Antimicrobial Barrier Dressing Versus Closed-incision Negative Pressure Therapy in the Obese Primary Total Joint Arthroplasty: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Antimicrobial Barrier Dressing Versus Closed-incision Negative Pressure Therapy in the Obese Primary Total Joint Arthroplasty (NCT03345771), a randomized phase 3 study. The primary outcome is Visual Analog Scale (VAS) Pain Score.

2. Clinical Question

The registry record reports mean scores of 64.0 for the antimicrobial barrier dressing arm and 61.0 for the closed-incision negative pressure therapy arm. The reported mean difference for negative pressure therapy relative to the antimicrobial dressing is -3.0, with a confidence interval from -10.79 to 4.785 and an unadjusted p-value of 0.45. The scale description states that higher scores indicate better scarring. These figures describe the posted score comparison; they do not establish whether either dressing changes infection risk, wound complications, or costs.

3. Trial Design

The registry identifies this as a Phase 3 study with randomized allocation and no masking. It lists 230 participants enrolled and a completed status. The study started on 2017-11-13, and primary completion is listed as 2021-04-01. Osteoarthritis is the stated condition. The study title identifies obese patients undergoing primary total joint arthroplasty as the setting.

Two approaches to postoperative incision care are described. The antimicrobial barrier dressing was placed in the operating room and remained in use from surgery through postoperative day 7. The portable negative pressure wound therapy device was likewise placed in the operating room and used through postoperative day 7. The brief study summary describes intended enrollment of approximately 240 participants, with 120 planned for total knee arthroplasty and 120 for total hip arthroplasty. The registry separately reports enrollment of 230; it does not report the actual knee and hip distribution in the posted primary outcome.

There is an internal design inconsistency in the registry record: allocation is described as randomized, while the study model is listed as single group. The record does not explain this mismatch, so the design classification cannot be reconciled from the registry information alone. It also does not describe the randomization procedure, concealment, masking of outcome assessment, or the protocol's analysis populations.

The posted primary outcome is a Day 8 Visual Analog Scale score. The registry lists two outcome measures, but the available primary result is the score comparison between the two dressing groups. No secondary outcome results are posted. The study summary states that wound complications, infection rates, and cost benefit were among the study aims, but results for those aims are not reported in the registry record.

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.

Overall Study

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 Visual Analog Scale (VAS) Pain Score, measured over Day 8.

The primary endpoint is the Visual Analog Scale (VAS) score assessed at Day 8. The registry describes a patient marking perceived scar severity along a 100 mm horizontal line, with the distance from the left edge recorded as the score. It gives the score range as 0 to 100 and says that a higher score indicates better scarring. The endpoint title, however, calls this a “VAS Pain Score,” while the description refers to scar severity. The registry does not clarify the difference in terminology, so the result should be understood according to the stated scale description while recognizing that inconsistency.

A continuous score can summarize participants’ ratings and support a comparison of average scores between groups. The registry reports the mean and standard deviation for each arm. The posted means are 64.0 for the antimicrobial barrier dressing and 61.0 for closed-incision negative pressure therapy. The reported comparison is a mean difference of -3.0 for negative pressure therapy relative to the antimicrobial dressing.

A Day 8 score captures a rating at one specified postoperative time. It does not, by itself, measure the incidence of infection or other wound complications over a longer follow-up period. Although those outcomes are part of the study’s stated purpose, their results are not reported in the registry record. The record also does not report why Day 8 was selected as the assessment time or how the score was collected beyond the scale description.

7. Results Posted in the Registry

• Closed-incision Negative Pressure Therapy: mean 61.00 versus 64.00 on control; difference -3.00 (95% CI -10.79 to 4.79), standardized effect d = -0.12; z-test p = 0.450.

8. Statistical Interpretation

9. Statistical Methods Explained

The primary outcome is continuous: it is a score on a 0-to-100 scale. The registry reports a mean and standard deviation for each arm, then gives a mean difference and confidence interval for the comparison. These summaries answer related but distinct questions. The group means describe the average scores; the mean difference describes the contrast on the original score scale; the standard deviations describe within-arm variability; and the confidence interval expresses uncertainty about the estimated contrast.

What does the mean difference represent?

