power steering in cars, regulatory control of a boiler for central heating
Open loop: ‘timer’ devices e.g. toasters
What is a ‘transducer’
Something that measures a variable and its changes
Proprioceptors are the human transducer. What are the main 2 types of proprioceptor
Muscle spindles
Golgi tendon organs
What are 2 disadvantages of body-powered prostheses
Rely on sequential control because of the lack of suitable control sites
Range of operation around the body is restricted due to cables
What is a volar plate
A channel-like gutter of thermoplastic strapped to the wrist. Used for holding small objects
What is an electromyogram
A noise-like signal that is the bi-product of muscle contraction. These spike of energy can be amplified and averaged to give a proportionally variable signal that allows for myoelectric control
Why can electromyographic control be said to have proportional control
The speed of the prosthetic motor can be varied according to the rate of change of the muscle signal
What are the main disadvantages of myoelectric control
Presence of artefact noise which leads to involuntary movement
Lack of muscle groups to attach the electrodes to (esp. in children with congenital absence)
How to 3-state control mechanisms of myoelectric control work
Low signal level operates one function
Increased signal level operates another antagonistic function
The third state is no action/off
What is extended physiological proprioception control
The ability to perceive the tip of a prosthetic limb. Achieved by a 5-functional arm system device with a powered hand, wrist, elbow and shoulder rotation and elevation
What is used to drive a prosthetic motor in a servo control mechanism
The difference between input and output
What are Force Sensitive Resistor material pressure switches
Thin flexible pads which can be placed in sockets and used as a method of switch control
When are servo-controlled prostheses considered useful
When reliable EMG sites are not available
What are the main components of a trans-radial body powered upper limb prosthesis
Terminal device Wrist unit Socket Harness Motivating cord
What are the main components of a trans-humeral upper limb prosthesis
Terminal device Wrist unit Forearm unit Elbow unit Socket Motivating cord and pulleys Harness
What are some key properties of a harness
Tailored to the user's body Comfortable Easy to don and doff Easy to clean Lightweight Broad (to spread load)
What is an advantage and a disadvantage of using leather as a harness material
Benefit: soft and excellent for covering sensitive areas
Disadvantage: very absorbent and difficult to clean
What method of suspension is usually used in trans-radial prostheses
Elastic or Velcro cuff
What are the 4 main types of body powered terminal device
The volar plate
The split hook
The mechanical hand
The mechanical elbow
What are the benefits of using a volar plate
Lightweight, portable and good sensory feedback from the residual limb
Describe a split hook
2 curved tines (1 fixed, 1 movable)
Extension projecting medially from the movable tine which ends in a rounded hole for cable attachment#
A pretensioned rubbed band that holds the tines together
How does a split hook work
The user provides the motive power to open the hook
The elastic bands provide the restoring force
What materials are tines made from
Aluminium for light work
Steel for heavy duty use
What are 2 major advantages of split hooks
Good quality feedback from the cable
Visually minimal: good visual feedback
What are the main disadvantages of split hooks
Functional appearance
Rigid and limited sized gripping areas
What are the benefits of mechanical hands
Good cosmesis
Better for gripping large or rounded objects
Disadvantages of mechanical hands
Heavier
Require more energy to operate
Design may restrict visual cues
What movement do mechanical hands typically allow
Uniplanar movement between the first and second fingers, and the opposing thumb
What are the components of an electric hand device
Outer cosmetic glove made from PVC
Shaping mechanism of plastic/foam/laminate material
Uniplanar grip between the 1st and 2nd fingers and the thumb
Battery power which drives the motors for finger movement
In mechanical terms, Power = Torque x ?
Angular speed
What is a problem associated with the stall point of a direct current motor (if a locking feature is not incorporated)
The electric motor continues to generate its stall torque = consumes its stall current = rapidly drains electrical energy
What are 3 examples of externally powered elbows
Boston
Hosmer
Utah elbows
What is the only commercially available elbow devices for children
The Variety Ability elbow
What are the 3 main battery types
Nickel Cadmium (NiCad)
Nickel Metal Hydride (NiMH)
Lithium
How is the capacity of a battery quoted
In milli-amp hours
What are the benefits of NiMH over NiCad
Better energy-to-weight ratio
Considered more environmentally friendly
What are the benefits of lithium batteries over NiCad
Much better energy-to-weight ratio
Reduces the need to change the batteries during the day
What are problems associated with lithium batteries
Safety: cell rupture
Geometry: cylindrical shape which limits where it can be placed in the prosthesis
What is a problem with NiCad batteries in terms of charging
Tends to develop a memory of the most frequently used charge histories. If energy is only partially used and frequently charged then in time the battery will retain a reduced amount of charge
How do smart chargers differ from conventional batteries
Discharge the battery first before starting the charging process. Prevents the battery from developing a ‘memory’ of its charge history
What are the 3 main control components for externally-powered prosthesis devices
Switches
Proportional transducers
Myoelectric processors
Explain the differences between traction, touch, and pressure switches
Traction: different functions are activated by different relative body movements
Touch: activated by residual body movement
Pressure: made from Force Sensitive Resistor material, mounted within sockets to give switch control or proportional control
What concept do portable transducers apply to drive prosthetic function
The magnitude of the error signal between the displacement of the activation cables and the displacement of the prosthetic finger/thumb unit
Boston elbow devices use linear potentiometers. What does this mean
Resistance varies with cable displacement
What are myoelectric transducers
Electrodes and amplifiers that are mounted within the socket over the controlling muscle site. Detect muscle electrical signals which are used to drive the electronics of the prosthesis
What material has traditionally been used to cover a prosthetic hand
PVC
What is a disadvantage of a PVC cosmetic glove
Easily stains and colour/strength is influenced by UV light
What are the benefits and disadvantages of silicone cosmetic gloves
Benefits: readily moulded, takes good skin details, realistic pigmentation, resistant to staining
Disadvantages: poor tensile strength, prone to tearing
