Comparison of Outcomes in Mobile and Fixed-bearing Total Knee Arthroplasty: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Comparison of Outcomes in Mobile and Fixed-bearing Total Knee Arthroplasty (NCT01027819), a randomized phase 4 study. The primary outcome is Rotational Angle Between Femur and Tibia.

2. Clinical Question

The clinical question is whether the two bearing designs differ in this postoperative rotational-angle measure. The registry’s broader stated purpose is to compare clinical and radiological outcomes, but the primary endpoint posted for this record is specifically the femur-to-tibia rotational angle. Knee Society Score is listed as a secondary outcome; further details about its measurement or results are not reported in the registry record.

This is a question about a measured outcome at a specified follow-up point, not a complete comparison of every clinical outcome or later course. The available result describes group averages and their uncertainty. It does not by itself establish what happened to each participant, how the designs compare on unreported outcomes, or whether any difference is important to patients. The registry does not report eligibility details, baseline characteristics, or additional follow-up results, so the scope of the comparison is limited to what is posted.

3. Trial Design

The registry describes a phase 4, randomized, parallel-group study with quadruple masking. It records an enrollment of 80 and lists Seoul National University Hospital as the sponsor. The listed start is 2009-11, and primary completion is recorded as 2013-12. The record’s status is UNKNOWN.

Participants were assigned to one of two device arms: mobile-bearing or fixed-bearing total knee arthroplasty. The registry lists 40 participants in each arm for the primary outcome. The allocation field says randomized, but details of sequence generation, allocation concealment, and implementation are not reported in the registry record. The masking designation is quadruple; which people or roles were masked and how masking was maintained are not reported in the registry record.

The primary outcome is a continuous measurement reported as a mean and standard deviation. It is the rotational angle between the femur and tibia, in degrees, assessed 2 weeks after surgery by postoperative CT. The record also lists Knee Society Score as a secondary outcome and records two outcome measures, with one secondary outcome posted. It does not provide a posted analysis for the primary outcome in the registry’s analysis count, which is recorded as zero.

The posted record does not report the trial’s original sample-size assumptions, a detailed statistical analysis plan, participant flow, analysis populations, missing-data procedures, or protocol deviations. It also does not give further descriptions of the study setting or participant characteristics. These omissions prevent a detailed assessment of how the conduct and analysis were planned beyond the design labels and outcome information recorded.

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

Analysis population for the primary outcome, as defined in the registry: The rotational angle between the femur and tibia was collected by postoperative CT and asssessed between the transepicondylar angle in the femur and the vertical line of the AP axis of the tibia(the line between the insertion of the PCL and the medial 1/3 point of the tibial tuberosity).

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 Rotational Angle Between Femur and Tibia, measured over 2 weeks.

The primary endpoint is the rotational angle between the femur and tibia, measured in degrees. The registry says it was collected using postoperative CT and assessed between the femoral transepicondylar angle and the vertical line of the tibial anteroposterior axis. It describes the tibial reference line using the insertion of the posterior cruciate ligament and the medial one-third point of the tibial tuberosity. The assessment timeframe is 2 weeks.

This is a continuous endpoint: its values can vary along a numeric scale rather than being limited to categories such as event or no event. The record summarizes it with a mean and standard deviation for each arm. In general, an average describes the central value of a group, while the standard deviation describes how spread out participant measurements are around that average. Reporting both helps distinguish the group-level average from the variation among individual measurements.

A postoperative imaging measurement provides a way to compare the alignment-related outcome between the two bearing designs at the specified follow-up. The registry’s stated purpose includes clinical and radiological outcomes, and this endpoint represents the radiological component posted as primary. The record does not report a threshold for a clinically important difference, a target range, or how this measure relates to later outcomes. Therefore, the result can describe the measured angle and its precision, but the clinical meaning of a particular value cannot be determined from the posted endpoint description alone.

7. Results Posted in the Registry

• Fixed Bearing: mean -0.66 versus -0.48 on control; difference -0.18 (95% CI -1.00 to 0.64), standardized effect d = -0.10; z-test p = 0.668.

8. Statistical Interpretation

9. Statistical Methods Explained

The primary outcome is continuous, so the registry reports a mean and standard deviation for each arm. In general, the mean is the arithmetic average of the observed values. The standard deviation describes the dispersion of values around that average. A mean difference compares the two group means in their original measurement units, here degrees. The reported contrast is fixed-bearing minus mobile-bearing: -0.18 degrees. A negative value means the fixed-bearing group mean is numerically lower on this scale than the mobile-bearing group mean. The result describes a difference in averages, not the difference experienced by every individual participant.

