Ken Mac Lecture Flashcards
What is an external limb prosthesis
An externally applied device used to replace, wholly or in part an absent or deformed limb segment
Stats on NHS Tayside Prosthetics Service
- covers 750k population
- 100 amputations per year of which 60% are fitted with a prosthetic
- mean age 69 years
- 80% peripheral arterial disease
- 95% lower limb
- case load ~ 750 prosthesis users
- trans tibial 55%, trans fermoral 37% 8% other
- Multidisciplinary team approach
- majority LL
Objectives of prosthetic replacement
comfort
- socket fit, force transmissin function
- stability, CONTROLLED movement
cosmesis
- static (colour, shape, texture)
- dynamic (gait pattern)
list the 5 prosthetic components
interface alignment functional cosmetic structural
Describe the interface component
support force
- axial and proximal direct forces associated with the bearing of body weight
stabilisation forces
- act perpendicular to the longitudinal axis of the lim and are associated with the resistance to moments acting around the joint
suspension Forces
- anatomical
- pressure differential (suction)
Alignment component
allow the relative positions and angles of components to be changed so that the magnitude and lines of action of forces can be altered and the moments acting round joints to be changed to ensure an efficient, comfortable and cosmetic gait patten
Magnitude and distribution of forces at the residual limb/socket interface can also be influenced by …..
alignment changes
what is the commonest alignment system
pyramid system
- uses four adjustable grub-screws in an outer collar which bear on an inverted pyramid and allow angular adjustments of up to ~15 degrees in ANY direction
What is the remedy for a prosthetic foot set too near to the mid-line of the body?
shift the foot laterally to bring the line fo action of the GRF through mid-line socket
What is the remedy for a knee forced into hyperextension?
A) if the p. is able to bear weigh equally between heel and forefoot = dorsiflex foot to allow knee to adopt a more normal posture
B0 is the p. is bearing weight excessively on the heel = shift foot posteriorly to move GFR closer to knee and reduce hyper-flexion MOMENT
Structural component
need to be able to withstand the force and moments to which they are subjected to.
e.g. shin tube, connects the knee joint to the angle-foot device, may have an integral alignment unit built into it
cosmetic component
usually consist of a foam “tube” shaped internally to fit the component and externally to match the limb
surface treatment of the foam cover can vary depending upon the patients requirement sand the cost but usually consists of a fabric, PVC or silicone “skin” which is applied over the foam
Wood and medal surface can be treated in the same way
Ankle-foot device requirements
- absorb shock just after heel strike
- allow a smooth transition to foot-flat and through mid-stance
- resist dorsiflexion and, if possible, store energy from mid-stance to heel-off
- provide push off- through energy return during late stance
Normal heel strike
foot is plantigrade –> ankle begins to plantar flex due to the GRF being behind the ankle joint
pretibial muscles contract to eccentrically absorbing energy
Normal foot flat
ankle is in ~10 degrees plantar flexion
- plantar flexion moment reduces as the point of application of the GRF moves anteriorly
- pretibial muscle activity falls off
- as the GRF starts to pass anterior to the ankle joint, the external moment becomes one of dorsiflexion
Normal mid-stance
ankle continues to dorsiflex
- plantar flexors (soles & gastrocnemius) control rate by contracting eccentrically
Normal heel off
ankle dorsiflexes to ~15 degrees and calf muscles contract strongly to counteract the increasing dorsiflexion moment and provide active push-off to propel the subject forward
Prosthetic Ankle-foot device
Heel off
allows controlled dorsiflexion to ~10 degrees during which time some devices store energy which is then returned to provide some active push-off
Prosthetic Ankle-foot device
Toe off
reruns to plantigrade position under the action of the energy-storing leaf-spring or compressed dorsiflexion “bumpers”
Prosthetic Ankle-foot device
Swing phase
device remains plantigrade
During normal toe off what angle has the ankle acheived?
~20 degrees plantar flexion
Swing phase
- what does the ankle do and why?
flexes slightly to improve toe-clearance at mid-swing
Prosthetic Ankle-foot device
Heel Strike
ankle device must stimulate the energy absorption of the GRF
Prosthetic Ankle-foot device
Foot Flat
device must allow a smooth controlled transition
Prosthetic Ankle-foot device
Mid-stance
device must stimulate smooth, controlled dorsiflexion
Prosthetic Ankle-foot device
Mid-stance
device must stimulate smooth, controlled dorsiflexion
Name two examples of a articulated multaxial ankle-foot device
Greissinger
Multiflex (15 degree P/ 10 degrees D)
Name an example of a non-articulated flexible ankle-foot device
flexfoot
Name an example of a non-articulated heel spring and flexible keel ankle-foot device
quantum foot
Name an example of a non-articulated stiff keel ankle-foot device
SACH
Name an example of a non-articulated flexible keel ankle-foot device
seattle
Costsof ankle-foot devices
SACH 60
Pathfinder 1,800
Flexfoot 800 to 3k
Principle requirements of a prosthetic knee
Stability (instance phase to support the amputee’s weight)
Flexion (in swing to allow clearance)
Stance Stability of the knee
- what types of locks can be used?
- if the free knee method is employed how is it achieved?
Free knee- stability is achieved by a combination of A/P knee axis and residual muscle action (alignment stabilty)
locks- manual or semi-automatic
brakes- friction or hydraulic
polycentric
Knee locks
- Manual
- Semi-automatic
Manual
- can engage for stability over rough ground or can walk with the knee unlocked (stability is achieved by residual muscles)
Semi-automatic
- locks automatically lock on extension but can unlock manually for sitting down
Brakes- friction
weight is applied to the prosthesis, the contact surfaces are push together and friction locks the knee
during swing phase, the spring keeps the surfaces together
Brakes- hydraulic
weight activated
a mechanical linkage closes a valve in hydraulic cylinder circuit, stopping the flow of fluid and effectively locking the piston
polycentric knee mechanism
instantaneous centre of rotation (changes as the knee flexes)
with the knee straight the knee centre is high in the thigh, and posterior to the GRF, enhancing stability
Swing phase control requirements of the knee
- allow the knee to flex for toe clearance
- control the heel rise immediately after toe-ff
- allow forward acceleration of the shank to ensure full knee extension (and stability) at next heel-strike
- control the knee extension so that terminal impact is minimised
Knee swing phase control friction brakes
- pros and cons
pros - relatively light in weight reliable can be adjusted by patient to suit individual walking speed - cost (300) cons - cadence-specific
Knee swing phase control hydraulic and pneumatic
- pros and cons
pros -effective over a range of cadences - can be controlled by microprocessor cons - heavy -requires more maintenance - jolly expensive (~ 12k)
What is great about the Otto Bock C-leg?
automatic control of the valve “leak rate”, the amputee is able to walk down stairs and slopes “foot-over-foot”
Combined stance and Swing control
combine it in one hydraulic unit and can be configured to allow “yielding” knee flexion in stance-phase which enables the patient to walk foot over foot down stairs and slopes
can also incorporate a manual lock
Ossur power knee
microprocessor controlled and motorised active knee extensions ( costs 75k)
