Does Ultrasound Improve Procedural Time in the Lateral Popliteal Approach to the Sciatic Nerve in Obese Patients: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Does Ultrasound Improve Procedural Time in the Lateral Popliteal Approach to the Sciatic Nerve in Obese Patients (NCT01579747), a randomized phase 4 study. The primary outcome is Procedural Time.

2. Clinical Question

The clinical question is specifically about the time needed to complete a procedure, rather than a broad comparison of postoperative pain or other patient outcomes. The registry lists postoperative pain as a condition and number of redirections as a secondary outcome, but it does not post results for that secondary outcome in the record.

The study rationale notes that ultrasound had been shown to improve nerve localization and block time in non-obese populations, while its utility in obese patients was uncertain. The record also notes that ultrasound can produce poorer-quality images in obese populations. These points motivate the comparison; they are not results from this trial. The trial's reported primary outcome addresses procedural time, not image quality, pain relief, block success, or the full range of possible consequences of either localization technique.

3. Trial Design

The registry describes a completed Phase 4, randomized, double-blind, parallel-group trial sponsored by the University of New Mexico. The listed interventions are ultrasound and nerve stimulation, both described as procedures for a sciatic nerve block through the lateral popliteal approach. The stated enrollment is 25 participants. For the primary outcome, results are posted for 12 participants in the nerve stimulation arm and 12 in the ultrasound-guided arm. Why the posted outcome group counts differ from the stated enrollment is not reported in the registry record.

The record lists the study start as 2012-07 and primary completion as 2012-12. It is marked completed. Although the design is described as double-blind, the record does not identify which people were masked or explain how masking was maintained during a procedure involving different localization techniques. The method used to generate or conceal random assignments is also not reported in the registry record.

The primary outcome is procedural time, expressed as a mean in seconds with standard deviation. The stated timeframe is less than 30 minutes. The registry also lists number of redirections as a secondary outcome, but no redirection results are posted in the record. The trial's original sample-size justification, intended analysis model, handling of interim analyses, and prespecified analysis populations are not reported in the registry record. The planning illustrations associated with the posted results are based on the observed effect and should not be confused with information about how the trial was originally designed.

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: 1 excluded because not meeting inclusion criteria. Patient lost weight prior to surgery

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 Procedural Time, measured over less than 30 minutes.

Procedural time is the time taken to complete a sciatic nerve block using the lateral popliteal approach. It is recorded in seconds, and the primary outcome is summarized as a mean with a standard deviation. The comparison is between ultrasound guidance and nerve stimulation. The stated outcome timeframe is less than 30 minutes.

Time is used to address the trial's focused question about how quickly each localization approach permits completion of the procedure. As a continuous measure, it retains information about the magnitude of times rather than reducing each participant's result to a category such as fast or slow. The mean describes the group's average time; the standard deviation describes variation among participant times around that average.

Procedural time does not by itself measure every aspect of a nerve block. In particular, it does not establish block success, pain outcomes, image quality, or the number of redirections. Number of redirections is listed separately as a secondary outcome, but its results are not reported in the registry record. The endpoint therefore answers a limited procedural question and should be interpreted within that scope.

7. Results Posted in the Registry

• Ultrasound Guided Sciatic Nerve Block: mean 206.00 versus 577.00 on control; difference -371.00 (95% CI -410.40 to -331.60), standardized effect d = -7.53; z-test p = <0.001.

8. Statistical Interpretation

9. Statistical Methods Explained

The primary outcome is continuous: each participant has a procedural-time measurement in seconds, and each arm is summarized by a mean and standard deviation. The registry reports 577.0 seconds with a standard deviation of 57.0 in the nerve stimulation arm, and 206.0 seconds with a standard deviation of 40.0 in the ultrasound-guided arm. These summaries describe the central value and within-group spread, respectively.

What does a mean difference represent?

A mean difference subtracts one group's mean from the other according to a specified order. In this comparison, the reported estimate is -371.0 seconds for ultrasound guidance relative to nerve stimulation. The negative sign indicates that the ultrasound group's reported mean is lower under that ordering. The difference is an estimate based on the participants with posted outcome measurements; it is not a claim that every participant had a time reduction of that size.

In general, a group mean can be affected by unusually high or low observations, and the mean alone does not show the full distribution of individual times. A standard deviation provides one measure of dispersion around the mean, but it also does not reveal the shape of the distribution or whether there were outliers. The registry does not post individual participant times or distributional diagnostics.

