Silicone Adhesive Multilayer Foam Dressings to Prevent Pressure Ulcer: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Silicone Adhesive Multilayer Foam Dressings to Prevent Pressure Ulcer (NCT03442777), a randomized na study. The primary outcome is Number of Participants Who Developed a New Pressure Ulcer Category 2 or Worse on Sacrum, Heels or Greater Trochanter.

2. Clinical Question

The endpoint is a participant count, rather than a count of every ulcer. It focuses on new ulcers at the specified sites and does not describe outcomes at other body locations. The registry’s primary outcome compares the intervention groups pooled together with standard prevention alone; it does not report a separate primary comparison for each dressing brand.

3. Trial Design

This was a randomized, multicentric, open-label, parallel-group medical-device trial. The registry lists no masking, so participants and study personnel were not blinded according to the record. The study is marked completed. Recruitment began on 2018-02-08, and primary completion was recorded as 2018-12-31. The registered enrollment was 1634.

There were two intervention groups and one control group. The intervention groups received Allevyn® or Mepilex® silicone adhesive multilayer foam dressings in addition to standard prevention. The control group received standard prevention alone, described as ongoing risk assessment, regular repositioning and skin care. For the primary outcome analysis, intervention groups 1 and 2 were pooled and compared with the control group.

The registry’s stated objective was to assess prevention of pressure ulcers at the sacrum, heels and greater trochanter among at-risk hospitalised patients. It notes that the study was intended to extend previous trial results in an intensive care setting, and that no more than 25% of participants would be recruited from intensive care. The actual distribution across care settings is not reported in the registry record.

The primary outcome was assessed during a study period of up to 14 days. The registry identifies the primary analysis population as intention to treat, with a reported population size of 1605. The randomisation procedure, allocation ratio planned by the protocol, detailed eligibility criteria, and the trial’s original sample-size assumptions are not reported in the registry record. The registry lists phase as not applicable.

4. Participant Flow and Accounting

The registry reports how participants moved through each stage of the study. All counts below are exactly as posted; missing counts are not inferred from other stages.

Before assignment: 47 participants were excluded; 1 asked to have their data excluded from the analysis and 46 were not randomised.

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: Intention to treat population (n=1605)

5. Baseline Characteristics

The table reproduces the baseline characteristics posted in the registry, which describe the participants analysed at baseline and help show whether the randomized groups were comparable.

6. Primary Endpoint

The registry lists the primary outcome as Number of Participants Who Developed a New Pressure Ulcer Category 2 or Worse on Sacrum, Heels or Greater Trochanter, measured over assessment during 14 days.

The primary endpoint counts participants who developed at least one new pressure ulcer of category 2 or worse on the sacrum, heels or greater trochanter. The registry specifies that ulcers were judged onsite and assessed during the study period, which could last a maximum of 14 days. Because the measure counts participants, a participant with more than one qualifying ulcer still contributes to the outcome as a participant with an event, rather than as multiple events.

This endpoint focuses on clinically identified new ulcers at locations where the dressings were intended to be applied. The registry’s classification note describes category I as non-blanchable erythema of intact skin, category II as partial thickness loss of dermis, category III as full thickness tissue loss, and category IV as full thickness tissue loss with exposed bone, tendon or muscle. It also notes that unstageable pressure ulcers and deep tissue injuries can be recognised. The primary outcome title specifies category 2 or worse.

A binary participant-level outcome allows a comparison of the proportion of participants with at least one qualifying event in each group. It does not capture the total number of ulcers, their duration, or outcomes at locations outside those listed. Details of any secondary endpoint definitions are not reported in the registry record.

7. Results Posted in the Registry

• Treatment Group (Defined in Protocol as Pooled Intervention Group 1 and Intervention Group 2): 4.0% (43/1066) versus 6.3% (34/539) on control; risk difference -2.3 percentage points (95% CI -4.6 to 0.1), risk ratio 0.64 (0.41–0.99); unadjusted two-proportion z-test p = 0.044.

8. Statistical Interpretation

9. Statistical Methods Explained

The primary outcome is binary at the participant level: a participant either developed at least one qualifying new pressure ulcer during the assessment period or did not. The posted group counts give the number with an event and the number assessed in each comparison group. A risk, in this context, is the proportion of participants in a group who experienced the event during the specified period. The registry’s analysis treats the two dressing groups as a pooled intervention group for the primary comparison.

What does the risk difference describe?

