Below-Knee Prosthetics

Below-knee, or transtibial, prosthetics are artificial limbs designed to replace the lower part of the leg, typically from below the knee joint to the foot. These prosthetic devices allow individuals who have undergone lower limb amputations to regain mobility and lead active lives.

The socket is the most critical component of a transtibial prosthetic limb, as it provides a custom-fit interface between the user's residual limb and the rest of the prosthesis. The socket's design and fit are crucial for comfort, stability, and proper weight distribution. It's typically made from durable materials like thermoplastics, carbon fiber composites, or a combination of materials, ensuring both strength and lightweight construction.

Socket fabrication often involves a meticulous process of taking impressions or scans of the residual limb to create a comfortable and secure fit. It can be designed with various features like locking mechanisms, adjustable components, and modular sections to accommodate changes in the user's limb shape or volume over time. The goal is to minimize pressure points, prevent skin irritation, and provide a secure connection for weight-bearing and mobility.

The pylon is a vertical strut that connects the socket to the foot or ankle component of the transtibial prosthetic limb. It serves several essential functions within the prosthetic system. Firstly, it provides structural support, helping to distribute the user's weight evenly and ensuring the prosthesis remains stable during various activities. The pylon's material choice, such as aluminum, titanium, or carbon fiber, impacts its strength-to-weight ratio, influencing the overall weight and performance of the prosthetic limb.

Secondly, the pylon allows for adjustments in the prosthesis's height. This feature is crucial for accommodating different footwear and optimizing the user's gait pattern. By modifying the pylon's length or using adaptive components, prosthetists can fine-tune the alignment to achieve the best possible fit and function.

The pylon's design may vary based on the user's specific needs, activity level, and the type of foot or ankle component used. Some pylons include shock-absorbing features to reduce impact forces during walking or running, while others may have built-in rotation mechanisms for more natural mobility.

The foot and ankle components are a critical part of a transtibial prosthetic limb, as it replicates the functions of a natural foot and ankle. Various types of foot and ankle components are available, each tailored to meet different user requirements and activity levels.

This basic foot component offers stability and limited movement in a single plane, suitable for individuals with lower activity levels.

A more advanced option, this component allows movement in multiple planes, resembling the natural range of motion of an ankle. It provides improved stability and adaptability for uneven terrain.

These prosthetic feet store and release energy during walking, simulating a more natural gait and providing greater energy efficiency, which is especially valuable for active individuals.

Some prosthetic feet are designed for specific activities, such as swimming or water sports, and are waterproof. There are also specialty feet for activities like running, hiking, or cycling.

The suspension system is an integral part of a transtibial prosthetic limb, responsible for securely holding the prosthesis in place on the user's residual limb. Proper suspension is crucial to prevent the prosthesis from slipping or moving during various activities. There are several common suspension methods:

1. Straps and Belts: Many prosthetic limbs use adjustable straps or belts that encircle the user's waist or thigh to hold the socket in position. These are often combined with a cushioning liner for comfort.
2. Silicone Liners: Some users prefer silicone liners that create a secure seal between the residual limb and the socket. These liners conform to the limb's shape and provide suction-based suspension, effectively locking the prosthesis in place.
3. Vacuum-Assisted Suspension: Advanced prosthetic systems incorporate vacuum-assisted suspension technology. A pump removes air from the socket, creating a vacuum seal for secure suspension. This method offers enhanced stability and comfort.

The choice of suspension method depends on the user's preference, lifestyle, and residual limb anatomy. Proper fitting and alignment are critical for effective suspension, as an ill-fitting suspension system can lead to discomfort and reduced mobility.

Achieving proper alignment in a transtibial prosthetic limb is essential for ensuring comfort, stability, and natural gait. The alignment mechanism allows prosthetists to fine-tune the orientation of the socket, pylon, and foot or ankle component to match the user's unique needs. Key aspects of alignment include:

1. Frontal and Sagittal Alignment: Adjusting the prosthesis's alignment in the front-to-back and side-to-side planes to ensure the user walks with proper posture and balance.
2. Vertical Alignment: Ensuring the prosthesis is at the correct height relative to the user's sound limb and optimizing weight distribution.
3. Rotational Alignment: Fine-tuning the rotation of the socket and foot component to align with the user's natural walking motion.
4. Dynamic Alignment: Considering how the prosthesis functions during walking, including factors like knee stability, foot roll-over, and gait symmetry.

Prosthetists use alignment tools and measurements, combined with feedback from the user, to optimize alignment. Accurate alignment minimizes excessive wear on components, reduces the risk of discomfort or pain, and promotes a more natural walking pattern.

Liners and padding play a critical role in ensuring comfort, reducing friction, and preventing skin irritation for transtibial prosthetic users. These components are placed between the residual limb and the socket. Key considerations include:

1. Cushioning: Liners are typically made from soft, cushioning materials like silicone or foam. They distribute pressure evenly, reduce shear forces, and protect the skin from excessive friction.
2. Proper Fit: Liners are available in various sizes and thicknesses to accommodate individual preferences and needs. A well-fitted liner enhances comfort and stability.
3. Skin Health: Regular cleaning and care of liners are essential to maintain skin health and prevent issues like pressure sores or blisters.
4. Socket Interface: Liners create a smooth interface between the residual limb and the socket, enhancing suspension and comfort.

Connectors and adapters are essential components that enable the assembly and adjustability of a transtibial prosthetic limb. These components allow prosthetists to connect the socket, pylon, and foot or ankle component while ensuring proper alignment. Key considerations include:

1. Quick-Release Systems: Some connectors feature quick-release mechanisms, making it easier to attach and detach the prosthesis or adjust its components.
2. Modularity: Many transtibial prosthetic limbs are designed with modular components, allowing for easy upgrades or replacements as needed.
3. Alignment Adjustments: Connectors and adapters often include features for fine-tuning alignment and optimizing fit.

• Amputee Coalition Official Website
• International Society for Prosthetics and Orthotics (ISPO) Website
• MIT Media Lab - Biomechatronics Group