Conservative Versus Surgical Treatment of Umbilical Pilonidal Disease: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Conservative Versus Surgical Treatment of Umbilical Pilonidal Disease (NCT01662765), a randomized na study. The primary outcome is Cure Rate.

2. Clinical Question

The trial’s stated aim was to compare conservative and surgical treatment, inform management, and learn more about the disease. Its investigators hypothesized that surgery would have advantages for recurrence, healing time, patient comfort and satisfaction, and cost effectiveness. That statement describes the investigators’ expectation; it is not itself evidence that any treatment is better.

3. Trial Design

The registry record identifies a prospective, randomized, multicenter clinical trial with a parallel-group model and triple masking. It lists the phase as NA and the status as completed. The recorded start is 2003-11, and primary completion is recorded as 2010-05. Enrollment was 84.

The trial compared two treatment groups: conservative treatment and surgery. The registry describes the conservative approach as hair extraction and curettage under local anesthesia, with postoperative management also mentioned. The available surgical description begins with an incision and excision but is incomplete. The registry record does not report which people were masked, how masking was maintained, how the random sequence was generated, or whether allocation was concealed before assignment. Those details should not be inferred from the label “triple.”

The primary outcome was cure at two years after initial treatment, defined as absence of recurrence during that period. For this outcome, the record says 41 participants in the conservative group and 40 in the surgical group were analyzed. The record does not explain why those analyzed counts differ from the enrollment figure or provide a participant-flow account. The safety figures use 42 people at risk in each group, while the secondary outcomes have their own reported group sizes. These should be read as outcome-specific denominators rather than assumed to describe one common analysis population.

The record does not report a sample-size justification, a prespecified statistical analysis plan, or details of interim analyses. The planning illustrations associated with the posted secondary results are explicitly based on observed effects and are not evidence of the trial’s original design assumptions. No sponsor-reported analyses are listed 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

Analysis population for the primary outcome, as defined in the registry: Of the 84 patients 41 patients in the CT group. and 40 patients in ST group were analyzed

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 Cure Rate, measured over 2 year after initial treatment.

The primary endpoint is cure, recorded as a participant count, at two years after initial treatment. Cure means that no recurrence occurred within two years. Recurrence was defined as a new active discharging sinus or granulation tissue deep in the umbilicus, with or without some hairs. The endpoint therefore represents a binary outcome: each analyzed participant was classified according to whether the stated recurrence criterion was met during the specified period.

This endpoint is used to capture whether the initial treatment was followed by a return of the condition during the registry’s follow-up window. A clear operational definition matters because the classification depends on what was considered recurrence and when it occurred. The registry gives both a time window and a clinical description of recurrence, which make the outcome more interpretable than an undefined label such as “success.”

The posted counts are 28 cures among 41 analyzed participants in the conservative group and 40 cures among 40 analyzed participants in the surgery group. The reported risk difference for surgery relative to conservative treatment is 0.317, with an interval from 0.175 to 0.460, and an unadjusted p-value of <0.001. These statistics describe the registry’s analyzed groups. The record does not specify whether the analysis followed an intention-to-treat approach, how participants without outcome information were handled, or whether the two-year assessment was complete for everyone enrolled.

7. Results Posted in the Registry

• Surgery: 100.0% (40/40) versus 68.3% (28/41) on control; risk difference 31.7 percentage points (95% CI 17.5 to 46.0), risk ratio 1.46 (1.19–1.80); unadjusted two-proportion z-test p = <0.001.

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.

Healing Time

• 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:
  • Conservative: mean 17.06 versus 12.81 on control; difference 4.25 (95% CI 2.47 to 6.03), standardized effect d = 1.02; z-test p = <0.001.
• Interpretation:
  • The 95% confidence interval excludes no difference, so the mean is statistically higher for Conservative than for the control. This is an unadjusted, secondary comparison, so it is suggestive rather than confirmatory.

Visual Analogue Scale for Patient Satisfaction (VAS-PS)

• 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:
  • Surgery: mean 71.00 versus 66.00 on control; difference 5.00 (95% CI 1.72 to 8.28), standardized effect d = 0.72; z-test p = 0.003.
• Interpretation:
  • The 95% confidence interval excludes no difference, so the mean is statistically higher for Surgery than for the control. This is an unadjusted, secondary comparison, so it is suggestive rather than confirmatory.

The two posted secondary outcomes point in a consistent descriptive direction across the group summaries: the surgery group has a lower mean healing time, while the surgery group has a higher mean satisfaction score. That pattern is directionally compatible with the primary comparison, in which the reported cure outcome is higher in the surgical group. It offers context across different aspects of the trial, but it does not make the outcomes interchangeable: cure, healing time, and satisfaction measure different things and have different follow-up descriptions.

