Lower limb biomechanics and prosthetics

Question 1

What is the definition of an external limb prosthesis?

Answer 1

An externally applied device used to replace, wholly or in part, an absent or deformed limb segment

Question 2

Objective of a prosthetic replacement?

Answer 2

Comfort - socket fit, transmission
Function - stability, controlled movement
Cosmesis - static (coulour, shape, texture) and dynamic (gait pattern)

Question 3

What 3 types of force are important in the interface component?

Answer 3

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)

Question 4

What are 3 general rules of socket design?

Answer 4

Determined by level of amputation
Avoid loading pressure-sensitive areas
Apply forces to pressure-tolerant areas

Question 5

Give 2 different levels of amputation?

Answer 5

Syme's amputation (disarticulation)
Transfemoral articulation (trans-section)

Question 6

What suspension devices are there?

Answer 6

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

Question 7

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?

Answer 7

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

Question 8

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?

Answer 8

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

Question 9

What is the normal smooth transition seen in foot flat?

Answer 9

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

Question 10

During mid-stance, the ankle continues to dorsiflex. What muscles control the rate of this?

Answer 10

The soleus and gastrocnemius control the rate by contracting eccentrically

Question 11

How does the prosthetic ankle provide “push-off”?

Answer 11

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

Question 12

At toe-off, the normal ankle reaches 20 degrees of plantarflexion. How do prosthetic ankle-foot devices return to the plantigrade position?

Answer 12

Due to the action of the energy storing leaf-spring or compressed dorsiflexion “bumpers”

Question 13

Classification of prosthetic ankle-foot devices?

Answer 13

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

Question 14

What are the principle requirements of prosthetic knee joints?

Answer 14

Stability - in stance phase to support the body weight

Flexion - in swing to allow foot clearance

Question 15

How is stance stability achieved?

Answer 15

Free knee
Locks
Brakes
Polycentric knee

Question 16

How does the free knee work to help stability?

Answer 16

Stability is achieved by a combination of antero-posterior knee axis position and residual muscle action (alignment stability)

Question 17

Knee locks can be either manual or semi-automatic. Discuss the differences between the two?

Answer 17

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

Question 18

What two types of knee brakes are there?

Answer 18

Friction and hydraulic

Question 19

How do frictional knee brakes work?

Answer 19

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

Question 20

How do hydraulic knee brakes work?

Answer 20

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

Question 21

How do polycentric knee mechanisms work?

Answer 21

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

Question 22

What must be achieved during the swing phase by a prosthesis?

Answer 22

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

Question 23

What 2 control types act in the swing phase?

Answer 23

Frictional (usually with extension assisted spring) and hydraulic or pneumatic controls

Question 24

What are the advantages and disadvantages of frictional swing phase control?

Answer 24

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)

Question 25

What are the advantages and disadvantages of hydraulic and pneumatic swing phase control?

Answer 25

Advantages:

• Effective over a range of cadences
• Can be controlled by a microprocessor

Disadvantages:

• Can be heavy
• May require more maintenance

Question 26

What is microprocessor controlled swing phase?

Answer 26

These are usually hydraulic or pneumatic devices which incorporate valves controlled by a motor. As the patient starts to walk faster, the change in cadence is detected by the electronics, and the motor closes the valve to stiffen the system. The range of cadence over which the device is effective is increased

Question 27

How does the Otto Bock C-leg work?

Answer 27

It automatically adapts to cadence by taking measurements from strain sensors and proximity switches 50 times per second and making up to 50 valve adjustments every second. By automatic control of the valve “leak-rate”, the amputee is able to walk down stairs and slopes “foot-over-foot”

Question 28

How do combined stance and swing control devices work?

Answer 28

These devices combine stance-phase and swing-phase control 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. They can also incorporate a manual lock for extra stance stability. E.g. Ossur Mauch XG Knee

Question 29

What is the function of “alignment components”?

Answer 29

These 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 around the joints may be changed to ensure an efficient, comfortable and cosmetic gait pattern.
The magnitude and distribution of forces at the residual limb/socket interface can also be influenced by alignment changes

Question 30

What is the commonest alignment system?

Answer 30

The pyramid system. This 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

Question 31

How can horizontal shift be achieved?

Answer 31

By using two pyramid alignment devices

Question 32

What is the remedy for the following mis-alignment:
The prosthetic foot is set too near the midline of the body causing the socket to try to rotate anticlockwise on the residual limb resulting in medial pressure proximally and lateral pressure distally?

Answer 32

Remedy - shift foot laterally to bring the line of action of GRF through the midline of the socket

Question 33

What is done if the knee is forced into hyperextension;

a) if the patient appears to be bearing weight equally between heel and forefoot?
b) if the patient appears to be bearing weight excessively on the heel?

Answer 33

a) Remedy is to dorsiflex the foot to allow the knee to adopt a more normal posture
b) Remedy is to shift the foot posteriorly to move the GRF closer to the knee and reduce the hyperextension moment

Question 34

Function of structural components?

Answer 34

They connect the various functional components. They must be able to withstand forces and moments to which they are subjected

Question 35

How are prosthetics designed cosmetically?

Answer 35

They usually consist of a foam tube shaped to match a normal limb. The surface treatment of the foam cover usually consists of PVC or silicone fabric