The risk difference is the intervention-group risk minus the control-group risk. A negative value means the observed event proportion is lower in the intervention group on this scale; a positive value means it is higher. The recalculated estimate is -0.023. Its 95% confidence interval runs from -0.046 to 0.001, so the interval contains zero. Zero represents no difference in the two risks on the risk-difference scale. This unadjusted calculation uses the posted summary counts and does not incorporate adjustments that may be part of the registry’s own analysis.

What does the risk ratio describe?

A risk ratio compares the intervention-group risk with the control-group risk by division. A value of 1 represents equal risks; values below or above 1 indicate a lower or higher risk in the intervention group, respectively. The registry reports a risk ratio of 0.64 and a 95% confidence interval from 0.41 to 0.99. The estimate is relative rather than absolute, so it does not directly state the difference in the number of participants with events. The posted confidence interval describes uncertainty around the relative estimate.

What is the Cochran-Mantel-Haenszel method?

In general, the Cochran-Mantel-Haenszel method can combine information across strata to estimate an association while accounting for those strata. The registry names this method for its reported risk-ratio analysis, but the strata and details of its implementation are not reported in the registry record. It is therefore not possible to specify from the record what variables, if any, defined the strata or how the analysis handled them.

The registry reports p = 0.04 for that analysis and labels it superiority. A p-value describes how compatible the observed data are with a specified null hypothesis under the assumptions of the statistical method; it is not the probability that the null hypothesis is true. The record does not provide further detail on the exact hypothesis-testing procedure beyond the method and result reported.

Why can the recalculation differ in presentation?

The recalculated risk difference is a simple, unadjusted estimate from posted summary statistics. The registry’s own result is a Cochran-Mantel-Haenszel risk ratio. These are different effect measures and may also reflect different analysis procedures. The unadjusted p-value associated with the recalculation is 0.044. It should not replace or be confused with the registry’s reported p-value of 0.04 for its own analysis. The registry does not flag a discrepancy between the reported result and the recalculation.

For a binary outcome, the comparison can be expressed using proportions, a difference between proportions, or a ratio of proportions. Each scale answers a related but distinct question. The confidence interval conveys the range of estimates compatible with the observed data under the method used. It is not a guarantee that the true effect lies in that particular interval, and its interpretation depends on the analysis assumptions.

Over-dispersion is a general concern when variation in data is greater than a model assumes; it can lead to uncertainty being understated if not handled appropriately. The registry describes a participant-level binary outcome and names the Cochran-Mantel-Haenszel method, but it does not report a model-based dispersion assessment. A separate over-dispersion adjustment is therefore not reported in the registry record. No additional modelling details should be inferred from the endpoint counts alone.

10. Confidence Intervals and Precision

The unadjusted risk-difference estimate is -0.023, with a 95% confidence interval from -0.046 to 0.001. The interval is the key measure of precision: it shows a range of values compatible with the posted data under the recalculation method. Its inclusion of zero means that the unadjusted result is compatible with no risk difference, while also allowing values on either side of zero. The interval does not establish that the groups are equivalent.

The registry’s reported risk ratio is 0.64, with a 95% confidence interval from 0.41 to 0.99. This interval is expressed on a relative scale, not the absolute risk-difference scale. Its upper endpoint is close to 1, the value representing equal risks on the risk-ratio scale. The two intervals should not be compared as though their endpoints measured the same quantity.

In general, a 95% confidence interval reflects the uncertainty of an estimate under the analysis method and its assumptions. A wider interval indicates less precision than a narrower one on the same scale. Precision depends on the amount of information in the data and on the method used to estimate the interval. The registry does not report details such as the strata used for its Cochran-Mantel-Haenszel analysis, so the reported interval cannot be independently reconstructed from that method using the record alone.

These intervals concern the specified participant-level outcome over a maximum of 14 days. They do not quantify uncertainty about longer follow-up or outcomes not included in the primary endpoint. The posted estimates should be interpreted with both the direction of the point estimate and the uncertainty around it in view.

11. Planning a Similar Trial: Sample Size

This is a planning illustration, not a re-evaluation of the original trial. It asks: if a new study wanted to detect a difference like the one observed, or a smaller one, how large would it need to be? It is deliberately not “observed power” of the finished trial, which is not a meaningful quantity.

The sample-size figures are planning illustrations based on the observed effect, not the trial’s original design assumptions. They give participants per arm, not total events. The assumptions are a two-sided significance level of 0.05, equal allocation between two arms, and a normal approximation. Each scenario uses the stated fraction of the observed effect as the effect to detect. The trial’s own original sample-size assumptions are not reported in the registry record.

