Reciprocating Medical Devices - a Study of a New Safety Device: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Reciprocating Medical Devices - a Study of a New Safety Device (NCT00651625), a randomized na study. The primary outcome is Pain Using VAS (Visual Analogue Pain Scale).

2. Clinical Question

The trial addressed a practical device comparison in procedures undertaken for usual care. The registry describes the RPD as a safety syringe and presents its intended design goals, including improved needle control and reduced tissue trauma. Those goals are hypotheses, not outcome findings. The primary result concerns reported pain-score change; it does not by itself establish differences in every aspect of device safety, procedure performance, or patient experience.

The conditions listed span different clinical settings, but the registry does not report how many procedures or participants belonged to each condition, or how results varied across them. The record also describes RPD use with and without ultrasound guidance without giving subgroup results. Consequently, the central comparison is between the randomized groups as reported, rather than a separate answer for each condition, procedure, or guidance approach.

3. Trial Design

The registry identifies NCT00651625 as a completed, randomized, parallel-group trial sponsored by the University of New Mexico. The posted enrollment is 437. The registry lists the phase as NA and describes masking as single; whose treatment assignment was masked is not reported in the registry record. The reported arm sizes are 219 for the conventional syringe and 218 for the RPD syringe.

The comparison was between the conventional syringe and the reciprocating procedure device in syringe-and-needle procedures. The RPD arm description includes use with and without ultrasound guidance. The registry does not explain how participants were assigned to those guidance approaches, whether guidance was randomized, or how guidance was handled in the comparison. The brief description says the study would randomize people requiring and assenting to a procedure to a conventional syringe or the RPD, with an RPD arm with and without ultrasound guidance.

There is a difference between the brief description's stated plan of 820 subjects over a five-year period and the posted enrollment of 437. The registry does not explain this difference, so the posted enrollment should not be read as evidence about why the target and enrollment differ. The record lists a start date of 2004-11 and a primary completion date of 2010-01. It does not report details of recruitment, exclusions, allocation concealment, adherence, or participant flow.

The primary outcome was pain-score change at two weeks. Secondary outcomes include aspirated fluid volume during the procedure, physician satisfaction during the procedure, and adverse outcomes. The record gives group summaries for the primary outcome and some secondary outcomes, but it does not provide a full analysis plan or detailed methods for handling the different procedures and guidance approaches.

4. Participant Flow and Accounting

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

Before assignment: patients recruited immediately at point of contact

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 Pain Using VAS (Visual Analogue Pain Scale), measured over 2 weeks.

The primary endpoint is the difference between the visual analogue pain score at two weeks and the score at the preprocedure assessment, defined as the two-week score minus the score at zero weeks. The scale runs from 0 to 10 cm, with 0 meaning no pain and 10 the worst pain imaginable. Because the measure is a difference from baseline, a positive value represents a score that is higher at two weeks than before the procedure; a negative value represents a lower score.

Using a change score can describe how pain at follow-up compares with each participant’s own starting point, rather than reporting only the follow-up score. It is a continuous outcome, so the registry reports group means and standard deviations. The comparison of mean changes summarizes the average contrast between groups, while the spread describes how variable individual scores are around each group mean. Neither summary describes every participant’s experience.

The endpoint is relevant to the registry’s question about procedural pain and subsequent pain, but it is limited to the specified assessment at two weeks. The record also mentions pain during the procedure and at six months in an arm description, but the primary outcome is the two-week change. Results for the primary endpoint do not answer whether pain differed during the procedure or at six months.

7. Results Posted in the Registry

• 2RPD Syringe: mean 6.20 versus 5.20 on control; difference 1.00 (95% CI 0.47 to 1.53), standardized effect d = 0.36; z-test p = <0.001.

8. Secondary Efficacy Endpoints

The registry also posts results for secondary outcomes. For each outcome below, the endpoint type, the test applied to the posted summary statistics, the result and its interpretation are given in turn. Secondary endpoints are usually not adjusted for multiple testing, so they support rather than confirm the primary finding.