The groups each contain 40 participants in the posted primary outcome results. The mobile-bearing mean is -0.48 degrees and its standard deviation is 2.18 degrees; the fixed-bearing mean is -0.66 degrees and its standard deviation is 1.52 degrees. In general, standard deviations help readers understand whether individual observations cluster closely around the group mean or show more spread. They are not uncertainty intervals for the group mean. The registry does not report distributions, outliers, or participant-level measurements, so the shape of the outcome data cannot be assessed from the posted summaries.

The reported interval for the mean difference runs from -1.004 to a positive upper bound. In general, an interval estimate expresses uncertainty around an estimated effect: values near the estimate may be more consistent with the observed data than values far from it, depending on the method and assumptions. An interval that includes zero does not rule out a zero difference under the interval procedure. It also does not prove that the true difference is zero. The confidence level and calculation details for this interval are not reported in the registry record.

The unadjusted p-value is 0.668. In general, a p-value is calculated under a null hypothesis and test assumptions; it indicates how unusual data at least as incompatible with that null would be if the null and assumptions held. It is not a measure of the probability that the null is true, nor does it quantify the probability that the observed result occurred by chance in an everyday sense. The registry does not report a full statistical analysis plan or identify the specific test, so the p-value should be read as the unadjusted value posted for this comparison.

The record also reports a standardized effect estimate of -0.096. In general, a standardized effect expresses a difference relative to a measure of variability, rather than in the original unit of degrees. Such a scale can assist comparisons across measures, but its interpretation depends on context and should not replace the original-unit difference. No benchmark for a clinically meaningful standardized effect is reported in the registry record.

The results are labelled unadjusted. In general, an unadjusted comparison uses the posted group summaries without accounting for additional covariates or other factors in a model. The registry does not report adjusted results or specify whether any adjustment was planned. Because the design is randomized, a group comparison may be intended to estimate the effect of assignment, but assessing that intention against the analysis requires information about the analysis population and missing outcomes that is not reported in the registry record.

Two outcome measures are recorded, and Knee Society Score is listed as a secondary outcome. The record reports one secondary outcome as posted, but it does not provide a secondary result in the registry record for this analysis. The number of analyses is recorded as zero. This means the available record does not give a detailed list of analyses or enough information to reconstruct the statistical plan. It should not be inferred that no analyses were performed; the scope of the posted analysis information is limited.

Multiplicity arises in general when several outcomes, time points, groups, or analyses are tested. Repeated testing can increase the chance of at least one apparently unusual result under a collection of null hypotheses. The record does not state whether multiplicity was considered or whether any adjustment was used. The primary outcome is identified, but that label alone does not reveal the prespecified hierarchy among all outcomes or tests.

For a continuous outcome, standard errors and intervals depend on assumptions about the sampling distribution and how variability is estimated. In general, larger samples can make estimates more precise when other factors are similar, while substantial variability can make them less precise. The posted means and standard deviations allow a summary comparison, but the registry does not give diagnostics or details needed to assess assumptions such as distributional shape, influential observations, or the specific interval method.

The trial’s original sample-size justification, interim analyses, and analysis populations are not reported in the registry record. No protocol-level assumptions should be inferred from the planning illustrations associated with the observed effect: those are retrospective illustrations, not evidence of the original design plan. Likewise, the posted result does not establish whether any interim monitoring or preplanned adjustment took place. The methods information therefore supports interpretation of the reported summaries, while leaving important design and analysis details unresolved.

10. Confidence Intervals and Precision

The estimated mean difference is -0.18 degrees, with the fixed-bearing mean compared against the mobile-bearing mean. The reported interval extends from -1.004 to a positive upper bound, so it spans values below and above zero. This indicates that the data do not pin down the direction of the underlying difference precisely: the result is compatible with a negative difference, a value near zero, and a positive difference under the reported interval procedure. The exact confidence level and interval method are not reported in the registry record.

Precision is distinct from whether a result crosses a statistical threshold. In general, a narrow interval indicates less uncertainty about the effect’s magnitude than a wide interval, while a broad interval leaves more possible values compatible with the data. The posted interval’s span across zero means the data do not provide a precise directional conclusion. It should not be read as proof of equivalence, since equivalence requires a prespecified margin and an analysis designed to assess whether the interval lies within that margin. No equivalence margin is reported in the registry record.