How should the p-value be read?

The posted p-value is <0.001. In general, a p-value describes how unusual data at least as incompatible with a specified null hypothesis would be if that null hypothesis and the test assumptions held. It is not the probability that the null hypothesis is true. It is also not a measure of the size or practical importance of a difference. The registry record does not identify the test, whether it was adjusted for any covariates, or the precise analysis assumptions used for this comparison.

The reported comparison also includes an interval from -410.4 to -331.6. In general, an interval estimate communicates uncertainty around an estimated effect: a narrower interval indicates greater precision under the method used, while a wider interval indicates less precision. The confidence level and method for this interval are not reported in the registry record, so the interval can be described by its posted bounds but its specific inferential properties cannot be confirmed from the record alone.

Why does variation matter?

In general, variability among participants affects how precisely a group mean and between-group contrast can be estimated. Standard deviations describe variation in observed times, whereas standard errors describe uncertainty in estimated means. The posted standard errors are 16.45 seconds for nerve stimulation and 11.55 seconds for ultrasound guidance. A standard error is not another description of the spread of individual measurements; it concerns the sampling uncertainty of an estimate. The registry does not explain the formula or model used to obtain these standard errors.

When data are more variable than a statistical model assumes, standard errors and p-values can be unreliable. For count outcomes, over-dispersion means that counts vary more than a chosen model expects; in general, accounting for it can change estimated uncertainty. That issue is relevant conceptually to outcomes such as number of redirections, but no results or analysis details for that secondary outcome are posted. There is no basis in the registry record to say whether over-dispersion was assessed or whether a particular count model was used.

What can randomization and masking contribute?

In general, random allocation aims to make treatment groups comparable at the start of a trial with respect to both measured and unmeasured characteristics, on average. The record labels the allocation randomized, but does not describe its sequence-generation or concealment procedures. The parallel-group design means the record assigns participants to one of the two listed approaches, rather than describing a crossover comparison.

Double masking can reduce some forms of bias, but its practical meaning depends on who was masked and how the masking was maintained. Those details are not reported in the registry record. Because the techniques differ, the record's double-blind label alone does not establish which study personnel or participants were unaware of assignment.

The record posts results for one primary comparison and lists one secondary outcome, number of redirections. It does not report a full statistical analysis plan, adjustment for multiple comparisons, missing-data methods, analysis-population definitions, or interim analyses. Those methodological details should not be inferred from the reported p-value or from the design label.

10. Confidence Intervals and Precision

The reported mean difference is -371.0 seconds, and its stated bounds are -410.4 and -331.6. The interval is entirely below zero, indicating that its full reported range corresponds to a lower mean time for ultrasound guidance under the comparison's direction. The registry record does not state the confidence level or interval method, so the bounds can be reported but should not be assigned a particular confidence level.

Precision concerns the uncertainty around an estimate, not whether the estimate is clinically important or whether the study has addressed every relevant outcome. In general, a narrower interval indicates a more precise estimate under the method used. The posted interval is relatively concentrated around the reported difference, but interpretation still depends on the unreported analysis details and the participants represented in the outcome results.

The standard deviations, 57.0 seconds for nerve stimulation and 40.0 seconds for ultrasound guidance, describe participant-to-participant variation. The posted standard errors are 16.45 and 11.55 seconds, respectively, and relate to uncertainty in the estimated group means. Neither the standard deviations nor standard errors are substitutes for the interval on the between-group difference.

The reported group sizes are 12 participants per arm for the primary outcome. A result estimated from these groups may not precisely characterize times in all obese patients or in settings beyond the trial. The record does not provide participant-level distributions, confidence intervals for the group means, or details needed to assess robustness to alternative modeling choices. Precision of the posted time contrast should therefore be distinguished from generalizability and from uncertainty about unreported outcomes.

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 entries are participants per arm, not total event counts. Event-count planning is relevant to time-to-event outcomes, but this trial's primary outcome is continuous procedural time, so the displayed quantities refer to participants in each arm.

The planning illustration assumes a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. It uses the observed effect as its starting point and considers detecting 100%, 75%, and 50% of that observed effect. The registry record does not report the trial's original sample-size assumptions or justification; these planning assumptions describe the illustration, not necessarily the design used for the trial.

