Prosthetic Knee Joint Variable Torque Stance Control
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Solution Overview
Problem
Polycentric knee joints for prostheses face challenges in securing the stance phase with a force curve that increases sharply at small angles, as existing solutions, such as springs, generate constantly increasing extension forces that impair swing phase control and are only required for small knee flexion angles.
Innovation Solution
A spring mechanism is coupled to the lower leg connection piece, allowing the cylinder to pivot and change its distance from the axis, generating a torque that counteracts initial flexion movements, thereby securing the stance phase without increasing forces at higher angles, using a pivot point remote from the axis to build up a torque that secures the stance phase.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If a spring is used to secure the stance phase by prestressing the piston towards the head part of the cylinder, then the knee joint extension is accelerated and stance phase is secured, but the spring generates constantly increasing extension forces that impair swing phase control
Solution Approach 1:
The patent applies the dynamics principle by making the spring prestressing force variable rather than constant. The spring is arranged such that its prestressing force changes dynamically with the piston's position, being high at small flexion angles to secure stance phase, and automatically reducing at larger angles to avoid impairing swing phase control. This dynamic characteristic allows the system to adapt the extension force according to the actual knee joint angle.
Solution Approach 2:
The patent implements parameter changes by varying the spring's prestressing force parameter based on the piston's position parameter. As the piston moves within the cylinder, the spring's prestressing force changes accordingly - high when the piston is at the head part (small flexion angles), and lower when the piston is at the lower leg part (larger flexion angles). This parameter variation resolves the contradiction between providing strong extension force and maintaining swing phase control.
2Reliability
If the spring prestressing force is increased to secure stance phase at small flexion angles, then the knee joint remains extended during foot contact, but the constantly increasing force acts against swing phase control function
Solution Approach 1:
The spring arrangement is designed to provide dynamic prestressing force that adapts to the piston position. At small flexion angles (when the piston is near the head part), the spring provides high prestressing force to securely maintain stance phase. As the piston moves to larger angles, the prestressing force automatically decreases, preventing the generation of harmful constantly increasing forces that would interfere with swing phase control.
3Speed
If a spring is used to generate extension force in the cylinder, then the knee joint extension is accelerated, but the spring function is only required for small knee joint flexion angles while polycentric joints need complex movement sequences
Solution Approach 1:
The spring arrangement with variable prestressing force serves multiple functions: it accelerates extension at small angles, secures stance phase during foot contact, and automatically reduces force at larger angles to accommodate the complex movement sequences required by polycentric joint geometry. This multi-functional design allows a single spring mechanism to support both the extension acceleration need and the complex polycentric movement requirements without adding excessive complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution ensures the knee joint remains extended during foot contact by generating a strong prestressing force at low angles, maintaining the stance phase without impairing the swing phase control with increasing forces, and allows for adjustable pretension to adapt to individual patient needs.
Implementation Method 1
a spring means (27) which allows the cylinder (12) to be pivoted in relation to the lower leg connecting piece (3), with which a change in the distance of the pivot point (16) far from the axis, exploits this to build up a torque counteracting the pivoting of the beginning flexion movement
Implementation Method 2
a change in the distance of the pivot point (16) far from the axis, exploits this to build up a torque counteracting the pivoting of the beginning flexion movement
Data Source
Figure 1
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AI summary
The knee joint (1) has a momentum phase control (11) with a piston movable with a thigh connecting piece (2) coupled opposite to a cylinder (12). The cylinder is coupled over a distal coupling point (16) at the spring unit (27) supporting lower leg connecting piece (3).