Lower limb biomechanics and prosthetics Flashcards
What is the definition of an external limb prosthesis?
An externally applied device used to replace, wholly or in part, an absent or deformed limb segment
Objective of a prosthetic replacement?
Comfort - socket fit, transmission
Function - stability, controlled movement
Cosmesis - static (coulour, shape, texture) and dynamic (gait pattern)
What 3 types of force are important in the interface component?
Support - generally axial or proximally directed associated with bearing body weight
Stabilisation - Act perpendicular to the longitudinal axis of the limb and are associated with resistance to. or modification of, moments acting around joints
Suspension - anatomical or pressure differential (suction)
What are 3 general rules of socket design?
Determined by level of amputation
Avoid loading pressure-sensitive areas
Apply forces to pressure-tolerant areas
Give 2 different levels of amputation?
Syme's amputation (disarticulation) Transfemoral articulation (trans-section)
What suspension devices are there?
Cuffs and straps - e.g. pelvic belt for Transfemoral prosthesis, supracondylar cuff for transtibial prosthesis, elasticated sleeve
Socket liners - e.g. ICEROSS silicone socket liner with “shuttle-lock” for Transfemoral and transtibial amputation
Prostheses must functionally be able to weight bear and allow controlled movement. Give some requirements which are specific to the function of ankle-foot devices?
Absorb shock just after heel strike
Allow a smooth transition to foot-flat and through mid-stance
Resist dorsiflexion
Provide push-off through energy return during late stance
During heel strike, the ankle-foot device must simulate the energy absorption of pretibial muscles in a normal individual. How does it normally occur in a normal individual?
The ankle begins to plantarflex due to the GRF being behind the ankle joint axis. In normal gait, the pretibial muscles contract eccentrically to absorb energy
What is the normal smooth transition seen in foot flat?
Ankle is in 10 degrees of plantarflexion. The plantarflexion moment reduces as the point of application of the GRF moves anteriorly. Pretibial muscle activity falls off. As GRF passes anteriorly to the ankle joint, the moment becomes one of dorsiflexion
During mid-stance, the ankle continues to dorsiflex. What muscles control the rate of this?
The soleus and gastrocnemius control the rate by contracting eccentrically
How does the prosthetic ankle provide “push-off”?
It allows controlled dorsiflexion to around 10 degrees during which time some devices store energy which is then returned to provide some active push-off
At toe-off, the normal ankle reaches 20 degrees of plantarflexion. How do prosthetic ankle-foot devices return to the plantigrade position?
Due to the action of the energy storing leaf-spring or compressed dorsiflexion “bumpers”
Classification of prosthetic ankle-foot devices?
Articulated:
Uniaxial
Multiaxial - e.g. “Greissinger” or “Multiflex”
Non-articulated:
Flexible ankle - e.g. “FlexFoot”
Solid ankle
- Heel spring and flexible keel e.g. “Quantum foot”
- Cushion Heel
- Stiff keel e.g. “SACH”
- Flexible keel e.g. “Seattle” foot
What are the principle requirements of prosthetic knee joints?
Stability - in stance phase to support the body weight
Flexion - in swing to allow foot clearance
How is stance stability achieved?
Free knee
Locks
Brakes
Polycentric knee
How does the free knee work to help stability?
Stability is achieved by a combination of antero-posterior knee axis position and residual muscle action (alignment stability)
Knee locks can be either manual or semi-automatic. Discuss the differences between the two?
Manual - amputee is able to engage the lock for stability (e.g. when walking over rough ground) or can walk with the knee unlocked, in which case the knee stability is achieved using residual muscles (like a free knee)
Semi-automatic - the knee locks automatically on extension (e.g. when the patient stands up) but can be unlocked manually for sitting down
What two types of knee brakes are there?
Friction and hydraulic
How do frictional knee brakes work?
When weight is applied to the prosthesis, the contact surfaces are pushed together and friction locks the knee. During the swing phase, the spring keeps the surfaces apart, allowing flexion
How do hydraulic knee brakes work?
They are weight activated - when the patient applies weight to the prosthesis, a mechanical linkage closes a valve in the hydraulic cylinder circuit, stopping the flow of fluid effectively locking the piston
How do polycentric knee mechanisms work?
4 bar linkage. The instantaneous centre of rotation of the knee joint moves as the angle of flexion changes. With the knee straight (e.g. at heel strike), the knee centre is high in the thigh, and posterior to the GRF which enhances stability
What must be achieved during the swing phase by a prosthesis?
Allow the knee to flex for toe clearance
Control the heel rise immediately after toe-off
Allow forward acceleration of the shank to ensure full knee extension at the next heel strike
Control the knee extension so that terminal impact is minimised
What 2 control types act in the swing phase?
Frictional (usually with extension assisted spring) and hydraulic or pneumatic controls
What are the advantages and disadvantages of frictional swing phase control?
Advantages:
- Relatively light in weight
- Reliable
- Can be adjusted by the patient to suit individual walking speed
Disadvantages:
- Cadence-specific (it only works effectively at one walking speed)
