Saphenous Nerve Block vs. Femoral Nerve Block for Total Knee Arthroplasty: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Saphenous Nerve Block vs. Femoral Nerve Block for Total Knee Arthroplasty (NCT01505374), a randomized na study. The primary outcome is Visual Analogue Scale Pain Score.

2. Clinical Question

This is a within-person clinical question in the sense that the brief summary describes each patient receiving both techniques, one on each leg. That design can make each person their own reference for comparing pain, because some characteristics are shared across the two legs. However, the registry record does not explain whether the reported statistical comparison accounts for the fact that the two leg measurements come from the same person.

The question is limited to the posted primary outcome: pain in each leg during the early postoperative period. The study also lists outcomes concerning opioid use, thigh strength, satisfaction, block duration, block success, and postoperative complications. Those outcomes may address other aspects of the comparison, but they do not change what the primary endpoint measures. The registry record does not report a broader primary question combining pain with muscle function or recovery.

3. Trial Design

The registry describes a randomized, double-masked, parallel study and lists the phase as not applicable. Its enrollment is 60, and its status is completed. The record identifies Hospital for Special Surgery, New York, as the sponsor. The start is recorded as 2012-03 and primary completion as 2013-09.

The brief summary describes a bilateral knee-replacement setting: participants were randomly assigned to receive a femoral nerve block on one leg and a saphenous nerve block on the other. Pain and thigh muscle strength were to be assessed before surgery, 6-8 hours following anesthesia administration, and on postoperative days 1 and 2. The primary outcome is described as postoperative pain in each leg within the first 24 hours. The registry’s parallel-model classification and its description of two techniques applied to different legs of the same patient are both relevant when interpreting the results. The record does not explain how the model label relates to the leg-by-leg assignment or how any within-person dependence was handled in analysis.

The outcome summaries report n=59 for the saphenous-block leg and n=59 for the femoral-block leg. The record does not explain why those outcome counts differ from the stated enrollment, or whether the counts refer to people, legs, or available observations. The reasons for any missing outcome measurements are not reported in the registry record.

The listed procedures are called Study Technique and Control Technique, and the arm descriptions do not add procedural detail. The brief summary gives a rationale for comparing the blocks: femoral block may cause thigh muscle weakness, while a saphenous block is hypothesized to numb the area around and below the knee while preserving thigh muscle function. This is the study’s stated rationale, not a reported finding. Specific details such as allocation concealment, the masking procedures, or the original sample-size justification 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.

Before assignment: Patients were excluded prior to randomization if they had contraindications to spinal or epidural anesthesia, had chronic opioid use, pre-existing neuropathies in the limbs, allergies to pre-operative medications, contraindication to assigned nerve blocks, allergy to local anesthetic, American Society of Anesthesia 4-5, and non-English speaking.

Overall Study

Reasons for not completing this period:

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 Visual Analogue Scale Pain Score, measured over Up to postoperative day 1.

The primary endpoint is the mean visual analogue scale pain score for each leg, measured up to postoperative day 1. The registry describes the primary outcome as pain in each leg within the first 24 hours after surgery. Scores could range from 0 to 10, with higher values representing a worse outcome. The registry reports the mean and standard deviation for each technique.

A continuous pain scale can retain gradations in reported pain rather than reducing each observation to a category such as pain present or absent. Comparing group means describes the average reported score, while the standard deviations describe how spread out the scores are around those averages. Neither measure alone describes every participant’s experience, and an average difference does not show how scores overlap between groups.

The endpoint is suited to the stated question because it directly measures postoperative pain in the leg receiving each block. It does not, by itself, measure thigh strength, opioid use, satisfaction, duration of blockade, block success, or complications. Those are listed as secondary outcomes. Nor does the primary score alone establish whether any observed pain difference is important to patients; the registry record does not report a threshold for interpreting a clinically important difference.

7. Results Posted in the Registry

• Study Technique: Saphenous Nerve Block: mean 2.81 versus 2.47 on control; difference 0.34 (95% CI -0.53 to 1.21), standardized effect d = 0.14; z-test p = 0.442.

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.

