PROSTHETIC LIMB COMPONENTS
Socket Design | Suspension Systems | Knee Units | Prosthetic Feet | Upper Limb Prosthetics
K-LEVEL FUNCTIONAL CLASSIFICATION
Critical Must-Knows
- Socket types: PTB (patellar tendon bearing) vs TSB (total surface bearing) for transtibial; quadrilateral vs ischial containment for transfemoral
- Suspension systems: Pin lock (mechanical), suction (seal-in liner), vacuum (active pump), sleeve (neoprene/gel)
- Knee units: Single-axis (simple, durable), polycentric (stability), hydraulic/pneumatic (cadence responsive), microprocessor (C-Leg, Genium)
- Prosthetic feet: SACH (solid ankle cushion heel), single/multi-axis, dynamic response (energy storing/returning)
- K-levels (K0-K4) determine component prescription - match complexity to functional capacity
Examiner's Pearls
- "TSB socket distributes load over entire residual limb vs PTB which concentrates on patellar tendon
- "Ischial containment socket provides better femoral control than quadrilateral
- "Microprocessor knees (C-Leg) reduce falls by 64% compared to mechanical knees
- "Energy-storing feet return 70-80% of stored energy during push-off
Critical Prosthetic Components Exam Points
Socket Fit Problems
Volume fluctuation is the most common socket problem - residual limb shrinks with weight loss, expands with edema. Pistoning (vertical movement) indicates poor suspension or socket looseness. Skin breakdown occurs at pressure points - check socket fit, liner condition, and hygiene. All require prosthetist review and socket adjustment.
Knee Unit Selection
Match knee complexity to K-level. K1-K2: Single-axis or polycentric mechanical knee. K3: Hydraulic or pneumatic (cadence responsive). K4 or high falls risk: Microprocessor knee (C-Leg, Genium). Microprocessor knees detect stumble and resist buckling - proven to reduce falls by 64%.
Prosthetic Foot Selection
SACH foot: Simple, durable, low activity (K1). Single-axis: Allows plantarflexion for knee stability (K1-K2). Multi-axis foot: Terrain adaptation (K2-K3). Dynamic response (energy-storing): Carbon fiber, returns energy for efficient gait (K3-K4). Match foot to activity level and terrain requirements.
Upper Limb Prosthetics
Body-powered: Cable-operated, reliable, provides proprioceptive feedback, lower cost. Myoelectric: EMG-controlled, cosmetically superior, higher grip strength. Terminal devices: Hooks (functional, durable) vs hands (cosmetic, complex). Higher rejection rates in upper limb than lower limb prosthetics.
Prosthetic Knee Unit Types - Comprehensive Comparison
| Knee Type | Mechanism | Best For (K-Level) | Advantages | Disadvantages |
|---|---|---|---|---|
| Single-axis | Simple hinge with friction control | K1-K2 (limited ambulators) | Durable, low maintenance, inexpensive | No cadence response, manual lock often needed |
| Polycentric (4-bar) | Multiple pivot points, shortens in swing | K2-K3 (stability needed) | Inherent stance stability, toe clearance in swing | Heavier, more complex mechanism |
| Hydraulic | Fluid resistance varies with speed | K3 (variable cadence) | Cadence-responsive, smooth gait at all speeds | Heavier, requires maintenance, more expensive |
| Pneumatic | Air resistance varies with speed | K3 (lighter option) | Lighter than hydraulic, cadence-responsive | Less resistance range than hydraulic |
| Microprocessor (C-Leg, Genium) | Sensors detect gait phase, adjust resistance | K3-K4 or falls risk | Stumble recovery, stairs descent, reduced falls | Expensive, battery dependent, requires training |
K-LEVELSK-Level Functional Classification
Memory Hook:K-LEVELS tell you what components to prescribe - higher level, higher technology
PTB vs TSBSocket Types for Transtibial
Memory Hook:PTB focuses on patellar tendon, TSB spreads load over Total Surface - both maintain full contact
SACH-MDRProsthetic Foot Types
Memory Hook:Start Simple with SACH, Add Motion with axes, go Dynamic for high activity
PINSSuspension Systems
Memory Hook:PINS hold the prosthesis on - choose based on activity level and residual limb
Overview and Prosthetic Fundamentals
Prosthetic limb components are the elements that make up an artificial limb system. Understanding these components is essential for orthopaedic surgeons involved in amputation surgery and post-operative rehabilitation planning.
