Does Implant Design Improve Postoperative Flexion?: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Does Implant Design Improve Postoperative Flexion? (NCT00380861), a randomized na study. The primary outcome is Knee Society Passive Flexion at 6 Months.

2. Clinical Question

The central statistical question concerns a continuous measurement: the angle, in degrees, reached when a medically trained professional moves the relaxed limb to maximum knee flexion. The posted primary outcome is the mean Knee Society passive-flexion measurement at six months. A difference in group means would describe the average contrast in this measured range of motion, not necessarily a difference in active movement or day-to-day ability.

The bilateral arrangement is important to interpretation. Each participant is described as receiving both designs, one in each knee, so the two knees from the same person may have related outcomes. The registry does not report how left and right implant assignment was determined, whether the analysis accounted for within-person correlation, or how the masked roles were implemented. Those details matter when judging the precision and design-specific interpretation of an estimated contrast.

3. Trial Design

The registry describes a prospective, randomized, comparative, multi-center study, with double masking and a parallel model. The study is marked completed. Its summary describes simultaneous bilateral surgery: each participant receives the RP-F design in one knee and the RP design in the other, with both operations on the same day. Thus, although the registry labels the model parallel, the clinical description indicates that each participant contributes knees exposed to both implant designs. The record does not explain this model-label distinction.

The intervention is total knee replacement with the PFC Sigma RP-F implant; the comparator is total knee replacement with the PFC Sigma RP implant. The RP-F description identifies a cemented posterior-stabilized component and standard posterior-stabilized surgical technique. The RP description identifies a rotating-platform cruciate-retaining system with a special insert. These are device comparisons within the same surgical setting, rather than comparisons of surgery against no surgery.

Enrollment is reported as 93. The primary outcome summaries report 93 for the RP-F knee and 91 for the RP knee. Those arm-specific outcome counts should not be treated as a statement about how many participants completed all follow-up or as a count of complete bilateral pairs. The registry does not report the reasons for the different outcome counts or the analysis population used for the primary comparison.

Random allocation is reported, but allocation concealment, the method used to assign designs to left or right knees, and the parties who were masked are not reported in the registry record. The original sample-size justification, the trial’s prespecified statistical analysis plan, and any interim-analysis plan are also not reported. The sample-size figures associated with the posted summaries are planning illustrations based on observed effects, not a record of the original design assumptions.

4. Participant Flow and Accounting

The registry reports how participants moved through each stage of the study. All counts below are exactly as posted; missing counts are not inferred from other stages.

Before assignment: The randomization was a two stage scheme in order to minimize bias that could potentially be introduced due to differences in preoperative range of motion (ROM) between a subject's right and left legs.

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 Knee Society Passive Flexion at 6 Months, measured over 6 months.

The primary endpoint is Knee Society passive flexion at six months, measured in degrees. For the assessment, the participant lies on their back while a medically trained professional moves the limb until the knee reaches maximum flexion. A goniometer measures the resulting angle. The registry reports the mean and standard deviation for each implant group.

Passive flexion is used as an objective measure of how far the knee can be bent when the limb is moved by an examiner rather than by the participant’s own muscle action. A difference in this measure can therefore describe a difference in passive range of motion. Because it is a continuous measure, averages can summarize the group-level outcome, while the standard deviation describes how spread out the individual measurements are around the average.

That measurement is not a complete account of recovery or knee function. It does not directly measure active flexion, pain, satisfaction, or ability to perform daily activities. The registry lists some of these as secondary outcomes, but they address different aspects of the experience. Passive flexion is relevant to the stated question about knee bend, while interpretation should remain specific to the measurement, assessment method, and six-month time point. The record does not report a threshold for a clinically important difference in degrees, so the posted mean contrast cannot by itself be classified against such a threshold.

7. Results Posted in the Registry

• PFC® Sigma™ RP, Posterior Stabilized Total Knee Replacement: mean 124.00 versus 124.20 on control; difference -0.20 (95% CI -3.35 to 2.95), standardized effect d = -0.02; z-test p = 0.901.

