The Influence of Silicone Oil on Nerve Fiber Layer Thickness After Pars Plana Vitrectomy: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for The Influence of Silicone Oil on Nerve Fiber Layer Thickness After Pars Plana Vitrectomy (NCT01255306), a registered phase not stated study. The primary outcome is Evidence of Retinal Nerve Fibre Layer Thickness Change Measured by Optical Coherence Tomography.

2. Clinical Question

The clinical question is about a measured change in retinal tissue thickness, not a comparison of randomized treatment assignments. The registry also names a secondary outcome concerning retinal nerve fiber layer thickness change in patients with raised intraocular pressure secondary to silicone oil endotamponade.

The record describes the reason for measuring thickness as a possible additional parameter for clinical decision making. It does not report a numerical primary-outcome estimate, a comparison between the raised-pressure groups, or uncertainty around any estimated change. As a result, the clinical question is identifiable, but the registry record does not provide the results needed to quantify the observed change or its precision.

3. Trial Design

The study, NCT01255306, is titled “The Influence of Silicone Oil on Nerve Fiber Layer Thickness After Pars Plana Vitrectomy.” It was sponsored by University Hospital Sestre Milosrdnice. The registry lists the study as completed, with a start date of 2010-04 and primary completion in 2011-11. Enrollment is listed as 60.

The record describes assessment of retinal nerve fiber layer thickness by optical coherence tomography after pars plana vitrectomy. Its two arm descriptions distinguish patients without raised intraocular pressure from patients with raised intraocular pressure. This is a grouping by pressure status, not a stated randomized allocation. The allocation method is not stated, and the registry specifically indicates that the arms are not a like-for-like control comparison.

Optical coherence tomography is listed as an other intervention, and local medical treatment of raised intraocular pressure is listed as a drug intervention. The record does not establish that every participant received both, or describe treatment schedules, dosing, or timing in enough detail to reconstruct the study procedures. Such details are not reported in the registry record.

The primary outcome is mean change in retinal nerve fiber layer thickness, measured in micrometers, over 6 months. The outcome population is described as patients who completed necessary follow-up visits and had optical coherence tomography measurements at all visits. The number meeting that requirement is not reported in the registry record. The record lists two outcome measures and no analyses. It does not report the statistical analysis plan, a sample-size justification, or interim analyses.

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: We excluded one patient who developed severe inflammatory reaction 5 days following pars plana vitrectomy and silicone oile tamponade from the study.

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: We included all patients that have completed necessary follow up visits and have had optical coherence tomography measurements at all visits in the final analysis.

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 Evidence of Retinal Nerve Fibre Layer Thickness Change Measured by Optical Coherence Tomography, measured over 6 months.

The primary endpoint is a change in retinal nerve fiber layer thickness measured by optical coherence tomography. The registry specifies the mean as the summary measure, the standard deviation as the dispersion measure, micrometers as the unit, and 6 months as the timeframe. The stated population for the final analysis consists of patients who completed the required follow-up visits and had optical coherence tomography measurements at all visits.

The retinal nerve fiber layer is the tissue being measured, and the endpoint expresses its thickness as a continuous quantity. The registry describes a possible role for thickness change as an additional parameter that could inform clinical decision making for patients with silicone oil tamponade. This is the rationale stated in the record; it does not establish that the measure is a validated decision rule or that a particular change has a specific clinical meaning.

General: A continuous endpoint preserves the magnitude of measured values rather than reducing them to categories such as improved or not improved. A mean describes the central tendency of the measurements, while a standard deviation describes their spread. The mean alone does not show how variable individual observations were, and the standard deviation alone does not provide the precision of an estimated mean.

The registry record does not report the mean change, its standard deviation, a confidence interval, or a test result. It therefore does not allow the size or precision of the primary endpoint to be assessed from the record.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

What kind of outcome was measured?

The primary endpoint is continuous: retinal nerve fiber layer thickness change measured in micrometers. The registry says the outcome is summarized as a mean and that its dispersion is described by a standard deviation. The timeframe is 6 months. These details identify the intended form of the outcome, but not its observed value. The mean, standard deviation, and any associated estimates are not reported in the registry record.

General: For a continuous outcome, measurements can take a range of values rather than falling only into categories. A mean provides a summary of the average measurement or average change for the analyzed group. A standard deviation describes how dispersed individual measurements are around that mean. It is not the same as a confidence interval: the standard deviation describes variability among observations, while a confidence interval describes uncertainty in an estimated quantity.

How should the group structure be understood?

The record distinguishes patients without raised intraocular pressure from patients with raised intraocular pressure. It does not state randomized allocation, and it explicitly notes that the arms are not a like-for-like control comparison. The allocation method is not stated. Therefore the group labels should be read as descriptions of pressure status rather than as evidence that participants were assigned at random to otherwise comparable groups.