The standard deviations are 2.18 degrees for the mobile-bearing arm and 1.52 degrees for the fixed-bearing arm. These describe variation among participants, not the precision of the difference itself. The group sizes are 40 in each arm, and the interval reflects uncertainty in the comparison. The registry does not report an outcome-specific threshold for meaningful change, so the relation between statistical uncertainty and clinical importance remains undetermined.

The unadjusted p-value of 0.668 is consistent with the interval’s inclusion of zero, but neither statistic establishes that the groups are identical. A more precise interpretation would require the protocol’s target difference, interval confidence level, and analysis details, none of which are reported in the registry record. The safest reading is that the estimated group difference is small in the original unit and remains uncertain over a range crossing zero.

11. Planning a Similar Trial: Sample Size

This is a planning illustration, not a re-evaluation of the original trial. It asks: if a new study wanted to detect a difference like the one observed, or a smaller one, how large would it need to be? It is deliberately not “observed power” of the finished trial, which is not a meaningful quantity.

The sample-size illustrations are planning calculations based on the observed effect, not the trial’s original design assumptions. For this continuous outcome, the figures are participants per arm, not total events. The stated planning assumptions are a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. The calculations use an effect set to the stated fraction of the observed effect. They do not report the trial’s original sample-size justification, which is not reported in the registry record.

For planning at 100% of the observed effect, the illustration gives 1,711 participants per arm for 80% power and 2,291 participants per arm for 90% power. At 75% of the observed effect, it gives 3,042 participants per arm for 80% power and 4,072 participants per arm for 90% power. At 50% of the observed effect, it gives 6,844 participants per arm for 80% power and 9,162 participants per arm for 90% power.

In general, power is the probability that a planned study will detect an effect of a specified size under the assumed model and significance threshold, if that effect is present. The 80% and 90% columns therefore describe different planning targets: the latter aims for a greater chance of detecting the specified effect, under the same assumptions, and requires more participants in these illustrations. Smaller effects are harder to distinguish from background variability, so the planning figures increase as the assumed effect becomes smaller.

These calculations use the observed effect as an input and are not observed power. They are conditional illustrations, not evidence that the trial was adequately or inadequately planned, and they do not establish what effect the trial could reliably detect under its actual design.

12. How the Sample Size Is Derived

Power is the probability that a study of a given size rejects the null hypothesis when a specific true effect exists. It depends on four things: the effect to detect, the variability of the outcome, the significance level (here a two-sided α of 0.05, so z_1−α/2 = 1.960) and the desired power (z_1−β = 0.842 for 80% and 1.282 for 90%).

Formula

n is the number of participants in each arm, Δ is the difference in means to detect and σ is the common standard deviation.

Worked calculation for this trial

1. Observed difference Δ = -0.180; pooled standard deviation σ = 1.879.
2. n = 2 × (1.96 + 0.842)² × 1.879² / -0.180² = 1,711.0, rounded up to 1,711 participants per arm for 80% power.

The smaller-effect rows in the table above repeat the same calculation with the effect multiplied by 0.75 and 0.5 (for ratios, applied to the logarithm of the ratio).

R code

alpha <- 0.05
z_a   <- qnorm(1 - alpha / 2)          # two-sided
delta <- -0.18                      # observed difference in means
sigma <- 1.8792019582791                     # pooled standard deviation
n_per_arm <- function(power, frac = 1) {
  ceiling(round(2 * (z_a + qnorm(power))^2 * sigma^2 / (delta * frac)^2, 6))
}
for (f in c(1, 0.75, 0.5))
  cat(sprintf("%3.0f%% of observed effect: n per arm = %s (80%% power), %s (90%% power)\n", 100 * f,
              format(n_per_arm(0.80, f), big.mark = ","), format(n_per_arm(0.90, f), big.mark = ",")))

# Check against R's built-in function (t-based, so very slightly larger)
cat("power.t.test:", round(power.t.test(delta = delta, sd = sigma, power = 0.80)$n, 1), "\n")

