Pain Study of Rectus Muscle Closure at Cesarean Delivery: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Pain Study of Rectus Muscle Closure at Cesarean Delivery (NCT00505362), a randomized na study. The primary outcome is Post-operative Pain.

2. Clinical Question

The registry describes a randomized comparison of two approaches to rectus muscle management during primary cesarean delivery. The clinical question is specifically about the recorded composite pain score, rather than pain considered apart from opioid use. The score incorporates both opioid consumption and movement pain, so its group comparison reflects the combined measure as defined in the registry.

The study description also identifies adhesions at repeat cesarean delivery as an intended area of assessment. However, the posted secondary outcome is operative time, and the registry record does not report an adhesion result. That planned interest should therefore be distinguished from the outcomes with posted results.

3. Trial Design

The registry identifies this as a randomized, parallel-group trial with triple masking. It lists the study as completed, with enrollment of 63. The sponsor was Stanford University. The recorded study dates are 2006-06 for study start and 2014-08 for primary completion. The phase is listed as not applicable.

The comparison was between rectus muscle closure and rectus muscle non-closure at primary cesarean delivery. Both arm descriptions include two-layer uterine closure, peritoneal closure, and fascial and skin closure; they differ in whether the rectus muscles were reapproximated. In the closure group, reapproximation was described as using three-interrupted sutures. These descriptions clarify the procedural contrast recorded in the registry, but they do not provide details about the conduct of the operations beyond those stated.

The registry reports a mean and standard deviation for the primary continuous outcome in each arm. It also reports the corresponding two-sided t-test result. Operative time is the posted secondary outcome, likewise recorded as a continuous measure with group means and standard deviations. The registry record does not report the trial’s original sample-size justification, detailed randomization procedures, masking procedures, or statistical analysis plan. Those features should not be inferred from the design labels alone.

Enrollment is a study-level registry figure. The posted arm-specific outcome summaries list 35 participants for closure and 28 for non-closure. The record does not explain whether any differences between enrollment and outcome-specific analysis counts reflect missing observations, exclusions, or another reason. The analyzed population and the handling of unavailable measurements are not reported in the registry record.

Although the study description mentions assessment of adhesions at repeat cesarean delivery, adhesions are not listed among the posted outcomes with results. Their results are not reported in the registry record. The design information therefore supports describing the randomized comparison and the listed endpoints, but not reconstructing unreported methods or findings.

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 Post-operative Pain, measured over 72-hour study period.

The primary endpoint was postoperative pain measured with the Silverman Integrated Assessment (SIA) pain score over the 72-hour study period. The registry describes a composite that combines opioid use, expressed in morphine milligram equivalents, and movement-pain scores summarized as area under the curve. The component measures are rank ordered, averaged, expressed as percent differences from their respective means, and combined by adding those percent differences.

This construction is intended to represent pain and analgesic use together rather than treating either as a complete account on its own. A person with more pain may also use more opioid medication, so considering both dimensions can distinguish comfort from medication use alone. That is the rationale implicit in the registry’s description of the composite; it does not establish that the score captures every aspect of postoperative experience.

The score’s direction matters for interpretation. The registry states that a higher positive score indicates less comfort or more pain despite greater analgesic use, while a lower score indicates greater comfort or less pain despite less analgesic use. Thus, group means should be read with that direction in mind. The reported outcome is a mean continuous score with standard deviations, not a proportion of people meeting a pain threshold. The registry record does not report a threshold for clinical importance or describe validation properties of the measure.

7. Results Posted in the Registry

• Rectus Muscle Non-closure: mean -31.00 versus 15.00 on control; difference -46.00 (95% CI -89.96 to -2.04), standardized effect d = -0.51; z-test p = 0.040.

8. Secondary Efficacy Endpoints

The registry also posts results for secondary outcomes. For each outcome below, the endpoint type, the test applied to the posted summary statistics, the result and its interpretation are given in turn. Secondary endpoints are usually not adjusted for multiple testing, so they support rather than confirm the primary finding.

Operative Times

• Endpoint type: Continuous: a mean value per participant, compared as a difference in means.
• Test: Unadjusted two-sample z-test for the difference in means, using the posted standard deviations.
• Result:
  • Rectus Muscle Non-closure: mean 65.00 versus 63.00 on control; difference 2.00 (95% CI -5.45 to 9.45), standardized effect d = 0.13; z-test p = 0.599.
• Interpretation:
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between Rectus Muscle Non-closure and the control. The interval gives the range of effects that remain compatible with the data.

