Ref (type)	Population	Outcome, Interventions	Results and statistical analysis	Effect size	Favours
Prevention of pressure ulcers
Systematic review	368 people 2 RCTs in this analysis	Incidence of pressure ulcers 3/188 (2%) with alternating-pressure overlay 7 days post surgery 14/180 (8%) with gel pad during surgery, standard mattress 7 days post surgery	RR 0.21 95% CI 0.06 to 0.70 P = 0.01	Moderate effect size	alternating-pressure overlay 7 days post surgery
Systematic review	416 people who had had elective major general, gynaecological, or vascular surgery Data from 1 RCT	Incidence of postoperative pressure ulcers 8 days 22/205 (11%) with viscoelastic polymer pad 43/211 (20%) with standard table alone	RR 0.53 95% CI 0.33 to 0.85 P = 0.008	Small effect size	viscoelastic polymer pad
Systematic review	413 people Data from 1 RCT	Proportion of people with ulcers of grade 2 or worse 6/206 (3%) with experimental foam overlay 3/207 (1%) with standard table alone	Significance assessment not performed
Systematic review	175 people undergoing cardiac surgery Data from 1 RCT	Proportion of people with grade 1 to 2 pressure ulcers 13/85 (15%) with thermoactive 4-cm viscoelastic foam overlay plus standard operating table 9/90 (10%) with standard operating table alone	RR 1.53 95% CI 0.69 to 3.39 P = 0.3	Small effect size	thermoactive 4-cm viscoelastic foam overlay plus standard operating table