Patient Satisfaction With Nerve Blocks

• 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:
  • Study Technique: Saphenous Nerve Block: mean 8.00 versus 8.40 on control; difference -0.40 (95% CI -1.25 to 0.45), standardized effect d = -0.17; z-test p = 0.355.
• Interpretation:
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between Study Technique: Saphenous Nerve Block and the control. The interval gives the range of effects that remain compatible with the data.

The registry lists secondary outcomes that cover several different domains: opioid use, thigh muscle strength, satisfaction with nerve blocks, duration of motor and sensory blockade, block success, and postoperative complications. These outcomes address aspects of the study’s rationale beyond pain, particularly the proposed balance between sensory pain relief and preservation of muscle function.

Numerical summaries and an unadjusted recalculation are posted for patient satisfaction. The other listed secondary outcomes do not have numerical results reported in the registry record. As a result, the available secondary evidence does not establish a broad or consistent pattern across pain, muscle strength, opioid use, block characteristics, and complications. It also cannot be used to confirm or overturn the primary pain comparison as a whole.

Secondary outcomes can help explain whether a primary finding sits alongside other relevant changes, but they are not automatically independent confirmation. Testing several outcomes increases the chance of at least one apparently notable result arising through random variation. In general, multiplicity procedures or a prespecified hierarchy can address this concern. The registry record does not report whether such a strategy was planned or used, and it lists no reported analyses in its analyses listing.

Accordingly, the secondary material should be treated as supplementary and incomplete. The satisfaction information is a result for one aspect of the experience, not evidence about every secondary domain. Unadjusted secondary results carry less weight when the number of tests, analysis plan, and reporting of the remaining outcomes are unclear. Their absence from the registry record should not be interpreted as evidence that an outcome was unchanged.

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

• Tracking Total Opioid Usage
• Preoperative and Postoperative Thigh Muscle Strength in Both Legs
• Patient Satisfaction With Nerve Blocks
• Duration of Motor and Sensory Blockade
• Rating the Success of the Nerve Blocks
• Postoperative Complications

9. Statistical Interpretation

10. Statistical Methods Explained

The primary outcome is continuous, and the registry reports a mean and standard deviation for each block technique. A mean is the arithmetic average of the observed scores. A standard deviation describes the spread of those scores around the mean; a larger spread indicates more variation among observations. These statistics provide a compact description, but they do not show the full distribution or reveal how strongly each patient’s two leg scores are related.

The posted mean difference compares the saphenous-block mean with the femoral-block mean. The estimate is 0.34, calculated as the saphenous mean minus the femoral mean in the posted comparison. A positive estimate therefore points toward a higher average pain score for the saphenous-block leg, and higher scores are defined as worse. This direction describes the estimate, not proof of a causal effect. The recalculated estimate is unadjusted: it is based on the posted summary statistics rather than an adjustment for other variables or a reported participant-level analysis.

The registry reports a p-value of 0.442 for the unadjusted recalculated comparison. In general, a p-value describes how surprising data at least as inconsistent with a specified null hypothesis would be if that hypothesis and the statistical model were correct. It is not the probability that the null hypothesis is true, and it does not describe the size or importance of an effect. A p-value should be interpreted alongside the point estimate, its uncertainty, the design, and the outcome’s clinical meaning. The registry record does not report the sponsor’s own p-value or statistical test details.

A confidence interval expresses uncertainty around an estimate under the assumptions used to construct it. In general, intervals that include zero for a difference do not rule out no difference, while also allowing nonzero differences in either direction. The primary interval crosses zero. That means the data do not tightly identify one direction of the difference. It does not prove equality. The registry record does not state the confidence level for the posted interval, so no confidence level is attributed to it.

The study’s description presents an important design feature: the two blocks were used on different legs of a person undergoing bilateral surgery. In general, paired measurements can be analyzed using the within-person difference, which accounts for the association between measurements from the same individual. That association can affect the standard error and interval. The registry record does not report the correlation between leg scores, a paired analysis, or enough participant-level information to reconstruct one. The recalculated comparison should therefore be read as a simple unadjusted calculation from the posted arm summaries, not as confirmation of the original trial analysis.