# Simulation: exact two-sample t-test using simulated means and variances
set.seed(1027819)
n    <- n_per_arm(0.80)
reps <- 20000
m_diff <- rnorm(reps, delta, sigma * sqrt(2 / n))             # difference in sample means
s2     <- sigma^2 * rchisq(reps, 2 * (n - 1)) / (2 * (n - 1))   # pooled sample variance
tstat  <- m_diff / sqrt(s2 * 2 / n)
cat(sprintf("Simulated power at n = %s per arm: %.3f (%d replicates)\n", format(n, big.mark = ","), mean(abs(tstat) > qt(0.975, 2 * (n - 1))), reps))

Output from running the code

100% of observed effect: n per arm = 1,711 (80% power), 2,291 (90% power)
 75% of observed effect: n per arm = 3,042 (80% power), 4,072 (90% power)
 50% of observed effect: n per arm = 6,844 (80% power), 9,162 (90% power)
power.t.test: 1711.9 
Simulated power at n = 1,711 per arm: 0.803 (20000 replicates)

The simulation draws many sets of group means and variances at the planned sample size and counts how often the two-sample t-test rejects the null. It should land close to the target power; at very small required sample sizes it can fall below it, because the normal approximation is rough there.

13. Multiplicity, Missing Data and Analysis Populations

The registry identifies the rotational angle between the femur and tibia as the primary outcome and lists Knee Society Score as a secondary outcome. Two outcome measures are recorded, with one secondary outcome posted. However, the record does not state whether the primary outcome was selected before results were known, how the secondary outcome was analyzed, or whether additional outcomes or time points were assessed. The number of analyses is recorded as zero, and detailed analysis information is not reported in the registry record.

In general, multiplicity is a concern when several statistical tests are conducted. Testing more outcomes or time points can increase the chance that at least one result appears notable even when no real difference exists. Researchers may address this through a prespecified hierarchy, adjusted significance thresholds, or other strategies. Whether any such approach was used in this trial is not reported in the registry record. The unadjusted p-value should therefore be understood as an unadjusted result, not as evidence that multiplicity was handled.

Missing data can also affect estimates if outcome measurements are unavailable for some participants. The primary results list 40 participants in each arm, but the registry does not report participant flow, the number randomized, the number with missing scans, reasons for missing measurements, or methods used to handle missing outcomes. It is not possible to determine from the posted summaries whether the analysis included all randomized participants or only participants with observed measurements.

The registry also does not identify analysis populations, such as an intention-to-treat or per-protocol population, or explain whether exclusions occurred after randomization. In general, different analysis populations can answer different questions and may yield different estimates. Without those details, the result should be treated as the comparison represented by the posted group summaries, rather than assigned to an analysis population that the record does not specify.

14. 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.

The registry lists 40 participants at risk for serious adverse events in each arm and records zero serious adverse events for both mobile-bearing and fixed-bearing groups. It also lists 40 participants at risk for other adverse events in each arm and records zero other adverse events for both groups. These are the event counts posted in the registry record.

A recorded zero means that no events of that category are entered for the listed group in this record. It does not by itself establish that no events occurred outside the definitions, observation period, or reporting process used. The registry does not provide further details about adverse-event definitions, collection methods, follow-up duration for safety, severity grading, or causal attribution. Death counts are not reported in the registry record.

In general, safety counts should be interpreted in relation to how events were defined, how completely they were collected, and how long participants were observed. Those details are not available in the posted information. The figures therefore describe the adverse-event entries in the record and do not support broader conclusions about comparative safety.

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

The posted primary result describes a small numerical difference between the group means in the rotational-angle measure at 2 weeks. The fixed-bearing mean was -0.66 degrees and the mobile-bearing mean was -0.48 degrees, with a reported mean difference of -0.18 degrees. The reported interval extends across zero, and the unadjusted p-value is 0.668. Together, these statistics indicate uncertainty about the direction and magnitude of the underlying difference; they do not establish that the group means are identical.

The result is specific to one radiological measure at one recorded timeframe. The registry states a broader aim of comparing clinical and radiological outcomes, but the posted primary result does not settle the comparison on Knee Society Score or other outcomes. Nor does it show how the measurement changes beyond 2 weeks, since further time points are not reported in the registry record.

Statistical compatibility with zero is not the same as evidence of equivalence or non-inferiority. Those conclusions require a prespecified margin and methods suited to the question. The registry does not report such a margin or a corresponding analysis. Similarly, a p-value is not a measure of clinical importance, and the record does not supply a threshold for a meaningful change in the rotational angle.