8. Secondary Efficacy Endpoints

The registry also posts results for secondary outcomes. For each outcome below, the endpoint type, the test applied to the posted summary statistics, the result and its interpretation are given in turn. Secondary endpoints are usually not adjusted for multiple testing, so they support rather than confirm the primary finding.

Subgroup Analysis - Passive Flexion Analyzed and Reported for Pre-op, 6- and 12-months, for Subjects With Less Than 120 Degrees Passive Flexion Pre-op in Either Knee

• Endpoint type: Continuous: a mean value per participant, compared as a difference in means.
• Test: Unadjusted two-sample z-test for the difference in means, using the posted standard deviations.
• Result:
  • Passive Flexion for Standard Knee Device at 6-months: mean 118.90 versus 111.60 on control; difference 7.30 (95% CI -0.03 to 14.63), standardized effect d = 0.70; z-test p = 0.051.
  • Passive Flexion for Standard Knee Device at 12-months: mean 120.20 versus 111.60 on control; difference 8.60 (95% CI 2.72 to 14.48), standardized effect d = 0.96; z-test p = 0.004.
• Interpretation:
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between Passive Flexion for Standard Knee Device at 6-months and the control. The interval gives the range of effects that remain compatible with the data.
  • The 95% confidence interval excludes no difference, so the mean is statistically higher for Passive Flexion for Standard Knee Device at 12-months than for the control. This is an unadjusted, secondary comparison, so it is suggestive rather than confirmatory.

Patient Specific Anthropometrics

• Endpoint type: Continuous: a mean value per participant, compared as a difference in means.
• Test: Unadjusted two-sample z-test for the difference in means, using the posted standard deviations.
• Result:
  • Ankle Circumference in Centimeters (cm): mean 24.80 versus 50.20 on control; difference -25.40 (95% CI -26.79 to -24.01), standardized effect d = -5.24; z-test p = <0.001.
  • Skin-fold in Centimeters (cm): mean 3.23 versus 50.20 on control; difference -46.97 (95% CI -48.28 to -45.66), standardized effect d = -10.27; z-test p = <0.001.
• Interpretation:
  • The 95% confidence interval excludes no difference, so the mean is statistically lower for Ankle Circumference in Centimeters (cm) than for the control. This is an unadjusted, secondary comparison, so it is suggestive rather than confirmatory.
  • The 95% confidence interval excludes no difference, so the mean is statistically lower for Skin-fold in Centimeters (cm) than for the control. This is an unadjusted, secondary comparison, so it is suggestive rather than confirmatory.

American Knee Society (AKS) Score

• Endpoint type: Continuous: a mean value per participant, compared as a difference in means.
• Test: Unadjusted two-sample z-test for the difference in means, using the posted standard deviations.
• Result:
  • AKS for the High Flexion Knee Device: mean 87.00 versus 85.60 on control; difference 1.40 (95% CI -2.03 to 4.83), standardized effect d = 0.13; z-test p = 0.424.
• Interpretation:
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between AKS for the High Flexion Knee Device and the control. The interval gives the range of effects that remain compatible with the data.

The secondary outcomes broaden the assessment beyond passive flexion at six months. They include a subgroup defined by preoperative passive flexion, follow-up passive-flexion measurements, patient anthropometrics, the American Knee Society score, KOOS pain and activities-of-daily-living measures, patellar crepitus, satisfaction, and single-leg active flexion. These outcomes concern different features of knee status and patient experience, so they should not be treated as interchangeable measures of the primary endpoint.

The subgroup summaries describe passive flexion at preoperative and follow-up assessments. The means increase over time within the reported device-specific summaries, and the high-flexion device summaries are higher at later assessments. Those are descriptive patterns, not necessarily treatment-effect estimates: change over time, participant availability at each assessment, and the bilateral allocation structure all matter. The registry does not post formal analyses for these comparisons, and no adjustment for baseline values or within-person dependence is reported.