Operative time is the posted secondary outcome, measured from the start to the end of cesarean delivery and assessed up to two hours. The registry includes group means and standard deviations and reports a two-sided t-test. The recalculated interval spans values on both sides of zero, and the sponsor-reported p-value is 0.61. This secondary result does not establish a clear mean difference in operative time.

The primary and secondary outcomes address different aspects of the procedure: the primary endpoint is a composite pain and opioid-use score over the 72-hour study period, while the secondary outcome concerns operating time. Their estimates should not be treated as interchangeable evidence or as a single combined measure of benefit or burden. The operative-time result does not explain the primary composite result, and the primary result does not determine the operative-time contrast.

The study description mentions an intention to assess adhesions at repeat cesarean delivery, but the registry does not post an adhesion outcome result. The posted secondary outcome information therefore does not answer that stated question. No conclusions about adhesions can be drawn from the operative-time comparison.

Secondary outcomes generally carry less confirmatory weight when they are considered alongside a primary outcome, particularly when multiple outcomes or comparisons are examined. Each additional statistical test creates another opportunity for a result to appear unusual by chance. General statistical reasoning: accounting for multiplicity can help control the overall false-positive risk, but the appropriate approach depends on the analysis plan and inferential goals. The registry record does not report a multiplicity adjustment or a hierarchy for interpreting secondary outcomes. Consequently, the secondary result is best treated as descriptive and supportive context rather than as an independent confirmatory finding.

Secondary outcomes registered for this trial, for reference (only those above were recalculated):

• Operative Times

9. Statistical Interpretation

10. Statistical Methods Explained

The primary outcome is continuous, and the registry summarizes it with a mean and standard deviation in each randomized arm. A mean describes the arithmetic center of the observed scores. A standard deviation describes how spread out individual scores are around that mean. Neither quantity alone describes the full distribution, and a mean difference can be influenced by the shape and variability of the observed scores.

What does the mean difference represent?

The recalculated contrast is defined as the non-closure mean minus the closure mean. For the primary endpoint, the estimate is -46.0. A negative value means that the observed average score was lower in the non-closure arm under this subtraction order. Since the registry defines lower SIA scores as indicating more comfort or less pain despite less analgesic use, the direction can be related to the score’s stated interpretation. It remains a comparison of group averages, not a claim that every participant had a lower score in one group.

The estimate is an unadjusted comparison based on the posted summary statistics. It does not account for covariates or other factors beyond the two-arm contrast. No adjusted primary analysis is reported in the registry record. The recalculation is not a substitute for analyses that may have been specified or conducted by the sponsor; in this case, the registry’s own p-value is also reported as 0.04.

How should the t-test be understood?

The registry reports a two-sided t-test for the primary comparison. In general, a two-sided t-test assesses whether the data are compatible with a difference in means in either direction, rather than only one prespecified direction. It compares the observed difference with the variability and sample information under a model for the means. A p-value of 0.04 indicates that results at least as discordant with equal means would be relatively unusual under the test’s null model and assumptions. It does not quantify effect size, clinical importance, or the probability that a treatment effect is real.

The unadjusted recalculation uses the reported group means, standard deviations, and outcome counts to estimate the difference and its uncertainty. General statistical interpretation of this kind of comparison assumes that the observations are independent between participants and that the t-based inference is sufficiently appropriate for the data. The registry record does not report diagnostic checks or details about the score distribution, so those assumptions cannot be evaluated from the posted summary alone.

What does the confidence interval add?

A confidence interval gives a range of values compatible with the data under the method used, reflecting sampling uncertainty. It is not a range containing a known proportion of individual scores, and it is not a guarantee that the true effect lies within the interval. For the primary endpoint, the interval is -89.96 to -2.043. Its limits show that the estimate is imprecise enough to include values close to zero as well as more substantial negative differences on the score scale. The registry record does not identify which values would be clinically important.

The standard deviations are large relative to the arm means, so individual scores varied substantially around their group averages. General statistical reasoning: greater variability makes an estimate less precise when other features are held similar. The confidence interval reflects that uncertainty in the group contrast; it should be considered alongside the point estimate rather than reduced to a yes-or-no interpretation based only on the p-value.

