A Retrospective Investigation of Local Anesthetic Concentrations in Femoral Nerve Block: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for A Retrospective Investigation of Local Anesthetic Concentrations in Femoral Nerve Block (NCT02501135), a registered phase not stated study. The primary outcome is Concentration of Local Anesthetic Injected for Femoral Nerve Block.

2. Clinical Question

This is a retrospective investigation, so the clinical question concerns observations from care that had already occurred rather than a prospectively assigned treatment comparison. The registry description also names postoperative pain and safety as aims. It lists time to discharge from the post-anesthesia care unit (PACU), and postoperative pain scales using FLACC and VAS, among the secondary outcomes. The record does not explain how the pain scales were collected, which specific measurements were selected, or how comparisons between concentrations were intended to be made. Because allocation is not stated and the arms are not described as a like-for-like control comparison, the record does not establish that differences in outcomes can be attributed to concentration alone.

3. Trial Design

The registry describes a completed retrospective investigation of femoral nerve blocks in pediatric patients. It reports enrollment of 281 participants, a start date of 2015-06, and a primary completion date of 2016-05. The listed intervention is a femoral nerve block with ropivacaine or bupivacaine. Its single arm description says patients received one of those anesthetics during a femoral nerve block; it does not provide separate arm descriptions for particular concentrations or drugs.

The study purpose is to compare perioperative pain relief and safety in patients who received varying local anesthetic concentrations. However, the registry record does not state how patients came to receive a particular concentration, whether concentration decisions were made by clinicians, or whether any other assignment process was used. Allocation is not stated. The record therefore does not describe a randomized trial, and the listed arm is not a like-for-like control comparison. Differences among patients, procedures, anesthetic agents, or care practices could be related to the concentration groups, but the record does not describe information that would allow those factors to be assessed.

The primary outcome is listed as the concentration of local anesthetic injected for femoral nerve block, with a participant count as its measure and the length of surgery as its timeframe. The record also lists seven outcome measures overall. Named secondary outcomes are time to discharge from PACU and postoperative pain measured using FLACC and VAS. The record does not give the timing or detailed definitions for those outcomes beyond their names. The number of analyses listed is zero, and numerical results for the primary endpoint are not reported in the registry record.

The status is completed, but completion status alone does not establish that every planned measure was collected or analyzed. The protocol details, sample-size justification, analysis plan, and reasons for choosing concentrations are not reported in the registry record. The record also does not state how many patients contributed data to each outcome or whether all enrolled patients were included in a given analysis.

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 Concentration of Local Anesthetic Injected for Femoral Nerve Block, measured over length of surgery.

The primary outcome is titled “Concentration of Local Anesthetic Injected for Femoral Nerve Block.” Its unit is participants, its measurement type is a count of participants, and its stated timeframe is the length of surgery. As titled, it concerns the local anesthetic concentration used, rather than directly measuring pain or a complication. The registry record does not provide the concentration categories, a numerical result, or a description of how the count was organized. It is therefore not possible to tell from the record how participants were distributed across concentrations or how the primary outcome was used to evaluate the stated comparison.

Recording the concentration used can describe the exposure under investigation. In a retrospective comparison, relating that exposure to pain or safety outcomes would require clear definitions of the exposure groups and the outcomes, as well as a method for addressing differences between patients and procedures. Those details are not reported in the registry record. The listed timeframe, length of surgery, identifies the period associated with the primary measure, but it does not by itself define the timing of pain assessments or safety follow-up.

As a general statistical point, an endpoint is useful when it is defined in a way that can be measured consistently and that matches the research question. A count can describe how many participants meet a defined category, but interpretation depends on knowing what category is counted and the number of participants eligible to be counted. Here, the registry record does not provide a category-level result or the relevant outcome definitions. It also does not report a primary-effect estimate or uncertainty interval. The title and measure alone cannot establish whether one concentration was associated with different pain relief or safety.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

What does a retrospective comparison describe?

In general, a retrospective study examines information about care or outcomes that has already occurred. Unlike a randomized comparison, it does not necessarily assign participants to groups according to a prospective allocation scheme. The registry identifies this investigation as retrospective and states that allocation is not stated. It also indicates that the arms are not a like-for-like control comparison. Consequently, the recorded groups may differ in ways other than the anesthetic concentration. A statistical comparison would need to consider whether such differences could influence the outcomes, but the registry record does not report the planned methods or any adjustment results.