The reported comparison is negative pressure therapy relative to the antimicrobial barrier dressing. Its mean difference is -3.0, based on the reported arm means of 61.0 and 64.0. The negative sign indicates that the negative pressure therapy arm’s mean is lower on the score scale. Since the scale description says higher scores indicate better scarring, the direction can be described using that definition. The registry does not state a threshold for a difference that would be important to patients or clinicians, so statistical summaries alone do not indicate whether a difference of this size would matter in practice.

A mean is sensitive to all observed scores and gives a compact summary of the center of a distribution. It does not show how scores are distributed, whether there are unusually high or low values, or whether subgroups differ. The standard deviations of 26.0 and 24.0 indicate that individual scores vary around their respective means. A standard deviation is not a confidence interval for the mean, and it does not show the range containing every participant’s score.

How should the confidence interval be read?

The reported confidence interval for the difference runs from -10.79 to 4.785. In general, a confidence interval is a range of values compatible with the observed data under a specified statistical method and its assumptions. It communicates uncertainty about the estimated difference. A narrower interval indicates greater precision than a wider interval, all else being equal. The interval crosses zero, meaning that the range includes both negative and positive differences. It therefore does not isolate one direction of difference. The registry does not provide the confidence level explicitly in the outcome record, nor does it document the calculation method.

What does the p-value add?

The unadjusted p-value is 0.45. In general, a p-value describes how unusual data at least as incompatible with a stated null hypothesis would be if that hypothesis and the model assumptions were correct. It is not the probability that the null hypothesis is true, nor is it a measure of the size or importance of an effect. A larger p-value does not prove equality. Here, the p-value and the confidence interval both indicate substantial uncertainty about the mean difference, but neither supports a claim of equivalence or non-inferiority. No equivalence margin or non-inferiority margin is reported in the registry record.

The word “unadjusted” matters because it distinguishes the posted p-value from a value modified for multiple comparisons or other adjustments. The registry record does not report whether covariates were included, how the test was chosen, or whether any other analysis was planned. It also lists zero analysis entries. Thus, the detailed statistical procedure behind the reported comparison is not reported in the registry record. The result should not be attributed to a particular test, model, or adjustment strategy that the record does not describe.

Why does variability matter?

General statistical principle: greater variation in individual scores makes an average difference harder to estimate precisely when other design features are held constant. The standard deviations give a measure of that variation for these scores. They are 26.0 and 24.0 in the two arms, and the reported pooled standard deviation is 25.0. The standardized effect size of -0.12 expresses the observed difference relative to the pooled variability. It is a descriptive scale-free measure, not a probability of benefit and not a substitute for the original-scale estimate.

General statistical principle: continuous outcomes are often analyzed using methods that rely on assumptions about observations and score distributions. Independence of observations and the behavior of residuals may matter, depending on the method. Whether the chosen approach assessed these assumptions, used a robust method, or accounted for any design features is not reported in the registry record. The comparison should therefore be interpreted from the posted summaries without claiming a particular model or verified assumptions.

What do the arm counts tell us?

The primary outcome summaries list 78 participants for the antimicrobial barrier dressing and 81 for closed-incision negative pressure therapy. These are the counts associated with the reported arm summaries. The registry reports enrollment of 230, but it does not explain why the outcome counts differ from the enrollment figure, whether some scores were missing, or which participants were included in the analysis. The adverse-event risk counts are reported separately as 115 in each arm; the registry does not explain how those counts relate to the primary outcome analysis. These distinct counts should not be treated as interchangeable.

Random allocation, if implemented as the record states, is intended in general to balance prognostic factors between groups on average. It does not eliminate the need to report analysis populations, outcome availability, or the method used to handle missing data. The registry record does not describe those details. Moreover, it labels allocation randomized but the model single group, an unresolved internal inconsistency that limits a full design-level assessment.

These methods concepts help distinguish what the posted statistics can support: a group-level score comparison with uncertainty. They do not supply evidence about unreported endpoints, missing-data handling, protocol adherence, or the trial’s complete statistical plan.

10. Confidence Intervals and Precision

The reported mean difference is -3.0, with a confidence interval from -10.79 to 4.785. The interval is the clearest posted summary of precision for the group contrast: it shows that the point estimate is uncertain, with compatible values extending in both directions around zero. The width of the interval makes it important not to interpret the point estimate alone as a definitive estimate of the difference.