Why do recalculated results and registry results differ in presentation?

The registry provides p-values for the sponsor’s reported t-tests, while the posted summary statistics permit an unadjusted recalculation of differences and confidence intervals. The primary recalculated p-value is 0.04 and matches the registry’s p-value of 0.04. For operative time, the recalculated p-value is 0.599, while the registry reports 0.61. Small differences in displayed results can arise from rounding or details of the underlying analysis; the registry record does not explain the difference. The recalculation is based on rounded summary statistics and should be understood as a simple unadjusted calculation.

The statistical plan, the original analysis population, and any adjustments beyond the reported t-tests are not reported in the registry record. The record also does not say whether there were planned interim analyses. These absences limit the ability to judge whether the posted analyses match all prespecified decisions. The methods therefore distinguish what the registry reports from general explanations of how continuous-outcome comparisons are commonly interpreted.

11. Confidence Intervals and Precision

The primary mean difference is -46.0, with a 95% confidence interval from -89.96 to -2.043. The estimate points toward a lower mean SIA score in the non-closure group, following the recalculation’s non-closure-minus-closure order. The interval remains entirely below zero, but its span indicates meaningful uncertainty about the size of the difference. The registry does not report a clinical threshold that would indicate which values within this interval are important to patients or clinicians.

A confidence interval is best read as an uncertainty range for the estimated group contrast under the statistical method, not as a range for individual participants’ scores. It also does not account for every possible source of uncertainty, such as missing outcome data or analysis choices that are not documented. The registry record does not report how missing measurements were handled, so the interval’s interpretation depends on information that is unavailable in the record.

The operative-time difference is 2.0 minutes, with a 95% confidence interval from -5.454 to 9.454. This range includes zero and extends in both directions. It therefore does not pin down a single direction for the mean difference with high precision. The p-value and interval should be read together: neither establishes a clear between-group difference in operative time, while the interval indicates the range of contrasts compatible with the recalculation.

Precision is distinct from statistical significance. A result can have an interval that excludes zero yet still leave uncertainty about practical importance; conversely, an interval including zero does not prove that the groups are identical. The posted estimates provide information about the observed contrasts, but the registry record does not give a minimum important difference for either outcome.

12. 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 are participants per arm, not total events. They are illustrative calculations based on the observed effect, not the trial’s original design assumptions. The listed assumptions are a two-sided significance level of 0.05, equal allocation between the two arms, a normal approximation, and an effect to detect set to the stated fraction of the observed effect. The original sample-size justification and design assumptions are not reported in the registry record.

The 80% power column gives the estimated number of participants per arm for a future study to have an 80% chance of detecting the specified effect under those planning assumptions, if that effect and the calculation model were correct. The 90% power column describes the corresponding planning target for a 90% chance. These are design probabilities under assumed conditions, not guarantees about what a particular study will find.

The rows scale the assumed effect relative to the observed effect. For example, planning for 75% of the observed effect requires more participants per arm than planning for 100% of the observed effect; planning for 50% requires more again. This illustrates a general relationship: smaller effects are harder to distinguish from random variation, so more participants are generally needed to achieve the same planned power.

The displayed curve should be read as a visual presentation of how the planning requirement changes with the assumed effect, not as a record of observed trial performance. These figures are not observed power. They are planning illustrations derived from the observed contrast and the stated assumptions, and they should not be interpreted as evidence that the completed study was adequately or inadequately powered for its original purpose. The registry record does not report the original target sample size or the reasoning behind it.

13. 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 = -46.000; pooled standard deviation σ = 90.921.
2. n = 2 × (1.96 + 0.842)² × 90.921² / -46.000² = 61.3, rounded up to 62 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 <- -46                      # observed difference in means
sigma <- 90.9213315157129                     # 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(505362)
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 = 62 (80% power), 83 (90% power)
 75% of observed effect: n per arm = 110 (80% power), 146 (90% power)
 50% of observed effect: n per arm = 246 (80% power), 329 (90% power)
power.t.test: 62.3 
Simulated power at n = 62 per arm: 0.802 (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.

