X-3 Polyethylene Survival Outcomes Study: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for X-3 Polyethylene Survival Outcomes Study (NCT00493792), a randomized na study. The primary outcome is Prosthesis Survivorship Free of Revision.

2. Clinical Question

3. Trial Design

This was a randomized, parallel-group clinical trial with single masking. The registry records an enrollment of 518 and lists Mayo Clinic as the sponsor. The trial compared X3 polyethylene with N2Vac polyethylene when each was used in the Triathlon posterior stabilized total knee system. The two arms are labelled Treatment and Control in the registry, respectively; their posted sample sizes were 264 and 254.

The primary outcome concerned prosthesis survival free of revision from operation through 10 years post-operative. The registry also lists outcomes covering survivorship free of reoperation and complications, Knee Society Scores, physical and mental health scores from the SF-12, and radiographic success or failure. It lists eight outcome measures and three analyses, although details of the statistical plan for the secondary outcomes are not reported in the registry record.

The trial status is completed. The recorded start is 2007-02 and the primary completion date is 2023-11-01. The registry record does not report details of sequence generation, allocation concealment, the masking procedures, or the timing and conduct of follow-up visits. The description of single masking does not identify which people were masked. The record also does not specify whether the primary analysis followed an intention-to-treat principle or another analysis population. These omissions limit how fully the design and analysis can be reconstructed from the registry information.

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: Patients were recruited from three academic, tertiary-care referral centers within one healthcare network. Patients approached for enrollment received a primary unilateral total knee arthroplasty (TKA) for a diagnosis of osteoarthritis (OA), avascular necrosis (AVN) or posttraumatic arthritis.

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.

Population: Patients were recruited from three academic, tertiary-care referral centers within one healthcare network. Patients approached for enrollment received a primary unilateral TKA for a diagnosis of osteoarthritis (OA), avascular necrosis (AVN) or posttraumatic arthritis. Patients were randomized into the control or treatment group. A Modified Intent-to-Treat (MITT) population was used.

6. Primary Endpoint

The registry lists the primary outcome as Prosthesis Survivorship Free of Revision, measured over From operation to 10 years post-operative.

The primary endpoint is prosthesis survivorship free of revision: whether a knee prosthesis remains without revision after the operation, assessed through 10 years post-operative. The registry reports the outcome as a percentage of prosthesis survivorship and gives a 95% confidence interval for each arm. The posted values are 96.0 for N2Vac, with a confidence interval from 93.0 to 98.0, and 97.0 for X3, with a confidence interval from 94.0 to 99.0.

Revision is a clinically relevant event for evaluating the durability of an implanted prosthesis. A time-to-event approach can account for when revisions occur and for participants whose observation ends before a revision is recorded. In general, a survival estimate describes the proportion remaining free of the event over time, while its confidence interval conveys uncertainty around that estimate. The registry does not report the detailed event and censoring data or the exact method used to calculate the arm-specific percentages and intervals.

The endpoint is specifically freedom from revision, not freedom from every subsequent procedure, complication, or symptom. Reoperation and complications are listed separately as secondary outcomes. Their presence in the registry does not make them part of the primary endpoint. The endpoint definition therefore gives a focused measure of revision-free prosthesis survival, while leaving other aspects of patient experience to other outcomes.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The primary outcome is a time-to-event outcome: the event of interest is revision, and the reported survival measure is the probability of remaining free of revision over time. In general, time-to-event methods are useful when follow-up duration can vary and when some participants have no recorded event before observation ends. Such participants can contribute information up to their last known follow-up through censoring. The registry does not provide the individual event times, censoring information, or a detailed description of the method used for the arm-specific survival percentages.

What does a confidence interval communicate?

A confidence interval expresses uncertainty around an estimated quantity under the assumptions of the statistical method. For the arm-specific estimates, the reported 95% intervals are 93.0 to 98.0 for N2Vac and 94.0 to 99.0 for X3. These are ranges of values compatible with the data and method at the stated confidence level; they are not guarantees that the true value lies in the interval. The extent of overlap between separate arm intervals is not itself a formal test of the between-arm difference. The registry reports a separate model-based comparison for that purpose.

What does the Cox regression estimate?

Cox regression is a commonly used method for comparing time to an event. It models the relative event hazard over follow-up while allowing the baseline hazard to remain unspecified. The posted Cox regression result is a hazard ratio of 0.72, with a 95% confidence interval from 0.29 to 1.77 and p = 0.47. A hazard ratio compares event rates at a given time under the model; it is not a ratio of the percentages who experienced revision by the end of follow-up. The interval includes the no-effect reference value, and its breadth indicates uncertainty in the estimated relative hazard. The registry does not report the covariates, model diagnostics, proportional-hazards assessment, or group coding. Those details matter for understanding the estimate and its direction.

How should the p-value be read?

