Nixon 1998.

Study characteristics
Methods	Study objective: to compare the postoperative pressure sore incidence in patients positioned on the standard operating table mattress with those positioned on the dry visco‐elastic polymer pad Study design: randomised controlled trial Study grouping: parallel group (sequential design) Duration of follow‐up: 8 days Number of arms: 2 Single centre or multi‐sites: multi‐sites Study start date and end date: not described; recruited from November 1994 to June 1996 Setting: operating rooms of hospitals
Participants	Baseline characteristics Inclusion criteria: patients aged ≥ 55 years, admitted for elective major general, gynaecological or vascular surgery in supine or lithotomy position and free of preoperative pressure damage greater than grade 1 Exclusion criteria: liver, urology and breast surgery; pressure damage of Grade 1a or above observed preoperatively; ward staff provision of preoperative alternating pressure mattress; dark skin pigmentation which precludes reliable identification of Grade 0 and Grade 1a skin assessments; skin conditions over the sacrum, buttocks or heels which preclude reliable identification of Grade 0 and Grade 1a skin assessments Sex (M:F): 119:101 in dry visco‐elastic polymer pad; 116:107 in standard operating theatre table mattress Age (years): 124 participants between 55‐69 years and 98 participants 70+ years in dry visco‐elastic polymer pad group; 128 participants between 55‐69 years and 96 participants 70+ years in standard operating theatre table mattress group Baseline skin status: categories of risk scores reported; free of pressure ulcers greater than grade 1 Group difference: no difference Total number of participants: n = 446 Unit of analysis: individuals Unit of randomisation (per patient): individuals
Interventions	Intervention characteristics Dry visco‐elastic polymer pad on operating table Description of interventions: dry visco‐elastic polymer pad on operating table NPIAP S3I classification: non‐powered, reactive gel surface Co‐interventions: warming mattress provision for both groups Number of participants randomised: n = 222 Number of participants analysed: n = Standard operating theatre table mattress Description of interventions: standard operating theatre table mattress plus Gamgee heel support NPIAP S3I classification: standard hospital surface Co‐interventions: warming mattress provision for both groups Number of participants randomised: n = 224 Number of participants analysed: n =
Outcomes	Proportion of participants developing a new pressure ulcer Outcome type: binary Time points: 8 days Reporting: partially reported Measurement method (e.g. scale, self‐reporting): defined by Torrance scale Definition (including ulcer stage): pressure sore at any of the 5 skin sites most likely to incur skin damage (sacrum, left and right buttocks, and left and right heels) Dro outs: 416 with complete data; 30 with incomplete data including 29 patients with lost forms and 27 having incomplete skin assessment records Notes (e.g. other results reported): 22 of 205 in dry polymer group; 43 of 211 in standard mattress group Time to pressure ulcer incidence Reporting: not reported Support‐surface‐associated patient comfort Reporting: not reported All reported adverse events using allocated support surfaces Reporting: not reported Health‐related quality of life (HRQOL) Reporting: not reported Cost‐effectiveness Reporting: not reported
Notes
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "a telephone randomisation schedule was developed within random permuted blocks of 6, with a run‐in of 8" Comment: low risk of bias because study likely used a proper randomisation method.
Allocation concealment (selection bias)	Low risk	Quote: "a telephone randomisation schedule was developed, and managed by the Northern and Yorkshire Clinical Trials and Research Unit" Comment: low risk of bias because study likely concealed allocation.
Blinding of participants and personnel (performance bias) All outcomes	Unclear risk	Comment: no information provided.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Outcome group: pressure ulcer outcome Quote: "All pre and intra‐operative data were recorded by the research nurse, and post‐operative data recorded by recovery and ward staff who were blind to the intraoperative mattress allocation. The record pertaining to the intra‐operative randomised mattress allocation remained separate from the main data collection proforma to maintain the blind" Comment: unclear risk of bias because there is attempt to blind outcome assessment.
Incomplete outcome data (attrition bias) All outcomes	Low risk	Outcome group: pressure ulcer outcome Comment: low risk of bias because although intention‐to‐treat (ITT) analyses claimed by authors, low proportions of missing data (17 of 222 vs 13 of 224) occurred in analysis.
Selective reporting (reporting bias)	Low risk	Comment: the study protocol is not available but it is clear that the published reports include all expected outcomes, including those that were pre‐specified.
Other bias	Low risk	Comment: the study appears to be free of other sources of bias.