Prosthetic knees play a significant role in restoring mobility and improving the quality of life for individuals with limb loss. These sophisticated devices aim to replicate the natural movement of a biological knee. Among the available types, the polycentric knee prosthesis is an advanced option, unique for its multiple pivot points and distinct advantages over simpler systems.
Understanding Polycentric Knee Prostheses
A polycentric knee prosthesis, sometimes referred to as a multi-axis knee, is characterized by several pivot points or axes of rotation, often in a four-bar design. This multi-axis design fundamentally differs from a single-axis knee, which operates like a simple hinge with a single fixed pivot point. In a single-axis knee, movement occurs as a pure rotation around this singular axis.
The arrangement of multiple pivot points creates a shifting instantaneous center of rotation (ICR). Unlike a single-axis knee’s fixed axis, the polycentric knee’s ICR changes location as the knee flexes and extends. This dynamic shift allows the polycentric knee to mimic the complex, gliding motion of a natural human knee more closely, leading to enhanced stability and flexibility.
How Polycentric Knees Operate
The polycentric knee’s multiple axes, often configured as a four-bar linkage, allow the instantaneous center of rotation (ICR) to shift during different phases of the gait cycle. During the stance phase, the polycentric design positions the ICR higher and more posterior when the knee is fully extended.
This posterior placement of the ICR relative to the load line enhances stability, preventing unintended knee flexion or buckling during heel strike. As the user progresses into the swing phase, the shifting ICR improves toe clearance. By moving the center of rotation, the prosthetic limb’s effective length shortens at toe-off, lifting the prosthetic foot higher off the ground. This shortening reduces the risk of stumbling.
Categories of Polycentric Knee Systems
Polycentric knee systems are categorized by their control mechanisms, which dictate how the knee’s movement is managed during walking.
Mechanical Polycentric Knees
Mechanical polycentric knees rely on constant friction or weight-activated braking systems. These systems apply a continuous braking force when weight is placed on the prosthesis, preventing buckling, and then allow the knee to swing freely when unweighted. They typically provide an optimal single walking speed due to their fixed resistance.
Fluid-Controlled Polycentric Knees
Fluid-controlled polycentric knees, incorporating hydraulic or pneumatic components, offer greater adaptability. These systems utilize a piston within cylinders containing either fluid (hydraulic) or air (pneumatic) to control resistance during both swing and stance phases. Hydraulic systems adjust resistance by managing fluid flow, allowing for smoother transitions and varying speeds. Pneumatic systems operate similarly by compressing air, providing resistance during knee flexion and releasing energy for extension, leading to a more natural gait across different cadences.
Microprocessor-Controlled (MPC) Polycentric Knees
Microprocessor-controlled (MPC) polycentric knees are the most advanced. They integrate sensors and a computer to continuously adjust knee resistance in real-time. These sensors gather data on the user’s gait and environmental conditions, allowing the microprocessor to adapt the knee’s flexion and extension range and speed instantly. This sophisticated control, often described as an enhanced hydraulic system, dynamically opens and closes valves to regulate fluid flow, providing the most natural and adaptive motion across diverse activities and terrains.
Suitability for Diverse Users
Polycentric knee prostheses are suitable for a wide range of individuals with limb loss. The inherent stability from their shifting center of rotation makes them a good option for new amputees or those with weaker hip musculature, as they require less active muscle control to maintain balance during standing and initial weight bearing. This reduces concern of involuntary knee flexion or buckling, promoting a greater sense of security.
The design’s ability to shorten the effective limb length during the swing phase improves toe clearance, helping in navigating uneven surfaces and reducing fall risk. This feature also benefits individuals with longer residual limbs by minimizing knee protrusion when seated. More advanced polycentric systems, such as those with hydraulic or microprocessor controls, cater to individuals with higher activity levels by allowing for variable walking speeds and smoother transitions across different terrains, including slopes and stairs.