Comparing Post-Op Narcotic Usage in Patients Receiving Periarticular Exparel vs. Standard Periarticular Joint Inj: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Comparing Post-Op Narcotic Usage in Patients Receiving Periarticular Exparel vs. Standard Periarticular Joint Inj (NCT02682498), a randomized phase 4 study. The primary outcome is 48 Hour Post-surgical Opioid Use.

2. Clinical Question

The registry record describes a comparison of postoperative opioid use in opioid-tolerant patients undergoing total knee arthroplasty. The study’s stated hypothesis was that patients receiving Exparel liposomal bupivacaine would use less opioid during the first 48 hours after surgery than patients receiving the standard periarticular joint injection. The primary outcome was 48-hour opioid use, expressed in morphine equivalents and summarized as a group mean.

The posted results address the measured difference between the groups, but do not establish whether any difference is clinically important. The registry record does not report a threshold for clinical importance. It also does not provide enough detail to assess how opioid use was collected or converted to morphine equivalents.

3. Trial Design

This was a randomized, double-blind, parallel-group, phase 4 clinical trial. The registry record reports an enrollment of 38 participants. The Exparel arm had 20 participants in the primary outcome results, and the standard periarticular joint injection arm had 18. The study status is completed. The record lists 2013-10 as the start and 2015-07 as the primary completion.

The two groups received different periarticular injections after total knee arthroplasty. In the Exparel arm, the liposomal bupivacaine suspension was injected into soft tissues at the end of surgery in place of the standard injection. The comparator was a standard preparation containing clonidine, epinephrine, ketorolac, ropivacaine, and sodium chloride, with the listed quantities appearing in the arm description.

Random allocation is intended, in general, to reduce systematic differences between groups at the start of a trial. Double masking is intended, in general, to limit the influence of treatment knowledge on care or outcome assessment. The registry record identifies the trial as randomized and double blind but does not report how the random sequence was generated, how allocation was concealed, who was masked, or whether masking was assessed.

The registry record does not report the trial’s original sample-size assumptions or a detailed statistical analysis plan. It lists four secondary outcomes: recovery room opioid use, average daily opioid use during admission, average daily patient pain score, and postoperative complications. The primary outcome was 48-hour postoperative opioid use.

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 48 Hour Post-surgical Opioid Use, measured over 48 hours.

The primary endpoint was 48-hour post-surgical opioid use, measured in morphine equivalents and summarized as a mean for each group. It captures the total opioid use recorded over the first 48 hours after surgery, expressed on a common scale. The registry record does not report the collection procedures, conversion method, or any rules used to handle missing measurements.

In general, a continuous endpoint such as opioid use can be summarized by a mean, which describes the average observed value in a group. The standard deviation describes how spread out individual observations are around that mean. These summaries help characterize both the group’s average and the variability among participants; they do not show every individual’s value or the shape of the outcome distribution.

A fixed 48-hour window makes the comparison specific to an early postoperative period. It does not, by itself, describe use after that window or other aspects of recovery. The registry record also lists secondary measures, including opioid use during admission and daily pain scores, but the primary result discussed in the posted comparison is the 48-hour mean. The record does not report a threshold defining what size of difference would be clinically important.

7. Results Posted in the Registry

• EXPAREL® Bupivacaine Liposome Suspension: mean 5.32 versus 5.07 on control; difference 0.25 (95% CI -0.96 to 1.46), standardized effect d = 0.13; z-test p = 0.686.

8. Statistical Interpretation

9. Statistical Methods Explained

The primary result compares means for a continuous outcome. The posted group means are 5.32 morphine equivalents for Exparel and 5.07 for the standard injection. The reported mean difference is 0.25, with the Exparel mean minus the standard-injection mean as the comparison direction. A difference is easier to interpret when its sign convention is clear: positive values indicate a higher mean in the Exparel group, while negative values indicate a lower mean in that group.

What does a mean difference describe?

In general, the mean difference expresses the distance between two group averages in the original measurement units. Here the units are morphine equivalents. It describes the contrast in average recorded use, not the difference for every participant. Participants within each arm may have had values above or below their group mean. The registry record gives standard deviations of 1.72 and 2.05, which describe the observed spread around the respective means.

What does the confidence interval add?

In general, a confidence interval gives a range of estimates compatible with the data and the statistical procedure used, under its assumptions. It communicates uncertainty around the estimated difference rather than just a single point estimate. The reported interval includes zero, so it includes a value corresponding to no mean difference, as well as negative and positive values. The interval therefore does not pin down the direction of the difference. Its width matters: a wide interval allows a broad range of plausible differences and signals limited precision.

