Above-Knee Prosthesis: Components, Function, and Advanced Features

What are the components of the above knee prosthesis?

An above-knee (transfemoral) prosthesis is a sophisticated medical device designed to replace a limb lost above the knee, comprising an intricate assembly of components that work in concert to restore mobility, function, and quality of life for the user.

An above-knee amputation, medically termed a transfemoral amputation, necessitates a prosthetic limb that can replicate the complex functions of the lost biological knee, lower leg, and foot. Unlike below-knee prostheses, the above-knee variant must incorporate an artificial knee joint, making its design and functional requirements considerably more intricate. Understanding these components is crucial for appreciating the biomechanical challenges and engineering solutions involved in modern prosthetics.

The Core Components of an Above-Knee Prosthesis

A transfemoral prosthesis is a highly individualized device, custom-designed to meet the specific needs, activity levels, and anatomical characteristics of each user. Despite variations, all above-knee prostheses share several fundamental components:

The Socket

The socket is arguably the most critical component of any prosthesis, serving as the direct interface between the residual limb (stump) and the prosthetic device. Its primary function is to comfortably and securely contain the residual limb while efficiently transferring forces from the body to the prosthesis.

• Custom Fit: Sockets are meticulously custom-fabricated from a mold of the individual's residual limb. This ensures an intimate fit, crucial for comfort, stability, and preventing skin breakdown or tissue damage.
• Materials: Typically made from lightweight, durable materials such as carbon fiber, fiberglass, or thermoplastics.
• Design Principles: Modern sockets often incorporate specific features like ischial containment (to bear weight on the ischial tuberosity, similar to sitting) or subischial designs, aiming to maximize comfort and control while minimizing pressure points.
• Biomechanics: A well-designed socket optimizes load distribution, allowing the user to bear weight and control the prosthesis with minimal discomfort and maximum proprioceptive feedback.

The Suspension System

The suspension system is the mechanism by which the prosthesis remains securely attached to the residual limb during all activities, preventing slippage or "pistoning" (the residual limb moving up and down within the socket).

• Suction Suspension: Creates a vacuum seal between the socket and the limb, often aided by a one-way valve. This provides excellent control and proprioception.
• Pin/Lock System: A pin attached to the distal end of a liner engages with a locking mechanism in the bottom of the socket. This is a common and secure method.
• Sleeve Suspension: A silicone or neoprene sleeve rolled over the top of the socket and onto the thigh creates a seal.
• Vacuum Suspension: Active vacuum pumps (mechanical or electronic) continuously draw air out of the socket, creating a powerful and consistent suspension.
• Liner: Often a gel or silicone interface worn directly on the residual limb, providing cushioning and protecting the skin while facilitating various suspension methods.

The Knee Joint

The knee joint is the pivotal component that replaces the anatomical knee, enabling flexion and extension and facilitating a natural gait. Its complexity varies significantly based on the user's activity level and functional goals.

• Single-Axis Knee: A simple hinge joint allowing only flexion and extension. It's durable but offers less gait variability.
• Polycentric Knee: Features a moving center of rotation, which can enhance stability during stance phase and provide a more natural swing phase.
• Hydraulic/Pneumatic Knees: Utilize fluid (hydraulic) or air (pneumatic) resistance to control the speed of knee flexion and extension during the swing phase, allowing for a more fluid and adaptable gait at varying speeds.
• Microprocessor-Controlled Knees (MPK): These advanced knees use sensors to monitor gait parameters (e.g., speed, ground reaction forces) and adjust resistance in real-time. This provides superior stability, reduces the risk of falls, and allows for more natural movement across different terrains and activities. Examples include the C-Leg, Genium, and Rheo Knee.

The Shank (Pylon)

The shank, often referred to as the pylon, is the structural support that connects the knee unit to the foot-ankle assembly.

• Materials: Typically made from lightweight yet strong materials like carbon fiber, aluminum, or titanium.
• Function: It bears the body's weight, provides structural integrity, and often allows for adjustments in height and alignment.
• Cosmetic Cover: A foam or silicone cover can be placed over the pylon to give the prosthesis a more natural anatomical shape, often matched to the user's skin tone.

The Foot-Ankle Assembly

The foot-ankle assembly replaces the biological foot and ankle, crucial for shock absorption, balance, and propulsion during walking.

• SACH (Solid Ankle Cushioned Heel) Foot: A basic, durable, and cost-effective foot that provides limited ankle motion, primarily through the compression of a cushioned heel.
• Single-Axis Foot: Allows for dorsiflexion and plantarflexion, providing some shock absorption and smoother roll-over.
• Multi-Axis Foot: Allows for movement in multiple planes (dorsiflexion/plantarflexion, inversion/eversion, abduction/adduction), offering better adaptation to uneven terrain and improved balance.
• Energy-Storing (Dynamic Response) Feet: These feet incorporate carbon fiber or other flexible materials that store energy during mid-stance and release it during push-off, providing a more dynamic and energy-efficient gait, especially beneficial for active individuals.
• Microprocessor-Controlled Feet: Some advanced systems integrate microprocessor technology into the foot and ankle, offering active dorsiflexion, power push-off, and improved stability on slopes and stairs.

Advanced Features and Considerations

Modern prosthetic technology continues to evolve, integrating advanced electronics, robotics, and materials science. Features like integrated sensor systems within sockets can provide feedback on pressure distribution, while powered prostheses aim to actively assist movement rather than simply respond to it. The entire system is often interconnected, with components communicating to optimize overall function.

The Role of the Prosthetist and Rehabilitation Team

The successful use of an above-knee prosthesis is a collaborative effort involving the individual with amputation, a prosthetist (who designs, fabricates, and fits the prosthesis), and a physical therapist. The physical therapist plays a crucial role in gait training, strengthening, balance exercises, and teaching the user how to maximize the function of their specific prosthetic components. This interdisciplinary approach ensures optimal fit, function, and rehabilitation outcomes.

Conclusion

The above-knee prosthesis is a marvel of biomechanical engineering, a complex synergy of custom-fit components designed to restore mobility and independence. From the intimate interface of the socket to the dynamic capabilities of microprocessor-controlled knee and foot units, each part plays a vital role in mimicking human locomotion. As technology advances, these devices become increasingly sophisticated, offering users greater comfort, stability, and the ability to engage in a wider range of activities, fundamentally transforming lives.