Reliability of Sacral Slope, Pelvic Femoral Motion and Cup Ante-inclination on AP Pelvic Radiographs in Healthy Pts: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Reliability of Sacral Slope, Pelvic Femoral Motion and Cup Ante-inclination on AP Pelvic Radiographs in Healthy Pts (NCT04687306), a registered phase not stated study. The primary outcome is Intraclass Correlation Coefficient (ICC) for Consistency in Spinopelvic Radiographic Measurements..

2. Clinical Question

The clinical motivation in the registry is that validated anatomical landmarks for predicting risk of, and diagnosing, abnormal pelvic-motion patterns are not currently available. The study was intended to examine whether measurements of these angles can be reproduced between observers. Reproducibility is an important preliminary property of a measurement: if different observers obtain substantially different values from the same images, interpreting those values in clinical assessment becomes difficult.

The stated motivation mentions potential clinical utility in evaluating and treating patients with hip-spine syndrome. However, the study population is described as healthy individuals, not patients with that syndrome. The question addressed is therefore measurement consistency in the described healthy sample, not whether the angles diagnose a condition, predict an outcome, or guide treatment.

3. Trial Design

The record identifies a completed study sponsored by the University of Nebraska. Its title and summary describe an assessment of inter-observer reliability for sacral slope, pelvic femoral motion or angle, and cup ante-inclination measured on anteroposterior pelvic radiographs. The registry lists enrollment as 33 and the study status as completed. The start date is 2021-01-15, and primary completion is listed as 2024-01-31.

The summary describes healthy individuals without a history of spinal and/or lower extremity pathology. It says the measurements would be made in sitting and standing positions and that four individuals would evaluate the radiographs. The intervention entry describes “X-rays- total of 4.” The record does not clarify how that count relates to the different positions, measurements, or observers. The arm descriptions list Females as 15 females and Males as 15 Males, while the overall enrollment is listed as 33. The relationship between those figures is not explained in the registry record.

This is a measurement-reliability study rather than a randomized comparison of treatments. The registry does not describe allocation to treatment groups, and its arm labels do not establish a control group. The central comparison is between measurements made by observers, not between clinical interventions. The summary identifies the intended focus as inter-observer reliability, while the primary outcome description refers more broadly to reliability and consistency.

The primary outcome is an ICC for consistency in spinopelvic radiographic measurements, with a 95% confidence interval and a timeframe of the study visit, for up to 1 hour. The registry lists one outcome measure and zero analyses. It does not report an ICC estimate, a confidence interval, or detailed analysis results. A detailed protocol, recruitment method, eligibility criteria beyond the summary description, and a rationale for the enrollment target are not reported in the registry record.

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

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 Intraclass Correlation Coefficient (ICC) for Consistency in Spinopelvic Radiographic Measurements., measured over At study visit for up to 1 hour.

The primary endpoint is the intraclass correlation coefficient (ICC) for consistency in spinopelvic radiographic measurements. The record says it is used to evaluate consistency among four providers measuring sacral slope, pelvic femoral angle, and cup ante-inclination on radiographs in sitting and standing positions. A 95% confidence interval is specified for the outcome, and the timeframe is the study visit, for up to 1 hour.

In general, an ICC expresses how much measured variation is associated with differences among the subjects or objects being measured, relative to other sources of variation specified by the particular ICC formulation. Its interpretation depends on the measurement design and the exact model used. The registry describes ICC values as ranging from 0 to 1, with values closer to 1 indicating stronger agreement and values closer to 0 indicating weaker agreement. This is the registry’s stated interpretation; it is not a reported study result.

Reliability is relevant because a measurement that is not reproducible between observers may be difficult to interpret consistently. However, an ICC is not by itself evidence that a measurement is accurate, clinically useful, or capable of diagnosing or predicting a condition. Reliability and validity address different questions. The registry does not report the observed ICC values or confidence limits, so the precision and magnitude of the study’s reliability estimates cannot be assessed from the outcome information available in the record.

The precise ICC model, including whether it treats observers as fixed or random and whether the target is consistency or absolute agreement, is not reported in the registry record. The title emphasizes inter-observer reliability, but the description also contains a broader reliability statement. The exact operational definition used in the analysis therefore cannot be reconstructed from the record.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

What does an ICC describe?