For 100% of the observed effect, the entries are 1 participant per arm for 80% power and 1 participant per arm for 90% power. For 75% of the observed effect, the entries are also 1 participant per arm for 80% power and 1 participant per arm for 90% power. For 50% of the observed effect, the entries are 2 participants per arm for 80% power and 2 participants per arm for 90% power.

In plain language, the 80% power column gives the planning number per arm under which the calculation would have an 80% chance of detecting the specified effect if the assumptions and effect were correct. The 90% column uses a higher target chance of detection under the same planning framework. Smaller effects generally require more participants to detect because they are harder to distinguish from random variation. These unusually small illustrations reflect the large observed contrast used as the planning input and the stated approximation. This is not observed power.

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 = -371.000; pooled standard deviation σ = 49.239.
2. n = 2 × (1.96 + 0.842)² × 49.239² / -371.000² = 0.3, rounded up to 1 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 <- -371                      # observed difference in means
sigma <- 49.2392120164407                     # 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(1579747)
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 (80% power), 1 (90% power)
 75% of observed effect: n per arm = 1 (80% power), 1 (90% power)
 50% of observed effect: n per arm = 2 (80% power), 2 (90% power)
power.t.test: 1.8 
Simulated power at n = 1 per arm: NA (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 identifies procedural time as the primary outcome and number of redirections as a secondary outcome. It lists two outcome measures and one analysis, but posts quantitative comparison results only for procedural time in the record. Results for number of redirections are not reported in the registry record.

When more than one outcome or comparison is tested, the chance of at least one false-positive finding can increase. In general, multiplicity procedures can be used to manage that issue, but the appropriate approach depends on the analysis plan and the role of each outcome. The registry record does not report whether the trial prespecified any multiplicity adjustment, hierarchy, or testing strategy. The posted p-value should not be assumed to account for other outcomes or analyses.

The record does not describe how missing primary outcome data were handled, whether all randomized participants were included in the analysis, or how the analysis population was defined. The outcome group sizes are 12 per arm, while stated enrollment is 25; the reason for the difference is not reported in the registry record. The available record does not establish whether any participant was excluded, had a missing measurement, or was handled in another way.

The registry also does not report whether an intention-to-treat or per-protocol analysis was used. These terms describe different analysis populations in general: an intention-to-treat approach typically analyzes participants according to assigned group, while a per-protocol approach focuses on those meeting specified adherence criteria. Neither approach should be attributed to this trial without a registry statement or analysis plan.

Interim analyses, stopping rules, and missing-data sensitivity analyses are also not reported in the registry record. The double-blind design label does not supply those details. As a result, the posted comparison can be interpreted descriptively, but the record does not permit a full audit of multiplicity, missing-data handling, or population selection.

14. 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 5% reporting threshold.

For serious adverse events, the registry reports 0 affected participants among 12 at risk in the nerve stimulation group and 0 among 12 at risk in the ultrasound-guided group. For other adverse events, it likewise reports 0 affected participants among 12 at risk in each group. No serious adverse-event terms are listed.

These figures describe the adverse-event counts posted for the reported groups. They do not establish that adverse events are impossible or that the approaches have identical safety profiles. In general, zero observed events in a study does not rule out events that could occur in a larger population or under different circumstances. The groups are small, and the record's reporting is limited to the stated counts.

Deaths are not reported in the registry record; the death entries are not posted as numerical values. They should not be interpreted as evidence of zero deaths. The record also does not provide additional detail about adverse-event definitions, collection procedures, severity classifications beyond the serious/non-serious grouping, or follow-up duration for safety. The safety counts therefore need to be read as a narrow summary of the registry's posted adverse-event information, not as a comprehensive safety evaluation.

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

The posted primary outcome shows a lower mean procedural time in the ultrasound-guided group than in the nerve stimulation group: 206.0 seconds compared with 577.0 seconds. The reported mean difference is -371.0 seconds, with an interval from -410.4 to -331.6 and a p-value of <0.001. These are the trial's posted time results for the groups with outcome data.

The result addresses procedural duration for a sciatic nerve block using the lateral popliteal approach. It does not show whether either method improved postoperative pain, increased block success, reduced complications, or changed other outcomes. Although number of redirections is listed as secondary, its results are not reported in the registry record. The stated study rationale also discusses nerve localization and image quality, but the posted primary result is a time comparison.

