Prosthetic Knees! Explained! Polycentric

Summary

This video explores the fascinating world of knee prostheses, focusing on the functional principles of monocentric and polycentric knee joints. It explains the mechanics behind these joints, such as the shifting pivot points and their impact on stability and movement. The polycentric knee, with its multiple pivot points, offers advantages like improved ground clearance and flexibility in the swing phase. The video compares various models, including those with pneumatic systems and geometric locks, offering insights into their strengths and trade-offs. Ultimately, the video emphasizes the importance of choosing the right knee joint based on user mobility needs.

Takeaways

• 😀 Polycentric knee joints have multiple pivot points, which allow for more flexibility and stability compared to monocentric knee joints.
• 😀 The pivot point in polycentric knee joints moves dynamically as the knee bends, providing better stability during standing and more ground clearance when walking.
• 😀 Monocentric knee joints have a single pivot point, but polycentric joints can adjust their pivot points based on the knee's flexion, making them more adaptable.
• 😀 The polycentric knee joints have advantages when sitting down or during walking, with the ability to shorten the leg for more ground clearance.
• 😀 Pneumatic systems integrated into polycentric joints add adjustable resistance to flexion and extension, adapting to walking speed but can also introduce instability in certain situations.
• 😀 A combination of four axes and pneumatic systems in knee joints can lead to instability due to rebound effects from compressed air.
• 😀 The momentary pivot point in polycentric knee joints changes its position during flexion, allowing for a more natural movement and better support in different postures.
• 😀 Some polycentric knee joints include additional axes (like a fifth axis) that help with bending and provide more stability in the stance phase.
• 😀 Some knee joint designs use a geometric locking system, which ensures stability during standing but allows for free swinging during the swing phase.
• 😀 A microprocessor-controlled knee, although adaptable, might not always be the best option for all users. Different designs like the geometric lock or semi-active polycentric knee offer safer and more affordable alternatives.

Q & A

What is the key difference between monocentric and polycentric knee joints?

Monocentric knee joints have a single pivot point, while polycentric knee joints feature multiple pivot points, making them more adaptable and dynamic in their motion, especially during flexion and extension.

What is a polycentric knee joint's 'momentary pivot point'?

The momentary pivot point is the point around which the lower leg rotates at a specific moment. It shifts based on the knee's angle and is found by connecting the polycentric knee's multiple pivot points.

How does the pivot point move in a polycentric knee joint during flexion?

In a polycentric knee joint, the pivot point moves along a 'polar curve,' shifting its position both forward and backward as the knee bends.

Why does the polycentric knee joint improve stability during standing?

Polycentric knee joints are more stable in the standing position because their pivot points shift to a more dorsal position, improving the balance and preventing instability.

What is the advantage of a polycentric knee joint when sitting down?

Polycentric knee joints allow the leg to shorten during the sitting phase, reducing excess length and improving comfort, unlike monocentric knees which cannot achieve this effect.

What are the benefits and drawbacks of adding pneumatics to a polycentric knee joint?

Pneumatic systems provide adjustable resistance for flexion and extension, which adapts to walking speed. However, they can cause instability in certain situations due to bouncing effects from air chambers.

How does the fifth axis in a knee joint's mechanism affect its function?

The fifth axis adds another rotational point, which helps the knee bend more naturally, especially in creating a neutral zone for flexion without losing stability during the stance phase.

What is 'bouncing' in the context of prosthetic knee joints?

'Bouncing' refers to a limited compression of the knee that causes slight flexion in the stance phase, which can help with a natural gait but may also introduce instability in some cases.

What does the 'geometric locking' system in some knee joints do?

The geometric locking system stabilizes the knee during the stance phase by ensuring that the joint doesn't bend. It allows for quick release before the swing phase, providing safety and efficiency.

What might the ideal prosthetic knee joint look like in the future?

The ideal prosthetic knee could combine the benefits of polycentric motion, microprocessor control, and active features, offering both adaptability and stability. While it doesn't exist yet, advancements in technology may make this possible.