Prosthetic Limb Components

PTB socket. The patellar tendon bearing socket is the traditional design, developed in the 1950s and still widely used. It concentrates weight-bearing on the patellar tendon, with the medial tibial flare and popliteal area as the other pressure-tolerant areas, and relieves the pressure-sensitive fibular head, tibial crest and distal tibia.

TSB socket. The total surface bearing socket is the modern design and has no specific weight-bearing focus. A gel liner distributes pressure uniformly on the hydrostatic principle, equal pressure throughout, so there is less point loading, and many patients find it more comfortable. It is often combined with suction or vacuum suspension, the liner creating the seal, or with a pin.

Total contact. Both designs keep the entire residual limb in contact with the socket, to prevent distal oedema.

PTB variations. Three variations on the PTB socket:

• PTB-SC (supracondylar): extended medial and lateral walls for rotational control
• PTB-SCSP (supracondylar suprapatellar): a higher anterior trim for suspension
• KBM (Kondylen Bettung Munster): intimate medial-lateral contouring

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Quadrilateral socket. The traditional design (1950s-1980s). Its square brim creates four distinct walls: the ischial tuberosity sits on a posterior shelf, a channel accommodates adductor longus anteriorly, and the lateral wall lies at the greater trochanter. It is less anatomical and controls the femur in adduction less well than ischial containment.

Ischial containment (IC) socket. The modern design (1980s to present) contains the ischial tuberosity and ramus within the socket and has a narrow medial-lateral dimension. It gives more anatomical femoral alignment and superior femoral control compared with the quadrilateral design, and is the modern standard for active transfemoral amputees. Its advantages:

1. Maintains the femur in adduction (anatomical alignment)
2. Better gluteal muscle function
3. Improved stance phase stability
4. More efficient gait pattern
5. Better proprioception and control

CAT-CAM socket. The Contoured Adducted Trochanteric-Controlled Alignment Method is a variant of ischial containment that emphasises femoral adduction alignment. It is narrow medial-laterally and wider anteroposteriorly, with improved muscle function and gait efficiency.

Whether the high ischial walls are essential is now questioned: a small randomised crossover of brimless against ischial ramus containment sockets (Kahle and Highsmith) is carded in the evidence section and discussed under controversies.

Every socket problem needs prosthetist review and socket adjustment.

Volume fluctuation is the most common socket problem. Residual limb volume changes throughout the day, and the limb is smallest in the morning after a night without the prosthesis. It shrinks with weight loss and swells with oedema, and weight gain and muscle atrophy also change it. Management is sock ply adjustment, a liner change or socket modification.

Pistoning is vertical movement of the residual limb within the socket, and indicates a socket that is too large or inadequate suspension. It causes skin shear, blistering and reduced control. Add socks, check the suspension and adjust the socket.

Skin breakdown takes several forms:

• Pressure ulcers at bony prominences
• Shear injuries from pistoning
• Friction blisters from socket movement
• Folliculitis from poor hygiene

Identify the cause, checking socket fit, liner condition and hygiene, then modify the socket and treat the wound.

Residual limb pain has its own list of causes:

• Neuroma at a weight-bearing point
• Bone spur
• Heterotopic ossification
• Poor socket fit

Investigate the cause and modify the socket; some patients need surgery.

At K1-K2 the choice is a mechanical knee, single-axis or polycentric.

Single-axis knee. The simplest design: a single pivot, with friction or a manual lock controlling motion, and weight-activated stance control in some models. It is durable, low-maintenance and inexpensive, but it has no cadence response, giving a single walking speed.

Polycentric knee. Multiple pivot points, typically a four-bar linkage, make the instantaneous centre of rotation move during flexion. In stance that centre lies posterior to the weight line, giving inherent geometric stability, so the knee is more stable than a single-axis design. It shortens functionally in swing, improving toe clearance, and its posterior displacement gives a cosmetic sitting position. It is good for longer residual limbs and ideal for knee disarticulation or nervous ambulators needing stability.

Manual locking knee. Locked in full extension during stance and unlocked by hand for sitting, it gives maximum stability for weak or nervous ambulators at the price of a stiff-legged gait. It is limited to K0-K1 function.

Cadence response. Hydraulic and pneumatic knees vary their resistance with the speed of movement. Faster walking brings more resistance, which prevents knee collapse; slower walking brings less, which allows flexion. The knee mimics natural gait across speeds, the major advantage over mechanical knees and essential for K3 ambulators who walk at varying speeds.

