Intraoperative Ocular Pressure in Lumbar Spine Fusion Patients: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Intraoperative Ocular Pressure in Lumbar Spine Fusion Patients (NCT02342288), a randomized na study. The primary outcome is Change in Intraoperative Ocular Pressure in Lumbar Spine Fusion Patients Head Raised 10 Degrees or Kept in Neutral Position.

2. Clinical Question

The trial examined whether head position during prone lumbar spine fusion surgery was associated with a different change in intraocular pressure (IOP). The registry background notes that prone positioning and a head-down tilt can increase IOP, and describes concern about a rare postoperative vision-loss complication. Those statements provide the clinical motivation; they are not outcomes tested by the primary measure.

IOP was assessed before, during, and after surgery according to the brief summary. The primary outcome definition focuses on the maximum two-eye average relative to the baseline two-eye average obtained at the first prone measurement. Measurements were scheduled while prone, after the head was raised, and at intervals during surgery. The key comparison is between the assigned head positions, not simply whether IOP changed over time within either group.

The registry results report a mean change for each group, with confidence intervals. They do not report an estimable two-arm comparison for the continuous endpoint. The available group summaries therefore describe the observed outcome in each arm but do not themselves provide a direct estimate of the between-group contrast or its uncertainty.

3. Trial Design

The registry identifies NCT02342288, titled “Intraoperative Ocular Pressure in Lumbar Spine Fusion Patients.” It describes a randomized, single-masked, parallel-group study. The registry lists an enrollment of 79 and a completed status. The recorded study start is 2008-07, and primary completion is recorded as 2014-02. The phase is listed as NA.

Participants undergoing elective lumbar spine surgery were assigned to head elevation or a neutral head position while prone. The intervention description specifies that the head was raised 10 degrees from neutral using Gardner Wells tongs. The neutral-position group kept the head in neutral position. The brief summary calls this the standard-care position. A parallel design compares groups assigned to different approaches during the same trial rather than having each participant receive both positions.

The registry labels masking as single but does not identify which individuals were masked. It also does not report additional information about concealment of allocation, the method used to generate the random assignment, or whether the assigned position was maintained throughout each operation. These details should not be inferred from the design labels alone.

The primary outcome is continuous: a mean change in IOP, expressed in mm Hg. The record lists two outcome measures and one secondary outcome concerning change and correlations in intraoperative IOP. It lists no analyses. For the primary continuous outcome, the posted group summaries give an analysis count of 25 for the raised-head group and 27 for the neutral-position group. These reported outcome counts are not interchangeable with the registry enrollment figure, and the record does not explain their relationship.

Neither a phase classification beyond NA nor a rationale for the enrollment target is reported in the registry record. The record also does not report a detailed protocol or a statistical analysis plan.

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: Seventy-nine patients were consented and enrolled. Sixty-three were randomized. Twenty-seven patients did not complete the study. Of the 27, 16 were never randomized. The number of patients who were randomized, started the study and received full treatment or placebo was 52. Reasons are listed below for those who did not complete the study.

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: Subjects who were undergoing elective lumbar spine surgery.

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: no significant differences in baseline demographics between the groups

6. Primary Endpoint

The registry lists the primary outcome as Change in Intraoperative Ocular Pressure in Lumbar Spine Fusion Patients Head Raised 10 Degrees or Kept in Neutral Position, measured over Prone; 5 minutes after head raised to 10 degrees; every 15 minutes; 1 hr until end of surgery.

The primary endpoint is mean change in intraoperative ocular pressure, in mm Hg. It is defined as the maximum average IOP across the two eyes minus the average across the two eyes at baseline, with baseline taken at the first prone measurement. This measure summarizes how far the maximum recorded average pressure rose or fell relative to that baseline during the monitored period.