Aspirated Fluid Volume

• Endpoint type: Continuous: a mean value per participant, compared as a difference in means.
• Test: Unadjusted two-sample z-test for the difference in means, using the posted standard deviations.
• Result:
  • 2RPD Syringe: mean 14.50 versus 7.20 on control; difference 7.30 (95% CI 6.62 to 7.98), standardized effect d = 2.01; z-test p = <0.001.
• Interpretation:
  • The 95% confidence interval excludes no difference, so the mean is statistically higher for 2RPD Syringe than for the control. This is an unadjusted, secondary comparison, so it is suggestive rather than confirmatory.

Physician Satisfaction

• Endpoint type: Continuous: a mean value per participant, compared as a difference in means.
• Test: Unadjusted two-sample z-test for the difference in means, using the posted standard deviations.
• Result:
  • 2RPD Syringe: mean 8.50 versus 6.40 on control; difference 2.10 (95% CI 1.60 to 2.60), standardized effect d = 0.79; z-test p = <0.001.
• Interpretation:
  • The 95% confidence interval excludes no difference, so the mean is statistically higher for 2RPD Syringe than for the control. This is an unadjusted, secondary comparison, so it is suggestive rather than confirmatory.

The secondary outcomes point in different directions relative to the primary pain-change result. The RPD group has higher posted averages for aspirated fluid volume and physician satisfaction, while its primary pain-change average is also higher. These measures capture distinct aspects of the procedure and should not be collapsed into a single overall judgment about the device. A difference on one measure does not determine the result on another.

The secondary outcomes address procedure output and physician experience, whereas the primary outcome concerns participant pain change at two weeks. Their relationship is therefore contextual rather than confirmatory: secondary findings can help describe the broader pattern, but they do not replace the primary endpoint. The registry also lists adverse outcomes separately; those safety figures are not a substitute for a formal analysis of the other outcomes.

Unadjusted secondary results carry less weight because multiple outcomes and comparisons create more opportunities for apparently notable findings to occur by chance. General explanation: multiplicity adjustment or a prespecified hierarchy can help control the chance of false-positive conclusions across a family of tests. The registry does not report such a strategy. The secondary outcome results are therefore best treated as descriptive or exploratory unless a more complete analysis plan establishes otherwise.

There is also uneven reporting across outcomes. A comparison is posted for aspirated fluid volume without a reported analysis entry; for physician satisfaction, the registry lists both a summary comparison and a separate t-test result. The record does not explain the difference between those entries. This limits how strongly the secondary outcomes can be used to support a single overall interpretation. The secondary pattern may motivate further study, but it cannot establish that every intended device advantage was achieved.

Secondary outcomes registered for this trial, for reference (only those above were recalculated):

• Aspirated Fluid Volume
• Adverse Outcomes (Hemorrhage, Infection, Hematoma, Extravasation)
• Physician Satisfaction

9. Statistical Interpretation

10. Statistical Methods Explained

These results concern a continuous measure: a pain-score change on a 0-to-10 cm scale. For a continuous outcome, the mean gives a group’s average, and the standard deviation describes the spread of observations around that mean. A mean difference compares the group averages in the stated order, here RPD minus conventional syringe. General explanation: a positive mean difference means the first-named group has the higher average measured value; what that means clinically depends on how the endpoint is defined.

What does randomization contribute?

Random allocation is intended to make the groups comparable, on average, with respect to factors that could affect outcomes. It does not guarantee that the groups are identical, and it cannot by itself resolve problems such as missing outcome measurements, deviations from assigned treatment, or outcome measurement issues. The registry describes the design as randomized, but the sequence-generation and allocation-concealment methods are not reported in the registry record. The analysis population and handling of treatment changes are also not reported in the registry record.

How should the mean difference be read?

The primary contrast is reported as a mean difference of 1.0 for RPD versus conventional syringe. Because the outcome subtracts the preprocedure score from the two-week score, the sign and size refer to differences in average change, not to a difference in the groups’ raw two-week scores alone. The record provides group means and standard deviations, as well as an interval estimate and an unadjusted p value. It does not identify the primary test method, state whether the comparison adjusted for baseline values, or give a model specification.

What does a p value say?

