A prosthetic leg is a device that replaces a portion of a lower limb lost due to trauma, disease, or a congenital condition. The limb remaining after the amputation is known as the residual limb, or sometimes the stump. Attaching this artificial limb to the body requires a stable, comfortable, and secure connection. This attachment is achieved through the creation of a highly customized interface and a reliable suspension system.
The Socket and Liner Interface
The prosthetic socket functions as the foundational interface, serving as the direct point of contact between the residual limb and the rest of the prosthetic device. Because no two residual limbs are identical, the socket is custom-molded to fit the individual’s unique shape precisely. This precise fit is necessary to distribute the body’s weight and the forces of movement evenly across the tissue, preventing concentrated pressure points that can cause skin breakdown or pain.
A liner is worn directly over the residual limb, acting as a soft, protective intermediary layer between the skin and the rigid socket wall. These liners are typically made from silicone, gel, or urethane materials, which provide cushioning and reduce friction. The liner also plays a crucial role in suspension by creating an airtight seal or providing a connection point for various locking mechanisms.
Non-Invasive Suspension Systems
Once the residual limb is seated within the socket, a suspension system is required to keep the entire prosthetic limb securely attached during all activities, preventing movement between the limb and the socket.
A common method is the mechanical pin/lock system, which uses a pin attached to the distal end of the liner. When the user pushes the residual limb into the socket, this pin slides into and engages a shuttle lock mechanism located at the bottom of the socket. The mechanical connection provides a positive, audible click that confirms the prosthesis is securely fastened. To release the limb, the user simply presses a button on the outside of the socket, which disengages the mechanical lock.
Another widely used approach is passive suction suspension, which relies on creating a vacuum seal to maintain the attachment. In this system, a one-way expulsion valve allows air to be pushed out as the limb is fully inserted. The seal is often maintained by a soft silicone sleeve that rolls up the residual limb and over the top edge of the socket, or by sealing rings integrated into the liner itself. This method provides a secure fit by creating a partial vacuum, which minimizes the up-and-down movement, known as pistoning, between the limb and the socket.
The active vacuum system, sometimes referred to as elevated vacuum, involves a mechanical or electronic pump integrated into the prosthetic leg. The pump continuously draws air out from the space between the liner and the socket. This maintains a higher, controlled level of negative pressure compared to passive suction, which significantly improves the security of the attachment. This constant, regulated vacuum helps to stabilize the volume of the residual limb throughout the day and promotes better skin health by improving circulation.
Direct Skeletal Attachment
An entirely different method of attachment is direct skeletal attachment, known medically as osseointegration. This surgical procedure bypasses the need for a traditional socket altogether. It involves the implantation of a metal fixture, typically titanium, directly into the medullary canal of the residual bone.
Over a period of weeks to months, the living bone grows directly onto and into the implant surface, creating a rigid and permanent fusion. A component known as an abutment then extends from the integrated implant through a small opening in the skin, called a stoma. The external prosthetic limb is then mechanically connected directly to this abutment, allowing for direct load transfer from the prosthesis to the skeleton. This direct connection offers a greater sense of stability and allows the user to perceive ground textures and forces through the bone, a sensation called osseoperception.