The findings should not be read as proof that one approach is suitable for every patient or setting. The record identifies obese patients as the population of interest but does not provide detailed participant characteristics or broader eligibility information. It also does not describe the randomization procedure, masking implementation, analysis population, or missing-data handling. Those gaps limit assessment of how representative the reported participants are and how robust the estimate may be to analytic choices.

The trial's result is a statistical comparison, not medical advice. It does not establish a complete balance of benefits and harms, and the reported adverse-event counts do not replace a broader safety assessment. The data support a statement about the observed procedural-time contrast in this completed trial; they do not by themselves settle questions about clinical outcomes, implementation, or applicability beyond the participants and procedures represented in the registry record.

16. Why This Trial Matters Statistically

This trial is statistically interesting because its primary outcome is a directly measured procedural quantity and the reported group means are distinctly separated. The mean difference and its interval communicate both the direction and magnitude of the time contrast, while the standard deviations and standard errors distinguish participant-level variation from uncertainty in estimated averages.

The result also illustrates why a p-value should not stand alone. The posted p-value is <0.001, but the estimate and interval are needed to describe the size and precision of the observed contrast. A p-value does not report whether a difference matters clinically, whether it applies in other settings, or whether secondary outcomes move in the same direction. Number of redirections is listed but its results are not reported in the registry record.

The planning illustration provides another statistical lesson: calculations based on an observed effect can produce very small planning numbers when that observed contrast is large. Such calculations depend on the chosen effect fraction and assumptions, and they are not a record of the trial's original design rationale. The trial's own sample-size justification is not reported.

Finally, this record demonstrates the distinction between a randomized design label and a fully documented analysis. Randomization is important for comparative inference, but details about allocation, masking, missing data, analysis populations, and multiplicity are needed to evaluate the full inferential process. The posted time result is informative, while unanswered design and reporting questions define the limits of what can be concluded statistically.

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.

The posted outcome groups contain 12 participants each, while the stated enrollment is 25. The reason for this difference is not reported in the registry record, and the record does not explain whether it reflects missing measurements, exclusions, or another circumstance. No participant-level procedural times or distributional information are posted, limiting independent assessment of outliers and the form of the data.

The record identifies the study population as obese patients but does not report detailed demographic or clinical characteristics, eligibility criteria, or the range of obesity represented. It therefore offers limited information for assessing how closely the participants match other populations or clinical settings. The record's double-blind label is not accompanied by an explanation of who was masked or how masking was implemented for the two different procedural techniques.

Randomization is reported, but sequence generation and allocation concealment are not described. The statistical test, model, confidence level for the interval, missing-data approach, analysis population, and multiplicity plan are also not reported in the registry record. The original sample-size justification and any interim analyses are not reported.

The primary outcome is procedural time. The registry lists number of redirections as secondary, but does not post its results. Safety reporting includes zero serious and other adverse events in each group, while deaths are not reported as numerical values. The scope of the posted results is consequently narrower than a complete comparison of efficacy, safety, and procedural performance.

18. Related Tutorials

19. Related Statistical Calculators

20. Sources

• ClinicalTrials.gov: Does Ultrasound Improve Procedural Time in the Lateral Popliteal Approach to the Sciatic Nerve in Obese Patients — NCT01579747.
• Registry record last updated 2014-01-24; status last verified 2013-12.
• Results first posted 2013-12-19.
• 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 completed Phase 4 trial randomized participants in a double-blind, parallel-group design to ultrasound-guided or nerve stimulation sciatic nerve block using the lateral popliteal approach. The sponsor is the University of New Mexico. The registry lists enrollment of 25, with study start in 2012-07 and primary completion in 2012-12.

The primary outcome is mean procedural time in seconds, with a timeframe of less than 30 minutes. Posted results include 12 participants per arm: mean time was 577.0 seconds with nerve stimulation and 206.0 seconds with ultrasound guidance. The reported mean difference is -371.0 seconds, the interval is -410.4 to -331.6, and p < 0.001. Number of redirections is listed as a secondary outcome, but its results are not reported in the registry record.

The adverse-event record reports no serious or other adverse events among the 12 participants at risk in each group. Deaths are not reported as numerical values. Several design and analysis details, including the original sample-size justification and missing-data methods, are not reported in the registry record.