General explanation: a p value evaluates how unusual the observed data, or more extreme data, would be under a specified null hypothesis and statistical model. A small p value is not the probability that the null hypothesis is true, and it does not measure the size or practical importance of a difference. The primary comparison’s unadjusted p value is < 0.001. The registry does not report enough detail to identify all assumptions used to obtain that value, so it should be interpreted alongside the estimated difference and confidence interval rather than on its own.

Why do spread and variability matter?

Participants can have quite different pain changes even when their group means are close. Standard deviations provide a measure of this within-group variability, but they do not show the shape of the score distribution or reveal whether there are unusual values. General explanation: when data are more variable, an estimate of a group contrast is commonly less precise if other features are unchanged. If outcomes are strongly skewed, bounded, or clustered by procedure or clinician, a simple mean comparison may not capture every feature of the data. The registry does not report distributional checks, clustering adjustments, or alternative analyses.

General explanation: over-dispersion occurs when observed variability exceeds what a statistical model expects. In that situation, a model that assumes less variability can understate uncertainty and make results appear more precise than warranted. The primary outcome is continuous and the record does not identify a count model, an over-dispersion assessment, or a specific model for the outcome. This concept is therefore general context, not a claim about an analysis the trial performed.

How should the secondary analysis entries be understood?

The registry gives comparison summaries for aspirated fluid volume and physician satisfaction, but does not list a reported analysis for aspirated fluid volume. For satisfaction, it lists a two-sided t-test with a p value of 0.05. The separately posted unadjusted summary comparison for satisfaction gives a p value < 0.001. These are distinct entries in the record; the record does not explain their relationship or provide details that would reconcile them. They should not be treated as interchangeable results. The registry does not report methods for adjusting secondary comparisons for multiple testing.

The terms “recalculated” and “planning illustration” refer to calculations based on posted summary statistics, not a substitute for a full trial analysis. The sample-size illustrations use stated planning assumptions and the observed effect as their starting point. They do not reveal what the investigators originally planned. The original sample-size justification, trial-specific assumptions, interim analyses, and complete statistical analysis plan are not reported in the registry record.

11. Confidence Intervals and Precision

The primary mean difference is 1.0, with a confidence interval from 0.475 to 1.525. General explanation: a confidence interval gives a range of estimates compatible with the observed data and the method’s assumptions. It describes uncertainty in the estimated group contrast, not the range within which every participant’s outcome lies, and it does not guarantee that the true value is inside the interval.

The interval does not include zero, so the posted estimate is separated from no average difference under the interval calculation used. Its endpoints are positive, consistent with a higher average change in the RPD group under the definition of RPD minus conventional syringe. The registry does not identify the method used to calculate the primary interval, so the interpretation should not imply details about the model or adjustments that are not recorded.

The group standard deviations, 2.7 and 2.9, indicate individual outcomes vary around their respective means. They are not confidence intervals and should not be read as uncertainty about the means. The comparison’s confidence interval addresses precision of the mean difference, whereas the standard deviations describe dispersion among observations. These answer different questions.

Precision is not the same as importance. The confidence interval quantifies statistical uncertainty but does not establish whether a difference is important to participants or clinicians. The registry does not report a threshold for a clinically meaningful pain-score change. It also does not provide subgroup intervals, intervals for pain during the procedure, or a six-month primary comparison. Thus, the primary interval supports interpretation only of the reported two-week mean change contrast.

12. 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 illustrations based on the observed effect, not the original trial’s design calculation. The trial’s own sample-size assumptions and justification are not reported in the registry record. The listed planning assumptions are a two-sided significance level of 0.05, equal allocation between the two arms, a normal approximation, and an effect to detect set to the stated fraction of the observed effect. These figures are not observed power.

For the primary outcome, planning for 100% of the observed effect gives 124 participants per arm for 80% power and 165 participants per arm for 90% power. Planning for 75% of the observed effect gives 220 participants per arm for 80% power and 294 participants per arm for 90% power. Planning for 50% of the observed effect gives 493 participants per arm for 80% power and 660 participants per arm for 90% power.