An intraclass correlation coefficient is a class of statistics used when measurements are grouped or repeated, such as measurements of the same subjects by different observers. It is intended to characterize reliability or agreement under a defined measurement design. The registry describes an ICC range from 0 to 1 and states that values closer to 1 indicate stronger agreement, while values closer to 0 indicate weaker agreement. These statements describe the intended scale, not the study’s findings.

The term ICC does not identify a single interchangeable calculation. Different ICC forms answer different questions. Some focus on consistency, meaning whether observers rank or distinguish subjects similarly even if their measurements have systematic offsets. Other forms focus on absolute agreement, where such offsets also count against agreement. Models also differ in how they treat observers and whether the result applies to the particular observers studied or a broader observer population. The registry uses the word “consistency” in the primary outcome title, but the specific ICC model and operational definition are not reported in the registry record.

Why does the observer design matter?

Reliability analysis must reflect how measurements were collected. The study summary describes four individuals evaluating radiographs, and its stated aim is inter-observer reliability. To estimate that reliably, the analysis would need to define which images each observer measured, whether observers measured the same images, whether readings were independent or repeated, and how measurements were combined. The registry does not report those procedural details. It also does not specify whether intra-observer or test-retest reliability was analyzed; the outcome description includes a bracketed reference to inter-rater, intra-rater, and test-retest reliability rather than identifying a specific form.

For continuous measurements, an ICC can be affected by the variability among subjects. In general, a sample with a wider spread of true values can yield a different ICC than a more homogeneous sample, even when the size of observer disagreement is similar. This is one reason an ICC should be interpreted with information about the population and measurement setting, rather than as a universal property of an angle or instrument. The record describes healthy individuals and sitting and standing radiographs, but does not provide the distributions of the measured angles or detailed sample characteristics.

What does the confidence interval add?

A confidence interval gives a range of values compatible with an estimate under the statistical model and its assumptions. In general, a narrower interval indicates greater precision, while a wider interval indicates more uncertainty. The interval is especially relevant for reliability results because an estimate alone may conceal substantial uncertainty. The registry specifies a 95% confidence interval for the primary outcome, but its limits are not reported. Neither precision nor the range of plausible reliability values can therefore be evaluated from the registry record.

Confidence intervals also depend on the analysis method and the data structure. Repeated measurements from the same participant are not independent observations. If the analysis treats them as independent when they are correlated, uncertainty may be misrepresented. Appropriate methods account for the clustering or pairing inherent in repeated ratings. The record does not describe its variance estimation, handling of repeated measurements, or other model assumptions, so these aspects cannot be assessed.

What is the distinction between consistency and agreement?

In general, consistency concerns whether observers measure subjects in a similar relative order, whereas absolute agreement additionally concerns whether they give similar numerical values. For example, a systematic tendency for one observer to record higher values could be treated differently under these two aims. The endpoint title says “ICC for Consistency,” but the registry does not specify enough detail to determine how systematic observer differences were handled. It is therefore appropriate to retain the registry’s stated endpoint description without asserting a more detailed interpretation.

Reliability is also distinct from validity. Reliability asks whether a measurement is reproducible under specified conditions. Validity asks whether it measures the intended construct or supports an intended inference. A highly reproducible measurement could still fail to represent the clinical phenomenon of interest. The registry’s motivation mentions potential clinical utility and the absence of validated anatomical landmarks, but the reported primary outcome is reliability, not a validation endpoint.

What can be concluded from the posted analysis information?

The record lists one outcome measure and zero analyses. The primary endpoint description states the ICC approach and confidence-interval type, but there are no analysis results, ICC values, confidence limits, or model specifications. As a result, the statistical method can be explained in general terms, while the study’s empirical reliability cannot be characterized.

There is no reported sample-size justification, so the enrollment figure cannot be interpreted as evidence that the study was designed to achieve a particular precision. In general, reliability-study precision depends on the number of subjects, observers, repeated measurements, and the structure of the data; the relevant details and a precision target are not reported in the registry record. The absence of a posted analysis also means that no recalculated estimates can be compared with a sponsor analysis.

Because the study is not a randomized treatment comparison, conventional treatment-effect language is not relevant to its primary question. The statistical target is measurement consistency. The interpretation should remain focused on whether observer measurements reproduce under the study conditions, while recognizing that the record does not publish the numerical result needed to answer that question.