In general, a p-value describes how unusual data at least as incompatible with a specified null hypothesis would be if that hypothesis and the model assumptions were true. The posted value is 0.47. It should not be read as the probability that the null hypothesis is true, as the probability that the finding is due to chance, or as a measure of clinical importance. The record labels the analysis as a superiority comparison, but the trial's prespecified significance threshold and detailed analysis plan are not reported in the registry record. The significance level of 0.05 appears in the planning assumptions for sample-size illustrations; it should not be assumed to be the trial's analysis threshold.

Why are survival and hazard summaries different?

An arm-specific survival percentage describes the estimated proportion remaining free of revision by a specified time. The Cox hazard ratio instead summarizes a relative event hazard across follow-up under model assumptions. Because these measures answer related but distinct questions, the Cox ratio cannot be recovered simply by comparing the two 10-year percentages. A hazard ratio also depends on event timing, not only on whether a revision occurred before the endpoint. The registry does not report enough detail to reproduce the Cox model from the arm summaries.

What can the planning illustrations establish?

In general, sample-size planning illustrates how the required study size depends on assumptions about the effect to detect, significance level, allocation, and statistical method. The registry's planning assumptions describe a two-sided significance level of 0.05, equal allocation, a normal approximation, and effects defined as fractions of the observed effect. For time-to-event outcomes, the illustrations are specified in total events; for other outcomes, they are specified as participants per arm. These are planning illustrations, not observed power and not proof that the trial was designed around those assumptions. The planning values and the trial's own sample-size justification are not reported in the registry record.

Overall, the analysis combines arm-specific survival estimates and a model-based time-to-event comparison. The registry's limited reporting means that a reader can interpret the estimates and uncertainty as posted, but cannot independently verify all analysis choices or assess assumptions from participant-level data.

10. Analyses Reported by the Sponsor

The registry reports the Cox regression result as the trial's between-group analysis: hazard ratio 0.72, 95% confidence interval 0.29 to 1.77, and p = 0.47. It also posts arm-specific 10-year survivorship estimates and confidence intervals. These summaries are not interchangeable: the arm percentages describe estimated survival at the stated time, while the Cox model compares event hazards over follow-up.

No separate recalculated hazard ratio or other recalculated comparison is reported in the registry record, so a numerical comparison between the reported model and an independent recalculation cannot be made from the posted statistics. The record does not flag a discrepancy between the reported analysis and any recalculation. The Cox result should therefore be described as the posted model-based analysis, without implying that it has been independently reproduced from the arm summaries. The record does not describe the model coding, covariate adjustment, or full analysis specification.

11. Multiplicity, Missing Data and Analysis Populations

The registry lists seven secondary outcomes in addition to the primary outcome. They include survivorship free of reoperation and complications, Knee Society Scores, SF-12 mental and physical health scores, and radiographic success or failure. Multiple outcomes create multiple opportunities to observe apparently notable results. In general, multiplicity procedures can address the increased chance of false-positive findings across a family of tests, but the registry does not report whether any adjustment was planned or used.

The record lists three analyses but does not provide a complete statistical analysis plan, a hierarchy of outcomes, or a definition of which comparisons were confirmatory. It also does not report whether subgroup analyses or interim analyses were conducted. These matters are not reported in the registry record, so conclusions about prespecification and control of multiple testing are limited.

Missing data and analysis populations are also not described in enough detail to assess their potential influence. The registry does not report follow-up completeness, reasons for missing observations, censoring counts, or how missing outcome information was handled. It does not specify whether the primary analysis included all randomized participants, only participants with available follow-up, or another population. For time-to-event analysis, censoring can be appropriate when the assumptions behind it are reasonable, but the registry does not provide details needed to evaluate those assumptions.

The enrollment and arm counts are reported, but those counts alone do not establish how many participants contributed information to each endpoint or model. Without the analysis population and missing-data procedures, the posted estimates cannot be fully audited. This uncertainty is distinct from the confidence intervals around the reported estimates: those intervals reflect statistical uncertainty under the analysis method, while incomplete reporting makes it harder to judge whether the method and data adequately represent the randomized groups.

12. 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's adverse-event fields report zero serious events and zero other events for each arm. The fields list 254 at risk in Control and 264 at risk in Treatment. These are the posted counts; they should not be interpreted as proof that no adverse events occurred during the trial. The registry does not provide event definitions, ascertainment methods, reporting periods, or a narrative explaining the zero counts.

Deaths are listed separately: 55 in Control and 50 in Treatment. Death counts are not interchangeable with serious adverse-event counts, and the registry does not describe their causes, timing, or relationship to the study devices. Nor does it report how deaths were handled in the time-to-event analysis. Consequently, the posted death counts do not explain the primary survival result and should not be combined with the adverse-event fields.

The trial also lists reoperation and complications as secondary outcomes, but their detailed results are not included in these safety counts. The zero values therefore describe only what is recorded in the adverse-event fields, not a comprehensive account of safety or all clinical events.

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

The posted results describe two arm-level estimates of prosthesis survival free of revision through 10 years: 96.0 for N2Vac and 97.0 for X3. Each estimate has a 95% confidence interval, and the Cox regression comparison reports a hazard ratio of 0.72 with a 95% confidence interval from 0.29 to 1.77 and p = 0.47. Together, these results do not identify a statistically clear between-group difference in the reported superiority analysis.