Measuring change from baseline can help distinguish a participant’s intraoperative change from the starting value. Averaging across both eyes produces a participant-level summary, rather than treating each eye as an independent participant. The maximum value is intended to capture the greatest measured pressure during the specified monitoring period. These are general features of such an endpoint; the registry does not provide further justification for choosing this particular definition.

Measurements were scheduled when prone, five minutes after the head was raised to 10 degrees, and every 15 minutes for one hour until the end of surgery. The endpoint description identifies the maximum and baseline values, but it does not state in more detail how measurements across those scheduled times were handled in the analysis.

The posted group means are 9.26 mm Hg for the head-raised group and 13.79 mm Hg for the neutral-position group. Their 95% confidence intervals are 6.65 to 11.87 mm Hg and 10.6 to 16.97 mm Hg, respectively. These are within-group summaries. The registry does not report an estimable two-arm comparison for this continuous endpoint, so the posted means should not be mistaken for a direct estimate of the difference between assigned positions.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The primary outcome is a continuous measure, and the registry reports a mean for each randomized group. A mean condenses a set of measurements into an average. It is useful for describing the central level of a quantitative outcome, but it does not show how much individual results vary. The registry does not post a standard deviation or another measure of participant-level spread for these outcomes.

What do the confidence intervals describe?

The record reports 95% confidence intervals for each arm’s mean. In general, a confidence interval conveys the precision of an estimate under the statistical method used: intervals that are narrower indicate greater precision, while wider intervals indicate greater uncertainty. A 95% confidence interval is not a statement that 95% of individual participants had values inside its bounds. Nor is either arm’s interval a confidence interval for the difference between arms.

The interval for the raised-head mean is 6.65 to 11.87 mm Hg, and the interval for the neutral-position mean is 10.6 to 16.97 mm Hg. The registry does not state the specific method used to construct these intervals. It also does not give a model-based or unadjusted contrast between the groups. As a result, the arm-specific estimates can be described, but the uncertainty around a comparative effect cannot be reconstructed from the reported information without making additional assumptions.

Why does the randomized comparison matter?

Random allocation is intended, in general, to make treatment groups comparable on average with respect to measured and unmeasured factors at the point of assignment. This is why a direct comparison of outcomes by randomized assignment can support a causal interpretation when the design, follow-up, outcome ascertainment, and analysis are appropriate. Randomization by itself does not remove uncertainty, address missing outcomes automatically, or demonstrate that all participants were analyzed as assigned.

The trial is described as parallel-group and single-masked. The registry does not identify who was masked or report the allocation procedure. Since head position is part of the intervention, the design label alone does not establish how masking was achieved for participants, clinicians, outcome assessors, or analysts. The registry record does not report these particulars.

What can be said about the sample-size illustrations?

The sample-size illustrations are planning scenarios based on the observed effect, not the trial’s own design. The stated assumptions are a two-sided significance level of 0.05, equal allocation, and a normal approximation. The illustrations consider effects defined as fractions of the observed effect and report participants per arm for 80% or 90% power. These are hypothetical planning quantities: they do not describe the power the completed trial actually had, and they are not observed trial outcomes.

In general, a planning calculation asks how much information a future study might need to detect an assumed effect under specified statistical assumptions. Its result depends on the assumed effect and variability as well as the chosen error rate and allocation. An illustration anchored to an observed effect is especially dependent on that estimate, which is uncertain. It should therefore not be read as proof that the observed effect is true or as a retrospective measure of the completed study’s success.

Why can over-dispersion matter?

General explanation: over-dispersion occurs when outcome variability is greater than the statistical model assumes. If a model understates variability, standard errors can be too small and confidence intervals too narrow. The registry does not report a model for these data or evidence of over-dispersion, and the continuous endpoint’s posted summary does not supply enough detail to assess this issue. It would be inappropriate to assert that over-dispersion occurred in this trial.

