interACTION: A Portable Joint Function Monitoring and Training System for Remote Rehabilitation Following TKA: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for interACTION: A Portable Joint Function Monitoring and Training System for Remote Rehabilitation Following TKA (NCT02646761), a randomized na study. The primary outcome is Value Was Evaluated as the Ratio of Change in the Activities of Daily Living Scale Score to the Costs of Rehabilitation..

2. Clinical Question

3. Trial Design

The registry describes a completed, randomized, parallel-group study with no masking. Its title concerns a portable joint-function monitoring and training system for remote rehabilitation following total knee arthroplasty. The study description says participants were followed for 10 weeks during outpatient physical therapy. The listed enrollment is 38. That enrollment figure should not be treated as the denominator for every posted outcome: the primary outcome results list 10 participants in each arm, and the registry record does not explain the relationship between the enrollment figure and those outcome denominators.

The InterACTION arm combined physical therapy scheduled 1 time per week with a home exercise program using the device. The standard physical therapy arm had visits scheduled 2 times per week and a standard home exercise program. Thus, the comparison concerns two rehabilitation approaches that differ in more than the presence or absence of a monitoring device: the described visit frequency and home program also differ. The registry record does not report how closely participants followed either program.

The study began in 2016-04, had primary completion in 2017-12, and is listed as completed. The registry record does not report the method used to generate or conceal random allocation, the intended sample size, or the protocol’s analysis plan. It also does not report whether outcome assessors were masked. Randomization can support a fair comparison when implemented and analyzed appropriately, but the registry description alone does not establish how those procedures were carried out. The posted primary group summaries and comparison are available, while the record does not list reported analyses for the secondary outcomes.

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: A total of 47 subjects were assessed for eligibility for the study. Nine were deemed ineligible and 38 were randomized into one of the 2 study arms. Subjects were recruited and randomized prior to surgery \& the baseline assessment was done approximately 2 to 4 weeks after surgery.

Overall Study

5. Baseline Characteristics

The table reproduces the baseline characteristics posted in the registry, which describe the participants analysed at baseline and help show whether the randomized groups were comparable.

Population: Baseline assessments were performed approximately 2 to 4 weeks after subjects underwent TKA.

6. Primary Endpoint

The registry lists the primary outcome as Value Was Evaluated as the Ratio of Change in the Activities of Daily Living Scale Score to the Costs of Rehabilitation., measured over 10 weeks.

The primary endpoint is the mean value, at 10 weeks, of the ratio of change in the Activities of Daily Living Scale score to charges for physical therapy treatment sessions. The change is defined as the 10-week ADLS score minus the baseline score. The scale is a 14-item measure of symptoms and activity limitations for people with a range of knee impairments. Each item is scored from 0 to 4; the item scores are summed, divided by 70, and multiplied by 100. Scores range from 0 to 100, with higher scores representing fewer symptoms and greater activity.

The ratio relates change in the ADLS score to the total charges for physical therapy treatment sessions. A higher ratio is defined in the registry record as greater value. It combines an activity-related outcome with rehabilitation charges, so its interpretation depends on both components. A group difference in the ratio does not by itself show whether ADLS change, charges, or both account for the difference. The record does not provide the component-level group results or the monetary unit for the charges. The posted endpoint results are means with standard deviations, rather than participant-level changes or a breakdown of cost and score contributions. This endpoint is used to frame the comparison around value per charge, but it should not be read as a measure of clinical improvement alone.

7. Results Posted in the Registry

• Rehabilitation With InterACTION: mean 0.01 versus 0.02 on control; difference -0.00 (95% CI -0.01 to 0.00), standardized effect d = -0.62; z-test p = 0.166.

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.

Performance Based Outcome Measures

• 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:
  • Rehabilitation With InterACTION: mean -0.01 versus 0.03 on control; difference -0.04 (95% CI -0.78 to 0.71), standardized effect d = -0.04; z-test p = 0.927.
• Interpretation:
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between Rehabilitation With InterACTION and the control. The interval gives the range of effects that remain compatible with the data.

