VisuMax Femtosecond Laser Small Incision Lenticule Extraction for the Correction of High Myopia: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for VisuMax Femtosecond Laser Small Incision Lenticule Extraction for the Correction of High Myopia (NCT02528123), a na na study. The primary outcome is Variability of the Refractive Predicatibility.

2. Clinical Question

3. Trial Design

The registry record describes a completed, single-group study of small incision lenticule extraction for high myopia. It lists an enrollment of 114 and identifies London Vision Clinic as the sponsor. The condition fields name myopia and refractive errors. The record gives a study start of 2016-04 and a primary completion date of 2020-12-07. Phase and allocation are listed as NA, and masking is listed as NONE.

The intervention is a SMILE procedure using the VisuMax femtosecond laser. The arm description names proxymetacaine 0.5% and oxybuprocaine 0.4% for anaesthesia during the procedure, followed by tobramycin and dexamethasone and ofloxacin after the procedure. These are the registry's intervention descriptions; they do not provide individual treatment records or confirm actual medication use.

A single-group design can describe outcomes among participants who underwent a procedure, but without a separate comparison group it cannot, on its own, estimate how outcomes would differ under another treatment or under no treatment. The record does not report random assignment, a control group, or masking. It also does not report a protocol-level account of how enrollment was determined, the planned analysis populations, or whether any interim analyses were conducted.

The primary endpoint is a continuous measure concerning refractive predictability at 1 year postop. The record also names secondary outcomes concerning uncorrected and corrected distance visual acuity, refractive correction and astigmatism, stability of spherical equivalent refraction, night vision disturbances, and corneal higher order aberrations. Their presence indicates a broader set of outcome domains, but the registry record does not report numerical results for them in the registry record for this account.

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: Calculate the standard deviation of the postoperative spherical equivalent, adjusted relative to the intended target, after the SMILE procedure. Analysis performed on the data from the 1 year postoperative visit. An accurate outcome is where the postoperative spherical equivalent is equal to the intended target spherical equivalent. A smaller standard deviation represents a more accurate and more predictable outcome.

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 Variability of the Refractive Predicatibility, measured over 1 year postop.

The primary endpoint is the standard deviation of postoperative spherical equivalent adjusted relative to the intended target, measured using data from the 1 year postoperative visit. Spherical equivalent is the refractive value used in the endpoint description; adjustment relative to target makes the measure about deviation from the intended correction rather than the postoperative value alone.

The endpoint description defines an accurate outcome as one in which the postoperative spherical equivalent equals the intended target. It also explains the direction of the mean difference: a value below zero would indicate undercorrection, while a value above zero would indicate overcorrection. The primary endpoint, however, is framed around variability, with a smaller standard deviation described as indicating greater predictability. The standard deviation captures the spread of individual outcomes around their mean; it is not itself the mean direction of any undercorrection or overcorrection.

As a general statistical concept, variability matters when the question is whether outcomes are consistent across individuals, not only whether their average is close to a target. A mean can conceal a broad spread, so a measure of dispersion adds information about how tightly outcomes cluster. The registry record does not report the observed standard deviation, the mean difference, the number of participants contributing to this endpoint, or an uncertainty interval. The intended interpretation is therefore clear from the endpoint definition, but the magnitude and precision of the study's result cannot be assessed from the registry record.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

What does the primary endpoint measure?

The primary endpoint is a standard deviation for postoperative spherical equivalent adjusted relative to the intended target. In general, a standard deviation describes how dispersed observations are around their mean. A smaller standard deviation means less spread in the measured outcomes, while a larger one means greater spread. Here, the registry's endpoint description uses that spread as an indicator of refractive predictability: outcomes clustered more closely together would be described as more predictable than outcomes with wider variation.

The standard deviation addresses consistency, not whether the group average exactly matches the target. The endpoint description separately explains the mean difference relative to target: a negative mean difference implies undercorrection, and a positive mean difference implies overcorrection. Those are distinct aspects of a distribution. A group could have a mean near target but substantial variability, or a more consistent spread with an average that is not exactly at target. These are general statistical distinctions; the registry record does not report either the observed standard deviation or the mean difference for this study.