That finding is not the same as demonstrating equal performance. A superiority analysis that does not show a clear difference cannot, by itself, establish equivalence or non-inferiority. Such conclusions require an appropriate design, a prespecified margin, and an analysis targeted to that question. The registry describes an equality hypothesis but does not report an equivalence or non-inferiority margin or analysis.

The results also do not show that the two materials have identical outcomes, nor do they establish that clinically important differences are ruled out. The confidence interval for the hazard ratio includes values on either side of the no-effect reference and is broad. The amount of uncertainty should be considered alongside the arm-specific survival intervals and the missing details about events, censoring, and analysis populations.

These findings concern revision-free prosthesis survival in the registry's described population: patients receiving primary unilateral total knee arthroplasty for osteoarthritis, avascular necrosis, or posttraumatic arthritis at three academic, tertiary-care referral centers within one healthcare network. The results do not, on their own, answer questions about all complications, reoperations, patient-reported outcomes, or radiographic findings. Those outcomes are listed separately, and their full results are not included in the primary comparison presented by the registry.

Accordingly, the statistical conclusion should remain narrow: the posted analysis does not establish a clear superiority difference, while leaving uncertainty about the range of possible differences and about the broader outcomes. It is not a treatment recommendation or a claim that either material works or fails.

14. Why This Trial Matters Statistically

This trial is statistically informative because it separates an intuitive comparison of end-point survival percentages from a time-to-event model. The posted percentages, 96.0 for N2Vac and 97.0 for X3, describe survival free of revision through 10 years. The Cox regression result uses event timing across follow-up and reports a hazard ratio with its own confidence interval and p-value. These statistics address related but distinct aspects of the data.

The result also illustrates why a non-significant superiority analysis should not be interpreted as proof of equivalence. The registry describes an equality hypothesis, yet the reported analysis is labelled as superiority. Establishing equivalence would require a defined margin and analysis that the registry does not report. This distinction is important when an observed comparison appears small but its uncertainty still permits a range of effects.

Another statistical lesson is that an estimate and its precision are both needed. The arm-level intervals and the model-based interval communicate uncertainty, while the absence of event counts, censoring details, model coding, and analysis-population definitions limits independent assessment. A p-value alone cannot convey these features or determine clinical importance.

Finally, the record lists secondary outcomes beyond revision-free survival, including reoperation, complications, function, health status, and radiographic results. These outcomes broaden the clinical questions but also raise issues of multiplicity and completeness of reporting. The registry does not give enough detail to determine how they were analyzed or prioritized. Taken together, the study's posted results are best read as a focused time-to-revision comparison with meaningful reporting gaps, rather than as a comprehensive statistical account of every outcome.

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.

Several limitations are specific to the information recorded for this trial. The registry gives arm-specific 10-year survival estimates and a Cox regression comparison, but does not report the number of revisions, event timing, censoring, or losses to follow-up. These omissions prevent assessment of how much direct event information underlies the estimates and how incomplete follow-up may have affected them.

The Cox model's reference-group coding, covariates, diagnostics, and proportional-hazards assessment are not reported in the registry record. Without the coding, the direction of the posted hazard ratio cannot be assigned confidently to X3 or N2Vac. Without model details, readers cannot assess whether adjustment was used or whether the model assumptions were examined.

The registry does not identify which people were masked or describe the masking procedures. It also does not report allocation concealment, the primary analysis population, or the approach to missing outcome data. These gaps limit evaluation of possible bias and prevent reconstruction of the primary analysis.

The stated hypothesis concerns equal performance, while the posted comparison is labelled as superiority. The registry does not report an equivalence margin or a corresponding equivalence analysis. In addition, it lists secondary outcomes but does not report the multiplicity strategy, outcome hierarchy, or complete secondary results in the record summarized by the primary comparison. Finally, adverse-event fields contain zero counts, but event definitions and ascertainment details are not reported. These limitations call for a cautious interpretation of both the efficacy comparison and the safety entries.

16. Related Tutorials

17. Related Statistical Calculators

18. Sources

• ClinicalTrials.gov: X-3 Polyethylene Survival Outcomes Study — NCT00493792.
• Registry record last updated 2024-11-06; status last verified 2024-10.
• Results first posted 2024-11-06.
• 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

NCT00493792 is a completed randomized, single-masked, parallel-group study comparing X3 polyethylene with N2Vac polyethylene in a Triathlon posterior stabilized total knee system. The registry records 518 participants and lists Mayo Clinic as sponsor. The primary endpoint was prosthesis survivorship free of revision from operation through 10 years post-operative.

Posted survival estimates were 96.0 for N2Vac and 97.0 for X3. The reported Cox regression gave a hazard ratio of 0.72, a 95% confidence interval of 0.29 to 1.77, and p = 0.47. The registry also lists secondary outcomes covering reoperation, complications, scores, health status, and radiographic results.

The posted comparison does not establish a clear superiority difference and does not demonstrate equivalence. Important details about event counts, censoring, model specification, missing data, and analysis populations are not reported in the registry record.