14. Analyses Reported by the Sponsor

The registry reports that the sponsor’s primary analysis used a two-sided t-test and gave p = 0.04. The unadjusted recalculation from the posted arm summaries gives a mean difference of -46.0, a 95% confidence interval from -89.96 to -2.043, and p = 0.04. Thus, the recalculated primary p-value matches the sponsor-reported value at the displayed precision. The recalculation also makes the contrast’s direction and uncertainty explicit using the posted group means and standard deviations.

For operative time, the sponsor-reported p-value is 0.61, while the recalculation gives p = 0.599, with a mean difference of 2.0 minutes and a 95% confidence interval from -5.454 to 9.454. The registry record does not explain the small difference between the displayed p-values. The recalculation is based on rounded posted summaries and is an unadjusted check, not a replacement for the sponsor’s analysis.

The record does not provide the sponsor’s full statistical analysis plan, analysis dataset, or detailed analysis population. The recalculated estimates should therefore be read as simple comparisons reconstructed from the posted summaries. For the primary outcome, they agree with the reported p-value; they do not establish that every methodological detail of the sponsor’s analysis can be reproduced.

15. Multiplicity, Missing Data and Analysis Populations

The registry lists a primary outcome and a secondary outcome, and it reports analyses for both. The record does not report whether the investigators prespecified a hierarchy among outcomes, adjusted for multiple testing, or used another strategy to address multiplicity. Without that information, the two reported p-values should not automatically be interpreted as if each were the only statistical test conducted.

General statistical explanation: when several hypotheses are tested, the chance of at least one apparently unusual result can increase if each test is judged at the same threshold without adjustment. This does not mean that every secondary result is false or uninformative. It means that the number of analyses and their prespecification matter when judging evidential strength. The registry record does not report the complete set of analyses considered, so the scope of multiplicity cannot be determined.

Missing-data procedures are also not reported in the registry record. The outcome summaries list 35 participants for closure and 28 for non-closure, while enrollment is listed as 63. The record does not explain the relationship between these counts, whether any outcome measurements were unavailable, or whether participants were excluded from an analysis. It does not report imputation, sensitivity analyses, or reasons for missingness. Therefore, it is not possible to assess from the registry entry whether missing outcomes could have affected the estimates.

The analysis population is not reported in the registry record. In particular, the record does not state whether the primary comparison followed an intention-to-treat approach, a per-protocol approach, or another population definition. Nor does it provide details about treatment adherence or deviations from the assigned procedure. Random allocation is reported, but the methods used to preserve the benefits of randomization in the analysis cannot be fully assessed without this information.

The statistical analysis plan, including any covariate adjustment or interim analyses, is not reported in the registry record. These omissions do not invalidate the posted summaries; they limit how fully readers can evaluate prespecification, missingness, and the role of other analyses in the overall evidence.

16. Safety Results

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

Other (non-serious) adverse events

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

The registry lists zero serious adverse events in the closure group among 35 participants at risk and zero in the non-closure group among 28 participants at risk. It also lists zero other adverse events in each group, with the same respective counts at risk. These are the recorded adverse-event counts for the groups in the registry entry.

Zero recorded events should not be read as proof that no adverse events occurred outside the registry’s reporting definitions or observation process. The record does not provide additional details about event collection, definitions, or completeness. The deaths field is not populated with a numeric result, so deaths are not reported in the registry record.

These safety figures are counts, not comparative estimates with confidence intervals. They should be considered alongside the limited amount of safety detail in the registry record and do not resolve questions about rare events or longer-term outcomes.

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

The posted primary results show a lower mean SIA score in the non-closure group than in the closure group, with a recalculated mean difference of -46.0 and a 95% confidence interval from -89.96 to -2.043. The registry reports p = 0.04 for the two-sided primary comparison. Under the score’s stated direction, lower values indicate more comfort or less pain despite less analgesic use. This describes the observed group contrast on the composite measure; it is not a statement about every participant or about pain considered separately from opioid use.

The result does not establish the clinical importance of the difference. The registry does not report a threshold for a meaningful change in the SIA score, and the composite combines two components without posting separate group results for them. Readers cannot determine from the posted composite summaries how much opioid use or movement pain individually contributed to the contrast.

The operative-time result does not establish a clear mean difference between groups. It also does not answer the study description’s question about adhesions at repeat cesarean delivery, because an adhesion result is not reported in the registry record. The adverse-event counts are limited registry summaries and do not establish the absence of all possible harms.

