Definition
A prosthesis replaces part or all of an absent limb and aims to restore function, stability, appearance and energy-efficient mobility.
Major components of a lower-limb prosthesis
- socket
- suspension system
- structural connection or pylon
- joint components where required
- foot
- cosmetic covering when desired
Socket
The socket is the interface between residual limb and prosthesis.
A good socket should:
- distribute load safely
- provide control of the prosthesis
- avoid excessive pressure on vulnerable areas
- accommodate residual-limb volume changes where possible
Modern sockets often use total-surface-bearing concepts and liners, although specific designs depend on level and patient factors.
Suspension
Suspension maintains attachment during swing and reduces pistoning.
Methods include:
- anatomical suspension
- suction
- vacuum-assisted systems
- liners with locking mechanisms
- straps or belts in selected patients
Transtibial prosthesis
Key goals:
- stable weight transfer
- control of rotation
- comfort at the tibial crest and fibular head
- preservation of knee function
Transfemoral prosthesis
In addition to socket and suspension, the prosthesis requires a knee mechanism.
Knee options range from:
- simple locked or stance-control systems
- mechanical polycentric or monocentric designs
- microprocessor-controlled knees
Choice depends on:
- activity level
- cognition
- balance
- strength
- terrain
- safety
- cost and serviceability
Prosthetic feet
SACH foot
Simple, stable design using a cushioned heel and solid ankle.
Dynamic-response foot
Stores and returns some elastic energy and can improve rollover and higher-level function in suitable users.
Feet vary from simple cushioned-heel systems to dynamic-response feet that store and return energy. Selection depends on activity level, terrain, stability requirements and goals.
Alignment
Alignment affects:
- socket pressure
- knee stability
- gait symmetry
- energy expenditure
- comfort
Static alignment is followed by dynamic gait assessment.
Level of amputation
More proximal amputation generally increases the energy cost of walking because:
- more joints and muscle mass are lost
- lever arms shorten
- prosthetic control becomes more complex
Preserving functional length is valuable, but only if it leaves a durable, painless residual limb with adequate soft-tissue coverage.
Prescription
Consider:
- amputation level
- residual-limb condition
- range of motion
- muscle power
- cognition
- hand function
- cardiovascular reserve
- home and work demands
- expected activity level
The best prosthesis is the one that matches the patient's functional goals and ability, not the most technologically complex device.
Rehabilitation goals after amputation
Successful prosthetic rehabilitation starts before prosthesis fitting. Priorities include:
- wound healing
- pain control
- oedema management
- preservation of joint range
- strengthening
- prevention of contracture
- psychological and social support
- cardiovascular conditioning
The final functional result depends on the person, residual limb, amputation level and prosthetic system together.
Energy expenditure and level
More proximal lower-limb amputations generally require greater energy expenditure for walking because more native joints and muscle power are lost. Preserving useful length is therefore desirable, but length should not be preserved at the cost of non-viable tissue, painful scars or poor prosthetic fit.
Residual-limb principles
A good residual limb aims for:
- durable soft-tissue cover
- a stable scar away from major pressure zones where possible
- balanced muscle
- preserved joint motion
- absence of painful bony prominence
- adequate length for leverage and prosthetic control
Myodesis fixes muscle to bone; myoplasty sutures opposing muscle groups to each other. Muscle stabilisation can improve shape and control.
Socket biomechanics
The socket is the interface between patient and prosthesis. It must:
- distribute load
- provide suspension
- control rotation
- avoid excessive focal pressure
- permit comfortable donning and doffing
Different socket concepts are used according to amputation level and anatomy.
Lower-limb components
A lower-limb prosthesis may include:
- socket
- suspension system
- pylons or structural elements
- knee unit where required
- foot/ankle unit
- cosmetic cover
Microprocessor knees can improve control in selected transfemoral amputees but require appropriate patient selection, training and funding.
Upper-limb prostheses
Upper-limb systems may be:
- cosmetic/passive
- body powered
- externally powered
- hybrid
Function depends heavily on level of amputation and training. Sensory feedback remains limited compared with the native limb.
Common residual-limb problems
Evaluate:
- neuroma
- skin breakdown
- infection
- heterotopic ossification
- phantom pain
- socket mismatch
- volume fluctuation
- joint contracture
- overuse of the contralateral limb
A “prosthesis problem” may actually be a residual-limb or rehabilitation problem.
FRCS synthesis
When discussing prosthetics, avoid focusing only on components. Start with the patient’s functional goals and residual-limb biology, then select a socket and component system that safely meets those goals.