The hinge mechanism cannot be secured effectively using the plastic knobs designed to do this job. Often the top will fall backward or forwards causing a dangerous, potentially litigious, situation. I was hit once while moving the unit to its site. Fortunately I wasn't hurt seriously. It is my belief that there should be a locking device once a desired attitude is selected in the position of the upper section. I've temporarily drilled and bolted the adjustment mechanisms to prevent the upper portion collapsing when in use.