The secondary results should carry less weight than a prespecified primary outcome when the registry record does not report how the analyses were planned or whether multiplicity was addressed. When several outcomes are examined, unadjusted p-values do not account for the possibility of chance findings across those comparisons. The registry lists two secondary outcomes and no reported analyses for them, so the posted summaries alone do not establish whether these comparisons were prespecified, exploratory, or selected for reporting.

Outcome-specific denominators also matter. The posted healing-time groups each include 42 participants, while the satisfaction groups include 33 in conservative treatment and 42 in surgery. These are not the same analyzed counts as those reported for the primary outcome. The registry record does not explain the differences or describe missing-data handling. As a result, the secondary pattern can be described, but the record does not support a detailed assessment of how complete or comparable the measurements were across outcomes.

Overall, the secondary summaries are broadly aligned in direction with the primary result, but they do not remove uncertainty about the primary analysis population or fill gaps in reporting. They are best read as additional descriptive evidence whose inferential weight is limited by the lack of a reported analysis plan and multiplicity details.

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

• Healing Time
• Visual Analogue Scale for Patient Satisfaction (VAS-PS)

9. Statistical Interpretation

10. Statistical Methods Explained

What does the risk difference describe?

General statistical explanation: for a binary outcome, a risk difference compares the outcome proportions in two groups by subtracting one group’s proportion from the other. It expresses the contrast on an absolute scale, unlike a relative measure, which compares proportions as a ratio. The registry reports a risk difference of 0.317 for surgery relative to conservative treatment, with an interval from 0.175 to 0.460. The direction is positive because the reported surgery-group cure proportion is higher. The posted counts are 40 of 40 in the surgery group and 28 of 41 in the conservative group.

The risk difference is an estimate from the analyzed participants, not a fixed property of the treatments. Its precision depends on the amount of information in the groups and the observed outcomes. The analyzed group sizes and the interval should be considered together: the point estimate gives the estimated contrast, while the interval conveys uncertainty around that estimate. The registry does not identify the interval’s confidence level or calculation method, so those should not be presumed.

What does the p-value contribute?

General statistical explanation: a p-value describes how unusual data at least as inconsistent with a specified null hypothesis would be if that hypothesis and the statistical model were true. It is not the probability that the null hypothesis is true, and it is not the probability that the observed effect is due to chance. The primary comparison has an unadjusted p-value of <0.001. “Unadjusted” indicates that the reported value is not adjusted for multiple comparisons; the record does not report a multiplicity procedure.

A p-value does not describe the size or practical importance of a contrast. For that, the estimate and interval are more informative. Nor does a small p-value address potential bias from missing outcomes, deviations from the planned analysis, or limitations in how the groups were formed. Those issues require information about trial conduct and analysis that is not reported in the registry record.

How should the secondary mean comparisons be read?

General statistical explanation: a mean summarizes the average observed value in a group, while a standard deviation describes the spread of individual values around that group mean. A mean difference compares group averages. For healing time, the posted contrast is expressed as conservative treatment minus surgery; for satisfaction, the comparison is surgery minus conservative treatment. The registry reports unadjusted comparisons, and the outcome-specific group sizes differ. These are summaries of observed measurements, not adjusted estimates that account for other participant characteristics.

A standard deviation is not the same as a confidence interval for the mean. It describes variation among observations, whereas an interval for a mean difference describes uncertainty in the estimated group contrast. The registry posts intervals for the secondary mean differences, but does not state their confidence level or calculation method. The posted p-values likewise do not establish whether the outcomes, analysis methods, and comparison directions were prespecified.

What do the planning illustrations mean?

General statistical explanation: sample-size planning asks how many participants per arm would be needed under stated assumptions to have a specified chance of detecting an effect of a chosen size at a chosen significance level. The posted illustrations use the observed effect as their starting point, then consider planning for 100%, 75%, and 50% of that observed effect. They use a two-sided significance level of 0.05, equal allocation between arms, and a normal approximation. The values are participants per arm for 80% power and participants per arm for 90% power. For healing time, the listed values are 16 and 21 when planning for 100% of the observed effect; 27 and 36 for 75%; and 61 and 81 for 50%. For satisfaction, they are 31 and 41 for 100%; 54 and 73 for 75%; and 122 and 163 for 50%.

These figures are planning illustrations, not observed power and not evidence of the trial’s original sample-size target. Because they are anchored to effects observed in this trial, they depend on those estimates and the stated assumptions. Planning for a smaller effect requires more participants per arm in these posted illustrations. The registry does not report the trial’s own sample-size justification or show that its enrollment was selected using these calculations.

Why does analysis population matter?