In plain terms, the 80% power column represents the per-arm sample size estimated to give an 80% chance of detecting the specified effect if the planning assumptions hold. The 90% column uses a higher target chance of detecting that effect, and therefore calls for more participants in these illustrations. These are prospective planning quantities, not a measure of how likely the completed trial was to detect its result after the data were observed.

The smaller the effect targeted for detection, the more participants are generally needed when the other assumptions stay the same. That is why the figures increase when the assumed effect is reduced from the full observed effect to 75% and then 50%. The calculations depend on the observed estimate, which itself is uncertain, and on assumptions that may not match the trial’s actual design or analysis. They should be read as planning scenarios, not as proof that the trial was adequately or inadequately sized.

13. 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 = 1.000; pooled standard deviation σ = 2.802.
2. n = 2 × (1.96 + 0.842)² × 2.802² / 1.000² = 123.2, rounded up to 124 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 <- 1                      # observed difference in means
sigma <- 2.80155539721744                     # 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(651625)
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 = 124 (80% power), 165 (90% power)
 75% of observed effect: n per arm = 220 (80% power), 294 (90% power)
 50% of observed effect: n per arm = 493 (80% power), 660 (90% power)
power.t.test: 124.2 
Simulated power at n = 124 per arm: 0.796 (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.

14. Multiplicity, Missing Data and Analysis Populations

Several outcomes appear in the record: the primary pain-change measure, aspirated fluid volume, physician satisfaction, and adverse outcomes. The record does not report a multiplicity strategy, such as a hierarchy for testing outcomes or an adjustment for making several comparisons. Without such a strategy, unadjusted secondary p values do not account for the chance of false-positive findings across multiple tests. They should consequently be interpreted more cautiously than a prespecified primary outcome.

The registry provides a primary comparison and secondary summaries, but does not report whether all planned outcomes or analyses are included. It does not identify prespecified subgroup analyses, interim analyses, or a complete statistical analysis plan. The absence of those details does not show that such procedures were or were not used; those details are not reported in the registry record.

Missing-data information is also limited. The record does not report how many pain assessments were missing, how missing outcomes were handled, or whether participants were analyzed according to randomized assignment. It does not describe an intention-to-treat or per-protocol population, or give participant flow through the trial. Without these details, readers cannot assess whether the reported group summaries include all randomized participants or how incomplete follow-up might affect the comparison.

The record reports arm sizes for the posted outcome summaries, but does not explain how those analysis counts were determined or whether outcome-specific denominators differed. The safety summaries give numbers at risk for each group, but that does not answer missingness questions for the pain endpoint. The brief study description’s planned enrollment and the posted enrollment also differ, with no explanation recorded. These gaps limit assessment of prespecification, analysis populations, and the completeness of outcome reporting; they are not evidence on their own that the analyses are biased.

15. 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.

Most frequent non-serious adverse event terms

The registry reports zero serious adverse events in the conventional syringe group, with 219 participants at risk, and zero in the RPD group, with 218 participants at risk. It also reports zero other adverse events in each group, using the same respective numbers at risk. The serious adverse event terms list is empty. Death counts are not reported in the registry record.

These figures describe what was recorded in the registry, not proof that no adverse event could occur. When no events are recorded, the summaries cannot show the range or frequency of possible event types beyond what the registry lists, and they do not establish equivalence between devices. The record does not give event definitions, follow-up duration for safety, severity grading beyond the serious-versus-other grouping, or details about how events were collected. The adverse-outcome figures should therefore be read as reported counts, with those reporting limits in mind.

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

The primary result describes a higher mean two-week pain-score change in the RPD group than in the conventional syringe group. The reported difference is 1.0, and its confidence interval lies above zero. This is evidence about the average of the specified change score in the reported groups. It is not a statement that every participant’s pain changed in the same direction or by the same amount.

The finding does not establish whether the difference is clinically important. The registry does not report a threshold for a meaningful change in this scale, nor does it provide participant-level outcomes or an account of how participants valued the difference. Statistical evidence against a zero difference and clinical importance are related but separate questions.