10. Multiplicity, Missing Data and Analysis Populations

The registry lists one outcome measure, but that outcome concerns three measurements—sacral slope, pelvic femoral angle, and cup ante-inclination—made in sitting and standing positions. Four individuals are described as evaluating the radiographs. The record does not explain whether separate ICCs were calculated for each angle, position, observer combination, or measurement occasion. It also lists zero analyses, so no analysis-specific results or multiplicity approach are available.

In general, multiplicity arises when several statistical questions or estimates are tested or interpreted together. Repeated testing can increase the chance of at least one apparently notable finding, depending on the analysis plan. A plan may address that concern through a prespecified hierarchy or adjustment, or it may present estimates descriptively without formal hypothesis testing. The registry record does not describe any multiplicity strategy, adjustment, or hierarchy. Since no analyses are listed, it is not possible to determine how the multiple measurement dimensions were handled.

Information about missing data is also not reported in the registry record. There is no account of whether all planned radiographs were available, whether all observers completed all measurements, or how incomplete readings would be handled. The number analyzed for each reliability estimate is not reported. The enrollment entry and sex-specific arm descriptions do not resolve these analysis-population questions.

The record does not define an analysis population, such as all enrolled participants or only those with complete measurements. It also does not report exclusions, withdrawals, or the handling of measurements that could not be made. These omissions matter because reliability estimates depend on which participants and ratings contribute to the calculation. The study’s completed status does not supply those details. No inference about completeness or the treatment of missing observations should be made from the registry record alone.

11. 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 lists separate safety entries for Females and Males. For each group, the serious-adverse-event figures list 15 at risk and 0 affected; the other-adverse-event figures also list 15 at risk and 0 affected. Deaths are listed as 0 for each group. No serious-event terms are listed.

These are the figures recorded for the groups, not an analysis of treatment safety. The study is described as involving radiographs and observer measurement rather than a randomized treatment comparison. The record does not explain the safety-monitoring procedures, the definitions used for adverse events, or whether the safety counts correspond to all enrolled participants. It also does not reconcile the group figures with the overall enrollment entry.

Zero affected in the registry entries means no events are recorded in those entries; it does not establish that an exposure has no risk. In general, safety interpretation depends on the population observed, the duration and completeness of follow-up, how events are collected, and the precision of the resulting estimates. Follow-up detail beyond the study-visit timeframe for the primary endpoint is not reported in the registry record. The adverse-event figures should therefore be read narrowly as the posted counts, not as proof of safety or as a comparative safety result.

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

The study addresses a measurement question: whether observers can measure sacral slope, pelvic femoral angle, and cup ante-inclination consistently from anteroposterior pelvic radiographs taken in sitting and standing positions. The registry identifies the ICC as the primary reliability measure and specifies a 95% confidence interval. This is a relevant preliminary question when measurements are being considered for clinical assessment, because inconsistent measurement can make comparisons and interpretation less dependable.

The registry record does not report the ICC estimates or confidence intervals. It therefore does not establish whether reliability was high, low, or estimated precisely. The stated interpretation of the ICC scale explains the direction of the measure, but without the study values it cannot answer the study’s empirical question. Completion status alone does not fill that gap.

The study also does not answer whether these angles diagnose abnormal pelvic motion, predict risk, identify hip-spine syndrome, or improve treatment decisions. Those are distinct questions from reproducibility. The registry’s summary presents potential clinical utility as a motivation for studying the measurements, but the listed primary outcome is reliability. No diagnostic or patient-outcome results are reported.

The described population is healthy individuals, with no history of spinal and/or lower extremity pathology according to the summary. Findings from such a population, if reported, would pertain directly to the study conditions and population; they would not automatically establish performance in patients with spinal or lower-extremity conditions. The record does not provide results that would permit even a population-specific reliability judgment.

Thus, the record supports understanding the intended purpose and broad measurement framework, but not a conclusion about the reproducibility achieved. An interpretation would require the numerical ICC results, confidence intervals, analysis model, and sufficient detail about which participants and ratings were included. Without them, claims about reliability or clinical usefulness would go beyond the posted evidence.