The registry lists five secondary outcome categories: rehabilitation-program compliance, satisfaction with the InterACTION device, performance-based outcome measures, extension range of motion, and flexion range of motion. The posted numerical comparison available for interpretation is for performance-based outcome measures at 10 weeks. The registry record does not list reported analyses for the other secondary outcomes, so their results cannot be characterized from the recorded information.

The performance-based measure has a mean difference estimate of -0.035 in the posted comparison, with a confidence interval that crosses zero and an unadjusted p-value of 0.927. This result does not provide a clear separation between the groups on that measure. Its uncertainty is consistent with a range of possible group differences, rather than a precise conclusion that the groups are the same.

The performance measure is related to the primary question in a broad sense because both concern outcomes of rehabilitation, but it is not the same as the primary value ratio. The primary endpoint combines ADLS change with charges; the secondary performance measure is reported as a z-score. The secondary result cannot explain the primary ratio, because the registry record does not provide a component-level analysis linking performance, ADLS change, and charges.

Unadjusted secondary results carry less weight when several outcomes are considered because each additional comparison creates more opportunity for a seemingly notable result to arise by chance. A multiplicity adjustment or a prespecified hierarchy can help control how the collection of tests is interpreted, but the registry record does not report such a plan. The secondary results should therefore be treated as exploratory context, not as independent confirmation of the primary finding. This caution is especially important when several listed secondary outcomes have no posted numerical analyses.

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

• Number of Participants That Complied With Rehabilitation Program
• Number of Subjects Satisfied With InterACTION Device
• Performance Based Outcome Measures
• Range of Motion (Extension)
• Range of Motion (Flexion)

9. Statistical Interpretation

10. Statistical Methods Explained

The primary outcome is a continuous measure, and the registry reports each group with a mean and standard deviation. A mean describes the average observed value in a group. A standard deviation describes how spread out individual values are around that group’s mean; it is not a confidence interval and does not directly express uncertainty about the difference between groups. The primary group standard deviations are 0.007 for InterACTION and 0.009 for standard physical therapy.

What does the mean difference describe?

The comparison is expressed as a mean difference, with the InterACTION group first and standard physical therapy as the reference group. Its estimate is -0.005. This is a contrast between the reported group means, not a relative percentage change. The sign indicates that the observed mean was lower in the InterACTION group. Because the endpoint defines higher ratios as greater value, the direction of this descriptive contrast favors a higher observed ratio in the standard physical therapy group. A direction in a sample does not establish a reliable population difference.

What does the confidence interval add?

A confidence interval gives a range of values that are compatible with the estimate under the statistical procedure used. It communicates precision: a narrower interval indicates less uncertainty about the estimated contrast, while a wider interval indicates more. For the primary contrast, the reported interval spans zero. This means the results do not clearly distinguish a negative difference from no difference or a positive difference. The interval should be considered alongside the estimate, rather than treating the point estimate as an exact value.

How should the p-value be read?

The unadjusted p-value is 0.166. In general, a p-value describes how unusual results at least as extreme as those observed would be under a specified null model. It is not the probability that the null hypothesis is true, and it is not a measure of the clinical importance of a difference. The registry record does not report a prespecified significance threshold or a multiplicity plan for interpreting the primary and secondary outcomes. The p-value is therefore best read as a descriptive inferential result, not as a standalone decision about the rehabilitation approaches.

What does randomization contribute?

In general, random allocation is intended to make treatment groups comparable with respect to both measured and unmeasured baseline factors. That benefit depends on appropriate implementation and on the analysis preserving the randomized comparison. The registry record identifies the study as randomized, but does not report the allocation-generation or concealment procedures, baseline group characteristics, or the population included in the primary analysis. It is not possible to assess those aspects from the registry entry alone.

Why might dispersion matter?