Hydraulic or pneumatic. A hydraulic knee has more resistance range and smoother control and gives a smooth, natural gait, but it is heavier than a mechanical knee and needs periodic maintenance; it is best for K3 community ambulators. A pneumatic cylinder is lighter and simpler, with less resistance range and less precise control, and makes a good intermediate option.

How they work. Sensors detect the gait phase and the knee adjusts its resistance in real time. It detects a stumble and resists buckling, and RCT and systematic-review evidence shows significantly fewer stumbles and uncontrolled falls than with mechanical knees.

The devices. Four are worth knowing:

• C-Leg (Otto Bock), the first commercially successful microprocessor knee. Sensors detect gait phase 50 times per second to adjust hydraulic resistance, stumble recovery resists sudden flexion, and step-over-step stair descent is possible.
• Genium (Otto Bock), with sensors that include a gyroscope and accelerometer. Pre-swing intuition anticipates the swing phase, natural stair ascent is possible, it adjusts to inclines automatically and the gait pattern is more natural.
• Rheo Knee (Ossur), which uses magnetorheological fluid: particles align in a magnetic field to change resistance, giving a very fast response and smooth, intuitive control.
• Power Knee, with active (motorised) knee extension that assists stair climbing and standing from sitting and gives higher energy availability, but it is heavier, more complex and battery dependent.

SACH foot. The solid ankle cushion heel foot is the simplest design: solid construction with no ankle joint and no moving parts. Ankle motion is simulated through the gait cycle:

• Heel strike: the compressible foam heel cushion compresses, simulating plantarflexion and absorbing impact
• Midstance: the rigid structure provides stability
• Push-off: the stiff forefoot keel provides the lever for propulsion

It is durable, waterproof, low-maintenance and inexpensive, but gives no energy return, and suits K1 limited ambulators.

Heel durometer. The hardness of the heel is selected for body weight: softer for lighter or less active patients, firmer for heavier or more active ones.

SAFE foot. The stationary ankle flexible endoskeleton is a SACH variant with a flexible keel. It still has no moving parts, but allows some forefoot flexibility and a smoother rollover than the rigid SACH.

Single-axis foot. One pivot point allows plantarflexion and dorsiflexion, with bumpers controlling the range. Faster plantarflexion at heel strike promotes knee stability, and the transition is smoother than with a SACH, for slightly more weight and maintenance. It suits K1-K2.

Multi-axis foot. Movement in several planes (plantarflexion, dorsiflexion, inversion, eversion and rotation) lets the foot adapt to uneven terrain. It is more complex, requires maintenance and suits K2-K3 ambulators. The benefits:

1. Terrain adaptation, accommodating slopes and uneven ground
2. Reduced shear forces on the residual limb
3. More natural gait on varied surfaces
4. Improved comfort outdoors

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Dynamic response feet. Made of carbon fibre composites, they store energy during stance and release it at push-off, and are lightweight for their performance. They suit K3-K4 active ambulators. The categories:

• Flex-Foot (Ossur), the original carbon fibre design
• Carbon fibre keel feet from various manufacturers
• Split-toe designs, for improved terrain adaptation
• Running-specific blades for sprinting

What "70-80%" means. Load in stance deflects the carbon keel, storing elastic energy that is released at push-off as a propulsion assist. The quoted 70-80% is a bench property of the spring: the proportion of mechanical energy the component gives back under laboratory loading. It is not a reduction in the patient's metabolic cost, and the two must never be quoted as if they were the same number.

The metabolic gain is small. In the Hsu trial carded below, the best-performing foot improved energy cost by about 5% walking and 11% running, and the Flex-Foot showed no significant advantage over the SACH foot at all. The gap exists because returned energy is not necessarily released at the right instant, in the right direction or at the joint that needs it, and because amputee walking cost is dominated by factors the foot cannot fix: absent active plantarflexor push-off, altered hip and trunk work, and asymmetric loading. The clinical case for a dynamic response foot is therefore comfort, terrain capability and higher-speed performance for an active walker, not a large metabolic saving.

Who should have one. Carbon fibre dynamic response feet are indicated for:

1. K3-K4 ambulators (community, variable cadence)
2. Patients wanting efficient gait
3. Young, active individuals
4. Athletes (specialised designs available)

They are not appropriate for K1-K2, because of their stiffness and lack of terrain adaptation.