In general, over-dispersion means that observed variation is greater than a statistical model expects. When a model does not accommodate the actual amount of variation, its uncertainty estimates may be too narrow or too wide. The registry does not report a model for over-dispersion, and the posted continuous endpoint is summarized by means and standard deviations. The main design-specific uncertainty documented is the unreported handling of the two leg measurements from each participant, rather than a reported over-dispersion problem.

Planning illustrations use a different purpose from the outcome comparison. They show how many participants per arm would be required under stated planning assumptions to detect fractions of the observed effect at specified power levels. They are not a description of the trial’s original design, and the original design assumptions are not reported in the registry record. The illustrations use a two-sided significance level of 0.05, equal allocation between arms, and a normal approximation. A smaller target effect requires more participants because a subtle difference is harder to distinguish from random variation. Those calculations do not alter the observed outcome or supply evidence about whether the intervention worked.

The registry lists six secondary outcomes, including opioid use, thigh muscle strength, satisfaction, duration of motor and sensory blockade, block success, and postoperative complications. It reports no analyses in its analyses listing. The absence of a reported analysis does not mean that no analysis was conducted; it means the registry record does not describe one. The record also does not report whether the primary analysis was prespecified in a statistical analysis plan, how multiplicity was handled, or how missing observations and analysis populations were defined. Those details would be needed to assess the full analysis strategy.

11. Confidence Intervals and Precision

The primary mean difference is 0.34, with a confidence interval that extends from below zero to above zero. This interval includes no difference and also allows values in both directions. It therefore does not resolve whether the underlying average pain score is higher with the saphenous block or higher with the femoral block. The interval’s breadth is important: the point estimate alone can look more definite than the range of values compatible with the posted summaries.

In general, a confidence interval conveys precision by indicating how narrowly an estimate is located under the statistical assumptions used. A narrower interval indicates greater precision; a wider interval indicates greater uncertainty. An interval that crosses zero is not proof of equality, and it does not say that all differences within the interval are equally likely. The registry record does not report the confidence level or enough detail about the original analysis to assess all assumptions behind its interval.

The standard deviations, 2.42 for the saphenous-block scores and 2.38 for the femoral-block scores, are substantial in relation to the mean difference. This indicates that participant-level scores vary more than the difference between the two reported averages. Group summaries cannot reveal how often an individual’s two leg scores differed substantially, or whether a subset experienced a different pattern.

The uncertainty should also be considered in light of the bilateral design. Because the two techniques were applied to different legs of the same patient, within-person pairing may affect precision. The registry record does not report whether that relationship was incorporated in the posted comparison. Consequently, the reported interval is useful for describing the posted recalculated estimate, but its interpretation as a complete account of the trial’s design-specific uncertainty is limited.

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.

The sample-size table and curve are planning illustrations, not a report of the trial’s original design calculations. The listed figures are participants per arm, not total events. The assumptions are a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. The effect to detect is defined as the stated fraction of the observed effect. The registry record does not report the trial’s own original sample-size assumptions or justification.

The 80% power column gives the planned participants per arm for a design with an 80% chance of detecting the target effect under the stated assumptions, if that effect and model assumptions are correct. The 90% power column expresses the corresponding planning target for a higher chance of detecting that effect. These are design probabilities, not probabilities that the observed result is true. The figures do not establish the power actually achieved in this trial.

The planning illustrations use the observed difference as their reference effect and also consider planning for 75% and 50% of that observed effect. For the primary outcome, the participants-per-arm figures are 783 and 1,048 for 100% of the observed effect; 1,391 and 1,862 for 75%; and 3,129 and 4,189 for 50%, for 80% and 90% power respectively. As the target effect becomes smaller, more participants are needed to distinguish it from random variation. The curve represents this planning relationship rather than additional observed trial results.

This is not observed power. It is a forward-looking illustration based on the posted effect estimate and stated assumptions; it should not be used to infer that the trial did or did not detect an effect because of its sample size.