General statistical explanation: randomized trials are often analyzed according to a prespecified population and assignment strategy. An intention-to-treat analysis, for example, generally retains participants in their randomized groups, but that label should only be used when the record confirms the approach. Here, the registry describes the primary outcome as analyzed in 41 conservative-group participants and 40 surgical-group participants. It does not report the analysis strategy, the reasons for any outcome data being unavailable, or how missing values were handled. The primary comparison should therefore be understood as the posted analyzed-group result, without assuming a particular missing-data method.

11. Confidence Intervals and Precision

The primary estimate is a risk difference of 0.317 for surgery relative to conservative treatment. Its reported interval runs from 0.175 to 0.460. The interval does not include zero, and the unadjusted p-value is <0.001. These results indicate that the posted analysis estimates a positive difference in cure proportions and gives a range of estimates above zero under the interval procedure used. The registry record does not report the confidence level or the procedure used to construct the interval, so its exact coverage interpretation cannot be confirmed from the record.

General statistical explanation: an interval is useful because a point estimate alone can give a misleading impression of certainty. A narrower interval indicates more precision under the method used; a wider interval indicates greater uncertainty. The interval is not a range that guarantees the true value lies within it, and it does not account automatically for problems such as missing outcome data or departures from the analysis plan. Those are separate questions about data quality and study conduct.

The posted group counts help explain the evidence base for the estimate: the primary outcome includes 41 analyzed participants in the conservative group and 40 in the surgery group. The interval should not be read as resolving the uncertainty about why those numbers differ from enrollment or how participants without a posted primary outcome were handled. The registry does not report those details.

Intervals are also posted for the secondary mean differences: 2.467 to 6.033 for the healing-time contrast and 1.722 to 8.278 for the satisfaction contrast. These are unadjusted secondary comparisons. Their precision should be interpreted alongside their outcome-specific denominators and the absence of reported multiplicity handling. The intervals communicate statistical uncertainty under their respective methods, but they do not establish that the secondary findings were prespecified or independently confirm the primary result.

12. Multiplicity, Missing Data and Analysis Populations

The registry lists one primary outcome, cure rate, and two secondary outcomes: healing time and patient satisfaction measured with a visual analogue scale. It posts unadjusted comparisons, but does not report whether the outcomes or specific comparisons were prespecified in a protocol or statistical analysis plan. It also does not report any adjustment for multiple testing or explain how the primary and secondary outcomes were intended to be interpreted together.

General statistical explanation: when several outcomes are tested, the chance of at least one apparently unusual result can increase if each test is considered separately at its own threshold. Multiplicity procedures can address that issue, but the appropriate approach depends on the study’s objectives and analysis plan. Without a reported plan, the unadjusted secondary p-values should not be treated as if they were corrected for the number of outcomes examined. The registry record does not report an adjustment, so none should be assumed.

The primary outcome is described for 41 conservative-group participants and 40 surgical-group participants. Enrollment was 84, but the record does not provide a participant-flow account, reasons for the difference in analyzed counts, or a missing-data method. It does not report whether participants were analyzed according to randomized assignment, whether all randomized participants were included in the stated analysis, or whether any sensitivity analyses were conducted. The outcome-specific secondary denominators also vary, and the record does not explain why.

The safety record reports 42 people at risk in each treatment group, which differs from the primary-outcome analyzed counts. This may reflect a different outcome-specific reporting population; the record does not explain the relationship. The death fields are not reported in the registry record. No interim-analysis information, analysis-plan details, or sponsor-reported analysis output is listed. These omissions limit evaluation of missingness, selective reporting, and the robustness of the posted results.

13. 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 reports zero serious adverse events among 42 people at risk in each group and zero other adverse events among 42 people at risk in each group. Death counts are not reported in the registry record. These are the adverse-event figures posted for the groups; they are distinct from the primary-outcome analyzed counts.

General statistical explanation: a zero count in a study group means no event of that reported type was recorded in that group under the registry’s reporting. It does not prove that the true event risk is zero, and it does not show that rarer events could not occur. Interpretation also depends on how events were defined, collected, followed, and classified. The registry record does not provide those details or explain the difference between the safety denominators and the primary-outcome denominators.

The safety figures therefore describe what the registry reports, rather than establishing a complete comparison of all possible harms. The absence of reported events should not be read as a guarantee of safety or as a substitute for fuller safety reporting.

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

The registry’s primary results show a higher observed cure outcome in the analyzed surgery group than in the analyzed conservative group. The reported risk difference is 0.317, with an interval from 0.175 to 0.460 and an unadjusted p-value of <0.001. This is evidence of a between-group difference in the posted analysis, not a guarantee that the same contrast would occur in other populations or settings.

The result does not establish why the groups differed, nor does it settle the effect of differences in analysis populations or missing data. The primary outcome is reported for 41 conservative-group participants and 40 surgical-group participants, while enrollment was 84. The record does not explain the difference or describe how unavailable outcomes were handled. The method used for randomization and allocation concealment is not reported, and the label “triple” does not identify who was masked. These details affect how fully the result can be assessed.

