Van Leen 2013.

Study characteristics
Methods	Study objective: to evaluate the clinical efficacy of a combination of a standard 15 cm viscoelastic foam mattress with a static air overlay mattress vs a standard 15 cm viscoelastic foam mattress alone in preventing pressure ulcers Study design: randomised controlled trial Study grouping: cross over design (data at the first stage extracted) Duration of follow‐up: 6 months Number of arms: 2 Single centre or multi‐sites: single centre Study start date and end date: not described Setting: nursing home
Participants	Baseline characteristics Inclusion criteria: age > 65, a Braden score between 6 and 19, and informed consent of the patients or their representatives in case of dementia or other mental disorder Exclusion criteria: patients with an existing pressure ulcer Sex (M:F): 14:6 in static air; 18:3 in foam Age (years): mean 79.1 (no SD) in static air; 80.8 in foam Baseline skin status: at risk and without existing ulcers. Braden scores classified into 2 subgroups and reported accordingly; not extracted Group difference: no difference Total number of participants: n = 41 Unit of analysis: individuals Unit of randomisation (per patient): individuals
Interventions	Intervention characteristics Standard viscoelastic foam mattress Description of interventions: standard viscoelastic foam mattress NPIAP S3I classification: non‐powered, reactive foam surface; viscoelastic foam Co‐interventions: when out of bed, all patients sat on a static air pillow Number of participants randomised: n = 20 Number of participants analysed: n = 20 Static air overlay Description of interventions: a combination of a standard visco‐elastic foam mattress with a static air overlay NPIAP S3I classification: non‐powered, reactive air surface Co‐interventions: when out of bed, all patients sat on a static air pillow Number of participants randomised: n = 21 Number of participants analysed: n = 21
Outcomes	Proportion of participants developing a new pressure ulcer Outcome type: binary Time points: 6 months Reporting: partially reported Measurement method (e.g. scale, self‐reporting): not reported; probably measured by the primary investigator Definition (including ulcer stage): the development of category 2, 3, or 4 pressure ulcers (PUs) (EPUAP‐classification) Dropouts: no missing participants Notes (e.g. other results reported): 1 of 20 in static air; 3 of 21 in foam 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 Outcomes that are not considered in this review but reported in trials: Treatment data on the new ulcers reported but not extracted
Notes
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "Patients were randomised into 2 groups using numbered envelopes" Comment: low risk of bias because, although the randomisation method is not sufficiently presented in the paper, author response suggests remote computer randomisation sequence generation.
Allocation concealment (selection bias)	Unclear risk	Comment: unclear risk of bias because author responded that sealed envelopes were opened by nurse but its unclear if envelopes were sequentially numbered and opaque.
Blinding of participants and personnel (performance bias) All outcomes	Unclear risk	Outcome group: all outcomes (primary outcome) Comment: no information provided.
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	Outcome group: all outcomes (primary outcome) Quote: "Patients’ skin was inspected weekly to assess the possible occurrence of a skin lesion" Comment: no information provided on the blinding of outcome assessment.
Incomplete outcome data (attrition bias) All outcomes	Low risk	Outcome group: all outcomes (primary outcome) Comment: no attrition identified; 2 cases were transferred to low‐air‐loss bed treatments after they developed category III ulcers.
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.