What would an uncertainty interval add?

In general, a confidence interval expresses uncertainty around an estimate. It indicates which values are reasonably compatible with the data under the method used to construct the interval. It is not a guarantee that the true value lies inside the interval, and it does not replace the estimate itself. For a standard deviation, an interval would help readers judge the precision of the estimated variability. The registry record does not report a confidence interval for the primary endpoint, so its precision cannot be assessed from this record.

How does the design affect interpretation?

The study is listed as single-group, with allocation recorded as NA and masking as NONE. In general, a single-group study can describe outcomes observed after an intervention, but does not provide a concurrent control group for estimating a between-group treatment effect. Changes or outcomes can be described, but attributing them specifically to the intervention requires care because the design does not supply a randomized comparison. The registry record does not report a comparator, a randomized allocation process, or a masking procedure.

The endpoint is assessed at 1 year postop according to its timeframe. A time-specific outcome describes the measurement at that assessment point; it does not by itself explain the course of outcomes at other times. The record does not report additional primary endpoint assessments or the timing of any other analyses.

What is known about the analysis plan?

The registry record describes the endpoint and its intended interpretation, but does not report the statistical analysis method used to estimate the primary result. It does not specify an analysis population, a missing-data method, or whether any covariate adjustment was performed. The number of analyses is recorded as 0. These omissions mean that readers cannot determine from the registry record how the planned endpoint was operationalized beyond the description given, or how the final estimate would have been calculated.

General statistical practice distinguishes a prespecified analysis from analyses chosen after outcomes are known, because the latter can increase the risk of selectively emphasizing favorable findings. The record does not report enough detail to assess prespecification or any interim analyses. No particular approach should be inferred from the endpoint wording alone.

How should planning illustrations be read?

The registry's sample-size assumptions describe planning illustrations based on the observed effect, rather than the trial's own design. They specify a two-sided significance level of 0.05, equal allocation between two hypothetical arms, a normal approximation, and effects set as stated fractions of the observed effect. For outcomes other than time-to-event outcomes, the illustrations are expressed as participants per arm. These are assumptions for planning calculations, not a description of the actual single-group study design.

A planning illustration asks how many participants per arm might be needed under stated assumptions to detect a chosen effect with a specified power. It does not describe how many participants were actually enrolled in this single-group study, nor does it establish the study's observed power. Sample-size calculations depend on assumptions about the target effect, variability, significance level, allocation, and analysis. If an observed effect is used as the basis for an illustration, that effect is an input to a hypothetical planning exercise, not evidence that the result is precise or reproducible. The numerical planning estimates are not reported in the registry record, so none can be interpreted or repeated.

Over-dispersion is a general concern when a statistical model assumes less variability than is present in the data. If it is ignored, standard errors may be too small and uncertainty may appear narrower than warranted. The registry record does not indicate that over-dispersion was relevant to this continuous endpoint or report a model for the analysis, so this concept should not be mistaken for a feature of the study's reported methods.

10. Confidence Intervals for Each Arm

No two-arm comparison is shown for this trial, but each arm's mean can be given with a t-based 95% confidence interval computed from the posted mean, spread and number of participants.

R code and output

arm <- c("Small Incision Lenticule Extraction")
n   <- c(113)
m   <- c(-0.22)
sp  <- c(0.48)
sd  <- sp      # posted standard deviations
for (i in seq_along(n)) {
  se <- sd[i] / sqrt(n[i]); h <- qt(0.975, n[i] - 1) * se
  cat(sprintf("CARM|%s|%d|%.3f|%.3f|%.3f|%.3f\n", arm[i], n[i], m[i], sd[i], m[i] - h, m[i] + h))
}

CARM  Small Incision Lenticule Extraction  113  -0.220  0.480  -0.309  -0.131

11. Multiplicity, Missing Data and Analysis Populations

The registry record names a primary endpoint and several secondary outcome domains. Considering multiple outcomes can create a multiplicity issue: as a general statistical matter, testing many questions can increase the chance of at least one apparently noteworthy result arising by chance. Researchers may address this through a prespecified hierarchy, an adjustment procedure, or a clearly stated distinction between confirmatory and exploratory analyses. The registry record does not report a multiplicity strategy, an outcome hierarchy beyond identifying the primary endpoint, or adjusted results.

