Aspirin and Compression Devices for VTE Prophylaxis in Orthopaedic Oncology: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Aspirin and Compression Devices for VTE Prophylaxis in Orthopaedic Oncology (NCT01696760), a randomized na study. The primary outcome is DVT Incident Rate.

2. Clinical Question

3. Trial Design

This was a randomized, parallel-group study with no masking reported; the registry lists masking as none. It is marked completed and lists a total enrollment of 12. The registry record identifies Ohio State University Comprehensive Cancer Center as the sponsor. The listed study start is 2010-10, and primary completion is listed as 2012-12. The phase is recorded as NA.

The study compared two medication-and-device regimens after surgery for pelvic or lower-extremity malignant tumors. Participants in Arm I received acetylsalicylic acid orally BID and wore a pneumatic compression device on days 1-28 after surgery. Participants in Arm II received enoxaparin subcutaneously SC QD and wore the device on days 1-28. The registry describes the device as an inflatable sleeve that periodically squeezes the calf. Thus, the randomized contrast was between medication strategies while both groups also used compression devices.

The posted primary outcome is DVT incident rate, measured as the count of participants, with a timeframe of up to 3 months. The results list 9 participants in Arm I and 3 in Arm II, with zero DVT events in each. The registry also names pulmonary embolism, other complications including bleeding, readmission, hematoma, excessive wound drainage, and death as secondary outcomes. Numerical results for those outcomes are not reported in the registry record. Details about randomization procedures, concealment, adherence, follow-up completeness, and the sample-size justification are not reported in the registry record.

4. Participant Flow and Accounting

The registry reports how participants moved through each stage of the study. All counts below are exactly as posted; missing counts are not inferred from other stages.

Overall Study

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 DVT Incident Rate, measured over Up to 3 months.

The primary endpoint counts participants who develop deep vein thrombosis (DVT) during follow-up up to 3 months. DVT is a blood clot in a deep vein; the registry’s study description focuses on prevention of this complication after surgery for malignant tumors of the pelvis or lower extremity. The endpoint is expressed as a count of participants, rather than a continuous measurement. The stated comparison concerns the proportion of participants with DVT in the aspirin-plus-device arm versus the enoxaparin-plus-device arm.

For a prevention comparison, an event count makes the observed occurrence of the targeted complication explicit in each arm. The registry describes a one-sided question: whether the DVT rate with aspirin and a compression device is not more than the rate with enoxaparin and a compression device. That directional formulation is the stated objective; it does not by itself establish that the treatments are equivalent or that a difference of any particular size is acceptable.

The posted results list zero DVT events for Arm I and zero for Arm II. No confidence interval, test statistic, or p-value for the primary comparison is reported in the registry record. The event counts therefore describe what was recorded, while leaving the precision of the between-arm comparison unresolved.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

What does a two-proportion test address?

General statistical explanation: a two-proportion test compares the share of participants with an event between two groups. Here, the event is DVT, and the groups are the randomized medication-plus-device regimens. The registry describes the intended test as one-sided, meaning it focuses on the stated directional question: whether the aspirin-plus-device DVT rate is not more than the enoxaparin-plus-device rate. A test result is interpreted against a chosen significance threshold, which the registry states as p < 0.05.

A p-value summarizes how incompatible the observed data are with a specified null hypothesis under the assumptions of the test. It does not measure the probability that a treatment works, nor does it quantify the size or clinical importance of an effect. The registry does not report a p-value for the primary comparison, so the stated threshold cannot be used to claim a statistically significant result.

Why do the event counts matter?

General statistical explanation: binary outcomes are recorded as whether an event occurred or did not occur for each participant. The reported DVT counts are zero in both arms. When events are absent in both groups, the data do not supply a direct observed contrast in event counts. Statistical methods may handle such sparse data differently, and estimates can be imprecise. The registry says a two-arm binary comparison is not estimable from the posted results. It does not give details of a method that might overcome this limitation.

The same observed counts do not prove equal underlying event rates. A study can observe no events while uncertainty about the risk remains, especially when the number of participants is limited. Confidence intervals are one way to express this uncertainty around an estimate. No confidence interval for the primary comparison is reported in the registry record.

What does the stated test direction imply?

General statistical explanation: a one-sided test addresses a direction specified in advance, rather than treating departures in either direction as the focus of the test. The registry’s stated aim is that the DVT rate with aspirin and a compression device be not more than the rate with enoxaparin and a compression device. The record does not report an acceptable margin for a non-inferiority claim, nor does it provide a confidence interval or test result. Without those details, the stated objective should not be converted into a conclusion that the regimens are equivalent or that a clinically meaningful disadvantage has been excluded.

How should significance and precision be separated?

General statistical explanation: statistical significance and precision answer different questions. A significance test evaluates evidence against a specified null hypothesis under a model; precision describes how narrowly the effect is estimated. A result can be statistically inconclusive because the data are too sparse to distinguish between possibilities. Conversely, a p-value alone does not communicate the range of effects compatible with the data. The registry provides neither a primary p-value nor an interval estimate, so neither statistical significance nor precision can be assessed from the posted primary results.