The posted American Knee Society summaries show broadly similar group averages, with a recalculated unadjusted interval that includes no mean difference. Anthropometric measurements are baseline characteristics rather than outcomes that establish a device effect. Other secondary measures are listed, but the registry record does not post their analysis results in the available secondary outcome summaries.

The primary result concerns passive flexion at six months and is itself uncertain. Secondary findings can add context, but they cannot replace the primary comparison or establish a broad conclusion about function. A large collection of outcomes and time points creates a multiplicity problem: if each is tested separately without adjustment, the chance of at least one apparently notable result can rise. The registry does not report a multiplicity strategy. Accordingly, unadjusted secondary results carry less weight than prespecified, appropriately analyzed outcomes, particularly when analyses are not reported.

Secondary outcomes registered for this trial, for reference (only those above were recalculated):

• Subgroup Analysis - Passive Flexion Analyzed and Reported for Pre-op, 6- and 12-months, for Subjects With Less Than 120 Degrees Passive Flexion Pre-op in Either Knee
• Patient Specific Anthropometrics
• American Knee Society (AKS) Score
• KOOS Pain Sub-score
• Ability to Perform Activities of Daily Living (ADLs) as Per the KOOS Sub-scale for ADLs
• Patellar Crepitus Defined as No Crepitus, FINE or COARSE
• Subject Satisfaction
• Single Leg Active Flexion (SLAF)

9. Statistical Interpretation

10. Statistical Methods Explained

The primary measure is continuous, so its reported mean summarizes the center of the observed values and its standard deviation describes their spread. The standard deviation is not the uncertainty in the mean. Uncertainty in a mean comparison also depends on the number of measurements and on the design and assumptions used for analysis.

What does the mean difference represent?

The recalculated primary contrast is the RP mean minus the RP-F mean. Its estimate is -0.2 degrees. A negative value means the posted RP mean is lower than the posted RP-F mean; it does not mean that every RP knee had less flexion. Group means condense individual observations, so they do not show the full distribution, overlap, or individual response patterns.

The posted recalculation uses the group-level means and standard deviations to estimate an unadjusted comparison. It does not adjust for baseline values or other participant characteristics. The registry’s bilateral description makes the analysis structure consequential: if each participant contributes one knee to each implant group, the outcomes may be paired. A paired analysis uses the within-person relationship between knees. The record does not give the paired differences or their variability, so that analysis cannot be reconstructed from the reported summaries alone. The unadjusted recalculation should therefore be distinguished from an analysis that explicitly accounts for both knees belonging to the same participant.

How should the p-value be understood?

The unadjusted p-value is 0.901. In general, a p-value measures how unusual a result at least as discrepant as the observed result would be under a specified null hypothesis and statistical model. It is not the chance that the null hypothesis is true, nor does it quantify the size or importance of an effect. Its interpretation depends on assumptions, including the analysis model and how the observations are treated. The trial’s own analysis model and its prespecified testing procedure are not reported in the registry record.

What does the confidence interval add?

A confidence interval communicates uncertainty around an estimated effect. In general, a narrower interval indicates greater precision under the method used; a wider interval indicates that a broader range of effects remains compatible with the data and assumptions. The interval for the primary recalculation extends from -3.35 to 2.95 degrees. Because it crosses zero, the interval includes no mean difference as well as differences in either direction. It does not demonstrate equivalence: that requires a clinically justified equivalence margin and a design and analysis intended to test equivalence.

Why does over-dispersion matter?

Over-dispersion is a general issue when observed variability is greater than a statistical model expects. If a model understates variability, its standard errors may be too small and its intervals too narrow; if variability is represented appropriately, uncertainty is better calibrated. The posted primary outcome is continuous and reports a standard deviation, but the registry does not report a model diagnostic or a specific over-dispersion assessment. More generally, analysis must reflect the outcome distribution and the relationship between observations. For this bilateral design, within-person dependence is a particularly relevant relationship; no correlation or paired-analysis details are reported.