The record lists the number of analyses as 0 and does not report numerical results for the primary or secondary outcomes. It also does not report the number of participants with available primary endpoint measurements, reasons for missing measurements, exclusions, or a missing-data approach. Enrollment is listed as 114, but that does not establish how many observations were analyzed at the 1 year postop visit.

Analysis populations are also not described in the registry record. It is not reported whether all enrolled participants were included in an analysis, whether participants were excluded according to prespecified criteria, or how incomplete follow-up was handled. These details matter because an estimate based on a subset can differ in meaning from one based on all enrolled participants, particularly if outcome availability is related to participant characteristics or outcomes.

Without these details, readers cannot judge the extent of missingness, the potential for selective availability of results, or whether any reported analyses account for multiple outcomes. No missing-data mechanism or multiplicity adjustment should be inferred from the outcome list alone.

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.

Most frequent non-serious adverse event terms

The registry record lists 0 serious adverse events among 114 participants at risk and 0 deaths. It also lists 10 other adverse events among 114 participants at risk. These are the recorded counts and denominators; they should be read as registry safety summaries, not as estimates recalculated from individual participant data.

The serious-event and other-event categories are distinct entries in the record. The label “other adverse events” does not, by itself, specify the event types, severity, timing, duration, or whether each event was judged related to the procedure or another intervention. The record does not provide those details in the registry record for this account. The serious adverse event terms are not listed, and the record does not describe how adverse events were collected or adjudicated.

Zero recorded events does not establish that an event is impossible or that no event occurred outside the recorded reporting period or definitions. Likewise, the other-event count alone does not establish causation. The registry counts should therefore be described as reported, without treating them as a complete clinical safety profile or as a comparison with another intervention. The record does not report a control group for a comparative safety assessment.

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

The registry record identifies a study question about refractive predictability after SMILE for high myopia and defines the primary endpoint as the standard deviation of target-adjusted postoperative spherical equivalent at 1 year postop. This is useful for understanding what the study intended to measure. The record does not report the primary estimate, its uncertainty, or the number of participants contributing data to it, so the observed degree of predictability cannot be judged from the information available.

The design also determines the scope of any interpretation. Because this is a single-group study, it does not provide a reported randomized comparison with another procedure or a control condition. A within-group outcome can describe what was recorded after the procedure, but it cannot by itself isolate the effect of the procedure from other explanations for the observed outcomes. The record does not report a comparative treatment effect.

The endpoint definition distinguishes the average direction of deviation from the spread of outcomes. Its description says that a mean difference below zero would imply undercorrection and a mean difference above zero would imply overcorrection, while a smaller standard deviation indicates greater predictability. Neither the mean difference nor the standard deviation is reported as a result. Those interpretive definitions therefore should not be converted into claims about the study's actual outcome.

The study's listed secondary outcome domains broaden the questions to visual acuity, refractive outcomes, stability, night vision disturbances, and corneal higher order aberrations. Numerical results for those outcomes are not reported in the registry record. The safety entries list 0 serious adverse events, 10 other adverse events, and 0 deaths, with 114 at risk for each adverse-event category; the event descriptions and attribution are not reported.

Accordingly, the record supports a description of the population focus, intervention, design, endpoint, and recorded safety counts. It does not support a conclusion about the magnitude or precision of the primary result, a comparison with another treatment, or the completeness of follow-up.