This distinction matters because an association is not automatically a causal effect. If patients receiving different concentrations also differ in procedure characteristics, clinical status, clinician decisions, or other factors, a difference in outcomes may reflect those factors, the concentration, or a combination. This is a general limitation of observational comparisons. The registry record does not specify which covariates were available or how they were handled.

What does a participant count tell the reader?

The primary measure is a count of participants, and the record characterizes its data type as binary. In general, a count reports the number of people meeting a stated condition; it is not by itself a rate or a comparison. Interpreting a count requires a clear definition of the condition, the group to which it applies, and the number of people whose data could be assessed. Although the endpoint is titled as the concentration injected, the registry record does not state the concentration categories or provide counts for them. The meaning and distribution of the primary measure therefore remain unclear from the posted outcome information.

Counts can be useful for describing events or categories, but comparisons usually require both the count and its denominator in each relevant group. An estimate comparing groups may also need an uncertainty interval. The registry record does not report concentration-specific counts, denominators, comparative estimates, or uncertainty intervals for the primary endpoint. The enrollment total should not be treated as the denominator for every outcome without confirmation that all participants contributed data to each measure.

How should estimates and confidence intervals be read?

As a general statistical explanation, an effect estimate summarizes the observed contrast between groups on a defined scale. A confidence interval describes the precision of that estimate under the assumptions used to calculate it. A narrow interval indicates more precision than a wide interval, while an interval spanning values that would imply materially different conclusions leaves greater uncertainty about the effect’s magnitude. Neither an estimate nor an interval resolves bias caused by confounding, measurement problems, or incomplete data.

No concentration-comparison effect estimate, confidence interval, or p-value is reported in the registry record. There is therefore no basis to describe a statistically significant or nonsignificant result, or to characterize the precision of an outcome comparison. A missing test result is not evidence of no difference. Likewise, the absence of a reported estimate is not evidence that groups were equivalent.

What can be learned from planning calculations?

In general, sample-size planning calculations describe the number of participants that might be needed under specified assumptions about an effect, allocation, significance level, and statistical model. They are planning illustrations, not observed findings and not “observed power.” The record’s stated planning assumptions use a two-sided significance level of 0.05, equal allocation between two arms, a normal approximation, and a fraction of the observed effect as the target effect. The record says that time-to-event calculations use a total event count and other calculations use participants per arm. These assumptions do not establish the investigation’s actual design or analysis plan.

The registry record does not report the study’s own sample-size justification. Nor does it provide a randomized two-arm design to which the stated equal-allocation illustration can be assumed to correspond. Planning calculations should therefore be read only as conditional illustrations, not as evidence that the study was designed prospectively for a particular power or that its groups followed equal allocation. Because no numerical planning results are stated in the registry record, no specific required sample size can be interpreted from the record.

Why do outcome definitions and data distributions matter?

Different outcomes require different statistical methods. A count of participants with a defined event, a continuous pain score, and time until discharge are distinct data types. In general, an analysis of time until an event may need to account for participants whose event time is not observed during follow-up. A continuous score requires information about its scale, distribution, and measurement timing. Event counts may require attention to the number at risk and whether participants can experience more than one event. These are general methodological considerations; the registry record does not report enough detail to determine the models or assumptions used for the named outcomes.

Over-dispersion is a general issue for count outcomes: it occurs when the observed variability is greater than a model assumes. If present, it can make conventional standard errors too small and uncertainty appear understated. Nothing in the registry record establishes that the outcomes were over-dispersed, and no model diagnostics or outcome distributions are reported. It would not be appropriate to assume a count model, a particular distribution, or a correction for over-dispersion from the endpoint title alone.

The registry record lists seven outcome measures and zero analyses. That listing does not provide an analysis specification, effect estimates, or a record of which outcome data were analyzed. The primary endpoint’s timeframe is the length of surgery; the secondary outcome names include PACU discharge time and postoperative pain scales. Detailed definitions, assessment schedules, statistical models, missing-data procedures, and analysis populations are not reported in the registry record. Interpretation must therefore remain at the level of the stated study aims and recorded counts rather than an appraisal of an unreported statistical analysis.