Key Principles of Prosthetic Prescription:
- Match components to functional level - K-level classification guides selection
- Socket fit is paramount - the interface between residual limb and prosthesis
- Suspension must be reliable - prevents pistoning and skin breakdown
- Knee and foot selection affects gait efficiency - higher function = more advanced components
- Early prosthetist involvement - ideally preoperative for optimal stump planning
The Prosthetic Prescription Hierarchy
Components are prescribed based on the K-Level functional classification. K0: No prosthetic ambulation. K1: Basic components for limited household use. K2: Intermediate components for limited community ambulation. K3: Advanced components for unlimited community ambulation with variable cadence. K4: High-activity components for athletes. Over-prescribing wastes resources; under-prescribing limits function.
Basic Prosthetic Anatomy:
Lower Limb Prosthesis Components
- Socket: Interface with residual limb (most critical component)
- Liner: Cushioning layer between skin and socket
- Suspension system: Keeps prosthesis attached
- Pylon/shank: Connects socket to foot (or knee to foot)
- Knee unit (transfemoral): Controls swing and stance
- Prosthetic foot: Ground contact and energy return
Upper Limb Prosthesis Components
- Socket: Interface with residual limb
- Suspension: Harness or suction
- Elbow unit (transhumeral): Controls flexion/extension
- Wrist unit: Allows pronation/supination, quick disconnect
- Terminal device: Hook or hand for function/cosmesis
- Control system: Body-powered cables or myoelectric
Socket Design Principles
The socket is the most critical component of any prosthesis - it is the interface between the residual limb and the artificial limb. Poor socket fit leads to skin breakdown, pain, and prosthetic rejection.
Socket Design Goals
- Comfortable weight distribution - pressure on tolerant areas, relief over sensitive areas
- Stable suspension - prevents pistoning and rotation
- Proprioceptive feedback - allows control of prosthesis
- Cosmesis - acceptable appearance
- Durability - withstands daily use
Transtibial Socket Designs
PTB (Patellar Tendon Bearing) Socket:
- Traditional design developed in 1950s
- Weight-bearing concentrated on patellar tendon
- Pressure-tolerant areas: patellar tendon, medial tibial flare, popliteal area
- Pressure-sensitive areas (relieved): fibular head, tibial crest, distal tibia
- Total contact maintained for edema control
TSB (Total Surface Bearing) Socket:
- Modern design distributing pressure uniformly
- No specific weight-bearing focus
- Uses gel liner to distribute pressure
- Hydrostatic loading principle - equal pressure throughout
- Often combined with suction or vacuum suspension
Socket Variations:
- PTB-SC (Supracondylar): Extended medial-lateral walls for rotational control
- PTB-SCSP (Supracondylar Suprapatellar): Higher anterior trim for suspension
- KBM (Kondylen Bettung Munster): Intimate medial-lateral contouring
PTB vs TSB Socket Design
PTB: Focuses weight on patellar tendon with relief areas. Traditional, still widely used. TSB: Distributes pressure over entire surface using gel liner. More comfortable for many patients. Both maintain total contact - the entire residual limb touches the socket to prevent distal edema.
Suspension Systems
Suspension keeps the prosthesis securely attached to the residual limb. Inadequate suspension leads to pistoning, reduced control, and skin problems.
Prosthetic Suspension Systems Comparison
| Suspension Type | Mechanism | Advantages | Disadvantages | Best For |
|---|---|---|---|---|
| Pin lock (shuttle lock) | Pin on liner engages lock in socket | Simple, secure, easy don/doff | Pistoning possible, milking effect on tissues | K1-K2, limited dexterity |
| Suction (seal-in liner) | Sealing lip on liner creates vacuum | Intimate fit, good suspension | Difficult don/doff, requires intact liner | K2-K3, good hand function |
| Vacuum (elevated vacuum) | Active pump maintains negative pressure | Excellent suspension, volume management | Expensive, battery dependent, complex | K3-K4, volume fluctuation issues |
| Sleeve suspension | Neoprene or gel sleeve over socket rim | Simple, inexpensive, adds stability | Hot, may irritate skin, stretches over time | Additional suspension, K1-K2 |
| Anatomical suspension | Socket contour locks over bony prominences | No additional hardware needed | Requires specific residual limb anatomy | Knee disarticulation, Syme |
Suspension Selection Principles
Active Patients (K3-K4)
- Suction or vacuum suspension preferred
- Intimate fit maximizes control
- Vacuum systems help with volume management
- Worth the complexity for high-activity users
- Consider elevated vacuum for variable activity
Less Active Patients (K1-K2)
- Pin lock often most practical
- Easy donning/doffing
- Simple mechanism to understand
- Less reliance on hand dexterity
- Sleeve suspension as adjunct
Elevated Vacuum Suspension Benefits
Vacuum suspension with active pump provides:
- Consistent negative pressure maintaining fit
- Reduces volume fluctuation effects
- Decreases pistoning significantly
- Improves proprioceptive feedback
- May improve residual limb health
Ideal for active amputees with volume management issues. More expensive and complex than passive systems.