General: In a randomized comparison, random assignment is intended to balance measured and unmeasured baseline characteristics on average. In a non-randomized grouping, differences between groups may reflect the characteristic used to define the groups, other participant characteristics, clinical decisions, or combinations of these factors. Statistical adjustment can sometimes address measured differences, but cannot guarantee removal of confounding. The registry record does not report an adjustment strategy or a model for comparing the groups.

What can the reported analysis status establish?

The registry lists two outcome measures and zero analyses. The primary outcome description gives the intended endpoint and population, but the record does not include numerical findings. There is no reported group estimate, standard error, confidence interval, or p-value for the primary endpoint. No recalculated estimate is available from the posted summary statistics.

General: A confidence interval is useful because it expresses uncertainty around an estimate in the units of the outcome. A narrow interval indicates greater statistical precision than a wide interval, although precision alone does not determine clinical importance. A hypothesis test and its p-value answer a different question and should be interpreted alongside an effect estimate and its uncertainty. Without the underlying results, these methods cannot be used to characterize the study’s observed effect.

What does the follow-up requirement imply?

The final-analysis population is defined as patients who completed necessary follow-up visits and had optical coherence tomography measurements at all visits. This is a complete-measurement requirement as described by the registry. It does not reveal how many enrolled patients satisfied the requirement. Nor does it state how patients with incomplete follow-up were handled, whether measurements were imputed, or whether other analysis populations were considered.

General: Restricting an analysis to participants with complete measurements can reduce the data available for analysis and may introduce bias if completion is related to outcome or participant characteristics. Imputation and other missing-data methods rely on assumptions about why data are missing. The appropriate method depends on study procedures and available information; no method can be attributed to this study based on the registry description.

What can be said about sample-size planning?

The registry record does not report a sample-size justification for the original study. It also does not report the assumed effect, variability, significance level, or target power used in the study’s design. Any sample-size illustrations associated with the observed effect are planning illustrations, not observed power. Since the primary effect estimate is not reported in the registry record, no numerical planning result can be interpreted from the registry information alone.

General: A prospective sample-size calculation uses design assumptions to estimate how many participants may be needed to detect a specified effect under a chosen statistical procedure. It is not evidence that the study actually observed that effect. “Observed power” calculated from an observed estimate does not add a useful interpretation beyond the estimate and its uncertainty. Planning illustrations based on an effect size should therefore be treated as hypothetical design scenarios, not as trial findings.

Over-dispersion is relevant in general when outcome variability is greater than assumed by a statistical model, because the model may then understate uncertainty. The registry identifies a continuous mean and standard deviation but does not report the observed dispersion or the statistical model. There is no basis to assess over-dispersion in this study.

10. Multiplicity, Missing Data and Analysis Populations

The registry lists two outcome measures: the primary measure of retinal nerve fiber layer thickness change and a secondary outcome concerning thickness change in patients with raised intraocular pressure secondary to silicone oil endotamponade. It does not report a multiplicity adjustment, a hierarchy of hypotheses, or a prespecified strategy for interpreting multiple outcomes. These details are not reported in the registry record.

General: When several outcomes or comparisons are tested, the chance of at least one apparently unusual result can increase if each is assessed separately using the same decision threshold. Prespecified outcome hierarchies, adjusted testing procedures, or emphasis on estimates and confidence intervals can help clarify interpretation. The registry record does not state which approach, if any, was used. Because no analyses or outcome results are listed, the effect of multiplicity on reported findings cannot be assessed.

The primary analysis population is described as patients who completed necessary follow-up visits and had optical coherence tomography measurements at all visits. The record does not provide the number included in that analysis, the number with incomplete follow-up, the reasons measurements may have been missing, or the handling of missing data. It also does not report whether the analysis followed an intention-to-treat principle or another population definition beyond the stated complete-visit criterion.

General: Missing data can affect both the amount of information available and the validity of an estimate. If participants with missing measurements differ systematically from those with complete measurements, a complete-case estimate may not represent the full enrolled group. Assessing this concern usually requires details about follow-up, reasons for missingness, and the analysis method. Those details are not reported in the registry record.

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.

The registry lists serious and other adverse-event counts for study eyes and control eyes. For each group, 57 were at risk, and 0 were affected by serious adverse events; the other-adverse-event counts are also 0 in each group, with 57 at risk. The registry does not list top serious adverse-event terms. Death counts are not reported in the registry record.

These entries are event counts in the registry’s eye-group categories. They should not be read as the number of participants in the primary efficacy analysis: the record does not state that these groups define that analysis population, and the primary outcome requires measurements at all visits. The record also does not explain how the eye categories relate to individual participants.

General: A zero count in a registry field means no events were recorded in that field for the stated group; it does not establish that no adverse events of any kind occurred. Interpretation depends on what was collected, how events were defined, the period of observation, and the completeness of reporting. The registry entries do not provide enough detail to assess those aspects or to compare safety between groups. In particular, the unreported death counts should not be interpreted as zero.

