IV vs. Oral Acetaminophen as a Component of Multimodal Analgesia After Total Hip Arthroplasty: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for IV vs. Oral Acetaminophen as a Component of Multimodal Analgesia After Total Hip Arthroplasty (NCT03020966), a randomized phase 4 study. The primary outcome is Pain With Physical Therapy on Post-operative Day 1.

2. Clinical Question

3. Trial Design

The registry describes a phase 4, randomized, triple-masked, parallel-group trial comparing routes of acetaminophen administration after total hip arthroplasty. The study status is completed. Its listed enrollment is 154, and the sponsor is Hospital for Special Surgery, New York. The recorded study start is 2017-02-16, with primary completion on 2019-06-16.

The two groups received active acetaminophen by different routes, together with placebo by the other route. Oral Tylenol consisted of 1000mg oral acetaminophen and an intravenous placebo; Intravenous Tylenol consisted of 1000mg intravenous acetaminophen and an oral placebo. The record labels masking as triple but does not identify which groups of people were masked, or describe how masking was maintained.

The primary endpoint was pain with physical therapy on postoperative day 1, assessed 24 hours after the operation. The registry reports the result as a mean and standard deviation for each group. Three patients withdrew from the protocol but were followed using intention-to-treat principles, according to the endpoint description. The record also notes the reasons associated with these cases: one patient did not wish to answer postoperative questions, one could not swallow the pills, and one received a different anesthetic/analgesic protocol due to pre-existing expectations.

The trial's original sample-size assumptions, detailed eligibility criteria, and fuller statistical analysis plan are not reported in the registry record. The planning illustrations accompanying the result are based on the observed effect and should not be interpreted as the trial's original design rationale.

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

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: Three patients withdrew from the protocol, but were followed using intention-to-treat principles. One patient did not wish to answer the postoperative questions, 1 could not swallow the pills, and 1 received a different anesthetic/analgesic protocol due to pre-existing expectations.

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 Pain With Physical Therapy on Post-operative Day 1, measured over 24 hours after the operation (post-operative day 1).

The primary endpoint is pain with physical therapy on postoperative day 1, measured 24 hours after the operation. The measure is a numerical rating scale ranging from 0, defined as no pain, to 10, defined as pain as bad as imaginable. The registry reports a mean and standard deviation for each treatment group, so the result describes average reported pain and the spread of individual scores around each group mean.

Pain during physical therapy is a patient-reported outcome tied to activity during early recovery. A pain score can capture the experience of symptoms during movement or therapy rather than only at rest. The registry's summary frames the broader research interest as pain, opioid consumption, opioid-related side effects, and satisfaction after surgery. However, the primary posted endpoint is specifically pain with physical therapy; the registry record does not post results for secondary outcomes.

The scale's endpoints make direction interpretable: a higher score represents worse pain, while a lower score represents less pain. The endpoint is a continuous measure, and its group comparison is expressed as a mean difference. The registry does not report a threshold defining a clinically important difference, nor does it state a target score that would determine whether a difference matters to patients. The result therefore describes the observed average scores and their statistical uncertainty, not whether any observed difference crosses a predefined clinical threshold.

7. Results Posted in the Registry

• Intravenous Tylenol: mean 3.90 versus 3.60 on control; difference 0.30 (95% CI -0.47 to 1.07), standardized effect d = 0.12; z-test p = 0.442.

8. Statistical Interpretation

9. Statistical Methods Explained

The primary endpoint is continuous: each participant's pain score is recorded on a numerical scale, and the registry summarizes the group results using means and standard deviations. The reported comparison is a difference between group means. In this trial, the contrast is intravenous acetaminophen relative to oral acetaminophen, so a positive estimate indicates a higher mean pain score in the intravenous group. Since higher scale values indicate worse pain, the direction of the estimate can be translated into the direction of the measured outcome.

What does the mean difference represent?

A mean is a measure of the center of a set of scores. Comparing means gives a concise description of how the average reported pain score differed between groups. The posted means are 3.9 for intravenous acetaminophen and 3.6 for oral acetaminophen; the reported mean difference is 0.3. The standard deviation describes variation among scores within a group. Both groups have a reported standard deviation of 2.4, indicating that individual scores varied around their respective averages. The standard deviation is not the uncertainty of the difference itself; the confidence interval conveys uncertainty about the estimated group contrast.

What does randomization contribute?

