Types of knee braces

A simplified method for finding the instantaneous center of knee rotation is often used as shown in the diagrams below. Whilst this is not the absolutely true axis of rotation it is a very good approximation for a knee brace hinge.

Types of knee braces - White-Smoke Types of knee braces - White-Smoke

When this method is repeated for the complete range of leg flexion, we find that the instantaneous center moves in a distinctive curve downwards and posteriorly (rearwards) in relation to the tibia. This is shown in the diagram below.

Types of knee braces - White-Smoke

Knee brace manufactures have tried to imitate this rotational and translational movement in their hinge systems. Note in this simplified diagram we only have two degrees of freedom not the six degrees of freedom of a normal natural knee joint. From researching the basic types of brace hinges commercially available, two types of hinge systems seem to prevail. These are bicentric hinges (the most common type) and four bar link hinges. The properties of the two types are explored in more detail below.

Types of knee braces - White-Smoke Types of knee braces - White-Smoke

The instantaneous center of rotation for a bicentric hinge only approximately follows the required knee rotations and translations, but tend to follow the general trend. The instantaneous centers of rotation are marked by the black dots, with the line of action indicated in orange. This type of hinge, usually only has two degrees of freedom. The four bar linkage hinge system is shown below.

Types of knee braces - White-Smoke Types of knee braces - White-Smoke

It would seem that the four bar linkage hinge system follows the required knee rotations and translations much better than the bicentric hinge. Again, the instantaneous centers of rotation are marked with the black dots, with the line of action indicated in orange. This type o hinge would usually have two degrees of freedom, but if the ends of the links are fitted with spherical bearings, then other degrees of freedom could also be achieved.

So much for theory, it was now the right time to do some experimenting. The brace I was given at the hospital was of the bicentric hinge type. A friend of mine had recently bought a very expensive Don Joy brace with bicentric hinges. I contacted Asterisk to get one of their four link braces. Practical tests could now begin. I do have some considerable experience with the geometric and dynamic properties of four link systems. All formula one racing cars use it in their suspension. I was intrigued if I could apply the same principles that I use in my models and simulations of racing cars to a knee brace. After all, an ideal knee brace system would need to have all the attributes of a sophisticated suspension system for the knee. A sort of exoskeleton for the leg.

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