Running feet. A J-shaped blade for sprinters has no heel and gives maximum energy return. It is used in Paralympic competition and is not for everyday walking.

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Proprio foot (Ossur). A microprocessor controls ankle position. It detects terrain such as stairs and slopes and adjusts the ankle angle automatically. It improves toe clearance on slopes and reduces compensatory hip flexion.

BiOM (Ottobock). A powered ankle-foot system with motorised plantarflexion at push-off. It provides propulsive power, not just return, and gives near-normal walking biomechanics, but it is heavy, expensive and battery dependent.

Active ankles in the balance. Powered push-off reduces energy expenditure, and an active ankle adapts to stairs and slopes, gives more natural gait biomechanics and reduces compensatory movements. Against that stand the cost, concerns over battery life, weight and complexity, and the device is limited to high-level ambulators.

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How it works. A cable and harness system captures movement of the opposite shoulder or trunk (scapular abduction, humeral flexion or chest expansion), and a Bowden cable transmits it to the terminal device. The components:

• Figure-of-8 harness: standard for transradial
• Figure-of-9 harness: for transhumeral, adding elbow control
• Wrist unit: quick disconnect for terminal devices
• Terminal device: hook or voluntary-opening hand

Feedback. Cable tension provides proprioceptive feedback: the user feels how hard they are gripping, which matters for delicate tasks. Many long-term users prefer body-powered prostheses for this feedback.

Trade-offs. Body-powered limbs are durable and reliable, cost less than myoelectric ones, come in waterproof options and work in any environment. Against that, they require body motion (harness effort), grip strength is limited (typically 20-25 lbs), the harness is visible and it can be hot and uncomfortable.

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How it works. Surface EMG electrodes detect muscle contraction, the signals are amplified and processed, and motors drive the terminal device. Control is typically two-site, from antagonist muscles. The control strategies:

• Two-site: flexors open, extensors close (or vice versa)
• Co-contraction: simultaneous contraction switches modes
• Proportional control: signal strength controls speed
• Pattern recognition: advanced multi-movement control

Trade-offs. No harness is required (self-suspended or minimal), grip strength is higher (typically 25-35 lbs), the appearance is more cosmetic and less body movement is needed. Against that, the device is expensive (often greater than 50,000 USD), needs battery charging, is not waterproof in most models, requires myoelectric training and gives no proprioceptive feedback.

Rejection. Myoelectric devices are often said to be rejected more often than body-powered ones, but the Biddiss and Chau survey carded in the evidence section found rejection rates lower for electric than for body-powered devices, in both adults and children.

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Hooks. A voluntary-opening hook is closed by rubber bands and opened by the cable; a voluntary-closing hook is closed by the cable and opened by a spring. The split hook design is common. Hooks give a precise grip for small objects and visual access to the work area, and are durable in work environments.

Hands. Glove-covered hands are cosmetically superior and give a tridigital grasp (thumb, index and middle fingers). They are more complex and less precise than hooks, and the gloves wear and stain.

Choosing. Most experienced users prefer hooks for function: better visibility of grip, more precise control, durability for work and outdoor activities, and lower maintenance costs. Hands are preferred for social situations and cosmesis, office work and light activities, and when the patient prefers the appearance. Many users have both and switch with the activity.

Activity-specific devices. A quick-change wrist allows switching between them:

• Work hooks (heavy duty)
• Sports terminal devices
• Guitar and music holders
• Recreational attachments

Targeted muscle reinnervation (TMR). A surgical procedure transfers arm nerves to the chest, and the reinnervated muscles provide control sites. Myoelectric control becomes more intuitive, with multiple independent movements possible, and pattern recognition is combined with TMR.

Osseointegration. A titanium implant fused to bone gives direct skeletal attachment with no socket, and an abutment protrudes through the skin. It improves proprioception and control and eliminates socket problems; the risk is infection at the skin-abutment interface.

Multi-articulating hands. They allow more natural grip patterns but are very expensive and complex:

• i-Limb (Ossur): individual finger motors
• bebionic (Ottobock): multiple grip patterns
• LUKE Arm (Mobius): advanced shoulder-level options

Emerging technology. Approaches under development include:

• Sensory feedback systems (haptic feedback)
• Brain-computer interfaces (experimental)
• 3D-printed custom devices
• Soft robotics approaches