What do the planning assumptions describe?

The registry’s planning description says its illustrations are based on the observed effect, assume two-sided significance level 0.05, equal allocation, and a normal approximation, and consider detecting a stated fraction of the observed effect. It also distinguishes event totals for time-to-event outcomes from participants per arm for other outcomes. These are planning assumptions, not observed-power results and not evidence about the trial’s original design justification. The record does not report the original sample-size justification. The primary endpoint description specifies a one-sided test, whereas the planning description specifies a two-sided significance level; the record does not explain how these descriptions relate.

General statistical explanation: sample-size planning asks how much information a future study might need under specified assumptions. It is not a measure of the probability that a completed study has found a real effect. An illustration based on an observed effect can be especially unstable when the observed outcome data are sparse. No numerical planning results are reported in the registry record.

10. Exact Estimates for Each Arm

No two-arm comparison is shown for this trial, but each arm's result can still be summarized with exact interval estimates computed from the posted counts. The Clopper-Pearson interval is exact and conservative; the Wilson interval is a common closer-to-nominal alternative; the Bayesian credible interval uses a uniform Beta(1, 1) prior. For small groups the three can differ noticeably.

R code and output

arm    <- c("Arm I (Acetylsalicylic Acid and PCD)", "Arm II (Enoxaparin and PCD)")
n      <- c(9, 3)
events <- c(0, 0)
for (i in seq_along(n)) {
  ci <- binom.test(events[i], n[i])$conf.int          # Clopper-Pearson exact interval
  cr <- qbeta(c(0.025, 0.975), 1 + events[i], 1 + n[i] - events[i])   # Bayesian, uniform Beta(1,1) prior
  wi <- (function(x, n) { p <- x / n; z <- qnorm(0.975); d <- 1 + z^2 / n
    c((p + z^2 / (2 * n) - z * sqrt(p * (1 - p) / n + z^2 / (4 * n^2))) / d, (p + z^2 / (2 * n) + z * sqrt(p * (1 - p) / n + z^2 / (4 * n^2))) / d) })(events[i], n[i])
  cat(sprintf("ARM|%s|%d|%d|%.1f|%.1f|%.1f|%.1f|%.1f|%.1f|%.1f\n", arm[i], n[i], events[i], 100 * events[i] / n[i],
              100 * ci[1], 100 * ci[2], 100 * wi[1], 100 * wi[2], 100 * cr[1], 100 * cr[2]))
}

ARM  Arm I (Acetylsalicylic Acid and PCD)  9  0  0.0  0.0  33.6  -0.0  29.9  0.3  30.8
ARM  Arm II (Enoxaparin and PCD)  3  0  0.0  0.0  70.8  0.0  56.1  0.6  60.2

11. Multiplicity, Missing Data and Analysis Populations

The registry lists six secondary outcomes: pulmonary embolism rate; development of other complications, including bleeding complications; hospital readmission; hematoma formation; excessive wound drainage; and death rate. Numerical results for these outcomes are not reported in the registry record. The record does not describe a plan for controlling multiplicity, such as adjusting interpretation when several outcomes are tested. It also does not report which outcome was designated for any particular inferential analysis beyond the stated primary DVT comparison.

Missing-data methods are not reported in the registry record. The posted arm counts and DVT event counts do not establish whether every randomized participant completed follow-up or had the outcome assessed through the full timeframe. The record does not report how missing or unevaluable observations were handled, or whether any participants withdrew or were excluded from outcome analyses.

Analysis populations are not reported in the registry record. In particular, the record does not specify whether the posted results use all randomized participants, participants who received treatment, or another analysis set. It also does not provide a participant flow or explain deviations from assigned treatment. These omissions limit assessment of potential bias and make it difficult to understand exactly which participants contributed to the reported counts.

The registry indicates zero posted analyses for this comparison and states that a two-arm binary comparison is not estimable from the posted results. This should be distinguished from proof that no analysis was performed; the record does not provide enough information to determine that. The reported counts can be described, but the completeness and analysis basis of those counts are not documented.

12. 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 serious-adverse-event figures list zero affected participants in Arm I, with 9 at risk, and zero in Arm II, with 3 at risk. The other-adverse-event figures also list zero affected participants in both arms, with the same at-risk counts. These are the posted counts; they should not be read as evidence that either regimen has no safety risks. The registry record does not give detailed definitions of serious and other adverse events, event ascertainment procedures, or confidence intervals for these figures.

The registry names bleeding complications, hematoma formation, excessive wound drainage, and death among the secondary outcomes. Numerical results for the named secondary outcomes are not reported in the registry record. Death counts are recorded as not available in the posted adverse-event information; that is different from a reported count of zero deaths. The registry also does not provide serious-event terms. The absence of listed terms should not be treated as proof that no particular event was assessed or could occur.