In general, a confidence interval combines an estimate with uncertainty under a statistical method. It is not a guarantee that the true value lies within the particular interval, and it does not assign probabilities to individual values after the data have been observed. The registry does not describe the method or confidence level used for this interval. Interpretation is therefore limited to the interval as posted, rather than a reconstruction of its calculation.

The interval includes values below and above zero. Accordingly, the result does not establish a difference in one direction, but it also does not rule out differences across the full interval. Without a prespecified threshold for clinical importance, it is not possible to judge from the interval alone which included values would be meaningful. The registry does not report such a threshold or an equivalence or non-inferiority margin.

The group standard deviations are 26.0 and 24.0, and the pooled standard deviation is 25.0. They provide context for the variability of individual scores, while the confidence interval describes uncertainty in the estimated between-group contrast. Those are not interchangeable measures. The unadjusted p-value of 0.45 is consistent with uncertainty, but it does not communicate the range of plausible effect sizes as directly as the interval does.

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 planning figures describe participants per arm for 80% power and participants per arm for 90% power. They are planning illustrations based on the observed effect, not counts of participants enrolled or analyzed. The assumptions listed for these illustrations are a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. The effect to detect is set to a stated fraction of the observed effect. The trial’s own original design assumptions are not reported in the registry record.

For planning at 100% of the observed effect, the figures are 1,091 participants per arm for 80% power and 1,460 participants per arm for 90% power. For planning at 75% of the observed effect, they are 1,939 participants per arm for 80% power and 2,595 participants per arm for 90% power. For planning at 50% of the observed effect, they are 4,361 participants per arm for 80% power and 5,838 participants per arm for 90% power. These values illustrate how the required sample size changes with the assumed effect size.

In general, 80% power means that, under the specified planning assumptions and if the targeted effect is the true effect, the design would have an 80% chance of producing a statistically significant result. The 90% column describes the corresponding chance under that planning scenario. The higher power target requires more participants at each listed effect fraction. Smaller effects need more participants to detect than larger effects, which is reflected by the increasing figures as the planning effect becomes a smaller fraction of the observed effect.

These calculations use the observed effect as an input and are not observed power. They do not show the probability that the trial result is correct, and they do not retrospectively establish whether the trial was adequately planned. The registry does not report the original trial’s sample-size justification or assumptions.

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, Δ is the difference in means to detect and σ is the common standard deviation.

Worked calculation for this trial

1. Observed difference Δ = -3.000; pooled standard deviation σ = 25.001.
2. n = 2 × (1.96 + 0.842)² × 25.001² / -3.000² = 1,090.2, rounded up to 1,091 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
delta <- -3                      # observed difference in means
sigma <- 25.0008917038425                     # pooled standard deviation
n_per_arm <- function(power, frac = 1) {
  ceiling(round(2 * (z_a + qnorm(power))^2 * sigma^2 / (delta * frac)^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 (t-based, so very slightly larger)
cat("power.t.test:", round(power.t.test(delta = delta, sd = sigma, power = 0.80)$n, 1), "\n")

# Simulation: exact two-sample t-test using simulated means and variances
set.seed(3345771)
n    <- n_per_arm(0.80)
reps <- 20000
m_diff <- rnorm(reps, delta, sigma * sqrt(2 / n))             # difference in sample means
s2     <- sigma^2 * rchisq(reps, 2 * (n - 1)) / (2 * (n - 1))   # pooled sample variance
tstat  <- m_diff / sqrt(s2 * 2 / n)
cat(sprintf("Simulated power at n = %s per arm: %.3f (%d replicates)\n", format(n, big.mark = ","), mean(abs(tstat) > qt(0.975, 2 * (n - 1))), reps))

Output from running the code

100% of observed effect: n per arm = 1,091 (80% power), 1,460 (90% power)
 75% of observed effect: n per arm = 1,939 (80% power), 2,595 (90% power)
 50% of observed effect: n per arm = 4,361 (80% power), 5,838 (90% power)
power.t.test: 1091.2 
Simulated power at n = 1,091 per arm: 0.796 (20000 replicates)

The simulation draws many sets of group means and variances at the planned sample size and counts how often the two-sample t-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. Multiplicity, Missing Data and Analysis Populations

The registry record lists two outcome measures, but no secondary outcome results are posted. It reports the Day 8 VAS score as the primary outcome and provides the arm summaries and comparison for that measure. The record does not describe a full statistical analysis plan, the number of planned comparisons, or whether additional outcomes or subgroups were analyzed. It also lists zero analysis entries. The exact set of analyses conducted is therefore not reported in the registry record.