Prosthetic Knee Units
Knee units are required for transfemoral, knee disarticulation, and hip disarticulation amputees. The knee must provide stability in stance and controlled motion in swing phase.
Mechanical Knee Units
Single-Axis Knee:
- Simplest design - single pivot point
- Friction or manual lock controls motion
- Weight-activated stance control (some models)
- Durable, low maintenance, inexpensive
- Suitable for K1-K2 ambulators
- No cadence response - single walking speed
Polycentric (Multi-Axis) Knee:
- Multiple pivot points (typically 4-bar linkage)
- Instantaneous center of rotation moves during flexion
- Inherent geometric stability in stance
- Shortens in swing phase (improved toe clearance)
- Good for long residual limbs, knee disarticulation
- More stable than single-axis
Manual Locking Knee:
- Locked in full extension during stance
- Manually unlocked for sitting
- Maximum stability for weak or nervous ambulators
- Limited to K0-K1 function
- Stiff-legged gait
Polycentric Knee Advantages
Four-bar polycentric knees offer:
- Inherent stance stability (center of rotation posterior to weight line)
- Functional shortening in swing (toe clearance)
- Cosmetic sitting position (posterior displacement)
- Good for longer residual limbs
Ideal for knee disarticulation or nervous ambulators needing stability.
Prosthetic Feet
The prosthetic foot provides ground contact, shock absorption, and energy return during gait. Selection depends on activity level, terrain requirements, and patient goals.
SACH and Basic Prosthetic Feet
SACH Foot (Solid Ankle Cushion Heel):
- Simplest prosthetic foot design
- No moving parts - solid construction
- Compressible foam heel cushion
- Simulates ankle plantarflexion at heel strike
- Rigid forefoot (keel) for push-off
- Durable, low maintenance, inexpensive
- Suitable for K1 limited ambulators
SAFE Foot (Stationary Ankle Flexible Endoskeleton):
- SACH variant with flexible keel
- Allows some forefoot flexibility
- Smoother rollover than rigid SACH
- Still no moving parts
Key Features of SACH:
- Heel durometer (hardness) selected for body weight
- Softer heel for lighter/less active patients
- Firmer heel for heavier/more active patients
- Waterproof, minimal maintenance
SACH Foot Mechanism
The SACH foot has no ankle joint. Ankle motion is simulated:
- Heel strike: Compressible heel cushion plantarflexes to absorb impact
- Midstance: Rigid structure provides stability
- Push-off: Stiff forefoot keel provides lever for propulsion
Simple, durable, but no energy return. Appropriate for low-activity K1 patients.
Upper Limb Prosthetics
Upper limb prosthetics present unique challenges compared to lower limb. The hand's complexity (27 bones, 18 degrees of freedom) cannot be replicated. Prosthetic options provide partial function or cosmesis.
Body-Powered Prosthetics
Mechanism:
- Cable and harness system
- Movement of opposite shoulder or trunk
- Bowden cable transmits motion to terminal device
- Scapular abduction, humeral flexion, or chest expansion
Components:
- Figure-of-8 harness: Standard for transradial
- Figure-of-9 harness: For transhumeral, adds elbow control
- Wrist unit: Quick disconnect for terminal devices
- Terminal device: Hook or voluntary-opening hand
Advantages:
- Proprioceptive feedback through cable tension
- Durable and reliable
- Lower cost than myoelectric
- Waterproof options available
- Works in any environment
Disadvantages:
- Requires body motion (harness effort)
- Limited grip strength (typically 20-25 lbs)
- Visible harness system
- Can be hot and uncomfortable
Body-Powered Prosthetic Feedback
Proprioceptive feedback is a key advantage of body-powered prosthetics:
- Cable tension provides sensory information
- User feels how hard they are gripping
- Important for delicate tasks
- This is lost with myoelectric prosthetics
Many long-term users prefer body-powered for this feedback.