How should the p-value be read?

The reported unadjusted p-value is 0.686. In general, a p-value is calculated under a null hypothesis and statistical model; it describes how unusual the data, or data more extreme, would be if that model were correct. It is not a measure of the probability that a treatment is effective, nor a direct measure of clinical importance. A p-value larger than a conventional threshold does not prove there is no difference, and a p-value alone does not establish equivalence. The registry record does not report a multiplicity adjustment or a more detailed testing procedure.

Why do group sizes and variability matter?

In general, uncertainty in a difference between means depends on the amount of variation in the outcome and the number of observations. More variability makes a group mean less precise, while more observations can improve precision, if the other assumptions hold. The primary result lists 20 Exparel participants and 18 standard-injection participants. The reported standard deviations show that observations were not all concentrated at the means. Because the total enrollment was 38, the estimate should be interpreted with attention to its confidence interval rather than by treating the observed means as exact descriptions of all relevant patients.

What assumptions are not documented?

The registry record does not report the specific statistical test used for the primary comparison, checks of distributional assumptions, handling of missing outcomes, or analysis population. It also does not report whether an adjusted or covariate-based analysis was planned. General methods for comparing means often rely on assumptions about independent observations and the behavior of the outcome or its sampling distribution. Those are general considerations, not confirmation of the trial’s actual analytic procedure.

The displayed comparison is described by the registry as a group-mean comparison. The reported confidence interval, p-value, group means, and standard deviations are the relevant posted summaries. No alternative adjusted analysis is reported in the registry record. No trial-specific claim about how the numerical results were obtained beyond the registry’s description should be inferred from general statistical practice.

Mean-based summaries answer a particular question about average opioid use. They do not indicate whether the outcome distribution was symmetric, whether a small number of participants had unusually high use, or whether individual responses differed substantially. The registry record does not report medians, quartiles, participant-level values, or a distributional plot description in the registry record for this result. Those omissions limit how fully the distribution can be assessed.

Finally, statistical evidence and clinical relevance are separate considerations. Statistical uncertainty is represented by the confidence interval and p-value, while clinical relevance requires context and a meaningful-difference threshold. The registry record does not report such a threshold. The numerical estimate can therefore be described as a small observed difference in group means, but the result alone does not determine whether that difference matters to patients or clinical practice.

10. Confidence Intervals and Precision

The mean difference is 0.25 morphine equivalents, with the Exparel group mean higher than the standard-injection group mean. The confidence interval has a negative lower endpoint and a positive upper endpoint, so it includes zero. It therefore includes estimates in which the Exparel mean is lower, no mean difference is present, or the Exparel mean is higher. The reported endpoints are -0.960 and 1.46; the lower endpoint is shown as recorded.

In general, a confidence interval expresses uncertainty about an estimated quantity under the statistical method’s assumptions. It is not a guarantee that the true value lies in the interval, and it is not a range containing a specified proportion of individual participants’ outcomes. The interval applies to the group mean difference, not to the spread of individual opioid-use values.

The interval spans both directions around zero and is wide compared with the point estimate. This means the posted data do not precisely determine either the direction or size of the difference. The p-value of 0.686 is consistent with the interval’s inclusion of zero, but neither statistic should be interpreted as proof that the groups are equivalent. The registry record does not report a clinical threshold that would help determine which values in the interval would be important in practice.

Small group sizes and variation among participant outcomes are relevant to precision. The primary outcome results include 20 participants in the Exparel group and 18 in the standard-injection group, with standard deviations of 1.72 and 2.05. These summaries help explain why the estimate should be read as uncertain, but they do not reveal individual observations or fully characterize the outcome distribution.

11. Planning a Similar Trial: Sample Size

This is a planning illustration, not a re-evaluation of the original trial. It asks: if a new study wanted to detect a difference like the one observed, or a smaller one, how large would it need to be? It is deliberately not “observed power” of the finished trial, which is not a meaningful quantity.

The planning figures are participants per arm, not total event counts. They are planning illustrations based on the observed effect, not the original trial’s design assumptions. The listed assumptions are a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. The registry record does not report the trial’s own original sample-size justification.

The 80% power column gives the planned number of participants per arm for a design with 80% power to detect the specified fraction of the observed effect under those assumptions. The 90% power column gives the corresponding number per arm for 90% power. In general, power is the chance of detecting a specified effect under a particular design and set of assumptions if that effect is real; these columns are planning targets, not descriptions of the completed trial’s probability of finding a result.

