Stability of the Medial Pivot Total Knee Prosthesis: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Stability of the Medial Pivot Total Knee Prosthesis (NCT02577978), a randomized na study. The primary outcome is Oxford Knee Score (OKS) Pre-operative Through 2 Years Post-operative.

2. Clinical Question

The study is titled “Stability of the Medial Pivot Total Knee Prosthesis.” Its brief summary frames the research as a comparison of objective and subjective knee-stability measures after total knee arthroplasty with a medial pivot design versus a posterior stabilized design. The primary outcome listed is the Oxford Knee Score (OKS), a patient-completed measure of the patient's perspective on outcome after surgery. Its assessment schedule begins pre-operatively on the day of surgery and includes follow-up at 6 weeks, 3 months, 6 months, 12 months and 24 months.

The registry record also names secondary outcomes involving the KT-1000 Arthrometer at 30 and 90 degrees and a Get-up-and-Go test. These measures represent different kinds of information: a patient-reported score, an instrument-based assessment of knee stability, and a functional test. The record does not provide results for any of them. It also does not specify a single time-point comparison or a combined outcome that would resolve the broad research question into one treatment contrast.

3. Trial Design

This is a completed study sponsored by Northwestern University. The registry record lists a start date of 2014-04, primary completion in 2017-04 and enrollment of 61. It identifies the allocation as randomized and masking as single. The phase is listed as not applicable. The study model is recorded as single group, even though the interventions and arm descriptions identify two prosthesis groups. That mismatch makes the recorded design classification difficult to reconcile with the described comparison; the registry record does not explain it.

The two devices are the Medacta GMK Sphere prosthesis and the Medacta GMK posterior stabilized prosthesis. The arm descriptions characterize the Sphere design as ball-and-socket and the PS design as cam-and-post. The brief summary says that objective and subjective measures of knee stability following total knee arthroplasty were to be compared across medial pivot and posterior stabilized designs. The available record does not describe the randomization procedure, allocation concealment, or the practical details of masking.

The primary outcome is the Oxford Knee Score measured pre-operatively and at multiple postoperative assessments through 24 months. The secondary outcomes include KT-1000 Arthrometer assessments at two knee angles and the Get-up-and-Go test. The record lists 13 outcome measures overall, but does not provide their full descriptions in the information recorded for this study. It lists no analyses and flags the primary outcome as having several categories or time points that were not reduced to one comparison.

The registry record does not report the planned sample-size justification, a statistical analysis plan, interim analyses, or details about how repeated measurements were to be analyzed. Nor does it report outcome estimates, uncertainty intervals or hypothesis tests. Consequently, the design features that are recorded can be described, but the intended statistical comparison and the study's findings cannot be reconstructed from this 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.

Population: From the total 61 patients enrolled, 11 patients (7 sphere, 4 PS) were dropped from the study due to loss to follow-up. The remainder 50 had data collected.

6. Primary Endpoint

The registry lists the primary outcome as Oxford Knee Score (OKS) Pre-operative Through 2 Years Post-operative, measured over Pre-operatively on day of surgery, 6 weeks, 3 months, 6 months, 12 months and 24 months post-operatively.

The primary endpoint is the Oxford Knee Score (OKS), a patient-completed questionnaire intended to assess the patient's perspective on outcome after total knee arthroplasty. The record describes the score as ranging from 0-48, with a higher numerical value reflecting improvement in outcomes. It gives 48 as an example of a score corresponding to a “complete” return to pre-disease functionality. The recorded summary measure is a mean, with standard deviation as the measure of dispersion.

The OKS is relevant to this study because a prosthesis comparison is not limited to an instrument reading of joint movement or stability. A patient-reported score can capture the patient's reported experience of knee function and symptoms. That perspective complements the objective stability measures named among the secondary outcomes, although the registry record does not explain how these measures were intended to be combined or interpreted together.

Assessments are scheduled pre-operatively on the day of surgery and at 6 weeks, 3 months, 6 months, 12 months and 24 months post-operatively. This makes the primary outcome a series of measurements over time rather than a single endpoint at one specified follow-up. The registry record does not identify one time point as the primary comparison, give a summary measure of change, or state how the repeated measurements were to be analyzed. It also does not report any observed OKS values. Therefore, the endpoint definition and schedule are available, but the results and their precision are not.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

What do the mean and standard deviation describe?

General: For a continuous measure such as the Oxford Knee Score, a mean describes the average observed score in a group, while a standard deviation describes how spread out individual scores are around that average. They answer different questions: the mean indicates the group's central value, whereas the standard deviation reflects variation among participants. A standard deviation is not a confidence interval and does not, by itself, show how precisely a group mean or a difference between groups has been estimated.

The registry identifies the primary outcome as a mean with standard deviation. It does not provide the values of either summary for any group or assessment. Consequently, the record does not allow readers to assess the distribution of observed scores or the precision of a group comparison. The endpoint is recorded at several visits, so any reported means would need to be associated with their relevant assessment times to be interpretable.