Upper Limb Prosthetic Rejection
Rejection rates are higher for upper limb than lower limb prosthetics:
- Transradial: 20-30% rejection
- Transhumeral: 30-50% rejection
- Higher rejection with proximal amputation
Reasons for Rejection:
- Insufficient function compared to remaining abilities
- Discomfort with socket and harness
- Weight of prosthesis
- Appearance concerns
- Lack of sensory feedback
- Difficulty learning myoelectric control
Upper Limb Prosthetic Acceptance
Keys to successful upper limb prosthetic use:
- Early fitting (within 30 days if possible)
- Comprehensive training program
- Realistic expectations counseling
- Multiple device options for different activities
- Ongoing prosthetist and therapy support
Delay beyond 6 months significantly reduces acceptance rates.
K-Level Classification and Functional Outcomes
Medicare Functional Classification Levels (K-Levels)
K-Level Classification and Component Prescription
| K-Level | Functional Description | Prosthetic Components | Expected Outcomes |
|---|---|---|---|
| K0 | Non-ambulatory, cannot use prosthesis | Cosmetic prosthesis only, wheelchair | No prosthetic ambulation expected |
| K1 | Household ambulator, transfers | SACH foot, single-axis knee, pin suspension | Limited indoor walking, standing |
| K2 | Limited community ambulator | Multi-axis foot, polycentric knee | Community distances, low obstacles |
| K3 | Unlimited community ambulator, variable cadence | Dynamic response foot, hydraulic knee, vacuum suspension | Variable speed, terrain adaptation |
| K4 | Active athlete, high-impact activities | Carbon fiber foot, microprocessor knee, specialized sport components | Running, sports, exceeds basic ambulation |
Functional Assessment Tools
Amputee Mobility Predictor (AMP/AMPnoPRO):
- Validated tool predicting prosthetic mobility potential
- 21-item assessment (without prosthesis version available)
- Scores correlate with K-level classification
- Used preoperatively and during rehabilitation
Timed Up and Go (TUG):
- Standard mobility measure
- Rise from chair, walk 3m, return, sit
- Greater than 19 seconds suggests falls risk
6-Minute Walk Test:
- Endurance assessment
- Distance correlates with community ambulation
- Greater than 200m suggests community ambulatory potential
L-Test:
- Modified TUG with turns
- More challenging than TUG
- Better predicts community mobility
K-Level Determination
K-level is determined by:
- Prior functional level (pre-amputation)
- Current physical examination
- Comorbidities and healing
- Cognitive ability
- Motivation and goals
K-level can change - reassess at follow-up. K2 patient may progress to K3 with training. Deteriorating health may reduce K-level.
Evidence Base and Key Studies
Microprocessor Knee Units and Falls Prevention
- Prospective study of C-Leg microprocessor knee vs mechanical knee
- 64% reduction in stumbles and falls with C-Leg
- Improved stair descent confidence and speed
- Reduced cognitive load during ambulation
- Patients preferred microprocessor knee 12:1
Energy Cost of Ambulation with Prostheses
- Quantified energy expenditure by amputation level
- Transtibial: 40-60% increase above normal
- Transfemoral: 90-120% increase above normal
- Vascular amputees walk at self-selected comfortable speed
- Energy efficiency improved with better prosthetic components
Dynamic Response Feet Energy Return
- Compared SACH, single-axis, and dynamic response feet
- Energy-storing feet return 60-80% of stored energy
- 15-25% reduction in oxygen consumption vs SACH
- Improved gait symmetry with dynamic feet
- Higher satisfaction scores with carbon fiber feet
Socket Design: Ischial Containment vs Quadrilateral
- Comparative study of socket designs in transfemoral amputees
- Ischial containment provides superior femoral control
- Improved stance stability with IC sockets
- Better gait kinematics and reduced compensatory movements
- Higher patient satisfaction with ischial containment
Upper Limb Prosthetic Rejection Rates
- Systematic review of upper limb prosthetic use and rejection
- Overall rejection rates: 20-30% for body-powered, 23-35% for myoelectric
- Higher rejection with more proximal amputation
- Early fitting improves acceptance significantly
- Lack of sensory feedback major reason for rejection
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
"A 45-year-old active male has undergone transtibial amputation for trauma 3 months ago. He is a construction worker who wants to return to work. What prosthetic components would you recommend for him?"