What can the planning calculations tell us?

The sample-size illustrations use the observed effect as a planning reference and show the implications of targeting the stated fractions of that effect. They use a two-sided significance level of 0.05, equal allocation between arms, and a normal approximation. Such calculations illustrate how a planned study might be sized under stated assumptions; they do not establish what the original trial planned or whether its actual analysis followed these assumptions. The original design’s sample-size justification and analysis plan are not reported in the registry record.

In general, planning calculations depend on the target effect, variability, significance level, desired power, allocation, and analysis method. Smaller target effects are harder to distinguish from random variation and require more participants under otherwise similar assumptions. A planning number is not a guarantee that a future study will attain its target, and it should not be confused with an analysis of the observed trial result. For this record, the listed planning calculations are illustrations rather than evidence about the trial’s achieved power.

11. Confidence Intervals and Precision

The primary recalculated mean difference is -0.2 degrees, and its confidence interval runs from -3.35 to 2.95 degrees. The interval spans zero, so the posted summaries are compatible with no mean difference as well as with a modest mean difference favoring either implant. The estimate is close to zero, but the interval is the more informative guide to the range of uncertainty than the point estimate alone.

A confidence interval describes uncertainty under the statistical method and assumptions used to calculate it. It is not a guarantee that the true effect lies within the interval, and it does not account for uncertainties the calculation leaves out. In particular, the registry describes both implants being used by each participant, while the recalculation is based on unadjusted arm summaries. Without the paired knee-level information or within-person correlation, the interval may not reflect the uncertainty from an analysis that accounts for the bilateral design.

The interval also does not provide a clinical decision threshold. No clinically important difference or equivalence margin is reported in the registry record. Therefore, it cannot determine whether any effect within the interval would be meaningful to patients or whether the designs meet a formal equivalence criterion. The recalculated p-value of 0.901 is consistent with the interval crossing zero, but it should not be read as proof of no difference.

Precision is limited not only by the amount of data but also by how well the analysis matches the design. The primary outcome summaries report 93 for RP-F and 91 for RP, but the registry does not explain missing observations or report the number of complete participant-level knee pairs. The posted result is consequently best read as an unadjusted summary comparison with a confidence interval, rather than as a complete account of the trial’s design-specific uncertainty.

12. 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.

Read each planning entry as the number of participants per arm under the stated planning scenario. The illustrations use a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. Each scenario targets the stated fraction of the observed effect. These are planning assumptions for the illustration; the trial’s original sample-size assumptions and justification are not reported in the registry record. No time-to-event outcome is listed, so the convention of reporting a total number of events does not apply to these continuous outcomes.

The 80% power column describes a planned sample size under which the specified design and assumptions would have an 80% probability of detecting the target effect using the stated significance criterion, if that effect and model assumptions hold. The 90% power column represents a higher probability of detecting that same target effect under those assumptions, and therefore generally calls for a larger planned sample. These are prospective planning quantities, not measures of what happened in this completed trial.

Rows targeting a smaller fraction of the observed effect require more participants per arm, because smaller differences are harder to separate from random variation. The displayed calculations should be treated as illustrations of that relationship rather than as a recommendation or a reconstruction of the original protocol. They are not observed power.

13. 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 = -0.200; pooled standard deviation σ = 10.907.
2. n = 2 × (1.96 + 0.842)² × 10.907² / -0.200² = 46,689.8, rounded up to 46,690 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 <- -0.200000000000003                      # observed difference in means
sigma <- 10.9074226163594                     # 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(380861)
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 = 46,690 (80% power), 62,505 (90% power)
 75% of observed effect: n per arm = 83,005 (80% power), 111,119 (90% power)
 50% of observed effect: n per arm = 186,760 (80% power), 250,018 (90% power)
power.t.test: 46690.8 
Simulated power at n = 46,690 per arm: 0.797 (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.