For 100% of the observed effect, the figures are 891 participants per arm for 80% power and 1,193 participants per arm for 90% power. For 75% of the observed effect, they are 1,584 and 2,120 participants per arm, respectively. For 50% of the observed effect, they are 3,563 and 4,770 participants per arm, respectively. The pattern reflects a general planning principle: smaller effects require more participants to detect with the same power, all else equal.

These calculations use the observed effect as their reference and should not be confused with the study’s enrollment of 38. This is not observed power. The planning values do not show what the trial was originally designed to detect, because that original design information is not reported in the registry record.

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 = 0.250; pooled standard deviation σ = 1.883.
2. n = 2 × (1.96 + 0.842)² × 1.883² / 0.250² = 890.6, rounded up to 891 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 <- 0.25                      # observed difference in means
sigma <- 1.88305381406551                     # 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(2682498)
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 = 891 (80% power), 1,193 (90% power)
 75% of observed effect: n per arm = 1,584 (80% power), 2,120 (90% power)
 50% of observed effect: n per arm = 3,563 (80% power), 4,770 (90% power)
power.t.test: 891.6 
Simulated power at n = 891 per arm: 0.800 (20000 replicates)

The simulation draws many sets of group means and variances at the planned sample size and counts how often the two-sample t-test rejects the null. It should land close to the target power; at very small required sample sizes it can fall below it, because the normal approximation is rough there.

13. Multiplicity, Missing Data and Analysis Populations

The registry record lists one primary outcome and four secondary outcomes. The primary outcome is 48-hour post-surgical opioid use. The secondary outcomes are recovery room opioid use, average daily opioid use during admission, average daily patient pain score, and postoperative complications. Having several outcomes can raise a multiplicity question: when many statistical tests are conducted, the chance of obtaining at least one apparently notable result by chance may increase. In general, a prespecified testing hierarchy or adjustment can address some multiplicity concerns.

The registry record does not report whether the trial used a multiplicity adjustment, a hierarchy among outcomes, or a formal procedure for controlling type I error across tests. It also does not report interim analyses or stopping rules. These details should not be inferred from the list of outcomes or from the posted primary comparison.

The record lists no analyses, while reporting group summaries and a comparison for the primary endpoint. It does not provide a detailed statistical analysis plan, identify an analysis population, or state whether the primary analysis was intention-to-treat, per-protocol, or another approach. The number randomized to each arm and the number included in the outcome summaries are not explicitly reconciled in the registry record.

Missing-data handling is also not reported. The record does not describe the amount or pattern of missing primary outcome data, any imputation, or whether all enrolled participants contributed to the reported group means. Without that information, readers cannot assess whether missingness could have affected the comparison.

The primary comparison is therefore best read as the posted group-level result, with the analysis procedures and population details not reported in the registry record. This limits assessment of whether the reported p-value and interval account for all planned outcomes, missing data, or deviations from the intended analysis.

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 0% reporting threshold.

The registry record reports zero serious adverse events and zero other adverse events in each of the two safety groups. It lists 18 participants at risk in the Exparel safety group and 20 at risk in the standard-injection safety group. These at-risk counts differ from the primary outcome arm counts, which are 20 for Exparel and 18 for the standard injection. The registry record does not explain this difference.

In general, an adverse-event count describes events recorded in the stated group and does not by itself establish whether an intervention caused or prevented an event. A count of zero means no events of that category were reported for the listed at-risk group; it does not show that the risk is zero. The registry record does not provide event definitions, follow-up details for safety reporting, or further context for these counts.

Deaths are listed as null for both groups, which means the registry record does not report a death count in this entry; it should not be read as a reported count of zero. No serious adverse-event terms are listed. These sparse safety summaries do not support broader conclusions about safety.

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

The posted primary result describes mean opioid use over the first 48 hours after surgery. The Exparel group mean was 5.32 morphine equivalents, compared with 5.07 in the standard-injection group. The estimated mean difference was 0.25, with the Exparel mean minus the standard-injection mean. The confidence interval includes zero and extends in both directions, and the reported p-value is 0.686. Together, these figures show that the estimate is uncertain and does not precisely establish the direction or magnitude of a difference.

The result does not prove that the groups are the same. A failure to establish a difference is not the same as evidence of equivalence; equivalence requires a defined margin and an analysis designed to assess it. The registry record does not report an equivalence margin or a clinically important difference threshold.

The result also does not establish that either injection caused the observed mean or that the observed numerical difference is clinically meaningful. Randomization supports a comparative trial design in general, but the registry record does not report all details needed to evaluate conduct, outcome completeness, or analysis populations. The posted summary should therefore be interpreted in light of the missing methodological information.