The 80% power column gives the illustrative number of participants per arm for an analysis to have 80% power under those planning assumptions and for the effect size used in that scenario. The 90% power column gives the corresponding number per arm for 90% power. Power is a property of a planned study under specified assumptions; it is not a measure of the probability that the observed finding is true.

For planning for 100% of the observed effect, the illustration gives 1,488 participants per arm for 80% power and 1,991 participants per arm for 90% power. For planning for 75% of the observed effect, it gives 2,782 participants per arm for 80% power and 3,724 participants per arm for 90% power. For planning for 50% of the observed effect, it gives 6,567 participants per arm for 80% power and 8,791 participants per arm for 90% power.

The pattern reflects a general planning principle: smaller effects are harder to distinguish from chance variation, so detecting them at a chosen power generally requires more participants. These figures are illustrations derived from the observed effect and assumptions stated for the calculation. They are not observed power for this completed trial.

12. How the Sample Size Is Derived

Power is the probability that a study of a given size rejects the null hypothesis when a specific true effect exists. It depends on four things: the effect to detect, the variability of the outcome, the significance level (here a two-sided α of 0.05, so z_1−α/2 = 1.960) and the desired power (z_1−β = 0.842 for 80% and 1.282 for 90%).

Formula

n is the number of participants in each arm, p₀ and p₁ are the control and treatment proportions, and p̄ is their average.

Worked calculation for this trial

1. Control proportion p₀ = 0.063; treatment proportion p₁ = 0.040; average p̄ = 0.052.
2. First term: z_1−α/2 × √[2p̄(1−p̄)] = 1.96 × 0.3132 = 0.6138.
3. Second term: z_1−β × √[p₀(1−p₀) + p₁(1−p₁)] = 0.842 × 0.3127 = 0.2632.
4. n = (0.6138 + 0.2632)² / (-0.0227)² = 1,487.1, rounded up to 1,488 participants per arm for 80% power.

The smaller-effect rows in the table above repeat the same calculation with the effect multiplied by 0.75 and 0.5 (for ratios, applied to the logarithm of the ratio).

R code

alpha <- 0.05
z_a   <- qnorm(1 - alpha / 2)          # two-sided
p0 <- 0.0630797773654917                          # control proportion
p1 <- 0.0403377110694184                          # treatment proportion
n_per_arm <- function(power, frac = 1) {
  q1 <- p0 + (p1 - p0) * frac               # smaller effect = fraction of the observed difference
  pbar <- (p0 + q1) / 2
  z_b <- qnorm(power)
  ceiling(round((z_a * sqrt(2 * pbar * (1 - pbar)) + z_b * sqrt(p0 * (1 - p0) + q1 * (1 - q1)))^2 / (q1 - p0)^2, 6))
}
for (f in c(1, 0.75, 0.5))
  cat(sprintf("%3.0f%% of observed effect: n per arm = %s (80%% power), %s (90%% power)\n", 100 * f,
              format(n_per_arm(0.80, f), big.mark = ","), format(n_per_arm(0.90, f), big.mark = ",")))

# Check against R's built-in function (unrounded n per arm)
cat("power.prop.test:", round(power.prop.test(p1 = p0, p2 = p1, power = 0.80)$n, 1), "\n")

# Simulation: how often does the z-test reject at the 80%-power sample size?
set.seed(3442777)
n    <- n_per_arm(0.80)
reps <- 20000
x0 <- rbinom(reps, n, p0); x1 <- rbinom(reps, n, p1)
ph <- (x0 + x1) / (2 * n)
z  <- ((x1 - x0) / n) / sqrt(ph * (1 - ph) * 2 / n)
cat(sprintf("Simulated power at n = %s per arm: %.3f (%d replicates)\n", format(n, big.mark = ","), mean(abs(z) > z_a, na.rm = TRUE), reps))

Output from running the code

100% of observed effect: n per arm = 1,488 (80% power), 1,991 (90% power)
 75% of observed effect: n per arm = 2,782 (80% power), 3,724 (90% power)
 50% of observed effect: n per arm = 6,567 (80% power), 8,791 (90% power)
power.prop.test: 1487.1 
Simulated power at n = 1,488 per arm: 0.804 (20000 replicates)

The simulation draws many pairs of event counts at the planned sample size and counts how often the two-proportion z-test rejects the null. It should land close to the target power; at very small required sample sizes it can fall below it, because the normal approximation is rough there.