More broadly, a p-value below a conventional threshold is not proof of a causal mechanism, an estimate of the probability that a finding is true, or a measure of clinical importance. Randomization is reported, but the registry record does not provide all details needed to assess the analysis population, missing data, outcome completeness, or multiplicity. The findings should be interpreted within the recorded trial comparison and the limits of its posted information, rather than treated as a universal conclusion about procedures or outcomes beyond those reported.

18. Why This Trial Matters Statistically

This trial is statistically useful because it illustrates the difference between a point estimate and the uncertainty around it. The primary mean contrast is negative under the stated comparison order, and the confidence interval gives a range of contrasts compatible with the summary data and method. Reading both together is more informative than relying on a p-value alone: the estimate conveys direction and scale, while the interval conveys uncertainty about the size of the difference.

The endpoint also highlights a measurement issue. Postoperative pain was represented by a composite combining opioid use and movement pain, with a defined direction in which lower scores indicate greater comfort. Composite measures can summarize related dimensions, but their interpretation depends on how components are constructed and weighted. Because component-specific results are not posted, the group contrast cannot be attributed to one component from the registry summaries alone.

The study further distinguishes a primary outcome from a secondary outcome. A result for the pain composite and a result for operative time answer different questions, and the latter does not confirm or explain the former. Considering multiple outcomes requires attention to prespecification and multiplicity; those details are not reported in the registry record.

Finally, the record shows how statistical interpretation depends on reporting completeness. Enrollment, arm-specific outcome counts, a randomized design label, means, standard deviations, confidence intervals, and p-values provide useful information, but missing details about the analysis population, missingness, and original design assumptions constrain the conclusions. Recognizing both the quantitative findings and the boundaries of the record is central to responsible interpretation.

19. 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 entry does not provide detailed eligibility criteria or a fuller description of the analyzed population. It reports enrollment of 63 and outcome-specific arm counts of 35 and 28, but does not explain how those figures relate or why the outcome counts differ from the enrollment figure. Missing-data handling, reasons for unavailable measurements, and sensitivity analyses are not reported in the registry record.

The registry identifies randomization and triple masking, but does not report the methods used to generate or conceal allocation, who was masked, or how masking was maintained. The statistical analysis plan and analysis population are not reported, so it is not possible to determine whether the posted tests reflect all prespecified decisions or how deviations from assigned procedures were handled.

The primary endpoint is a composite score. Although its construction and direction are described, the registry does not post component-specific results or a threshold for clinical importance. The observed mean contrast therefore cannot be translated into a separate opioid-use effect, movement-pain effect, or clinical interpretation based on a stated meaningful-change threshold.

The study description mentions adhesions at repeat cesarean delivery, but no adhesion result is posted. The listed secondary outcome is operative time, so that outcome does not fill the gap. The registry record also does not report a multiplicity strategy, interim analyses, or the trial’s original sample-size justification. These limits make it difficult to judge the full prespecification and scope of the analyses.

Finally, the adverse-event information is summarized as counts, with no further detail about collection or follow-up. The record does not provide numeric deaths results. These limitations concern the information available in the registry record; they should not be replaced with assumptions about the study’s conduct or unreported outcomes.

20. Related Tutorials

21. Related Statistical Calculators

22. Sources

• ClinicalTrials.gov: Pain Study of Rectus Muscle Closure at Cesarean Delivery — NCT00505362.
• Registry record last updated 2017-11-06; status last verified 2017-10.
• Results first posted 2017-03-27.
• 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.

23. Record Summary

The registry describes a completed randomized, parallel-group, triple-masked study sponsored by Stanford University, with enrollment of 63. It compared rectus muscle closure with non-closure at primary cesarean delivery. The primary outcome was the mean SIA postoperative pain score over the 72-hour study period.

The non-closure group had a lower posted mean SIA score, and the unadjusted recalculation gives a mean difference of -46.0 with a 95% confidence interval from -89.96 to -2.043; the registry reports p = 0.04. The posted secondary outcome was operative time, for which the recalculated interval includes zero. The registry also lists zero serious and zero other adverse events in both groups, while deaths are not reported in the registry record.

The record does not report the original sample-size justification, missing-data procedures, analysis population, multiplicity approach, or adhesion results. Those omissions limit interpretation beyond the posted group summaries and analyses.