Aronovitch 1999.

Methods	Quasi‐randomised trial with 7 day follow‐up.
Participants	> 18 y; free of pressure ulcers; undergoing elective surgery under GA, of > 3 h operative time. No significant differences between groups for age, sex, race, weight, height, smoking status at baseline, but patients in conventional management group were at greater risk of pressure ulcer development as defined by Knoll score.
Interventions	1. AP system intra and postoperatively (MicroPulse) (n = 112). Micropulse is thin pad with over 2,500 small air cells in rows; 50% cells inflated at any time. 2. Conventional management (n = 105): consisted of use of a gel pad in the operating room and a replacement mattress postoperatively.
Outcomes	Occurance of pressure ulcer within 7 days of surgery: number/size/grade of ulcers on each postoperative day: 1. MicroPulse system 1% (1/90), however, ulcer was due to a foreign body and considered "not related to the bed". 2. Conventional management 9% (7/80) (7 patients developed 11 pressure ulcers; the stage of 6 of these could not be determined because of eschar). Grade 1: 1; Grade 2: 4.
Notes	1. MicroPulse system: device was inadvertently turned off during treatments of 4 patients. 4 patients asked to withdraw for various unreported reasons. 3 patients withdrew due to back pain. 12 patients assigned to this group were placed on another surface postoperatively for reasons unrelated to the surface. 2. Conventional management: 6 patients were placed on the MicroPulse postoperatively. Analysis was on an ITT basis.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Quasi‐randomised: "randomisation was performed by week rather than by patient to decrease protocol error".
Allocation concealment (selection bias)	Unclear risk	Not reported.
Blinding (performance bias and detection bias) Pressure ulcer incidence	Unclear risk	Not reported.
Incomplete outcome data (attrition bias) All outcomes	Low risk	All reasons/numbers for attrition/exclusions reported.
Selective reporting (reporting bias)	Low risk	All pre‐specified outcomes reported.
Free of other bias ‐ were groups similar at baseline regarding the most important prognostic indicators?	Low risk	It was stated, however, that all data were not available for all patients.
Free of other bias ‐ was the timing of the outcome assessment similar in all groups?	Low risk	Outcomes assessed on days 1, 4 and 7.