13. Why This Trial Matters Statistically

Measurement reliability is a foundational statistical concern. Before a measurement is used to compare people, monitor change, or support clinical reasoning, it is useful to know whether observers can obtain consistent values under defined conditions. This study focuses on that issue for sacral slope, pelvic femoral angle, and cup ante-inclination on pelvic radiographs in sitting and standing positions.

The registry’s motivation is the lack of validated anatomical landmarks for predicting risk of and diagnosing abnormal pelvic-motion patterns. Assessing inter-observer consistency can help clarify whether these radiographic angles are reproducible enough to merit further study. Reliability is not equivalent to validity or clinical benefit, but a measurement that cannot be reproduced would pose an obstacle to those later questions.

The design also highlights why statistical methods need to match the data structure. Multiple angles, positions, and observers create repeated and potentially related measurements. A suitable reliability analysis must define what counts as agreement, how observers are treated, and which readings contribute to each estimate. Those details affect what an ICC means and how precisely it is estimated.

The registry record states the intended endpoint but does not post numerical ICC results or confidence intervals. That absence limits the statistical contribution that can be assessed from the record: the question is clear, but the estimated reproducibility is not. The distinction matters because a protocol objective is not evidence of an observed result.

For biostatisticians and clinical researchers, this study is a useful example of the difference between a measurement-reliability objective and a treatment-effect objective. The relevant evidence is not a between-arm effect, but observer-level reproducibility with uncertainty quantified. Clear reporting of the ICC model, measurement process, included ratings, estimates, and confidence intervals would be necessary for readers to judge how the findings apply.

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 principal limitation for interpretation is that the registry record does not report the primary ICC estimates or their 95% confidence intervals. Without those values, the magnitude and precision of the reliability findings cannot be evaluated. The record lists zero analyses, so it provides no analysis-specific results that would clarify the endpoint.

The statistical model is not described in sufficient detail to identify the ICC formulation. Although the primary outcome title refers to consistency and the study summary emphasizes inter-observer reliability, the record does not state the model, the treatment of observers, or whether absolute agreement was assessed. The exact definition of the reliability target is therefore uncertain.

The measurement procedures are also incompletely described. The registry says the radiographs were to be evaluated by four individuals and refers to a total of 4 X-rays, but it does not explain the relationship between images, positions, measurements, observers, or repeated readings. It does not state whether each observer rated the same images or whether ratings were repeated. These details are needed to understand the data structure and the scope of any reliability estimate.

The enrollment is listed as 33, while the arm descriptions list 15 females and 15 males. The record does not reconcile those entries or report the number included in each analysis. It also does not describe recruitment, withdrawals, missing ratings, exclusions, participant-level measurement distributions, or analysis populations. The study summary describes healthy individuals without a history of spinal and/or lower extremity pathology, which limits the population context; the record does not provide results in patients with those conditions.

Finally, the primary outcome is reliability, not clinical validity or utility. No diagnostic, predictive, or patient-outcome results are reported. These limitations mean the record can identify what the study set out to examine, but cannot support a detailed appraisal of its empirical reliability findings or their broader clinical applicability.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Reliability of Sacral Slope, Pelvic Femoral Motion and Cup Ante-inclination on AP Pelvic Radiographs in Healthy Pts — NCT04687306.
• Registry record last updated 2026-05-26; status last verified 2024-09.
• Results first posted 2026-05-26.
• 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

NCT04687306 is a completed study sponsored by the University of Nebraska. The registry lists enrollment as 33, a start date of 2021-01-15, and primary completion on 2024-01-31. The study summary describes healthy individuals without a history of spinal and/or lower extremity pathology.

The stated aim was to assess inter-observer reliability for sacral slope, pelvic femoral angle, and cup ante-inclination measured on anteroposterior pelvic radiographs in sitting and standing positions. Four individuals were to evaluate the radiographs. The primary outcome is an ICC for consistency with a 95% confidence interval, measured at the study visit for up to 1 hour.

The record lists one outcome measure and zero analyses. It does not report the ICC estimates, confidence intervals, detailed statistical model, missing-data handling, or analysis populations. Safety entries list zero serious and other adverse events affected and zero deaths for the Females and Males groups, with 15 at risk in each group as recorded. The group descriptions and enrollment figure are not reconciled in the registry record.