Comparison of Acetaminophen and PRP Therapy for Knee OA: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Comparison of Acetaminophen and PRP Therapy for Knee OA (NCT01782885), a randomized na study. The primary outcome is Change in Western Ontario and McMaster Universities Arthritis Index (WOMAC).

2. Clinical Question

The registry describes a controlled randomized study intended to compare PRP with acetaminophen. The treatment question concerns patient-reported pain, stiffness, and functional limitation as captured by WOMAC. The summary also says investigators planned to evaluate pain, knee function, and quality of life before treatment and at follow-up. A secondary outcome is change in Visual Analog Scale (VAS).

The primary outcome entry names change in WOMAC but gives a timeframe of 0 weeks. Its description also refers to evaluation at 0, 6, 12, and 24 weeks after treatment is finished. Those entries do not identify a single follow-up comparison in the posted results. The registry record reports no analyses, and the primary outcome has several categories or time points that were not reduced to one comparison. The clinical question can therefore be described, but an answer based on comparative efficacy estimates cannot be drawn from the posted results.

3. Trial Design

The registry lists this as a randomized, parallel-arm study with no masking. Its condition is knee osteoarthritis, and its summary describes the intended population as patients with mild knee osteoarthritis. The registry lists enrollment as 543 and status as completed. It lists the phase as NA. The listed sponsor is Hospital Universitario Dr. Jose E. Gonzalez.

The intervention arm received autologous PRP by three intra-articular knee injections, one every two weeks. The comparator arm received acetaminophen at 500 mg/8 hours while PRP treatment lasted. The registry describes acetaminophen as first-line treatment in the study summary. Because masking is listed as none, the study is open-label according to the registry entry; it does not describe blinding of participants or investigators.

The record lists a start date of 2013-05 and primary completion of 2015-08. These dates describe the registry timeline; they do not provide the timing of publication or analysis. Randomization is reported, but the method used to generate the allocation sequence and the method for concealing assignments before enrollment are not reported in the registry record. Details of eligibility criteria, treatment adherence, co-interventions, and follow-up completion are also not reported in the registry record.

The summary says that outcomes in pain, knee function, and quality of life were to be evaluated before treatment and at 6, 12, and 24 weeks of follow-up. It also describes in vitro experiments using human osteoarthritic cartilage explants treated with PRP for histological and gene-expression assays. These laboratory experiments are distinct from the randomized clinical comparison and do not substitute for patient-level clinical outcome results. The registry lists 9 outcome measures and 0 analyses. Consequently, the design is identifiable, but the posted record does not supply comparative clinical estimates or an account of how the planned clinical analyses were conducted.

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: Only patients meeting inclusion criteria were assigned to a group (radiographic grade I and II osteoarthritis, mainly). Importantly, patients with previous surgical interventions, with concomitant diseases or with Hb values of \<11 g/dL and platelet values of \<150,000/µL were excluded prior to group assignment.

Overall Study

Reasons for not completing this period:

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 Change in Western Ontario and McMaster Universities Arthritis Index (WOMAC), measured over 0 weeks.

The stated primary endpoint is change in the Western Ontario and McMaster Universities Arthritis Index (WOMAC), expressed in units on a scale and summarized using a mean and standard deviation. WOMAC is a patient-reported measure covering pain, stiffness, and functional limitation. The record describes five pain items, two stiffness items, and 17 functional-limitation items. It says that total WOMAC is formed by summing the items across the subscales, with a score range of 0-96. Higher scores indicate worse pain, stiffness, and functional limitations.

In general, a measure such as WOMAC is useful in a treatment comparison because it translates patients’ reported symptoms and limitations into a structured outcome that can be compared between study groups. Change scores are intended to represent movement in the measure over time; interpretation depends on knowing the measurement occasions, the direction of change, and the analysis used. A group mean alone would not show how varied individual scores were, which is why dispersion is also relevant.