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

The registry record describes a study designed to measure retinal nerve fiber layer thickness by optical coherence tomography after pars plana vitrectomy with silicone oil tamponade. Its primary endpoint is mean change in thickness over 6 months, and the record states that the intended final-analysis population had completed necessary follow-up visits and measurements at all visits. The record also describes groups according to whether raised intraocular pressure was present.

What the record does not establish is the observed amount or direction of thickness change. It contains no numerical primary-outcome result, no estimate of its variability, and no uncertainty interval or test result. It therefore cannot show how precisely any change was estimated. Nor does it provide a numerical comparison between patients with and without raised intraocular pressure.

The group structure also limits what a comparison could mean. The record does not state that participants were randomly allocated and specifically warns that the arms are not a like-for-like control comparison. A difference between pressure-status groups, if reported, would not automatically represent a causal effect of pressure status, silicone oil, or local medical treatment. The registry record does not report an adjustment strategy that would support a causal interpretation.

General: An observed association is not necessarily a causal effect, especially when groups are defined by a clinical characteristic rather than randomized assignment. A numerical estimate also needs its uncertainty and a clear account of the population analyzed to judge how stable and generalizable it may be.

Thus, the record identifies a clinically motivated measurement question, but does not answer it with posted outcome results. It does not show that silicone oil increased or decreased retinal nerve fiber layer thickness, and it does not establish that thickness change predicts a clinical outcome. These conclusions cannot be drawn from the available registry information.

13. Why This Trial Matters Statistically

This study is statistically interesting because it focuses on a continuous structural measurement and describes change over a defined follow-up timeframe. Optical coherence tomography is the specified measurement method, and the endpoint is expressed as mean retinal nerve fiber layer thickness change in micrometers. That framing makes clear what quantity the investigators intended to assess, even though the registry does not post its numerical result.

The distinction between the study groups is also important. The record separates patients with raised intraocular pressure from those without it, but does not describe a randomized comparison and cautions that the arms are not like-for-like controls. This highlights a central interpretive issue in clinical research: a group difference can reflect baseline or clinical differences as well as the characteristic used to define the groups. The design description should therefore shape any interpretation, rather than treating the labels as equivalent to randomized treatment arms.

The registry’s stated complete-follow-up analysis population raises another general statistical issue. Requiring measurements at all visits may make the analyzed group different from the enrolled group if follow-up completion is selective. The number analyzed and the handling of missing measurements are not reported in the registry record, so that issue cannot be evaluated for this study.

Finally, the record illustrates the difference between a registered research question and interpretable posted results. Enrollment and an endpoint definition do not supply an effect estimate. Without numerical results and measures of precision, readers cannot assess the magnitude or uncertainty of the outcome. The study remains informative as a description of a measurement question and a non-randomized group structure, but the registry entry does not support a quantitative conclusion about retinal nerve fiber layer change.

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 statistical interpretation is that the registry record lists no analyses and does not report numerical results for the primary endpoint. The mean change, standard deviation, group estimates, confidence intervals, and p-values are not reported in the registry record. Without those quantities, the size, direction, and precision of the observed outcome cannot be assessed.

The record gives an enrollment of 60 but does not report the number included in the final analysis. Its stated analysis population requires completed follow-up visits and optical coherence tomography measurements at all visits. The number excluded by that requirement, reasons for incomplete data, and methods for handling missing measurements are not reported in the registry record.

The arm descriptions are based on raised intraocular pressure, and allocation is not stated. The registry notes that the arms are not a like-for-like control comparison. This limits causal interpretation and leaves open the possibility that group differences reflect factors other than pressure status. Baseline characteristics, adjustment methods, and details of any group-comparison model are not reported in the registry record.

The registry lists two outcome measures but does not report a multiplicity strategy, a detailed statistical analysis plan, interim analyses, or the study’s sample-size justification. The record also provides limited detail on outcome timing beyond the primary 6-month timeframe, measurement procedures, and the relationship between participant counts and the eye-based adverse-event groups. These gaps constrain assessment of analysis choices, precision, missingness, and the generalizability of any result.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: The Influence of Silicone Oil on Nerve Fiber Layer Thickness After Pars Plana Vitrectomy — NCT01255306.
• Registry record last updated 2012-03-23; status last verified 2012-03.
• Results first posted 2012-01-24.
• 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

NCT01255306 is a completed study sponsored by University Hospital Sestre Milosrdnice, with enrollment listed as 60, a start date of 2010-04, and primary completion in 2011-11. It examines retinal nerve fiber layer thickness after pars plana vitrectomy with silicone oil tamponade, using optical coherence tomography. The primary endpoint is mean thickness change in micrometers over 6 months.

The registry describes groups with and without raised intraocular pressure and notes that they are not a like-for-like control comparison. The final-analysis population is described as patients completing required visits and measurements at all visits, but its size is not reported in the registry record. Two outcome measures are listed, and no analyses are listed. Numerical primary and secondary outcome results, the original sample-size justification, and details of missing-data handling are not reported in the registry record.