In general, multiplicity arises when several outcomes, time points, groups, or statistical tests are considered. If many comparisons are made, the chance of at least one apparently notable result can increase. Statistical plans may address this through prespecification or adjustment, but the record does not report whether any such approach was used. The posted p-value is identified as unadjusted, and no multiplicity adjustment is reported. It would be inappropriate to assume either that an adjustment was planned or that no additional analyses occurred.

Missing-data handling is also not reported. The primary outcome counts are 78 for the antimicrobial dressing and 81 for negative pressure therapy, while overall enrollment is 230. The record does not explain the difference, identify missing Day 8 scores, or state whether imputation or another method was used. It does not report whether participants were analyzed in their assigned groups, whether adherence was considered, or whether a per-protocol analysis was conducted. The analysis population is not reported in the registry record.

General statistical principle: missing outcome data can affect interpretation when the likelihood of a score being unavailable is related to the score itself or to participant characteristics. Without information about the amount, reasons, and handling of missingness, the potential effect on the estimate cannot be assessed. The registry also gives adverse-event risk counts of 115 in each arm, but does not explain how those counts relate to the primary outcome population.

Finally, the record does not report interim analyses, stopping rules, or a plan for control of type I error across outcomes. Those details should be treated as unknown, not inferred from the endpoint listing or the single posted p-value.

14. 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 serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: irrigation and debridement (1.7%), revision total knee arthroplasty (0.9%).

The registry lists serious adverse-event risk counts of 115 in each arm. It reports 3 serious adverse events for the antimicrobial barrier dressing and 0 for closed-incision negative pressure therapy. The record lists 6 other adverse events for the antimicrobial dressing and 2 for negative pressure therapy. Deaths are listed as 0 in each arm. These are the posted counts; the registry does not explain whether the adverse-event entries represent events, participants with events, or a particular follow-up period.

The registry identifies “Irrigation and Debridement” at 1.7% and “Revision Total Knee Arthroplasty” at 0.9% among the top serious adverse-event terms. These terms and percentages are reported as listed and should not be interpreted as a complete description of all event types or their timing. The record does not provide detailed event narratives, severity grading, or a breakdown by procedure type.

Adverse-event counts are descriptive safety information. The small number of listed events and the absence of detail about event ascertainment, follow-up, and the analysis population limit comparisons between arms. The registry does not report a statistical comparison of these safety figures. These counts should not be treated as proof that one dressing is safer or less safe than the other.

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

The posted result describes average Day 8 VAS scores in two dressing groups. The antimicrobial barrier dressing arm had a mean of 64.0, and the closed-incision negative pressure therapy arm had a mean of 61.0. The reported mean difference for negative pressure therapy relative to the antimicrobial dressing is -3.0. The confidence interval, from -10.79 to 4.785, includes zero, and the unadjusted p-value is 0.45. Together, these statistics indicate that the estimated difference is imprecise and that the posted analysis does not establish a clear group difference.

This does not establish that the treatments are equivalent. Equivalence requires a defined range of differences considered acceptably small and an analysis designed to evaluate that margin; no such margin is reported. Nor does the result establish non-inferiority, for which a prespecified margin and suitable analysis would also be needed. Neither conclusion follows simply from a non-significant p-value.

The outcome itself has a wording inconsistency: the title calls it a pain score, while the description calls it perceived scar severity and states that higher scores indicate better scarring. This makes careful use of the registry’s scale description important. The result concerns this score at Day 8; it does not directly answer whether wound complications or infections occurred less often, or whether either approach had a cost advantage.

The study summary identifies wound complications, infection rates, and cost benefit as aims. Results for those aims are not reported in the registry record. The record also does not report secondary outcome results, missing-data handling, a statistical analysis plan, or the details needed to evaluate the full analysis. The score comparison therefore cannot stand in for the broader outcome assessment described in the study summary.

