Many specialized beds appear to be effective in reducing the development of pressure ulcers when compared with standard mattresses. For example, in 4 studies  cited by the systematic review  that compared standard hospital foam mattresses to enhanced foam alternatives, a summary relative risk was calculated, favoring the intervention group. Between-group comparisons of the previously defined low-technology constant low-pressure devices did not yield clear conclusions. Similarly, in 7 RCTs the comparison of alternating pressure devices with a variety of constant low-pressure devices (a water mattress, foam pad, static air bed and foam overlays) showed no significant difference in pressure ulcer development  However, a study of alternating pressure supports compared with standard foam mattresses did demonstrate lower pressure ulcer development in the intervention group  Comparing pressure-reducing devices among themselves (versus against a standard mattress) yields no significant differences in the prevention of pressure ulcers. These trials have been summarized in a recent review.