13. Analyses Reported by the Sponsor

The registry’s own analysis reports a Cochran-Mantel-Haenszel risk ratio of 0.64, with a 95% confidence interval from 0.41 to 0.99 and p = 0.04. It is labelled a superiority analysis. This is the sponsor’s reported analysis and should be kept distinct from the simple unadjusted re-calculation based on the posted group counts.

The re-calculation reports a risk difference of -0.023, with a 95% confidence interval from -0.046 to 0.001 and an unadjusted p-value of 0.044. A risk ratio and a risk difference use different scales, and the registry’s named method differs from the simple unadjusted calculation. Their estimates and intervals therefore should not be expected to match numerically or answer precisely the same statistical question.

The registry does not flag a discrepancy. The reported analysis remains the appropriate source for describing the sponsor’s prespecified or chosen result as recorded; the re-calculation is supplementary context based on the posted counts. Details of any analysis plan, adjustment variables or strata are not reported in the registry record.

14. Multiplicity, Missing Data and Analysis Populations

The registry records four outcome measures and one analysis, and identifies one primary outcome: the number of participants who developed a new pressure ulcer of category 2 or worse at the specified sites. It reports no posted secondary analyses. The record does not explain whether other outcomes were tested, how outcomes were prioritised beyond the primary designation, or whether any multiplicity procedure was planned or used.

Multiplicity is a general concern when several outcomes, comparisons or repeated tests are assessed. The more opportunities there are to find a result meeting a chosen statistical threshold, the more important it is to understand whether the analysis plan accounted for that testing. The registry record does not report a multiplicity adjustment or provide enough detail to determine how any additional testing was handled.

The primary outcome is reported for an intention-to-treat population of 1605. The registered enrollment is 1634. The registry does not explain the disposition of participants between enrollment and the reported analysis population, the amount or pattern of missing outcome data, or the handling of missing observations. No imputation method or sensitivity analysis for missing data is reported in the registry record.

The record names the intention-to-treat population but does not provide detail on how that population was defined in practice or whether other analysis populations were considered. Per-protocol or as-treated results are not reported. The randomisation procedure, stratification variables, covariate adjustments, interim analyses and detailed statistical analysis plan are also not reported in the registry record. These omissions limit what can be concluded about implementation of the analysis beyond the method and results explicitly recorded.

15. Safety Results

The registry reports serious adverse events as affected participants divided by participants at risk.

Deaths

Other (non-serious) adverse events

Participants with at least one non-serious event at or above the registry's 0% reporting threshold.

Most frequent non-serious adverse event terms

The posted serious adverse event figures cover the two intervention groups. Intervention group 1 reports 0 participants affected by serious adverse events among 539 at risk; intervention group 2 also reports 0 among 538 at risk. No serious adverse event terms are listed. These figures describe what is recorded for those groups and should not be read as evidence that no serious events occurred outside the registry’s recorded reporting.

The record also lists other adverse events affecting 16 participants in intervention group 1 and 12 in intervention group 2. Deaths are listed for 27 participants in intervention group 1 and 23 in intervention group 2. The registry does not provide corresponding control-group adverse-event figures in this record, nor does it report event descriptions, attribution, timing or detailed severity information for these other events.

These safety counts are separate from the primary pressure-ulcer outcome and cannot, by themselves, establish a comparative safety profile. The registry’s adverse-event presentation should be interpreted within the limited group and event information it records.

16. What the Result Does, and Does Not, Mean

The trial provides a randomized comparison of standard pressure-ulcer prevention alone with the same general prevention approach plus silicone adhesive multilayer foam dressings, with the two dressing groups pooled for the primary outcome. The posted counts and estimates describe the occurrence of new pressure ulcers of category 2 or worse at the sacrum, heels or greater trochanter during a study period of up to 14 days.

The registry’s own analysis reports a risk ratio of 0.64 and labels its result a superiority analysis, with a 95% confidence interval from 0.41 to 0.99 and p = 0.04. The unadjusted re-calculation reports a negative risk difference, with an interval that includes zero. These results need to be read in light of their different scales and methods. They describe an estimated comparison in this trial, not a certainty about outcomes in every setting.

The result does not establish a separate primary outcome effect for either dressing brand, because the registry pools the intervention groups for its primary comparison. It does not answer whether the findings apply to patients with different characteristics, to care settings not represented in the trial, to other pressure-ulcer sites, or to periods beyond the registered assessment window. The actual proportion of participants recruited from intensive care is not reported in the registry record.