The appropriate statistical reading is bounded: the posted means and uncertainty describe the observed Day 8 score comparison among the participants included in those summaries. They do not determine clinical importance, establish equivalence, or support conclusions about unreported outcomes. The unresolved mismatch between the randomized allocation label and the single-group model adds another limitation to interpreting the study design from the registry record alone.

16. Why This Trial Matters Statistically

This study is statistically informative because it illustrates the difference between an estimated group contrast and the certainty around that estimate. The mean difference is -3.0, but the confidence interval extends from -10.79 to 4.785. Looking only at the point estimate would hide the range of uncertainty. The unadjusted p-value of 0.45 is also not a measure of effect size or clinical importance; it should be read alongside the estimate and interval.

The example also shows why endpoint definitions matter. A numeric score is only interpretable in context: the registry’s endpoint title says pain, while its description concerns perceived scar severity and says that higher scores indicate better scarring. That wording affects how the direction of the mean difference is understood. Clear alignment between an outcome’s name, description, scale, and interpretation is important in clinical reporting.

The stated study aims extend beyond the posted score, including wound complications, infection rates, and cost benefit. The registry record does not report results for those aims. This distinction matters because a result for one endpoint cannot substitute for results on other outcomes, even when those outcomes motivate the study question. Readers need to know which estimand a posted statistic addresses and which questions remain unanswered.

Finally, the design information includes an unresolved inconsistency: allocation is labeled randomized, but the model is labeled single group. The outcome counts also differ from the overall enrollment, without an explanation of the analysis population or missing scores. These are not reasons to infer a particular explanation; they are reasons to keep design and data-availability uncertainty in view. Statistically sound interpretation depends not only on estimates, but also on transparent descriptions of the design, endpoints, and analysis.

17. 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.

Several limitations specific to this registry record constrain interpretation. First, the primary endpoint’s title refers to a VAS pain score, whereas its description measures perceived scar severity and defines higher values as better scarring. The record does not reconcile those descriptions. It also does not explain why Day 8 was selected or report further measurement details beyond the line-scale description.

Second, the posted primary endpoint is a score at one time point. The study summary describes wound complications, infection rates, and cost benefit as aims, but results for those outcomes are not reported in the registry record. No secondary outcome results are posted. This leaves the broader study questions unanswered by the available results.

Third, the registry reports 230 participants enrolled, while the primary outcome summaries list 78 and 81 participants in the two arms. The record does not explain this difference, report the number of missing Day 8 scores, or describe how missing data were handled. Adverse-event risk counts are listed separately as 115 in each arm, without an explanation of how those counts relate to the outcome population.

Fourth, allocation is described as randomized, but the model is listed as single group. The registry record does not resolve that discrepancy or report the randomization procedure, analysis populations, detailed statistical methods, or original sample-size justification. No equivalence or non-inferiority margin is reported. The summary also gives planned knee and hip enrollment but does not report the actual procedure mix for the posted outcome. These limitations restrict assessment of design implementation, precision, and generalizability from the registry information alone.

18. Related Tutorials

19. Related Statistical Calculators

20. Sources

• ClinicalTrials.gov: Antimicrobial Barrier Dressing Versus Closed-incision Negative Pressure Therapy in the Obese Primary Total Joint Arthroplasty — NCT03345771.
• Registry record last updated 2022-04-22; status last verified 2022-03.
• Results first posted 2022-04-22.
• 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.

21. Record Summary

The registry record for NCT03345771 describes a completed Phase 3 study of antimicrobial barrier dressing versus closed-incision negative pressure therapy in obese patients undergoing primary total joint arthroplasty. It reports randomized allocation, no masking, and enrollment of 230. Both dressing approaches were placed in the operating room and used through postoperative day 7.

The posted primary outcome is a Day 8 Visual Analog Scale score. Mean scores are 64.0 and 61.0, with a reported mean difference of -3.0 for negative pressure therapy relative to the antimicrobial dressing. The confidence interval is -10.79 to 4.785, and the unadjusted p-value is 0.45. The record’s endpoint title and description differ in whether the score is characterized as pain or scar severity.

Results for wound complications, infection rates, and cost benefit are not reported in the registry record. The record also contains an unresolved mismatch between its randomized allocation label and single-group model, and does not describe missing-data handling or the detailed statistical plan.