General statistical explanation: when observations vary more than a model assumes, standard errors and confidence intervals based on that model can be misleading. Over-dispersion is one example of such a mismatch and is relevant in settings where the outcome distribution is more variable than expected. The primary outcome is reported as a continuous measure with means and standard deviations. The registry record does not report a diagnostic assessment of the outcome distribution or an adjustment for over-dispersion, so no claim about whether it occurred is warranted.

The registry record does not report the statistical model, covariate adjustments, handling of missing data, or whether the primary comparison was specified before the results were observed. The displayed inferential comparison should therefore be interpreted with those reporting limits in mind. A simple unadjusted comparison can describe the difference in the posted group means, but it does not answer questions about adjustment, subgroup effects, or robustness to alternative assumptions.

11. Confidence Intervals and Precision

The primary mean difference is -0.005, and its confidence interval crosses zero. That interval is the key indication of uncertainty around the contrast: results compatible with a negative difference, no difference, and a positive difference remain possible under the reported analysis. The interval does not support a precise conclusion about the direction of a population-level difference.

The group standard deviations are 0.007 for InterACTION and 0.009 for standard physical therapy, with 10 participants in each posted outcome group. These describe observed variation, while the confidence interval describes uncertainty in the estimated difference. They answer related but distinct questions. The registry record does not provide a clinically meaningful threshold for the ratio, so the interval cannot be judged against a stated threshold for importance.

Precision is also limited by incomplete reporting about the analysis population and missing values. The listed enrollment is 38, whereas the primary outcome groups each report 10 participants. The registry record does not explain this difference, state whether the outcome denominators reflect all randomized participants, or describe any imputation or other missing-data approach. Without that information, the interval should be understood as applying to the posted comparison, rather than assumed to represent a particular analysis population.

A confidence interval does not assign probabilities to specific treatment effects after the data have been observed. In general, it summarizes uncertainty under the method’s assumptions. The posted interval is more informative than the p-value alone because it indicates that estimates on either side of no difference are compatible with the data. It does not establish equivalence, rule out a meaningful difference, or identify which part of the value ratio accounts for the observed contrast.

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 figures are planning illustrations expressed as participants per arm, not total events. The calculation assumptions listed in the registry-derived information are a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. Each planning row uses a stated fraction of the observed effect as the effect to detect. The original trial’s own sample-size assumptions and justification are not reported in the registry record.

The 80% power column gives the illustrative number of participants per arm that the calculation indicates would be needed for an 80% chance of detecting the specified effect under those assumptions. The 90% power column gives the corresponding illustrative number per arm for a 90% chance. These are prospective planning quantities under a model, not descriptions of the number enrolled in the trial or of what the trial actually achieved.

The rows consider planning for 100%, 75%, and 50% of the observed effect. The reported figures rise as the effect to be detected becomes smaller: for example, planning for 75% of the observed effect gives 73 participants per arm for 80% power and 98 per arm for 90% power; planning for 50% gives 164 and 219, respectively. Smaller effects require more participants because they are harder to distinguish from random variation.

This is not observed power. It is a planning illustration based on the observed effect, and it should not be used as a retrospective verdict on the study. The registry record does not report the assumptions behind the original trial design, so these figures do not establish what enrollment target investigators selected or why.

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 = -0.005; pooled standard deviation σ = 0.008.
2. n = 2 × (1.96 + 0.842)² × 0.008² / -0.005² = 40.8, rounded up to 41 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 <- -0.005                      # observed difference in means
sigma <- 0.00806225774829855                     # 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(2646761)
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 = 41 (80% power), 55 (90% power)
 75% of observed effect: n per arm = 73 (80% power), 98 (90% power)
 50% of observed effect: n per arm = 164 (80% power), 219 (90% power)
power.t.test: 41.8 
Simulated power at n = 41 per arm: 0.796 (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 one primary outcome and five secondary outcome categories. The primary comparison has an unadjusted p-value, and the performance-based secondary comparison also has an unadjusted p-value. The registry record does not report a procedure for adjusting inference across outcomes, a hierarchy for testing them, or which outcomes were designated for formal hypothesis testing. It also does not report whether the listed outcome set was complete or whether other analyses were conducted.