Random allocation is intended to create groups that are comparable, on average, with respect to factors that might affect the outcome. It supports a causal comparison more strongly than a simple comparison of people who selected their own treatment. Randomization does not guarantee that every characteristic is balanced in a particular trial, and the registry summary does not provide baseline characteristics or details of the allocation procedure. The registry identifies a parallel-group design, meaning participants were assigned to one of the groups rather than the comparison being described as a within-participant crossover.

How should the p-value be read?

The unadjusted p-value is 0.442. In general, a p-value describes how unusual data at least as incompatible with a specified null hypothesis would be under the assumptions of the statistical model. It is not the probability that the null hypothesis is true, and it does not measure the probability that the observed result occurred by chance. It also does not directly quantify clinical importance. The registry does not report a broader multiplicity adjustment or a detailed statistical analysis plan, so the p-value should be read as the reported unadjusted comparison rather than as evidence about an adjustment procedure.

Why report a confidence interval?

A confidence interval gives a range of values compatible with the estimate under the statistical method used, reflecting sampling uncertainty. For a mean difference, zero represents no difference in group means. The reported interval includes zero and values on both sides of it. That makes clear that the estimate is imprecise enough to leave more than one direction of difference plausible. A confidence interval is not a guarantee that the true effect lies in the interval, nor does it show the distribution of individual patients' scores. The registry record does not describe the exact interval-construction method.

What do the reported standard deviations tell us?

Standard deviations describe the spread of outcome scores within each treatment group. They help readers understand the variability behind the averages, but do not indicate whether the group means differ statistically. With continuous outcomes, variability affects the precision of the mean estimate: more score variation generally makes it harder to estimate a mean precisely for a given amount of information. Here, the two reported standard deviations are equal at 2.4. No adjustment for baseline measurements or other covariates is reported in the registry record.

What can be inferred from the analysis record?

The registry has no posted analysis records for this outcome comparison, while arm-level results and a comparison estimate are posted. The specific model, test implementation, and handling of any missing endpoint observations are not described in the record. The endpoint notes intention-to-treat follow-up for three protocol withdrawals, but that statement does not establish every detail of the analytic dataset or missing-data method. Accordingly, the explanation can interpret the reported mean difference, p-value, and interval, but cannot attribute unreported modeling choices to the investigators.

General statistical interpretation also requires separating a result's direction, magnitude, and precision. The direction is conveyed by the sign of the mean difference. Its magnitude is expressed on the original pain scale. Precision is reflected in the confidence interval. None of these elements alone establishes clinical importance, and the registry does not report a clinical-importance threshold. The posted result is best understood as an estimated average contrast with substantial uncertainty about its exact value and direction.

10. Confidence Intervals and Precision

The reported mean difference is 0.3 points, intravenous relative to oral acetaminophen. Its confidence interval runs from -0.466 to 1.066 when the lower limit is displayed to three decimal places. The interval includes zero and extends to both negative and positive differences. Negative values would correspond to a lower average pain score in the intravenous group; positive values correspond to a higher average score. The estimate therefore does not precisely establish the direction of the underlying mean difference.

In general, a confidence interval communicates sampling uncertainty around an estimate. A narrower interval indicates greater precision, while a wider interval leaves a broader range of effects compatible with the data and method. This interval includes no mean difference as well as differences in either direction. Its breadth matters: the point estimate alone should not be treated as the only plausible value, and the interval does not establish that the groups are equivalent.

The interval is reported for the difference in group means on the pain-score scale. It is not an interval for individual pain scores, and it does not show how much participants' scores overlapped. The group standard deviations, both 2.4, describe within-group variability, whereas the confidence interval concerns uncertainty in the comparison estimate. The registry does not report a prespecified equivalence margin or clinical-importance threshold. Thus, the interval can be interpreted statistically, but the record does not define which differences would be considered clinically negligible or important.

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 events. They are illustrative calculations based on the observed effect, not the original design assumptions for the trial. The listed assumptions are a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. The calculation considers detecting the stated fraction of the observed effect. The trial's own sample-size justification and original design assumptions are not reported in the registry record.

The 80% power column describes the number of participants per arm that the planning calculation estimates would be needed for an 80% chance of detecting the chosen effect under the stated assumptions, if that effect and model were correct. The 90% power column expresses the corresponding planning target for a 90% chance. These are design calculations, not guarantees about the outcome of a future study. They are also not observed power for the completed trial.

