A custom retainer is a thermoformed device created specifically to fit your mouth and is worn to continue to keep teeth straight after braces or other orthodontic treatment such as aligners. People prefer clear retainers because they find them to be more comfortable than traditional ones that have wires.After your braces are removed, teeth have the tendency to move back towards their previous position because the ligaments attached need time to heal. A custom retainer prevents the elasticity of the ligaments from moving your teeth, and allows time for your jaw bones to solidify around them.