There are many different types of knee braces.  The most common are the prophylactic, functional, patellafemoral, unloader or offloader, hinged, and the donjoy knee brace.  In addition to these types there are many more.

The prophylactic knee brace is designed to protect the knees from bowlegged stresses that could be harmful to the collateral ligaments.  These are primarily used during sports of physical contact like football or basketball.

The functional knee brace is specifically designed to stabilize the knee during rotational and anteroposterior forces.  They help provide cushion and support injured knees from sustaining any more damage.

A patellafemoral knee brace is used to cure anterior knee irregularity and it also offers moderate subjective improvements without many disadvantages.

The rehabilitative knee brace limits the functions of the knee so it can heal.  This is commonly used after damage or trauma has occurred to a knee or surgery has been performed.
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The unloader knee brace is also known as the offloader.  It is designed to provide total comfort for problems with arthritis of the knees.

A donjoy knee brace is the most  trusted knee brace in the industry.  This also includes Aircast braces, but those are more commonly associated with ankle injuries.

Another type of knee brace is the hinged knee brace.  This contains metal reinforcements on the sides of the brace which connect from the hinge in the center of the brace. 

There are other supports for the knees that provide the same as a basic spandex knee brace.  Some have side panels to give comfort over the standard knee support which allow for sideways movements and limit the stress on the knees.  Some supports contain flexible steel springs sewn into the fabric on the sides and have elastic straps surrounding the sides for additional support.

The basic knee supports do not have the metal pieces in them and they are primarily made of elastic and other fabric.  They are designed for protection for the knees against impacts and pressure from the ground when kneeling for long periods of time.  They can also provide protection against falling or landing on your knees.

A knee brace is designed to protect the knee from many different things.  Some knee braces have different purposes than others.