Multiplicity matters because interpreting several tests as if each were the only test can make chance findings more likely across the collection. This does not mean that any particular reported result is false; it means that the evidential weight of an isolated unadjusted secondary result is limited without knowing the analysis plan. The secondary results are best considered exploratory unless further documentation establishes how they were prespecified and interpreted.

Missing-data handling is not reported in the registry record. The primary outcome groups each list 10 participants, while enrollment is listed as 38. The record does not explain the denominators, describe reasons for missing observations, or say whether imputation, complete-case analysis, or another approach was used. It also does not report whether withdrawals or exclusions differed between groups.

The analysis population is likewise not reported in detail. The record does not say whether the primary comparison included everyone randomized, only participants with an observed endpoint, or a different defined population. The posted group summaries can be described as reported, but they should not be assumed to represent an intention-to-treat analysis. The registry record also does not report interim analyses, stopping rules, or a statistical analysis plan. These omissions limit assessment of selective reporting and of how closely the reported comparison follows the intended trial design.

15. Safety Results

The registry reports serious adverse events as affected participants divided by participants at risk.

Deaths

Other (non-serious) adverse events

Participants with at least one non-serious event at or above the registry's 0% reporting threshold.

The serious adverse event figures should be read with their group-specific at-risk denominators. For rehabilitation with InterACTION, 0 serious adverse events are listed among 13 at risk. For standard physical therapy, 0 serious adverse events are listed among 12 at risk. The record also lists 0 other adverse events and 0 deaths in each group, using those same group-specific at-risk figures.

These registry figures describe what was recorded; they do not establish that no adverse events occurred outside the collection or reporting process. The outcome denominators for safety differ from those listed for the primary endpoint, and the registry record does not explain the difference. It does not provide event-level details, follow-up completeness, severity definitions beyond the serious and other categories, or a comparative safety analysis.

With zero serious events recorded in each group, a relative comparison based on the event counts is not informative. The counts also do not demonstrate that the risks are identical. Safety interpretation is limited by the reported denominators and by the absence of further details about collection and follow-up.

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

The primary result describes the average value ratio recorded at 10 weeks in the two outcome groups. The observed mean was 0.013 with InterACTION and 0.018 with standard physical therapy. The estimated mean difference was -0.005, with an interval that includes zero. The results therefore do not establish a clear difference in the primary endpoint, and the interval leaves uncertainty about the direction and magnitude of any population difference.

This does not demonstrate equivalence. Equivalence requires a prespecified margin and an analysis designed to determine whether the uncertainty is contained within that margin; the registry record does not report such a margin or analysis. Nor does the result establish that either approach is superior or inferior in clinical terms. The registry record does not report a threshold for a meaningful difference in the value ratio.

The endpoint is a ratio of ADLS change to charges for physical therapy treatment sessions. Since the record does not give separate group results for ADLS change and charges, the primary finding cannot show whether the observed ratio reflects a difference in symptoms and activity, treatment charges, or both. It also does not directly answer whether participants complied with their rehabilitation programs, despite compliance being listed as a secondary outcome.

The study compares the described rehabilitation programs as a whole. The approaches differ in scheduled physical therapy frequency and in the home exercise program, as well as in use of InterACTION. The posted comparison cannot isolate the effect of the device from those other program differences. The registry record does not provide detailed adherence data or the numerical results for most listed secondary outcomes.

The appropriate interpretation is consequently narrow: the posted primary estimates are uncertain and do not clearly distinguish the groups on the stated value measure. They do not establish no effect, equivalence, a mechanism, or a conclusion about all patients undergoing total knee arthroplasty. Further interpretation would require information about the analysis population, missing data, component outcomes, and the prespecified analysis plan, none of which is reported in the registry record.