The record’s timeframe field says 0 weeks, while its description specifies WOMAC evaluations at 0, 6, 12, and 24 weeks after treatment is finished. The summary separately describes assessments before treatment and at follow-up. The record does not identify which assessment occasion or contrast was used for a single primary comparison. It also does not report group means, changes, standard deviations, confidence intervals, or a test of the endpoint. Thus the instrument and its interpretation are described, but the comparative result is not reported in the registry record.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The registry describes a randomized, parallel-arm comparison and specifies that masking was none. It does not report the statistical methods used to analyze the clinical outcomes. The concepts below are general explanations of how such methods are commonly interpreted; they should not be read as a description of an analysis performed in this trial.

What does randomization contribute?

In general, random allocation is intended to make treatment groups comparable at the point of assignment, including for factors that are measured and factors that are not measured. This supports a fair comparison, but it does not guarantee that groups will be identical in a particular trial. The registry does not report how the random sequence was generated or concealed. Concealment concerns whether an assignment can be anticipated before a participant enters a group; masking concerns whether people know the assigned treatment after allocation. The record lists no masking, but that entry does not establish whether allocation was concealed.

What would a mean and standard deviation describe?

The primary outcome is a continuous score and the record identifies the mean as its summary parameter and standard deviation as its dispersion measure. In general, a mean describes the average score or average change in a group. A standard deviation describes how spread out individual values are around that mean. It is not the same as the uncertainty in the estimated mean: uncertainty depends on the data and analysis, and is commonly expressed using a confidence interval. None of the primary outcome means, standard deviations, or confidence intervals is reported in the registry record.

For a comparison of change, an analysis needs to define which measurements contribute and how change is calculated. The record describes WOMAC at 0, 6, 12, and 24 weeks after treatment is finished, while the primary timeframe field says 0 weeks. It does not report whether the comparison used change from baseline, scores at follow-up, a model combining repeated measurements, or another approach. These choices can target different quantities, so the specific estimand and analysis cannot be inferred from the endpoint name alone.

How should a confidence interval be understood?

In general, a confidence interval gives a range of values compatible with the observed data under the method’s assumptions. It helps readers assess the precision and plausible magnitude of an estimated contrast, alongside the point estimate. It does not describe the probability that a particular treatment effect is true. Because no interval or treatment estimate is posted for the clinical outcomes, their precision cannot be assessed from this record.

What do significance tests and power mean?

In general, a significance test evaluates how compatible the observed data are with a specified null hypothesis, given the test assumptions. A p-value does not measure the size or clinical importance of an effect. The record does not report an analysis p-value or a statistical analysis plan, so neither the test nor its interpretation is available.

The record also describes a sample-size planning illustration based on the observed effect, with a two-sided significance level of 0.05, equal allocation, and a normal approximation; the intended effect is a stated fraction of the observed effect. Such an illustration is a planning calculation, not observed power and not evidence that a particular effect was found. The record says that non-time-to-event illustrations are expressed as participants per arm. The numerical planning results are not reported in the registry record, and the illustration is explicitly not the trial’s own design. The trial’s sample-size justification is not reported in the registry record.

Why can repeated measures require additional care?

In general, measurements from the same participant over time are related, rather than independent. A repeated-measures analysis can account for this relationship and use the trajectory across visits, but its interpretation depends on the model, the selected time points, and the treatment contrast. If many time points or outcomes are examined, the chance of at least one seemingly notable result can increase. Methods for addressing this multiplicity may include prespecifying a primary contrast or adjusting inference across comparisons. The registry does not report whether or how these methods were used.

General statistical methods can help explain what results would mean, but they cannot fill in absent estimates. The posted record does not provide the model, the variance assumptions, the handling of missing values, an analysis population, or the clinical outcome results. Any claims about those analytic choices would go beyond what the registry record reports.