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 = 0.340; pooled standard deviation σ = 2.400.
2. n = 2 × (1.96 + 0.842)² × 2.400² / 0.340² = 782.2, rounded up to 783 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 <- 0.34                      # observed difference in means
sigma <- 2.40008333188662                     # 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(1505374)
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 = 783 (80% power), 1,048 (90% power)
 75% of observed effect: n per arm = 1,391 (80% power), 1,862 (90% power)
 50% of observed effect: n per arm = 3,129 (80% power), 4,189 (90% power)
power.t.test: 783.2 
Simulated power at n = 783 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.

14. Multiplicity, Missing Data and Analysis Populations

The registry identifies one primary endpoint and lists six secondary outcomes. It does not report a statistical analysis plan, a hierarchy of hypotheses, or a procedure for controlling multiplicity. When several outcomes or comparisons are examined, the chance of a nominally notable result can increase. In general, multiplicity adjustments or a prespecified ordering of tests can help distinguish confirmatory questions from exploratory ones. Whether either approach was used is not reported in the registry record.

The record’s analyses listing reports zero analyses. This does not demonstrate that investigators performed no analysis; it indicates that analyses are not described in that part of the registry record. The posted unadjusted comparison is a recalculation from summary statistics, rather than a reported sponsor analysis. The registry does not provide enough detail to establish whether the original comparison was prespecified, whether it included covariate adjustment, or how it accounted for the bilateral leg-level measurements.

Missing-data procedures and analysis populations are also not reported in the registry record. Outcome summaries list n=59 for each block technique, while registry enrollment is 60. The record does not state whether all enrolled participants contributed data, why the listed outcome counts differ from enrollment, how incomplete measurements were handled, or whether an intention-to-treat or other analysis population was used. The safety summaries use 60 at risk for each technique, but that does not explain the population used for the pain endpoint.

These omissions matter because analytic choices can influence estimates and their uncertainty. Without a reported plan for multiplicity, missingness, analysis populations, and within-person dependence, the posted summaries are interpretable as descriptive and unadjusted results, but they do not provide a complete account of the trial’s inferential methods.

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 0% reporting threshold.

Most frequent non-serious adverse event terms

The safety summaries list 60 participants at risk for serious adverse events in each technique group. No serious adverse events are recorded for either group. The serious-event terms list is empty. The record also lists other, non-serious adverse events: one for the saphenous-block group and none for the femoral-block group. Death counts are not reported in the registry record.

These counts are a limited description of what is entered in the registry. They do not establish that no unrecorded event occurred, and they do not show that the techniques have identical safety profiles. The registry record does not provide event descriptions for the non-serious event, event timing, severity details beyond the serious-versus-other distinction, or an analysis comparing the groups.

The safety figures should be read separately from the pain outcome. The listed risk groups each contain 60, while the pain summaries list n=59 for each technique; the record does not explain the difference in these reporting counts. Small or absent counts in a registry entry are not, by themselves, conclusive evidence about uncommon harms.

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

The posted results describe mean pain scores of 2.81 for the saphenous-block leg and 2.47 for the femoral-block leg, with standard deviations of 2.42 and 2.38. Since higher visual analogue scale scores indicate worse pain, the observed average is higher for the saphenous-block leg. The unadjusted recalculated mean difference is 0.34, and the interval crosses zero. The data therefore do not precisely establish the direction or size of an underlying difference.

This result does not show that the blocks are equivalent. Demonstrating equivalence requires a prespecified margin defining how much difference would be acceptable and an analysis designed to test that question. Neither an equivalence margin nor an equivalence analysis is reported in the registry record. A nonsignificant p-value also does not prove that there is no difference. It indicates that the posted comparison does not provide strong evidence against the null under its assumptions.

The result does not establish that either block is clinically preferable, that one technique preserves muscle function, or that either technique improves recovery. Although the study rationale discusses possible thigh weakness and the secondary outcomes include muscle strength, numerical results for that secondary outcome are not reported in the registry record. The primary endpoint measures pain, not strength, recovery time, or every aspect of postoperative care.