The record also does not state whether the analysis adjusted for baseline IOP, repeated measurements, or other variables. Repeated measurements within a participant are generally related, so a model that uses the measurement sequence would ordinarily need to account for that dependence. This is a general methodological point, not a claim about the analysis used. The endpoint’s maximum-minus-baseline definition reduces the repeated measurements to a summary, but the registry does not detail the analytic steps used to produce the posted means and intervals.

10. Multiplicity, Missing Data and Analysis Populations

The registry record lists a primary outcome and one secondary outcome concerning change and correlations in intraoperative IOP. It lists two outcome measures, but no analyses. The record does not report whether more than one primary contrast, time point, subgroup, or statistical test was planned. It also does not report a multiplicity strategy, such as adjustment of significance levels or a prespecified hierarchy of tests. Without that information, the handling of multiple comparisons cannot be assessed.

The registry does not report a missing-data method, an imputation approach, or reasons why an outcome might be unavailable. It also does not specify an analysis population, such as an intention-to-treat population or a per-protocol population. The endpoint summaries report an analysis count of 25 for the raised-head group and 27 for the neutral-position group, while enrollment is recorded as 79. The registry does not explain the difference between these figures, so it is not possible to determine from the record whether it reflects missing measurements, exclusions, or another reason.

For repeated measurements, missing observations at particular times can affect both the maximum and the baseline-referenced change. General statistical practice distinguishes a missing outcome from an observed outcome that has a low or high value; missingness cannot be treated as no change without a stated method. The record does not give enough information to assess the amount, timing, or handling of missing IOP measurements.

The registry record also does not report a statistical analysis plan, prespecified handling of protocol deviations, or sensitivity analyses. These omissions limit assessment of whether the posted summaries follow the intended analysis and how robust they are to alternative assumptions. They do not establish that any particular analytical problem occurred.

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 0% reporting threshold.

Most frequent non-serious adverse event terms

The registry’s adverse-event figures report zero serious adverse events among the 25 participants at risk in the head-raised group and zero among the 27 participants at risk in the neutral-position group. These are counts recorded for the stated at-risk groups; they do not establish that no other safety outcomes occurred or that every possible event was assessed in the same way.

For other, non-serious adverse events, the record reports one event in the head-raised group and zero in the neutral-position group. The registry does not provide a list of serious-event terms, and it does not report death counts. The death values are not reported in the registry record. The event information should therefore be interpreted narrowly as the counts and categories listed, rather than as a complete account of all possible harms.

In general, zero reported events does not prove that an event is impossible; it describes what was recorded in the available safety data. The registry entry does not provide further detail about event definitions, severity grading beyond the serious/other categories, collection procedures, follow-up duration, or causal attribution. These safety counts are descriptive and do not establish whether one assigned position caused or prevented an event.

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

The registry results show that the reported mean maximum-minus-baseline IOP change was 9.26 mm Hg in the group assigned to a 10-degree head elevation and 13.79 mm Hg in the group assigned to a neutral position. The arm-specific 95% confidence intervals are 6.65 to 11.87 mm Hg and 10.6 to 16.97 mm Hg, respectively. These values describe the posted summaries, not a direct estimate of how much the groups differed.

The registry does not report an estimable two-arm comparison for the continuous endpoint. It therefore does not provide a comparative effect estimate, a confidence interval for that contrast, or a reported significance test. The numerical difference in the posted group means should not be treated as a complete statistical comparison, especially because the analysis population and the method used to calculate the interval are not described in detail.

The primary outcome is an intraoperative pressure measure. It is not an outcome of postoperative vision, visual function, or the frequency of vision loss. The registry background raises a concern about vision loss and discusses IOP as a possible contributor, but the posted primary results do not test whether head position changes the risk of that complication. No claim about prevention or clinical benefit can be drawn from the pressure summaries alone.

The record also does not report the statistical analysis plan, missing-data handling, or analysis population. The absence of those details limits interpretation of how the summarized results were obtained. The sample-size illustrations are planning scenarios based on the observed effect and stated assumptions; they are not observed power and do not remedy the lack of a reported comparative analysis.