17. Why This Trial Matters Statistically

This trial is statistically interesting because its primary endpoint combines a patient-reported activity measure with rehabilitation charges. Such a ratio poses an interpretation challenge: a difference in the ratio can arise from its outcome component, its cost component, or both. The registry record defines higher ratios as greater value but does not report component-level results. This makes it important to separate a value summary from claims about clinical change or cost alone.

The study also illustrates why an estimated difference should be read together with its uncertainty. The primary mean difference is -0.005, but the confidence interval crosses zero and the unadjusted p-value is 0.166. The point estimate conveys direction in the observed outcome groups; the interval indicates that the evidence does not pin down the population contrast precisely. Neither a nonzero point estimate nor a nonsignificant p-value settles the question of practical importance.

The outcome denominators make reporting context important. The registry lists enrollment of 38, while the primary results list 10 participants per arm. Without an explanation of participant flow and analysis populations, readers cannot tell how the primary result relates to enrollment or assess the possible impact of missing outcomes.

Finally, the listed secondary outcome categories invite a broader view of rehabilitation, including performance, range of motion, compliance, and satisfaction. Yet only limited numerical secondary analysis is posted. This highlights the importance of prespecification, transparent outcome reporting, and a clear multiplicity strategy when a study examines several outcomes. The record allows a focused reading of the posted primary contrast, while also showing why broader conclusions require more complete reporting.

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 limitations are specific to the information recorded for this trial. First, the primary outcome groups each list 10 participants, whereas the registry lists enrollment of 38. The registry record does not explain the difference, provide a participant-flow account, or identify the analysis population. It is therefore not possible to assess whether the posted groups include all randomized participants or how missing outcome data may affect the estimate.

Second, the primary endpoint is a ratio of ADLS change to physical therapy session charges, but separate group results for the ADLS change and charges are not reported. The unit for charges is not specified in the primary outcome description. This limits both replication of the ratio and interpretation of what accounts for the group means.

Third, although the study is described as randomized and unmasked, the registry record does not report allocation-generation or concealment procedures, assessor masking, baseline characteristics, or treatment adherence. The scheduled visit frequency also differs between arms, so the comparison does not isolate the device from all other features of the rehabilitation programs.

Fourth, numerical results are not reported for most listed secondary outcomes, and the record does not describe a multiplicity plan. There is no reported analysis plan, missing-data method, or explanation of whether the posted comparisons were prespecified. These gaps limit assessment of selective reporting and of the robustness of the findings.

Finally, the registry record does not give a clinically meaningful threshold for the primary ratio or detailed adverse-event collection information. The posted results should be interpreted as limited estimates from the registry record, not as a complete account of trial conduct or outcome analysis.

19. Related Tutorials

20. Related Statistical Calculators

21. Sources

• ClinicalTrials.gov: interACTION: A Portable Joint Function Monitoring and Training System for Remote Rehabilitation Following TKA — NCT02646761.
• Registry record last updated 2019-02-05; status last verified 2019-01.
• Results first posted 2019-02-05.
• 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

The record describes a completed randomized, parallel-group, unmasked study of rehabilitation following total knee arthroplasty. The intervention combined weekly physical therapy with an InterACTION-supported home exercise program; the comparison was standard physical therapy scheduled 2 times per week with a standard home program. Follow-up was described as 10 weeks, and listed enrollment was 38.

The primary endpoint was the ratio of ADLS change to physical therapy session charges. The posted means were 0.013 for InterACTION and 0.018 for standard physical therapy, with a mean difference of -0.005 and an unadjusted p-value of 0.166. The confidence interval crosses zero, leaving uncertainty about the group contrast. Outcome denominators, missing-data handling, and the analysis population are not explained in the registry record.

Five secondary outcome categories are listed, but numerical results are posted for the performance-based measure only. The record also lists zero serious adverse events in each group. These results are descriptive and do not resolve the limitations in outcome reporting or analysis details.