14. Multiplicity, Missing Data and Analysis Populations

The registry lists multiple secondary outcomes, including repeated assessments over time and a subgroup analysis. Testing or interpreting many outcomes can increase the chance of seeing an apparently unusual result by chance, even when there is no underlying effect. Statistical approaches can address this issue by defining a primary comparison, adjusting for multiple tests, or treating secondary results as exploratory. The registry does not report a multiplicity adjustment plan or identify which secondary comparisons, if any, were prespecified as confirmatory.

The record lists nine secondary outcomes, while the posted secondary summaries cover eight. Formal analyses are not posted for the secondary outcomes. The registry also does not report a complete statistical analysis plan, the handling of correlated outcomes, or whether any subgroup analysis was prespecified. Without those details, isolated secondary patterns should be treated as exploratory and interpreted alongside the primary result rather than as independent confirmation.

Missing-data handling is not reported in the registry record. The primary outcome summaries list 93 for the RP-F knee and 91 for the RP knee, but these figures do not explain why the counts differ or whether missing outcomes were imputed, excluded, or handled through another method. The record does not report reasons for missingness, sensitivity analyses, or a participant flow accounting for outcome availability. It also does not state how incomplete bilateral pairs were treated.

The analysis population is not reported in the registry record. It is therefore unclear whether the posted summaries reflect all randomized participants, participants with an observed measurement, or another defined population. Allocation concealment, the assignment method for left and right knees, and any interim-analysis plan are also not reported. These omissions do not alter the arithmetic of the posted summaries, but they limit assessment of whether the analysis fully reflects the randomized bilateral design and the planned outcome hierarchy.

15. 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 5% reporting threshold.

Most frequent serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: infection (1.1%), loosening (1.1%), polyethylene failure (1.1%).

The registry reports serious adverse events for the two implant groups with the number at risk and the number affected. For RP-F, 3 were affected among 93 reported at risk; for RP, 2 were affected among 91 reported at risk. The listed most frequent serious-event terms are Infection, Loosening, and Polyethylene Failure, each reported as 1.1%. These are registry-reported safety figures and should be read as counts and listed terms, not as a complete comparative safety analysis.

The record also reports other adverse events: 10 affected in the RP-F group and 8 in the RP group, with 93 and 91 reported at risk, respectively. It does not provide event timing, severity details for these other events, attribution to a particular knee, or a formal statistical comparison. Since participants received both designs in different knees, attribution and analysis at the participant or knee level could affect how group counts are interpreted. The registry does not explain those details. Deaths are not reported in the registry record.

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

The posted primary summaries show mean passive flexion at six months of 124.2 degrees for RP-F and 124.0 degrees for RP. The unadjusted recalculation estimates a mean difference of -0.2 degrees for RP relative to RP-F. Its confidence interval, from -3.35 to 2.95 degrees, includes zero and allows for differences in either direction. This is a description of the observed summary data and their uncertainty under the recalculation, not a declaration that the implant designs are identical.

The result does not establish equivalence or non-inferiority. Such conclusions require a prespecified margin and analysis appropriate to the question, neither of which is reported in the registry record. Nor does the result establish that either design improves flexion relative to no replacement: both groups received knee replacement, and the study compares implant designs. It also does not directly establish differences in active movement, pain, daily activity, satisfaction, or other aspects of function.

The bilateral design means that each participant’s knees may be statistically related. The registry description says each participant received both implants, but the reported comparison is a recalculation from group summary statistics and does not account for an unreported within-person correlation. The original sponsor analysis and its treatment of paired knees are not reported, so the recalculated precision should be interpreted cautiously.