In short, the registry provides arm-level descriptive results for a randomized study of head position during prone lumbar spine surgery. It leaves the key between-group statistical estimate and important analysis details unreported. The numbers can inform a description of the recorded IOP changes, but they do not by themselves establish a causal effect on IOP or a clinical effect on vision.

13. Why This Trial Matters Statistically

This trial illustrates why a randomized design and a reported randomized contrast are both important. Random assignment is intended to support a fair comparison, but the arm-specific means alone do not quantify the treatment contrast or its precision. For a continuous outcome, readers need the estimated between-group difference and an uncertainty measure for that difference to judge the magnitude and precision of the comparison.

The endpoint also highlights the distinction between a physiological measure and a patient-important clinical outcome. Intraoperative IOP may be of interest because the registry background connects pressure with concern about rare postoperative vision loss. But a pressure measurement is not equivalent to a visual outcome. A statistical association or change in an intermediate measure cannot be assumed to establish a change in a clinical complication.

The registry’s reported group-specific confidence intervals make uncertainty visible for each mean, while the absence of a reported comparative analysis leaves uncertainty about the randomized contrast unresolved. This is a useful reminder that separate estimates and intervals do not automatically answer the question posed by a two-group trial. The analysis should directly match the randomized comparison and the stated clinical question.

Finally, outcome counts and enrollment figures need careful interpretation. The registry records an enrollment of 79, while the primary endpoint summaries use group-specific counts of 25 and 27. Without an explanation of how participants moved from enrollment to analysis, readers cannot assess the role of missing data, exclusions, or analysis-population choices. Transparent reporting of these details helps readers distinguish the design of a study from the evidence actually contributing to its estimates.

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 registry record leaves several trial-specific questions unanswered. It does not identify which people were masked despite the single-masked designation, and it does not describe the randomization procedure or allocation concealment. The record also does not say how adherence to assigned head position was assessed. These omissions limit evaluation of implementation and potential sources of bias, but they do not show that a particular bias occurred.

The record reports an enrollment of 79 but gives primary outcome analysis counts of 25 for the head-raised group and 27 for the neutral group. It does not explain these figures or provide participant flow, reasons for unavailable measurements, or analysis-population definitions. Consequently, the representativeness of the analyzed outcomes cannot be assessed from the registry entry alone.

The primary endpoint is described as the maximum two-eye average minus baseline two-eye average. The registry does not explain how the maximum was selected across measurements, how measurements from both eyes were combined if observations were incomplete, or how the repeated schedule was incorporated into the analysis. It gives group means and confidence intervals but does not identify the interval method or report a direct between-arm estimate.

Further limitations include the lack of a posted analysis plan, missing-data methods, multiplicity handling, or results for the listed secondary outcome. The registry does not report the duration or detail of safety-event collection, and it does not provide results for a vision outcome. Each of these gaps narrows what can be concluded from the posted record; none should be filled by assumptions about what the investigators may have done.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Intraoperative Ocular Pressure in Lumbar Spine Fusion Patients — NCT02342288.
• Registry record last updated 2016-02-24; status last verified 2016-01.
• Results first posted 2016-02-24.
• 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

NCT02342288 is a completed randomized, single-masked, parallel-group study of intraoperative head position in elective lumbar spine surgery. The registry records a start of 2008-07, primary completion in 2014-02, and enrollment of 79. The compared positions were head raised 10 degrees from neutral using Gardner Wells tongs and head maintained in neutral position.

The primary outcome was mean change in IOP, in mm Hg, defined using the maximum two-eye average and the baseline two-eye average from the first prone measurement. The posted group means were 9.26 mm Hg and 13.79 mm Hg, with arm-specific 95% confidence intervals. The registry does not report an estimable two-arm comparison for this continuous outcome. It lists one secondary outcome, no analyses, and does not report the analysis plan, missing-data methods, or analysis populations.