Why does the assessment schedule matter?

General: Repeated assessments on the same participant are related observations, not independent pieces of information. An analysis that accounts for this relationship can distinguish patterns of change over time from differences between groups at a particular assessment. The choice of method can affect both the estimate and its uncertainty. A single visit comparison and an analysis of the full trajectory answer different statistical questions.

For this study, the registry lists pre-operative assessment and follow-up through 24 months. It does not report a longitudinal model, a primary comparison time point, or a prespecified summary of change. The record also states that the primary outcome has multiple categories or time points and was not reduced to one comparison. Thus, a reader cannot identify one primary statistical contrast from the registry record.

What does randomization contribute?

General: Random allocation is intended to make the groups comparable, on average, with respect to factors known and unknown at the time of assignment. Its value for interpretation depends on the allocation process being carried out as intended and on the analysis preserving the randomized comparison. Randomization does not eliminate uncertainty, guarantee identical groups, or replace reporting of the outcomes and methods.

The record lists randomized allocation and single masking, but does not describe the allocation procedure, concealment, who was masked, or the analysis population. It also lists a single-group model while naming two prostheses and describing two arms. These entries do not provide enough detail to resolve how the trial was organized or analyzed.

How should uncertainty be understood?

General: A confidence interval expresses the range of estimates reasonably compatible with the observed data under the assumptions of the statistical method. A narrow interval indicates greater precision than a wide interval, but an interval must be interpreted in relation to the outcome scale and the clinical question. A point estimate alone does not convey the full uncertainty around a comparison.

No confidence interval or other measure of uncertainty for the primary outcome is reported. The registry record also contains no group means, standard deviations, or hypothesis-test results. It is therefore not possible to describe the precision of the treatment comparison. No unadjusted recalculation is available from the posted primary outcome information, and the record reports no sponsor analysis to compare with a recalculation.

What does multiplicity mean for this study?

General: When several outcomes, time points or comparisons are examined, the chance of finding at least one apparently notable result can increase. A prespecified analysis plan can identify which outcome and contrast are primary and whether any adjustment or hierarchical testing is intended. Without those details, a collection of separate results may be difficult to interpret as a coherent test of one main question.

The registry identifies 13 outcome measures and multiple OKS assessment times, but does not report a multiplicity plan or analyses. It would be inappropriate to assume that an adjustment was or was not performed. The record also does not identify a single primary OKS time point.

What can be inferred from the planning assumptions?

General: Sample-size planning illustrations describe the number of participants that might be needed under stated assumptions about an effect and statistical method. They are not observed results and do not establish how well a completed study could detect its actual effect. A planning illustration based on an observed effect is especially distinct from the trial's original design justification.

The registry record does not report the trial's own sample-size justification. It also does not report the effect estimates needed to interpret a planning illustration based on observed effects. Thus, no conclusion about achieved power or the adequacy of the enrolled sample can be drawn from the available record.

10. Multiplicity, Missing Data and Analysis Populations

The primary OKS is recorded at several assessments, and the registry lists 13 outcome measures in total. Multiple outcomes and time points can create multiplicity: examining many contrasts can make selective emphasis on one result misleading unless the analysis plan clarifies which comparisons were primary and how other analyses were to be handled. The registry record does not report a multiplicity strategy, a hierarchy of outcomes, or a single primary OKS time point. It also reports no analyses, so it is not possible to determine what comparisons were actually conducted.

The record does not report how missing OKS assessments or missing secondary-outcome observations were handled. It does not state whether analyses used all available observations, imputed missing values, or restricted analysis to participants with complete follow-up. The number of participants with measurements at each visit is not reported. General statistical concern: missingness can affect an estimate when the probability of having an observed measurement is related to participant characteristics or outcomes. Without information on the extent and handling of missing data, the direction or size of any such effect cannot be assessed.

Analysis populations are also not described. The registry does not say whether results were intended to follow randomized assignment, reflect the prosthesis actually received, or use another population definition. The record lists randomized allocation, but provides no details of crossovers, adherence, exclusions or analysis sets. It is therefore not possible to establish whether a posted analysis would preserve the original randomized comparison.

These omissions are distinct from the absence of numerical results. Even if outcome values were available, the missing information about multiplicity, missingness and analysis populations would limit assessment of how robustly the results address the study question.

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

The registry record supports a description of the study's intended assessment: a comparison involving two prosthesis designs, with patient-reported OKS measured before surgery and at multiple postoperative visits, alongside named secondary measures of stability and function. It does not report the outcome results needed to describe the observed scores, estimate a difference between devices, or assess uncertainty. The scientifically appropriate interpretation is therefore limited to what was planned and recorded, not what the study found.