The entries are shown for planning to detect 100%, 75%, and 50% of the observed effect. A smaller target effect is harder to distinguish from random variation, so the planning calculation requires more participants per arm as the assumed effect becomes smaller. The figures are based on the observed estimate, which itself is uncertain; they should therefore be treated as illustrations rather than definitive recommendations. They do not demonstrate that the original trial was planned using these assumptions, and they do not supply information about unreported design choices such as any anticipated loss to follow-up.

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 = 0.300; pooled standard deviation σ = 2.400.
2. n = 2 × (1.96 + 0.842)² × 2.400² / 0.300² = 1,004.7, rounded up to 1,005 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 <- 0.3                      # observed difference in means
sigma <- 2.4                     # 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(3020966)
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 = 1,005 (80% power), 1,345 (90% power)
 75% of observed effect: n per arm = 1,787 (80% power), 2,392 (90% power)
 50% of observed effect: n per arm = 4,019 (80% power), 5,380 (90% power)
power.t.test: 1005.6 
Simulated power at n = 1,005 per arm: 0.801 (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 lists three outcome measures, but no secondary outcome results are posted. The study summary describes interest in opioid consumption, opioid-related side effects, and patient satisfaction, while the posted primary result concerns pain during physical therapy. Without posted secondary estimates, those outcomes cannot be compared between groups using the registry record. The record does not describe whether the listed measures were analyzed, how they were prioritized beyond the stated primary endpoint, or how any multiple comparisons were handled.

Multiplicity arises when several outcomes, time points, groups, or statistical tests are considered. If many tests are conducted, the chance of at least one apparently notable result can increase unless the analysis plan accounts for the number and structure of comparisons. This is a general statistical concern, not evidence that a particular adjustment was or was not used in this trial. The registry does not report a multiplicity strategy, and the reported p-value is identified as unadjusted. No adjustment should be assumed from the record.

The endpoint description states that three patients withdrew from the protocol but were followed using intention-to-treat principles. It gives the following reasons: one patient did not wish to answer postoperative questions, one could not swallow the pills, and one received a different anesthetic/analgesic protocol due to pre-existing expectations. This information provides context for protocol departures and willingness or ability to complete postoperative questions. It does not fully define which observations were available for the primary endpoint or how any missing values were handled.

An intention-to-treat approach generally analyzes participants according to their assigned group, regardless of treatment adherence, to preserve the comparison created by randomization. The endpoint note indicates intention-to-treat follow-up for the withdrawals, but the registry does not provide enough detail to confirm the exact analysis population, the handling of missing responses, or whether any imputation was used. The statistical analysis plan and further missing-data procedures are not reported in the registry record.

14. 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 reports zero serious adverse events in the oral group and zero in the intravenous group. It also reports zero other adverse events in each group, and zero deaths in each group. The serious-adverse-event terms list is empty. These are the adverse-event figures posted in the record; they are counts, not estimates of comparative risk.

A zero count means no events were recorded in the listed category for the group in this registry result. It does not establish that an event is impossible, or that the treatments have identical safety profiles. Rare events may not be observed in a trial, and the precision of a safety estimate depends on the amount of follow-up and other details. The registry record does not report a fuller adverse-event collection method, event definitions, or follow-up duration for safety outcomes, so those details cannot be inferred from the zero counts.

These figures should be read separately from the primary pain outcome. They do not provide evidence about opioid-related side effects specifically unless those events are separately defined and reported, which the registry record does not do. The absence of posted event terms also does not supply a detailed narrative of events or monitoring.

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

The posted primary result estimates a mean pain score of 3.9 for intravenous acetaminophen and 3.6 for oral acetaminophen during physical therapy on postoperative day 1. The mean difference is 0.3 points in the direction of a higher score in the intravenous group. Its confidence interval includes zero and spans values in both directions, and the unadjusted p-value is 0.442. The result therefore does not provide clear statistical evidence of a difference in this primary measure, but it does not prove that the treatments are equivalent.

Equivalence is a distinct statistical claim. It ordinarily requires a prespecified margin defining how much difference would be acceptably small, along with an analysis designed to test that claim. No equivalence margin or equivalence analysis is reported in the registry record. A non-significant difference test cannot substitute for such evidence. Similarly, the registry does not report a clinical-importance threshold for the pain scale, so the observed estimate cannot be classified as clinically important or unimportant based on a stated criterion.