14. Why This Trial Matters Statistically

This trial is statistically notable because its primary question concerns predictability rather than only the average refractive result. The endpoint is the standard deviation of postoperative spherical equivalent adjusted relative to the intended target. As a general matter, reporting both location and spread can give a fuller description of outcomes: an average indicates the central tendency, while variability indicates how consistent individual outcomes are around that average.

The study also illustrates why a clearly defined estimand and result matter. The registry explains the timeframe, target adjustment, and interpretation of the standard deviation, but does not report the numerical estimate or its uncertainty. A well-defined endpoint helps readers understand what a result would mean; without the result and precision, it cannot answer how much variability was observed or how stable that estimate may be.

Its single-group design provides a further statistical lesson. Descriptive outcomes after an intervention and comparative effects are different quantities. A single group can characterize recorded outcomes, but without a comparator it cannot estimate the outcome difference between alternative strategies under the study design. The registry record does not report a randomized comparison or a control arm.

The listed secondary outcomes span several domains, including visual acuity, refractive correction, stability, night vision disturbances, and corneal higher order aberrations. In general, multiple outcomes raise questions about which were primary, how analyses were selected, and whether multiplicity was addressed. The record identifies a primary endpoint but does not report a multiplicity strategy or numerical results for the secondary outcomes.

Finally, the record distinguishes enrollment and safety counts from analysis results. It lists enrollment and adverse-event counts, yet does not report how many participants contributed to the primary endpoint or the endpoint estimate itself. This makes transparent reporting of outcome availability, analysis populations, and uncertainty especially important for interpreting the study.

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.

The principal limitation for statistical interpretation is that the registry record does not report the primary endpoint result. No standard deviation, mean difference, or confidence interval is given. The endpoint's meaning can be explained, but its observed magnitude and precision cannot be evaluated from this record.

The record does not state how many participants contributed data at 1 year postop, whether any primary outcome measurements were missing, or how missing observations were handled. It lists enrollment as 114, but does not identify the primary analysis population or report exclusions. This prevents assessment of how representative the endpoint data may be of enrolled participants.

The single-group design has no reported comparator and no randomized allocation. Therefore, the registry record does not support a treatment-effect contrast against another approach. Masking is listed as NONE; the record does not elaborate on outcome-assessment procedures or the consequences of the unmasked design.

The registry lists secondary outcome domains but does not report their numerical results, their analysis populations, or how multiplicity was handled. The number of analyses is recorded as 0. The statistical analysis method, any covariate adjustment, and any interim analyses are not reported in the registry record.

The safety information is limited to counts: 0 serious adverse events, 10 other adverse events, and 0 deaths, each with 114 at risk in the corresponding entry. The record does not provide the event terms, timing, severity detail, attribution, or follow-up context. The planning assumptions describe hypothetical sample-size illustrations, not the actual design; the numerical illustrations are not reported in the registry record.

16. Related Tutorials

17. Related Statistical Calculators

18. Sources

• ClinicalTrials.gov: VisuMax Femtosecond Laser Small Incision Lenticule Extraction for the Correction of High Myopia — NCT02528123.
• Registry record last updated 2024-04-23; status last verified 2024-04.
• Results first posted 2024-04-23.
• 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

NCT02528123 is a completed, single-group study sponsored by London Vision Clinic. It evaluates small incision lenticule extraction using the VisuMax femtosecond laser for high myopia. The registry record lists enrollment of 114, a study start of 2016-04, and primary completion on 2020-12-07. Phase and allocation are listed as NA, and masking as NONE.

The primary endpoint is the standard deviation of postoperative spherical equivalent adjusted relative to the intended target at 1 year postop. The record describes a smaller standard deviation as indicating greater predictability but does not report the endpoint estimate or its uncertainty. Secondary outcome domains include visual acuity, refractive outcomes, stability, night vision disturbances, and corneal higher order aberrations; numerical results are not reported in the registry record.

The safety entries list 0 serious adverse events, 10 other adverse events, and 0 deaths, with 114 at risk for each entry. The record does not report a comparator, analysis population, missing-data method, or multiplicity strategy.