10. Multiplicity, Missing Data and Analysis Populations

The registry record lists seven outcome measures, including the primary measure concerning local anesthetic concentration and secondary outcomes involving time to discharge from PACU and postoperative pain scales using FLACC and VAS. It lists zero analyses. The record does not identify which outcomes were analyzed, how many comparisons were planned, or whether any outcome was selected as the sole confirmatory test. A statistical analysis plan is not reported in the registry record.

As a general statistical issue, testing several outcomes or several group contrasts can increase the chance of observing an apparently notable result by chance. Methods for addressing multiplicity can include defining a primary comparison in advance, adjusting inference across tests, or describing secondary analyses as exploratory. The registry record does not state whether any such approach was planned or used. Without results or a documented plan, it is not possible to assess how multiplicity might affect interpretation.

Missing-data procedures and analysis populations are also not reported in the registry record. It does not say whether all enrolled participants had data for each outcome, how incomplete records were handled, or whether participants were excluded from any analysis. The primary outcome’s population field is not specified in the record. The enrollment total of 281 therefore should not be assumed to be the denominator for every outcome or comparison.

These omissions limit assessment of selection and measurement issues. For example, if outcome availability differed among concentration groups, a comparison based only on observed records could represent a selected subset. This is a general possibility, not a reported feature of this investigation. The registry record does not provide information to determine whether it occurred or how it was addressed.

11. 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.

The registry record lists one group, “Femoral Nerve Blocks,” with 281 participants at risk for serious adverse events and zero affected. It also lists 281 at risk for other adverse events and zero affected, and records zero deaths. These are counts for the listed group, not results separated by anesthetic concentration or by ropivacaine versus bupivacaine.

Such counts should be read as the registry’s reported safety information. A count of zero recorded events does not establish that the risk is exactly zero or that no events occurred outside the recorded data. Interpretation depends on how events were defined, collected, and followed, and on whether the reporting covered all participants and the relevant observation period. The registry record does not give those details, list serious adverse event terms, or describe safety follow-up beyond the group counts.

As a general statistical point, zero observed events can still be compatible with uncertainty about the underlying event frequency, particularly when follow-up and ascertainment details are not available. A comparative safety assessment would also require outcome information by concentration group and a defined comparison. Neither is reported in the registry record. The listed counts therefore describe the entered safety data but do not establish that concentrations were equally safe or support a concentration-specific risk estimate.

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

The record identifies a clinically relevant question: whether perioperative pain relief and safety varied with local anesthetic concentration in pediatric femoral nerve blocks. It also documents that the investigation was retrospective, was marked completed, and enrolled 281 participants. The named outcomes include concentration injected, PACU discharge time, and postoperative pain measured using FLACC and VAS. These details show what the investigation set out to examine, not what the comparison found.

Numerical outcome results for concentration, discharge time, or postoperative pain are not reported in the registry record. There is no reported effect estimate, confidence interval, or statistical test comparing concentrations. Therefore, the record does not establish the direction or size of any association, how precise an estimate might be, or whether any observed difference would be compatible with chance under a particular analysis. It also does not establish equivalence between concentrations.

The single listed arm includes patients receiving ropivacaine or bupivacaine, while the stated purpose refers to varying concentrations. Concentration-specific groups and a separate control group are not described. Allocation is not stated, and the record notes that the arms are not a like-for-like control comparison. A retrospective comparison can be affected by differences in which patients received particular concentrations and by other aspects of care. The registry record does not provide an analysis that can show whether these issues were addressed.

The safety entry reports zero serious adverse events, zero other adverse events, and zero deaths among the listed counts at risk. These registry counts do not provide a concentration-level comparison or enough detail about event definitions and follow-up to support a complete safety assessment. They should be understood as reported counts, not as proof that adverse events could not occur.

In general, a completed study record and a reported enrollment total do not replace results. The information in the registry record supports describing the investigation’s question, design label, stated outcomes, and recorded safety counts. It does not justify a conclusion that a particular concentration improves pain relief, shortens discharge time, or changes safety. Any such conclusion would require outcome results and methods that are not reported in the registry record.

