Active air cushions are made up of a series of light weight cells filled with air. These cells are usually situated on top of a foam base. The entire structure is very light. If the cushion is not fixed securely to the chair the patients sacrum could fall towards the back of the cushion eventually resting directly on the seat. This could cause a pressure ulcer. It is therefore important to use the appropriate securing straps in conjunction with the instructions for use to tie the cushion into place.