Hoshowsky 1994.

Study characteristics
Methods	Study objective: to examine the effects of 2 operating room (OR) table mattresses and 1 mattress overlay on intraoperative pressure sore formation Study design: randomised controlled trial Study grouping: parallel group (split body design) Duration of follow‐up: not given Number of arms: 4 different treatment protocols (made up from 3 types of mattresses) tested in 6 different pairings Single centre or multi‐sites: single centre Study start date and end date: not described Setting: university teaching hospital
Participants	Baseline characteristics Inclusion criteria: patients in the study were placed in the supine or prone positions while undergoing surgery, older than 12 years of age, and possession of symmetrical lower limbs Exclusion criteria: not given Sex (M:F): overall 184:321 (across all 6 comparisons) Age (years): overall mean 47 years (SD 17.1) and range 13 to 86 (across all 6 comparisons) Baseline skin status: not given Group difference: no difference within each comparison (due to within‐person comparison made) Total number of participants: standard foam mattress (SFM) vs. foam and gel mattress (FGM): n = 91; VEO‐Action above SFM vs. FGM n = 92; SFM versus VEO above FGM n = 62; VEO above SFM versus VEO above FGM n = 113; SFM versus VEO above SFM n = 73; and FGM versus VEO above FGM n = 74 (overall 505 across 6 comparisons) Unit of analysis: treatment sessions of individuals Unit of randomisation (per patient): treatment sessions of individuals
Interventions	Intervention characteristics Standard foam mattress Description of interventions: a standard vinyl covered 2‐inch thick foam OR table mattress (SFM) NPIAP S3I classification: non‐powered, reactive foam surface Co‐interventions: not described Number of participants randomised: this intervention was involved in 3 comparisons and each had a different numbers of participants (see above) Number of participants analysed: not given Foam and gel mattress (FGM) Description of interventions: a nylon fabric covered 2‐inch thick foam and gel OR table mattress (FGM ‐ Akros®, American Sterilizer Co.) NPIAP S3I classification: non‐powered, reactive foam plus gel surface Co‐interventions: not described Number of participants randomised: this intervention was involved in 3 comparisons and each had a different numbers of participants (see above) Number of participants analysed: not given VEO‐Action® Description of interventions: a viscoelastic dry polymer mattress overlay (VEO‐Action®, Action Products Inc.) NPIAP S3I classification: non‐powered, reactive gel surface Co‐interventions: not described Number of participants randomised: this intervention was involved in 5 comparisons and each had a different numbers of participants (see above) Number of participants analysed: not given
Outcomes	Proportion of participants developing a new pressure ulcer Outcome type: not given Time points: not given Reporting: partially reported Measurement method (e.g. scale, self‐reporting): all skin changes noted; blanchable hyperaemic areas classified as skin changes and non‐blanchable hyperaemic areas classified as Stage I pressure sores, in accordance with the NPIAP staging system Definition (including ulcer stage): not specified with details; skin change and ulcer incidence Dropouts: not described Notes (e.g. other results reported): none of the 505 patients developed pressure sores of severity Stages II through IV; Stage I pressure sores in 85 patients (16.8%); skin changes that did not reach Stage I in 290 patients (57.4%). Odds of developing pressure ulcer with viscoelastic overlay (versus standard hospital mattress) 0.40 (95% CI 0.21 to 0.77); however, the related logistic regression as described does not appear to take into account the multiple measures per person. Time to pressure ulcer incidence Not reported Support‐surface‐associated patient comfort Not reported All reported adverse events using allocated support surfaces Not reported Health‐related quality of life (HRQOL) Not reported Cost‐effectiveness Not reported
Notes
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Comment: unclear risk of bias because each patient served as their own control but within the patient, the allocation of interventions was unspecified.
Allocation concealment (selection bias)	Unclear risk	Comment: no information provided.
Blinding of participants and personnel (performance bias) All outcomes	High risk	Outcome group: ulcer incidence. Quote: "Use of the overlay in this manner prevented the investigators from being blinded at the time of postoperative assessment whenever the overlay was used." Comment: high risk of bias because non‐blinding is clearly stated.
Blinding of outcome assessment (detection bias) All outcomes	High risk	Outcome group: ulcer incidence. Quote: "Use of the overlay in this manner prevented the investigators from being blinded at the time of postoperative assessment whenever the overlay was used." Comment: high risk of bias because non‐blinding is clearly stated.
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	Comment: no information provided.
Selective reporting (reporting bias)	Unclear risk	Comment: the study protocol is not available but it is clear that the published reports include all expected outcomes. No data are reported on the number or rate of pressure ulcers by group and this would be expected. Only statistically significant odds were reported.
Other bias	High risk	Comment: the study appears to consider parts of a person's body as unit of analysis. However, the logistic regression as described does not appear to take into account the multiple measures per person.