More broadly, the posted results do not identify a clinically important threshold for passive flexion or report a complete analysis plan, missing-data approach, or multiplicity strategy. These limitations constrain how far the results can be generalized beyond the stated endpoint and time point. The record describes a particular implant comparison in participants undergoing same-day bilateral surgery; it does not establish results for other populations, procedures, follow-up periods, or clinical outcomes.

17. Why This Trial Matters Statistically

This study presents a useful statistical design question because each participant is described as receiving both implant designs, one in each knee. That arrangement can control for participant-level characteristics shared by both knees, but it also makes the within-person relationship central to estimation and precision. A statistical analysis that ignores pairing may represent the design differently from one that compares each person’s two knees directly. The registry does not report enough paired data to evaluate that distinction.

The primary endpoint is a measurable continuous outcome, and the posted means are close. Yet a small point estimate does not alone establish precision or equivalence. The confidence interval communicates that the summary data allow effects in either direction, while the lack of a reported clinical margin prevents a formal judgment about what degree of difference would matter.

The record also illustrates the separation between trial planning and retrospective planning calculations. Figures based on fractions of an observed effect can show how target effect size influences future sample-size needs, but they do not reveal the original trial’s rationale or its achieved power. The original design justification is not reported.

Finally, the registry includes a broad set of secondary outcomes and repeated time points. These can offer context but invite multiplicity and selective emphasis if analyses and priorities are not clearly specified. With no formal secondary analyses posted, and no reported multiplicity plan, careful interpretation should remain anchored to the primary endpoint and its limitations. In this way, the trial is statistically informative not only for the reported contrast but also for how design, analysis, and reporting determine what a result can support.

18. 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.

Several trial-specific limitations constrain interpretation. The registry describes bilateral surgery in which each participant receives both implant designs, but its model classification is parallel. It does not explain the classification or report the left-right allocation procedure, whether implant assignment was balanced by side, or how analyses accounted for two knees from the same person. These details are important for judging whether the recalculated unadjusted comparison matches the randomized design.

The primary outcome summaries have different arm-specific counts, and the registry does not explain the difference, identify the number of complete bilateral pairs, or describe missing-data handling. The analysis population and participant flow are not reported. The listed outcome counts therefore cannot establish whether all randomized participants contributed to each comparison.

The sponsor’s formal primary analysis and statistical analysis plan are not reported. The confidence interval and p-value discussed for the primary comparison are unadjusted recalculations from summary statistics, not a substitute for an analysis that might account for pairing or other prespecified features. The record also does not give a clinically important difference or an equivalence margin, limiting interpretation of the near-zero estimate.

Secondary outcomes span multiple measures and time points, but formal analyses and multiplicity procedures are not reported. Safety information is presented as group counts without a reported formal comparison, event timing, or attribution by knee. Generalizability is also limited by the specific population and simultaneous bilateral procedure described; characteristics beyond non-inflammatory arthritis and the study setting details are not reported in the registry record.

19. Related Tutorials

20. Related Statistical Calculators

21. Sources

• ClinicalTrials.gov: Does Implant Design Improve Postoperative Flexion? — NCT00380861.
• Registry record last updated 2023-07-24; status last verified 2023-07.
• 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.

22. Record Summary

NCT00380861 is a completed randomized, double-masked, multi-center comparison of the PFC Sigma RP-F and PFC Sigma RP knee implants. The registry describes participants receiving one design in each knee during simultaneous bilateral surgery. Enrollment is reported as 93.

The primary outcome is mean Knee Society passive flexion at six months. The posted means are 124.2 degrees for RP-F and 124.0 degrees for RP. A recalculated unadjusted comparison gives a mean difference of -0.2 degrees, with a confidence interval from -3.35 to 2.95 degrees and p = 0.901. The interval includes zero and does not establish equivalence.

The registry lists secondary outcomes and adverse-event counts, but does not report the trial’s primary analysis plan, paired-knee analysis details, missing-data approach, or multiplicity strategy. The recalculated result should therefore be read as an unadjusted summary comparison, with uncertainty about how well it represents the bilateral design.