13. Why This Trial Matters Statistically

This investigation raises a useful statistical question about how an exposure that varies in clinical practice can be related to patient outcomes. The registry describes a retrospective comparison of local anesthetic concentrations in pediatric femoral nerve blocks, with pain relief and safety as the stated concerns. That setting highlights the distinction between describing treatment received and estimating the effect of changing treatment.

In general, treatment concentration may be selected in response to clinical or procedural circumstances. When the assignment process is not described as randomized, a comparison can reflect both the exposure and differences among the people or circumstances represented in each group. Clear group definitions, outcome measurement, and an appropriate analysis are needed to evaluate those possibilities. The registry record does not report those details, so the statistical challenge is apparent even though the comparison itself cannot be assessed from the posted outcome information.

The outcomes also illustrate that a single study may include measures with different statistical structures. Participant counts, pain scale measurements, and time to discharge do not necessarily call for the same model or interpretation. General methodological choices should follow the outcome definitions and data structure; the registry record does not state which methods were used. Listing outcomes is not the same as reporting an analysis.

Finally, the record’s zero adverse-event counts invite careful separation of reported observations from broader safety conclusions. Event counts are informative only in the context of definitions, follow-up, denominators, and group-level comparisons. The record lists an overall group but no concentration-specific safety results. This example underscores why registry entries are valuable for documenting aims and outcomes, while also showing why transparent results and analysis methods are necessary for estimating effects and their precision.

14. 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 most immediate limitation for interpretation is that the registry record does not report numerical primary or secondary outcome results. The primary outcome is named and its measurement unit is given, but concentration categories and participant counts by category are absent. Results for PACU discharge time and postoperative pain using FLACC and VAS are also not reported in the registry record. No estimate or uncertainty measure is available for judging the magnitude or precision of a comparison.

The design information is limited. The investigation is retrospective, allocation is not stated, and the registry indicates that the arms are not a like-for-like control comparison. The record does not describe how patients came to receive a concentration, whether concentrations differed alongside the anesthetic agent, or whether other patient and procedural characteristics were measured. These details are important when assessing possible confounding, but they are not reported in the registry record.

Outcome definitions and data collection details are also missing. The registry names FLACC and VAS but does not provide assessment timing, scoring details, or the specific contrasts intended. It does not describe how discharge time was defined, how adverse events were identified, or how long safety was monitored. The primary measure’s timeframe is the length of surgery, but that does not clarify follow-up for the secondary or safety outcomes.

The record lists seven outcome measures and zero analyses. It does not report a statistical analysis plan, multiplicity approach, sample-size justification, analysis populations, or missing-data procedures. It is therefore not possible to establish which participants contributed to each measure, how incomplete observations were handled, or whether analyses accounted for differences among groups. The reported enrollment of 281 does not resolve these questions.

Finally, the adverse-event counts are not separated by concentration or anesthetic agent. Zero recorded events in the listed group do not supply comparative safety estimates, and the registry record does not detail ascertainment or follow-up. These limitations concern what can be evaluated from the record and should not be mistaken for evidence that the underlying investigation necessarily lacked such information.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: A Retrospective Investigation of Local Anesthetic Concentrations in Femoral Nerve Block — NCT02501135.
• Registry record last updated 2019-03-06; status last verified 2018-11.
• Results first posted 2019-03-06.
• 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.

18. Record Summary

The registry describes a completed retrospective investigation of local anesthetic concentrations used in pediatric femoral nerve blocks. Enrollment was 281; the reported start date was 2015-06 and primary completion date was 2016-05. The listed intervention involved ropivacaine or bupivacaine during femoral nerve block.

The primary outcome is titled “Concentration of Local Anesthetic Injected for Femoral Nerve Block,” measured as participants over the length of surgery. Named secondary outcomes include time to discharge from PACU and postoperative pain scales using FLACC and VAS. The record lists seven outcome measures and zero analyses.

The listed group has 281 participants at risk for serious adverse events and zero affected, 281 at risk for other adverse events and zero affected, and zero deaths. Concentration-specific outcome results, comparative estimates, analysis methods, missing-data procedures, and detailed outcome definitions are not reported in the registry record. The record states that allocation is not stated and that the arms are not a like-for-like control comparison.