A prosthetic leg is routinely available and highly advanced for amputations that occur above the knee. Modern prosthetic technology has transformed the lives of individuals with this level of limb loss. Today’s devices are sophisticated systems that incorporate advanced materials and computer-controlled mechanics to restore a high degree of mobility and functional independence. These systems are custom-designed to interface precisely with the user’s body, manage complex joint movement, and allow for activities ranging from walking on level ground to navigating stairs and uneven terrain.
Defining the Transfemoral Prosthesis
The clinical term for an above-the-knee amputation is a transfemoral amputation (TFA), referring to the surgical procedure through the femur, the long bone of the thigh. The artificial limb designed to replace a leg lost at this level is called a transfemoral prosthesis. This prosthesis must replace the function of the missing knee joint, distinguishing it from a below-the-knee, or transtibial, prosthesis.
A complete transfemoral prosthesis is constructed from several interconnected components. These include a custom-fitted socket, a knee unit, a pylon, and a prosthetic foot. The socket serves as the interface with the residual limb, while the pylon acts as the structural shank, often made of lightweight material like carbon fiber.
The Role of the Socket and Suspension
The prosthetic socket is the interface between the residual limb and the artificial leg, determining comfort, control, and function. This component is custom-molded to fit securely over the residual limb, often made from laminated material or thermoplastic. A proper fit is necessary because the socket must distribute the forces of body weight and movement across the residual limb while preventing skin breakdown.
The goal of the socket design is to achieve a total contact fit, supporting the entire surface of the residual limb. This intimate contact ensures efficient energy transfer from the user to the prosthesis and is important for controlling the limb during walking. If the socket does not fit correctly, the limb can shift inside the socket, a problem known as “pistoning,” which leads to pain and blisters.
Securing the prosthesis to the body is managed by a suspension system, which prevents the artificial limb from falling off during activity. Common methods include mechanical locking systems and suction systems. Pin-locking suspension uses a liner worn on the residual limb that locks into the socket. Suction suspension creates a seal between the socket and the limb, often using a one-way valve to force air out and create a vacuum to hold the socket firmly in place.
An advanced suspension method is the elevated vacuum system, which uses a mechanical or electronic pump to actively draw air out of the socket. This continuous vacuum maintains a secure and stable connection, minimizing movement between the residual limb and the socket. It also helps manage volume fluctuations in the limb and promotes better fluid exchange, contributing to skin health.
Understanding Advanced Knee Joint Technology
The prosthetic knee joint is the defining mechanical feature of a transfemoral prosthesis, replacing the intricate, dynamic function of the natural knee. Prosthetic knees are broadly categorized into mechanical systems and advanced computerized systems. Mechanical knees, such as manual locking or polycentric knees, use simple pivots and fixed friction mechanisms to control movement. A manual locking knee offers maximum stability by remaining locked, while polycentric knees offer a smoother, more stable swing phase.
More sophisticated options include hydraulic or pneumatic systems, which use fluid or air pressure to control the speed and resistance of the knee. These systems provide variable resistance, allowing the user to walk at different speeds with a more natural gait pattern. The resistance automatically adjusts during both the stance phase (when the foot is on the ground) and the swing phase (when the foot is moving forward).
The most advanced technology is the microprocessor-controlled knee (MPK), which uses sensors and sophisticated algorithms to adjust resistance in real-time. MPKs measure parameters like knee angle and walking speed to provide immediate control over the hydraulic or pneumatic resistance. This real-time adaptation significantly improves stability on various terrains, including slopes and stairs, and offers stumble recovery technology to prevent falls.
The selection of a knee component is tied to the user’s expected functional level, categorized by the Medicare Functional Classification Level, or K-levels. K-levels range from K1 (household ambulator) to K4 (high-level activity). A higher K-level corresponds to the prescription of more technologically advanced components, such as MPKs. MPKs are more energy efficient and require less cognitive effort to operate than non-microprocessor knees, leading to a more symmetric gait for the user.
The Process of Fitting and Rehabilitation
The successful use of a transfemoral prosthesis involves a structured process of fitting and rehabilitation. Following surgical healing, the first step is often the fitting of a temporary or preparatory prosthesis. This initial device allows the residual limb to continue shaping and shrinking, which is common in the months following amputation. The temporary socket can be easily modified or replaced as the limb volume changes.
During this pre-prosthetic phase, the physical therapy team works to prevent joint contractures, maintain range of motion, and build muscle strength in the hip. This early rehabilitation prepares the limb and the body for the demands of walking with a prosthesis. The physical therapist also teaches the patient proper skin care and management of the limb’s volume using prosthetic socks or compression garments.
Once the residual limb has stabilized, the prosthetist designs the definitive, or permanent, prosthesis. The design is highly individualized, taking into account the user’s activity goals and K-level. After the definitive prosthesis is delivered, the user begins extensive gait training with a physical therapist. This training focuses on teaching the user how to control the prosthetic knee joint, manage balance, and develop an efficient walking pattern on different surfaces.
Learning to walk with a transfemoral prosthesis is significantly more challenging than with a below-the-knee device, requiring more energy expenditure and focus on hip control and balance. The physical therapist helps the user master skills such as standing up, sitting down, and safely navigating stairs, ramps, and uneven ground. The entire process, from surgery to mastering the definitive prosthesis, can take a year or more, requiring consistent practice and collaboration between the user, the prosthetist, and the rehabilitation team.