The findings also cannot establish that one bearing design improves outcomes, harms outcomes, or is preferable. This statistical summary describes the posted group averages, variation, and uncertainty; it is not medical advice. Interpreting broader clinical implications would require additional outcome results and details about the trial population, implementation, analysis, missing data, and follow-up. Several of these details are not reported in the registry record.

The randomized label describes the intended allocation design, but the record does not report enough information to independently assess how randomization, masking, participant retention, or the analysis were carried out. The numerical comparison should therefore be read as a result in the registry, with those design and reporting limits in mind rather than as a complete account of the study’s evidence.

16. Why This Trial Matters Statistically

This trial illustrates why a point estimate should be read together with its uncertainty. The difference in average rotational angle is -0.18 degrees, but the reported interval crosses zero. The estimate alone gives a single location; the interval communicates that the data remain compatible with a range of possible group differences. A small point estimate does not, by itself, establish either a negligible effect or equivalence.

The endpoint is continuous and reported in its original unit, degrees. This makes the group means and mean difference directly interpretable on the measurement scale, while the standard deviations reveal participant-level variability. The two arms have different posted standard deviations, which reinforces the importance of considering spread as well as average values. The record does not provide enough distributional detail to evaluate individual patterns or the shape of the measurements.

The sample-size illustrations also demonstrate the dependence of planning requirements on the effect size selected. Smaller assumed effects correspond to larger participant requirements, and a higher power target also calls for more participants under the stated assumptions. These are planning illustrations based on the observed effect, not a report of the trial’s original design justification or observed power.

Finally, this record shows the limits of interpreting registry results without a detailed analysis plan. The primary endpoint and study design are identified, but methods for missing data, analysis populations, multiplicity, and interval construction are not reported. Such details matter because they clarify what a reported estimate represents and how far it can be generalized. The statistical lesson is to separate the observed estimate, its uncertainty, and the broader clinical claims that the posted data do not establish.

17. 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 record provides limited information about eligibility criteria, baseline characteristics, participant flow, and the study setting beyond the named sponsor. Consequently, the extent to which the enrolled participants represent a broader population cannot be assessed from the registry information. The record lists an enrollment of 80 and 40 participants in each primary outcome arm, but it does not describe recruitment, exclusions, or retention.

Details of random sequence generation and allocation concealment are not reported in the registry record. The record labels masking as quadruple, but does not identify who was masked or how masking was maintained. These omissions prevent a detailed assessment of the procedures supporting the stated design.

The primary endpoint is measured at 2 weeks, and no further primary outcome time points are reported. The listed secondary outcome is Knee Society Score, but its result and measurement details are not included in the registry record for this comparison. The broader purpose mentions clinical and radiological outcomes, while the posted primary result addresses only one radiological measurement.

The record does not report the confidence level or method for the interval, the statistical test underlying the p-value, the original sample-size justification, interim analyses, analysis populations, or missing-data procedures. It also does not give a threshold for clinical importance or an equivalence margin. These limitations constrain conclusions about precision, clinical interpretation, and the extent to which the posted comparison reflects the planned analysis.

Safety information is also sparse: event counts are listed, but definitions, ascertainment procedures, and further follow-up details are not reported in the registry record. The status is UNKNOWN. These omissions do not alter the posted numerical estimates, but they limit the context available for evaluating the study as a whole.

18. Related Tutorials

19. Related Statistical Calculators

20. Sources

• ClinicalTrials.gov: Comparison of Outcomes in Mobile and Fixed-bearing Total Knee Arthroplasty — NCT01027819.
• Registry record last updated 2012-12-20; status last verified 2012-12.
• Results first posted 2011-04-05.
• 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.

21. Record Summary

NCT01027819 is a phase 4, randomized, parallel-group study comparing mobile-bearing and fixed-bearing total knee arthroplasty for osteoarthritis of the knee. The registry lists quadruple masking, enrollment of 80, Seoul National University Hospital as sponsor, a start of 2009-11, primary completion of 2013-12, and status UNKNOWN.

The primary outcome is the rotational angle between the femur and tibia, measured by postoperative CT at 2 weeks and reported as a mean in degrees. The posted means are -0.48 degrees for mobile-bearing and -0.66 degrees for fixed-bearing, with a mean difference of -0.18 degrees and an unadjusted p-value of 0.668. The reported interval spans zero. Knee Society Score is listed as a secondary outcome.

The record lists zero serious and zero other adverse events in each arm. Further details on analysis populations, missing data, interval methods, and the original sample-size rationale are not reported in the registry record.