The primary endpoint is limited to whether a participant had at least one qualifying new ulcer. It does not report the total number of ulcers per participant, duration, healing, or longer-term outcomes. Safety reporting is also incomplete for comparative interpretation because control-group adverse-event counts are not included in the record presented.

These findings are statistical evidence about a defined comparison and outcome. They are not medical advice, and the estimates alone do not determine how prevention should be managed in an individual patient or institution.

17. Why This Trial Matters Statistically

This trial is statistically informative because it separates the intervention into two dressing groups during the trial but pools them for its primary analysis. That choice answers a question about the pooled dressing strategy compared with standard prevention, rather than estimating a separate primary effect for Allevyn® and Mepilex®. Readers should align interpretation with that pooled comparison.

The outcome is a participant-level binary measure over a defined period and at specified anatomical sites. This makes the comparison relatively direct, but it also means the endpoint does not represent the full burden of ulcer events or their subsequent course. A participant with more than one qualifying ulcer still contributes as a participant with an event; the primary count is not a count of all ulcers.

The registry provides both a reported analysis and enough group-level information for an unadjusted re-calculation. Their distinct scales and methods illustrate why a statistical result should not be reduced to a single p-value. The risk difference conveys an absolute contrast, while the risk ratio conveys a relative contrast. The intervals and the method used to obtain them are important parts of the interpretation.

The record also identifies gaps that matter for reproducibility, including details of the statistical plan, the strata used in the reported method, missing-data handling and the original sample-size assumptions. Recognising those limits helps distinguish what the posted results support from what cannot be checked from the registry record alone.

18. Limitations

• The posted summary statistics are the only data these results rest on; patient-level data, full protocols and statistical analysis plans are not available.
• Recalculated results are simple, unadjusted calculations from posted summaries. They do not replace the sponsor's prespecified analysis.

The registry record does not report detailed eligibility criteria, the randomisation procedure, the allocation ratio planned by the protocol, or the original sample-size justification. Although the official title describes a multicentric trial, details about participating centres and how centre differences were handled are not reported. The intended limit on recruitment from intensive care is recorded, but the actual distribution of participants by setting is not.

The registry reports a Cochran-Mantel-Haenszel analysis but does not identify the strata or provide the analysis details needed to reproduce that result. It does not report the detailed statistical analysis plan, any interim analyses, adjustment for multiple testing, or a missing-data method. The disposition of participants between registered enrollment and the intention-to-treat population is not explained.

The primary outcome is limited to new pressure ulcers of category 2 or worse at the sacrum, heels or greater trochanter during a maximum 14-day period. The record does not provide longer-term outcomes, a count of all ulcers, or detailed results for each dressing brand separately. The primary intervention result pools those groups.

Safety reporting in the record lists adverse-event information for the intervention groups but does not provide corresponding control-group figures. Details on the nature, timing and attribution of the listed events are not reported. These limitations constrain independent assessment of the analysis and the scope of conclusions that can be drawn from the registry results.

19. Related Tutorials

20. Related Statistical Calculators

21. Sources

• ClinicalTrials.gov: Silicone Adhesive Multilayer Foam Dressings to Prevent Pressure Ulcer — NCT03442777.
• Registry record last updated 2021-04-26; status last verified 2021-03.
• Results first posted 2021-04-26.
• Data retrieved from ClinicalTrials.gov on 2026-11-09.
• Group sizes, means, event counts and sponsor-reported p-values are reproduced as posted. Effect estimates, confidence intervals and p-values not attributed to the sponsor are independently calculated from the posted summary statistics.

22. Record Summary

NCT03442777 is a completed, randomized, open-label, parallel-group medical-device trial sponsored by the Belgium Health Care Knowledge Centre. It compared standard pressure-ulcer prevention plus Allevyn® or Mepilex® silicone adhesive multilayer foam dressings with standard prevention alone in at-risk hospitalised patients. Registered enrollment was 1634.

The primary outcome counted participants with at least one new pressure ulcer of category 2 or worse on the sacrum, heels or greater trochanter during a maximum 14-day assessment period. The primary comparison pooled the two intervention groups and used an intention-to-treat population reported as 1605. The registry’s analysis reports a risk ratio of 0.64, a 95% confidence interval from 0.41 to 0.99 and p = 0.04.