10. Multiplicity, Missing Data and Analysis Populations

The record lists 9 outcome measures, including change in WOMAC as the primary outcome and change in VAS as a secondary outcome. The primary outcome description refers to assessments at 0, 6, 12, and 24 weeks after treatment is finished, while its timeframe entry says 0 weeks. The record also notes that the primary outcome has several categories or time points and was not reduced to one comparison. This leaves the primary comparison and its timing unclear in the posted results.

In general, evaluating several outcomes or time points creates a multiplicity issue: the more comparisons considered, the greater the chance that at least one result may appear notable by chance. Statistical plans can address this by specifying a primary contrast in advance or by using appropriate adjustments. The registry record does not report whether a multiplicity strategy was prespecified or applied. The analysis plan and the hierarchy among outcome measures are not reported in the registry record.

Missing-data methods matter because participants without an observed outcome may differ from those with complete follow-up. In general, an analysis may make assumptions about missing values, use a model that accommodates incomplete repeated measurements, or apply another prespecified approach. The registry record does not report outcome completion, reasons for missingness, imputation, or sensitivity analyses. It also does not report whether the efficacy analysis followed an intention-to-treat, per-protocol, or other population definition.

The record lists enrollment as 543, but it does not report the number contributing to each clinical outcome analysis. The safety denominators should not be used to infer efficacy analysis populations. These reporting gaps limit assessment of the risk of selective outcome reporting, the effects of missing data, and the extent to which the randomized comparison was preserved in analysis.

11. 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 5% reporting threshold.

Most frequent non-serious adverse event terms

The registry’s safety entries list 32 participants at risk for serious adverse events in the acetaminophen group and 33 in the PRP group. The serious adverse-event counts are 0 for each listed group. For other adverse events, the listed at-risk counts are 32 for acetaminophen and 33 for PRP; the affected counts are 0 and 4, respectively. The record lists no top serious adverse-event terms.

These figures should be read as the specific safety entries recorded, not as a complete safety profile. The registry does not define the “other” adverse-event category in the information, describe event types, or report event severity and timing. It also does not explain how the safety denominators relate to the enrollment entry of 543. The deaths field is null for both groups; null is not a reported count of zero.

In general, an event count alone does not establish that a treatment caused an event. A comparative safety assessment also depends on consistent event definitions, duration of observation, completeness of reporting, and the population at risk. The registry does not report a statistical comparison of adverse events. The listed figures therefore describe what was entered in the safety record, but do not by themselves establish a causal or comprehensive safety conclusion.

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

The registry record establishes that investigators registered a randomized, parallel-arm, unmasked comparison of PRP injections and acetaminophen for knee osteoarthritis. It identifies WOMAC change as the primary outcome and VAS change as a secondary outcome. It describes the intervention schedules, the WOMAC domains and scale, and planned follow-up. These details help define the intended clinical question and how an outcome was to be measured.

The record does not report an efficacy analysis. It lists 0 analyses, and no arm-specific WOMAC or VAS estimates, between-arm contrasts, confidence intervals, or p-values are given. The primary outcome also refers to multiple time points without identifying a single analyzed comparison. Consequently, the posted information does not establish the magnitude or direction of an observed clinical difference, or how precisely one was estimated.

The registry summary’s assertion about PRP effectiveness and its description of possible biological mechanisms are not substitutes for comparative outcome results. The in vitro work on cartilage explants is laboratory research, not a reported clinical treatment estimate. Likewise, completed status indicates the trial is marked completed in the record; it does not tell readers what the clinical findings were.

In general, a statistically detectable difference and a clinically meaningful difference are distinct questions. Judging either requires a defined contrast and observed results, with uncertainty and adequate context. Those elements are not reported in the registry record. The safety entries provide limited event counts, but the record does not establish causation or furnish a full comparative safety analysis. The appropriate reading is therefore descriptive: the record documents the trial structure and some safety data, while leaving the clinical efficacy comparison unanswered.