The result also does not settle every question raised by the trial’s rationale. It does not establish effects on pain during the procedure or at six months, and does not by itself answer whether the device changes bleeding, infection, hematoma, extravasation, or other outcomes. The safety summaries record no serious or other adverse events in either group, but they do not establish that the devices have identical safety profiles.

The registry’s descriptions of intended device advantages are design aims, not conclusions from the primary endpoint. A reader should distinguish those aims from observed results and avoid extending the two-week mean comparison to all procedures, conditions, patients, or ultrasound-guidance circumstances. The record does not report separate effects for these subgroups. The most bounded interpretation is that the posted data compare average change in pain score between the two reported groups at the specified follow-up, with uncertainty represented by the confidence interval.

17. Why This Trial Matters Statistically

This trial illustrates why a randomized device comparison needs to be interpreted through its defined endpoint, not just the device’s proposed mechanism. The RPD was designed with intended benefits in needle control and tissue trauma, while the primary outcome was a patient-reported pain-score change. A mechanism or design rationale can motivate a trial, but only outcome data address the measured question.

It also highlights the value of separating average effects from individual variability. The means summarize group-level results, the standard deviations describe spread, and the confidence interval conveys uncertainty in the contrast. Those quantities answer different statistical questions. Reading them together gives a more informative account than relying on a p value alone.

The record demonstrates a second important issue: primary and secondary outcomes do not have the same inferential status. Secondary endpoints can enrich the interpretation, but multiple comparisons and incomplete reporting make them less conclusive when no testing strategy is described. Differences between separate registry analysis entries also require care rather than selective emphasis.

Finally, the distinction between planning calculations and trial design assumptions matters. A planning illustration based on an observed effect cannot reconstruct the investigators’ original rationale or quantify the completed study’s observed power. The difference between the brief description’s intended enrollment and posted enrollment also cannot be explained from the record. These are common interpretive boundaries in registry-based analysis: clearly reported results can be described, while missing design and analysis details should remain unresolved.

18. 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 population and procedures are broad in description. The registry lists several conditions and describes syringe-and-needle procedures, but does not report how participants or procedures were distributed across those categories. The results therefore do not establish whether the comparison was similar across different conditions or procedure types.

The RPD arm includes use with and without ultrasound guidance, but the registry does not report how these approaches were assigned or provide separate results. That makes it difficult to distinguish device effects from any role of guidance. The record also does not specify who was masked, how outcomes were assessed, or whether the people recording pain scores knew the assigned device.

The posted enrollment of 437 differs from the brief description’s plan for 820 subjects. The registry does not explain the difference. Participant flow, outcome-specific missingness, analysis populations, allocation methods, and handling of missing data are not reported in the registry record. These omissions prevent a full assessment of attrition and analytic choices.

The primary comparison is presented without a reported test method or detailed model, and the record does not state a clinical-importance threshold. Secondary findings lack a reported multiplicity strategy; the satisfaction entries also include differing p values, without an explanation of their relationship. Finally, the safety summaries do not include death counts, event definitions, or detailed collection and follow-up methods. These are limitations of what can be concluded from the registry record, not proof that the underlying trial lacked procedures for addressing them.

19. Related Tutorials

20. Related Statistical Calculators

21. Sources

• ClinicalTrials.gov: Reciprocating Medical Devices - a Study of a New Safety Device — NCT00651625.
• Registry record last updated 2016-03-02; status last verified 2016-02.
• Results first posted 2016-03-02.
• 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.

22. Record Summary

NCT00651625 is a completed randomized parallel-group comparison of the reciprocating procedure device and a conventional syringe for syringe-and-needle procedures. The registry reports enrollment of 437 and identifies the primary endpoint as the two-week change in visual analogue pain score from the preprocedure score.

The posted primary mean difference is 1.0 for RPD versus conventional syringe, with a confidence interval from 0.475 to 1.525 and an unadjusted p value < 0.001. The mean change is higher in the RPD group. Secondary outcomes include aspirated fluid volume and physician satisfaction, with adverse outcomes also reported. The record does not provide a full analysis plan, missing-data account, multiplicity strategy, or explanation for the difference between the brief enrollment plan and posted enrollment.