The secondary summaries are directionally consistent with the primary comparison, but they are unadjusted and the registry does not report multiplicity handling or a statistical analysis plan. They provide additional context, not independent proof that the primary finding is robust. Likewise, zero reported adverse events in the registry groups does not establish that no adverse events can occur; the death counts are not reported in the registry record.

General statistical explanation: statistical evidence is one part of interpreting a trial. Estimates and intervals describe the observed data and their uncertainty under an analysis, while study conduct, outcome definitions, completeness of follow-up, and reporting determine how confidently the findings can be interpreted. The posted results support describing the observed contrast in these analyzed groups. They do not by themselves establish the result’s broader applicability, the completeness of safety information, or the certainty of effects on outcomes beyond those recorded.

15. Why This Trial Matters Statistically

This trial is statistically informative because its primary outcome is defined in concrete terms: cure means no recurrence within two years after initial treatment, and recurrence is described using clinical features. A stated time window and outcome definition make it possible to understand what the cure counts represent and to distinguish the endpoint from less precisely defined notions of treatment success.

The reported primary comparison also illustrates why an estimate, an interval, and a p-value answer different questions. The risk difference of 0.317 describes the estimated absolute contrast; the interval from 0.175 to 0.460 conveys uncertainty under the unreported interval method; and the unadjusted p-value of <0.001 addresses statistical evidence against a null comparison under the analysis. None of these alone answers questions about missing outcomes, analysis choices, or applicability.

The record also shows why analysis denominators need attention. The primary outcome has 41 analyzed participants in the conservative group and 40 in surgery, while secondary and safety outcomes use different group sizes. These figures should not be combined or treated as one common population. Because the record does not explain the denominator differences, readers cannot determine from the posted information whether they reflect unavailable data, distinct assessment populations, or another reporting reason.

Finally, the secondary planning illustrations are useful for understanding how assumed effect size affects a planning calculation, but they are not evidence of observed power or of the trial’s original sample-size planning. The record does not report that original justification. Taken together, the trial is a useful example of how a clear endpoint and a pronounced observed contrast can coexist with important gaps in reporting that constrain statistical interpretation.

16. 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 are specific to the registry information for this trial. The primary outcome is reported for 41 participants in the conservative group and 40 in the surgery group, while enrollment is 84. The record does not explain the difference, report a participant-flow account, give reasons for unavailable outcomes, or describe missing-data handling. The results should therefore be read as an analysis of the reported groups, not assumed to be an analysis of every randomized participant.

The outcome denominators are not consistent across the record: the healing-time summaries include 42 participants in each group, satisfaction includes 33 in the conservative group and 42 in surgery, and the safety figures use 42 at risk in each group. The record does not explain the relationship among these populations. It also does not report the statistical analysis plan, sample-size justification, interim analyses, randomization method, allocation concealment, or who was masked. The intervention descriptions are incomplete, limiting interpretation of exactly how the treatments were delivered.

The registry posts unadjusted comparisons and no sponsor-reported analyses. It does not state the confidence level or method for the reported intervals, or whether multiplicity was addressed. For safety, zero serious and zero other adverse events are reported in each group, but event definitions and collection procedures are not described, and death counts are not reported in the registry record.

These reporting limitations do not change the posted estimates, but they restrict assessment of bias, precision, completeness, and generalizability. They also make it difficult to determine whether the reported analyses followed prespecified procedures.

17. Related Tutorials

18. Related Statistical Calculators

19. Sources

• ClinicalTrials.gov: Conservative Versus Surgical Treatment of Umbilical Pilonidal Disease — NCT01662765.
• Registry record last updated 2017-02-16; status last verified 2016-12.
• Results first posted 2017-02-16.
• 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.

20. Record Summary

The registry record describes a completed, prospective, randomized, triple-masked, parallel, multicenter comparison of conservative treatment and surgery for umbilical pilonidal disease. Enrollment was 84. The primary outcome was cure, defined as no recurrence within two years after initial treatment. The record reports the primary outcome for 41 conservative-group participants and 40 surgical-group participants, with 28 and 40 cures, respectively. The reported risk difference for surgery relative to conservative treatment is 0.317, with an interval from 0.175 to 0.460 and an unadjusted p-value of <0.001.

Two secondary outcomes are posted, covering healing time and patient satisfaction. Their group summaries point in directions consistent with the primary comparison, but the registry does not report a statistical analysis plan or multiplicity handling. Safety figures report zero serious and zero other adverse events among 42 people at risk in each group; death counts are not reported in the registry record. The record does not explain the different outcome denominators, missing-data handling, or the trial’s sample-size justification.