Safety counts based on a small enrolled sample can describe only the events recorded during the study; they cannot by themselves characterize the frequency of uncommon harms or establish comparative safety. The record does not report the duration or completeness of safety follow-up separately from the primary endpoint timeframe.

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

The registry describes a randomized comparison of aspirin plus pneumatic compression devices and enoxaparin plus pneumatic compression devices in patients undergoing surgery for pelvic or lower-extremity malignant tumors. The primary outcome is the count of participants with DVT up to 3 months. The posted result is zero DVT events in each arm, with 9 participants listed in the aspirin arm and 3 in the enoxaparin arm.

These results establish what the registry reports as observed counts; they do not establish equal DVT risks. They do not show that either regimen is superior, non-inferior, or equivalent. The stated one-sided testing objective and significance threshold do not substitute for a reported test result. The registry does not report a primary p-value or confidence interval, and it states that a two-arm binary comparison is not estimable from the posted results.

The small and unequal arm sizes limit the information conveyed by zero event counts. Without interval estimates or a reported analysis, the reader cannot tell how precisely the underlying risks have been estimated or what range of differences remains compatible with the observations. The findings also should not be extended beyond the stated surgical and tumor context without additional evidence.

The record names several secondary outcomes and reports zero serious and other adverse events in each arm. However, numerical findings for the named secondary outcomes are not reported, and the death counts are not available. The safety information therefore does not establish the absence of harm or a comparative safety advantage. In statistical terms, this record offers descriptive observations but does not provide enough inferential detail for a conclusive claim about comparative effectiveness or safety.

14. Why This Trial Matters Statistically

This trial addresses a statistical challenge common in clinical research: comparing treatments for an event that may be uncommon in a small study. Randomization is intended to support a fair comparison of assigned groups, but randomization alone does not ensure that a study has enough outcome information to estimate a difference precisely. In this record, the primary event count is zero in both arms, and the arm sizes are limited and unequal.

The result illustrates why descriptive counts, estimates, and uncertainty measures serve different roles. Counts report the events recorded. A comparative estimate describes the size and direction of a difference, while an interval conveys uncertainty about that estimate. A hypothesis test addresses evidence relative to a specified null and analysis assumptions. The registry supplies the counts and a description of an intended one-sided test, but no test result or interval estimate. Those omissions prevent the stated objective from being resolved statistically using the posted results.

The study also highlights the importance of distinguishing the treatment contrast from the broader care context. Both arms used pneumatic compression devices, so the randomized comparison concerns aspirin versus enoxaparin as the medication paired with the device. It is not a comparison of medication-plus-device care against no prophylaxis.

Finally, registration records are most informative when they connect planned outcomes and methods to complete results. Here, several secondary outcomes are named, but numerical results and analysis-population details are not reported in the registry record. This limits interpretation of both efficacy and safety. These points make the study a useful example of how transparent reporting of event counts, estimands, analysis methods, and uncertainty is necessary for sound statistical interpretation.

15. 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 primary comparison has few posted outcome details: the registry reports zero DVT events in each arm but no p-value, confidence interval, or comparative estimate. It explicitly indicates that a two-arm binary comparison is not estimable from the posted results. The limited arm sizes, particularly the 3 participants listed for enoxaparin, constrain what can be learned from the absence of recorded events.

The record does not report participant characteristics, eligibility criteria in detail, the randomization procedure, allocation concealment, adherence, treatment discontinuation, or follow-up completeness. It does not state how missing outcomes were handled or identify the analysis population. These omissions make it difficult to assess potential selection, performance, or attrition concerns and to establish how the posted counts were generated.

The named secondary outcomes lack numerical results in the registry record, limiting assessment of pulmonary embolism, bleeding and other complications, readmission, hematoma, wound drainage, and death. Adverse-event counts are reported without event definitions or detailed methods, and death counts are not available. The record also does not report multiplicity procedures or the original sample-size justification. Together, these reporting gaps leave uncertainty about both the scope of analyses performed and the completeness of outcome reporting.

16. Related Tutorials

17. Related Statistical Calculators

18. Sources

• ClinicalTrials.gov: Aspirin and Compression Devices for VTE Prophylaxis in Orthopaedic Oncology — NCT01696760.
• Registry record last updated 2018-12-05; status last verified 2018-11.
• Results first posted 2014-07-04.
• 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.

19. Record Summary

The registry describes a completed randomized, parallel-group study comparing acetylsalicylic acid plus a pneumatic compression device with enoxaparin plus a pneumatic compression device after surgery for pelvic and lower-extremity malignant tumors. Masking is listed as none. Enrollment is 12, and the primary DVT outcome is measured up to 3 months.

The posted primary counts are zero DVT events in Arm I, with 9 participants, and zero in Arm II, with 3 participants. The registry describes a one-sided two-proportion test and a significance threshold of p < 0.05, but reports no test result or confidence interval and says the binary comparison is not estimable from the posted results. Secondary outcomes are named, but their numerical results are not reported in the registry record. Serious and other adverse-event figures list zero affected participants in both arms; death counts are not available.