"A 72-year-old woman with diabetes and peripheral vascular disease has had a transfemoral amputation. She was previously mobile with a walking frame indoors only. What knee unit and foot would you recommend?"
"A 35-year-old transfemoral amputee reports multiple falls over the past 6 months, including one resulting in a hip fracture. He is currently using a mechanical hydraulic knee. What would you recommend?"
Australian Context
In Australia, prosthetic limb services are provided through a combination of state-funded limb services, the National Disability Insurance Scheme (NDIS), and private health insurance. Understanding the Australian system is important for appropriate referral and patient counseling.
State and Territory Limb Services: Each state and territory operates a prosthetic limb service providing prosthetics to eligible amputees. These services have traditionally been the primary providers, though the NDIS has changed the landscape significantly. State services typically provide assessment, fitting, and ongoing maintenance. Wait times vary by jurisdiction and complexity of prescription.
National Disability Insurance Scheme (NDIS): The NDIS provides funding for prosthetics for eligible participants under age 65 at time of application. Prosthetics are considered assistive technology under NDIS plans. High-cost items like microprocessor knees require justification demonstrating functional benefit. NDIS funding allows greater choice of prosthetist and components, though the process can be complex and slow.
PBS Considerations: Medications for residual limb pain, phantom limb pain, and wound care are available on the Pharmaceutical Benefits Scheme. Gabapentin and pregabalin are PBS-listed for neuropathic pain. Amitriptyline and duloxetine are also available. Wound care products for socket-related skin issues may be available through Stoma Appliance Scheme for some patients.
Quitline (13 78 48): Smoking cessation is critical for amputation wound healing and long-term prosthetic outcomes. Referral to Quitline for free telephone coaching and support should be offered to all smoking amputees.
Prosthetic Limb Components - Exam Quick Reference
High-Yield Exam Summary
K-Level Classification
- •K0: Non-ambulatory - cosmetic prosthesis only
- •K1: Household ambulator - SACH foot, single-axis or manual lock knee
- •K2: Limited community - multi-axis foot, polycentric knee
- •K3: Unlimited community - dynamic response foot, hydraulic/microprocessor knee
- •K4: Active athlete - specialized high-activity components
Socket Types
- •Transtibial: PTB (patellar tendon bearing) vs TSB (total surface bearing)
- •PTB focuses weight on patellar tendon, TSB distributes evenly
- •Transfemoral: Quadrilateral (old) vs Ischial Containment (modern standard)
- •Ischial containment provides better femoral control and gait
- •All sockets should have total contact to prevent distal edema
Suspension Systems
- •Pin lock: Simple, reliable, easy don/doff (K1-K2)
- •Suction: Intimate fit, good for active (K2-K3)
- •Vacuum (elevated): Best volume management, complex (K3-K4)
- •Sleeve: Simple adjunct, can cause sweating
- •Poor suspension causes pistoning and skin breakdown
Knee Units
- •Single-axis: Simple, durable, no cadence response (K1-K2)
- •Polycentric (4-bar): Inherent stability, shortens in swing (K2-K3)
- •Hydraulic: Cadence-responsive, smooth gait (K3)
- •Microprocessor (C-Leg/Genium): 64% fall reduction, stumble recovery (K3-K4)
- •Match knee to K-level - dont overprescribe or underprescribe
Prosthetic Feet
- •SACH: Simple, no moving parts, compressible heel (K1)
- •Single-axis: Plantarflexion for knee stability (K1-K2)
- •Multi-axis: Terrain adaptation, inversion/eversion (K2-K3)
- •Dynamic response: Carbon fiber, 70-80% energy return (K3-K4)
- •Microprocessor feet: Active ankle control, stair/slope adaptation (K4)
Upper Limb Prosthetics
- •Body-powered: Cable control, proprioceptive feedback, durable
- •Myoelectric: EMG control, higher grip, no feedback, expensive
- •Terminal devices: Hooks (functional) vs Hands (cosmetic)
- •Rejection rates 20-30% for transradial, higher proximal
- •Early fitting (less than 30 days) improves acceptance
Socket Problems
- •Volume fluctuation: Most common - sock ply adjustment needed
- •Pistoning: Socket loose or suspension inadequate
- •Skin breakdown: Check fit, bony prominences, hygiene
- •All problems require prosthetist review
- •Socket is the most critical component - fit determines success