The bilateral study description may support a within-person comparison, but the registry record does not report how the paired nature of the leg measurements was reflected in the primary analysis. It also does not report a clinical threshold for an important pain difference, the original statistical plan, or missing-data procedures. Those limits make it inappropriate to treat the point estimate or p-value as a complete account of the evidence.

The most defensible reading is descriptive: the observed arm means are close, the saphenous-block mean is higher, and the uncertainty interval includes differences in either direction. These findings do not settle whether the techniques differ in pain or in the broader clinical outcomes named by the study.

17. Why This Trial Matters Statistically

This trial raises a useful statistical issue because its clinical comparison is described at the level of two legs within a person. When each participant receives both techniques, the resulting measurements may be paired rather than independent. Recognizing that structure is essential: the association between a person’s leg measurements can affect how uncertainty is estimated. The registry’s parallel-model label does not resolve this issue, and the record does not describe whether the analysis used the pairing.

The case also illustrates why an estimate, a p-value, and a confidence interval answer different questions. The mean difference gives the direction and size of the observed contrast; the p-value summarizes evidence against a specified null under a model; and the interval describes uncertainty around the estimate. A p-value that does not indicate a clear difference cannot establish equality, especially when the interval permits effects on either side of zero.

Another statistical lesson is that a pain endpoint cannot answer every question motivating a study. The rationale includes possible muscle weakness, while the primary outcome is pain and the secondary outcomes include strength and other domains. Without results across those domains, a reader cannot assess the full pattern of benefits and trade-offs described in the study rationale.

Finally, registry reporting supports interpretation only to the extent that analysis details are available. The absence of a reported statistical plan, multiplicity strategy, missing-data approach, and sponsor analysis makes it harder to determine how the posted summary relates to the trial’s intended confirmatory analysis. Clear reporting of design-specific analysis choices is especially valuable when the observations are structurally linked.

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.

A central limitation is that the study is described as assigning different blocks to the two legs of the same patient, but the registry record does not report whether the primary comparison accounts for within-person pairing. The posted recalculation uses arm-level summary statistics and cannot recover the association between the two legs. Without that information, the uncertainty estimate may not fully reflect the described design.

The record states an enrollment of 60, while the primary and satisfaction summaries list n=59 in each technique group. It does not explain this reporting difference, identify missing observations, or describe how missing data were handled. It also does not define the analysis population. These details are necessary to judge whether the reported summaries represent the intended participant group.

The primary outcome covers pain up to postoperative day 1, but the record does not give more detailed timing for the individual pain measurements used in the reported mean. The original statistical analysis plan, sponsor analysis, multiplicity strategy, and original sample-size justification are not reported in the registry record. The planning illustrations are not a substitute for the trial’s original design assumptions.

Secondary outcomes are listed, but numerical results are not reported for most of them. The available information therefore cannot describe the full set of outcomes relevant to the study rationale. In particular, the registry record does not provide numerical thigh-strength results, despite the stated interest in muscle function. The record also does not report participant characteristics or further eligibility details, limiting assessment of how broadly the findings might apply.

19. Related Tutorials

20. Related Statistical Calculators

21. Sources

• ClinicalTrials.gov: Saphenous Nerve Block vs. Femoral Nerve Block for Total Knee Arthroplasty — NCT01505374.
• Registry record last updated 2022-04-14; status last verified 2022-04.
• Results first posted 2017-05-01.
• 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

NCT01505374 is a completed randomized, double-masked study of saphenous versus femoral nerve block for bilateral total knee arthroplasty. The registry describes each patient receiving one technique on each leg. Its primary endpoint is mean visual analogue scale pain score up to postoperative day 1, with higher scores indicating worse pain.

The posted means are 2.81 for saphenous block and 2.47 for femoral block, with an unadjusted recalculated mean difference of 0.34 and p = 0.442. The confidence interval crosses zero. These findings describe the posted summaries but do not establish equivalence or a definitive difference. The registry record does not report the sponsor’s analysis, the handling of paired leg measurements, or the original statistical plan.

Safety entries record no serious adverse events in either group, one other adverse event for saphenous block, and none for femoral block; deaths are not reported. Numerical results for most listed secondary outcomes are not reported in the registry record.