13. Why This Trial Matters Statistically

This trial is statistically informative as an example of why a registered design and a reported result are not interchangeable. Randomization describes the intended basis for a comparative inference, while the clinical question ultimately depends on reported outcome data and a clearly defined analysis. In this record, the trial is marked completed, yet no analyses are listed and the primary outcome does not resolve into one reported comparison.

The outcome definition also illustrates why time points matter. WOMAC is described as a multi-domain score, and the registry refers to several assessment times. A change at a particular follow-up, an average trajectory, and an overall treatment-by-time comparison can answer different questions. Without a designated contrast and results, readers cannot know which question was analyzed or how precisely it was answered.

The entry also distinguishes clinical outcomes from mechanistic or laboratory claims. The summary describes cartilage explant experiments and a biological rationale, but those do not replace evidence from patient outcomes. Statistical interpretation requires linking the population, intervention, comparator, outcome, timing, and analysis to the reported estimate.

Finally, the record highlights the role of transparent reporting. Information about missing data, analysis populations, multiplicity, and safety definitions helps readers judge how much confidence to place in a result. When these details and results are absent, the right conclusion is not to infer a positive or negative finding, but to recognize the limits of what can be learned from the registry entry.

14. Limitations

• The posted summary statistics are the only data these results rest on; patient-level data, full protocols and statistical analysis plans are not available.
• Recalculated results are simple, unadjusted calculations from posted summaries. They do not replace the sponsor's prespecified analysis.

The main limitation for interpretation is that the registry record does not report clinical outcome analyses. Although the trial is marked completed and the record lists outcome measures, there are no posted comparative estimates for WOMAC or VAS. This prevents assessment of effect direction, size, and precision.

The primary endpoint information is not fully aligned: its timeframe is listed as 0 weeks, while the description refers to WOMAC evaluation at 0, 6, 12, and 24 weeks after treatment is finished. The summary also describes assessments before treatment and during follow-up. The record does not specify which time point or contrast was designated for the primary analysis.

Details needed to evaluate conduct and analysis are not reported in the registry record. These include eligibility criteria, random-sequence generation, allocation concealment, adherence, attrition, missing-data handling, the analysis population, and the statistical analysis plan. The record does not describe whether multiplicity was addressed across outcomes or time points. The relation between listed safety denominators and overall enrollment is also not explained.

Safety information is limited to event counts and at-risk entries. The record does not give event descriptions for the other adverse events, establish causality, or report a statistical comparison. The deaths fields are null rather than numerical counts. The registry’s enrollment figure should not be treated as the number analyzed for each outcome, because outcome-specific analysis counts are not reported.

These limitations concern what the registry record reports, not proof that particular procedures were or were not followed. Additional trial documents or results could contain details absent from the registry entry, but those details are not reported in the registry record.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Comparison of Acetaminophen and PRP Therapy for Knee OA — NCT01782885.
• Registry record last updated 2015-11-20; status last verified 2015-10.
• Results first posted 2015-11-20.
• Data retrieved from ClinicalTrials.gov on 2026-11-09.
• Group sizes, means, event counts and sponsor-reported p-values are reproduced as posted. Effect estimates, confidence intervals and p-values not attributed to the sponsor are independently calculated from the posted summary statistics.

18. Record Summary

The registry entry describes a completed, randomized, parallel-arm, unmasked trial of PRP injections versus acetaminophen for knee osteoarthritis. Enrollment is listed as 543. The intervention consists of three autologous PRP injections, one every two weeks; the comparator is acetaminophen at 500 mg/8 hours while PRP treatment lasts.

The primary outcome is change in WOMAC, a measure of pain, stiffness, and functional limitation. The record also lists change in VAS as a secondary outcome and describes follow-up assessments. It lists 9 outcome measures and 0 analyses, and does not report comparative clinical outcome estimates. Safety entries list no serious adverse events in either arm and 4 other adverse events in the PRP group, with no such events listed for acetaminophen. Further efficacy, missing-data, analysis-population, and statistical-method details are not reported in the registry record.