The outcome is specific: average pain with physical therapy at the stated postoperative time. It does not by itself describe pain at other times or circumstances. The study summary refers to opioid consumption, opioid-related side effects, and satisfaction, but no secondary results are posted in the registry record. Those outcomes cannot be inferred from the primary pain-score result.

The record also reports zero serious adverse events and zero other adverse events in each group, with zero deaths. These are observed counts in the registry entry, not proof that no safety differences could exist. Overall, the record offers a randomized comparison of routes and an estimate for one primary pain endpoint, but leaves uncertainty about the size and direction of the underlying difference and does not answer every outcome named in the study summary.

16. Why This Trial Matters Statistically

This trial is statistically informative because it makes the comparison between routes explicit and reports a continuous patient-reported endpoint with group means, standard deviations, a mean difference, and a confidence interval. Those statistics answer different questions: the means describe average pain, the standard deviations describe within-group variability, the difference expresses the contrast on the original scale, and the interval shows uncertainty around that contrast.

The result also illustrates why a point estimate and a test result should not be interpreted in isolation. The estimated difference is 0.3 points, but its interval includes zero and values in both directions. The p-value of 0.442 does not show that the groups are identical; rather, the posted data do not establish a difference under the reported unadjusted comparison. A reader can see both the estimated direction and the limits on what that estimate supports.

The registry record also makes some limitations visible. It describes protocol withdrawals and reasons, but does not provide a detailed missing-data method or a complete statistical analysis plan. It lists secondary outcomes without posting their results and does not report a multiplicity approach. These omissions matter because interpreting a primary finding requires knowing how it fits into the wider analysis and how incomplete observations were handled.

Finally, the planning illustrations show how design targets depend on the effect size chosen: detecting a smaller fraction of the observed effect requires more participants per arm under the stated planning assumptions. That is useful for understanding statistical precision and future study planning, but it is separate from the original trial design and does not describe the trial's observed power.

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 primary result is limited to pain with physical therapy at 24 hours after the operation. It does not establish how pain changed at other time points or during other activities. While the study summary identifies opioid consumption, opioid-related side effects, and satisfaction as interests, the registry record does not post secondary outcome results. The available result therefore cannot answer those parts of the research question.

The registry lists group means and standard deviations and a comparison estimate, but does not describe the exact statistical model or interval-construction method. It does not report baseline characteristics, detailed eligibility criteria, a clinical-importance threshold, or an equivalence margin. These omissions limit assessment of population context and the clinical meaning of the estimated difference.

The endpoint description notes three protocol withdrawals and says they were followed using intention-to-treat principles. It gives reasons for those cases, but does not specify how missing endpoint responses were handled, whether every participant contributed an outcome measurement, or how the analysis population was defined. The registry record also does not provide the trial's full statistical analysis plan or multiplicity strategy. The p-value is unadjusted, and no secondary results are posted.

Safety interpretation is limited to the adverse-event counts shown in the record. The registry does not report detailed collection procedures, safety follow-up duration, or event definitions. The zero counts should not be taken as proof that uncommon events could not occur. These limitations concern the information reported in the registry and do not establish what may have been collected or analyzed outside the record.

18. Related Tutorials

19. Related Statistical Calculators

20. Sources

• ClinicalTrials.gov: IV vs. Oral Acetaminophen as a Component of Multimodal Analgesia After Total Hip Arthroplasty — NCT03020966.
• Registry record last updated 2021-08-10; status last verified 2021-08.
• Results first posted 2021-08-10.
• 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

NCT03020966 is a completed phase 4 randomized, triple-masked, parallel-group trial of oral versus intravenous acetaminophen after total hip arthroplasty. The registry lists enrollment of 154 and identifies Hospital for Special Surgery, New York, as sponsor. The primary endpoint was pain with physical therapy on postoperative day 1, measured on a 0-to-10 numerical rating scale 24 hours after the operation.

The posted means were 3.6 for oral acetaminophen and 3.9 for intravenous acetaminophen. The reported mean difference was 0.3, with a confidence interval that includes zero, and the unadjusted p-value was 0.442. The registry reports zero serious adverse events, zero other adverse events, and zero deaths in each group. It lists three outcome measures but posts no secondary results. Details of the original sample-size assumptions, full statistical analysis plan, and missing-data procedures are not reported in the registry record.