The record's enrollment of 61 is a study descriptor, not a result. It does not establish how many participants contributed measurements at each visit or whether the analysis had adequate precision for a particular contrast. The listed completion status likewise does not establish that the outcome results are available in the registry. No sample-size justification, statistical plan or analysis is reported, so the enrolled number cannot be interpreted in relation to a prespecified detectable effect.

The listed zero adverse-event counts should not be interpreted as proof that no adverse events occurred. The registry lists zero participants at risk in each group, which means those entries do not describe event experience among an identified at-risk cohort. Other safety information is not reported in the registry record.

More generally, a planned endpoint is not a finding, and a missing registry result is not a null result. The absence of estimates does not demonstrate equivalence, benefit, lack of benefit, or lack of change. It means that the recorded evidence is insufficient to quantify how the groups compared on the listed outcomes. The mismatch between the single-group model classification and the two named intervention groups adds uncertainty about the recorded design description, but does not resolve the missing results.

Accordingly, the available record can inform readers about the question and measurement schedule, but it cannot support a statistical conclusion about comparative outcomes or safety. Any such conclusion would require outcome data and sufficient analysis details, neither of which is reported in the registry record.

12. Why This Trial Matters Statistically

This trial raises a useful statistical distinction between a broad clinical question and a clearly defined primary contrast. The brief summary describes comparing objective and subjective knee stability after two types of total knee arthroplasty, while the primary outcome is a patient-reported score collected at several time points. These measures can address related but distinct aspects of outcome. A clear analysis plan would specify how each contributes to the main question; that plan is not reported in the registry record.

The repeated OKS schedule also highlights why time matters in clinical research. A comparison at one follow-up visit may not represent the pattern over the full follow-up period. Conversely, a difference in trajectories requires a method that accounts for the repeated observations on the same participants. The registry identifies the visit schedule but does not report the analysis approach or select one time point as the primary comparison.

The record further illustrates that study design classifications need to agree with intervention descriptions. Randomized allocation and two named prostheses suggest a comparative framework, while the model is recorded as single group. Since this discrepancy is unexplained, readers cannot confidently resolve the structure of the study from those fields alone.

Finally, registration of outcomes is not a substitute for reporting results. Here the record identifies the primary measure and multiple secondary outcomes, but reports no analyses or outcome estimates. This limits assessment of effect size, precision, missing data and multiplicity. Statistically, the most important conclusion is not a claim about which prosthesis performed better; it is that the record does not provide the results required to answer that question.

13. 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 central limitation is that the registry record does not report results for the primary or secondary outcomes. There are no OKS means or standard deviations, no group comparison, and no uncertainty estimates or hypothesis tests. The primary endpoint spans several visits, but no single comparison is reported. Consequently, there is no quantitative basis in this record for interpreting differences between the prosthesis groups or changes over follow-up.

Several design and analysis details are also absent. Although allocation is recorded as randomized and masking as single, the record does not describe the randomization procedure, concealment, who was masked, or the analysis populations. The single-group model classification is inconsistent with the two named intervention groups and arm descriptions, and the record does not explain that discrepancy. The protocol's statistical plan, sample-size justification, interim analyses, multiplicity approach and handling of missing data are not reported.

The primary outcome is scheduled repeatedly, and the record lists 13 outcome measures. Without a prespecified primary contrast or analysis details, readers cannot determine how the study intended to distinguish a key finding from the range of possible outcomes and time-point comparisons. The registry does not provide participant counts for individual follow-up assessments, so completeness of longitudinal data cannot be evaluated.

The adverse-event entries list zero at risk and zero events for both groups. They cannot be treated as a complete safety result because the entries do not identify participants at risk. Finally, the registry record does not report characteristics of the enrolled participants or detailed eligibility criteria in the recorded information. These limitations restrict both interpretation of the study design and appraisal of its findings.

14. Related Tutorials

15. Related Statistical Calculators

16. Sources

• ClinicalTrials.gov: Stability of the Medial Pivot Total Knee Prosthesis — NCT02577978.
• Registry record last updated 2023-05-19; status last verified 2023-05.
• Results first posted 2019-05-29.
• 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.

17. Record Summary

The completed study, sponsored by Northwestern University, concerns stability after total knee arthroplasty with the Medacta GMK Sphere and Medacta GMK posterior stabilized prostheses. The record lists randomized allocation, single masking, a single-group model, enrollment of 61, and a study period from 2014-04 to primary completion in 2017-04. The model classification does not align clearly with the two named prostheses, and the registry record does not explain the inconsistency.

The primary outcome is mean Oxford Knee Score, with standard deviation, assessed before surgery and at 6 weeks, 3 months, 6 months, 12 months and 24 months post-operatively. Secondary measures include KT-1000 Arthrometer assessments at 30 and 90 degrees and a Get-up-and-Go test. The record lists 13 outcome measures and no analyses; it does not report outcome results, a statistical plan, or the handling of missing observations and multiple comparisons. It therefore documents the study question and assessments without providing evidence to quantify the comparative outcomes.