The findings concern the recorded 48-hour opioid-use endpoint in the trial population described as opioid-tolerant patients undergoing total knee arthroplasty. They do not answer questions about opioid use after 48 hours, every secondary outcome, other patient groups, or other clinical settings. The record lists secondary outcomes but does not provide their numerical results in the registry record for this analysis.

The enrollment of 38 and the observed variation in outcomes are important context for interpreting precision. The confidence interval communicates a range of differences compatible with the result, rather than a definitive treatment effect. The registry record does not report a threshold that would distinguish clinically important from unimportant values within that range. Accordingly, the posted data support a description of the comparison and its uncertainty, not a medical recommendation.

16. Why This Trial Matters Statistically

This trial illustrates why a point estimate should be read together with its uncertainty. The mean difference of 0.25 morphine equivalents is a compact description of the observed contrast, but the confidence interval includes zero and values in both directions. Looking only at the point estimate would obscure how imprecisely the difference is estimated.

It also illustrates the distinction between a statistical test and a clinical question. The p-value of 0.686 does not tell readers whether a difference would matter to patients, and it does not prove the groups equivalent. A clinically meaningful threshold would help put the magnitude of the estimate into context, but such a threshold is not reported in the registry record.

Planning calculations are another important statistical lesson. The posted planning illustrations use fractions of the observed effect and show that targeting smaller effects requires more participants per arm under the stated assumptions. These are not a reconstruction of the trial’s original design. The registry record does not report the original sample-size justification, so the planning figures should not be mistaken for what investigators anticipated when planning the trial.

Finally, registry summaries have limits. Group means and standard deviations communicate average outcomes and variability, but the record does not report individual data, missing-data procedures, analysis populations, or detailed methods. Readers can interpret the reported comparison while recognizing that these omissions constrain assessment of the analysis and its broader implications.

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 registry record contains limited information for evaluating the trial’s conduct and analysis. It identifies randomization and double masking but does not describe sequence generation, allocation concealment, who was masked, or whether masking was maintained. It does not report eligibility criteria beyond the description of opioid-tolerant patients presenting for total knee arthroplasty.

The primary outcome summaries list 20 participants in the Exparel group and 18 in the standard-injection group, while the adverse-event summaries list 18 and 20 at risk for those groups, respectively. The record does not explain the differing counts. It also does not report whether all randomized participants contributed to the primary outcome, the amount of missing data, or how missing outcomes were handled.

The record does not provide a detailed statistical analysis plan, a named analysis population, the specific test used, or any adjustment for multiple outcomes. It lists four secondary outcomes but the registry record does not report their numerical results in the registry record for this analysis. It also does not report the original sample-size justification or a threshold for clinical importance.

The safety entry reports zero serious and other adverse events for the listed at-risk groups, but deaths are null rather than reported as zero. Event definitions and safety follow-up details are not reported. These limitations mean the result can be described from the posted summary, while several aspects of the analysis, safety reporting, and interpretation remain unclear.

18. Related Tutorials

19. Related Statistical Calculators

20. Sources

• ClinicalTrials.gov: Comparing Post-Op Narcotic Usage in Patients Receiving Periarticular Exparel vs. Standard Periarticular Joint Inj — NCT02682498.
• Registry record last updated 2017-07-07; status last verified 2017-07.
• Results first posted 2017-07-07.
• Data retrieved from ClinicalTrials.gov on 2026-11-09.
• Group sizes, means, event counts and sponsor-reported p-values are reproduced as posted. Effect estimates, confidence intervals and p-values not attributed to the sponsor are independently calculated from the posted summary statistics.

21. Record Summary

The registry record describes a completed, randomized, double-blind, parallel-group phase 4 trial of Exparel liposomal bupivacaine versus a standard periarticular joint injection in opioid-tolerant patients undergoing total knee arthroplasty. Enrollment was 38. The listed start and primary completion are 2013-10 and 2015-07.

The primary endpoint was mean opioid use in morphine equivalents over 48 hours. The reported means were 5.32 for Exparel and 5.07 for standard injection, with a mean difference of 0.25 and a p-value of 0.686. The confidence interval includes zero and extends in both directions, indicating uncertainty about the direction and size of the difference.

The registry record does not report the original sample-size justification, detailed statistical plan, missing-data methods, or analysis population. Safety summaries report zero serious and other adverse events in both groups, with at-risk counts that differ from the primary outcome counts; deaths are not reported.