Comparing Total Hip Arthroplasty and Hemi-Arthroplasty on Secondary Procedures and Quality of Life in Adults With Displaced Hip Fractures: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Comparing Total Hip Arthroplasty and Hemi-Arthroplasty on Secondary Procedures and Quality of Life in Adults With Displaced Hip Fractures (NCT00556842), a randomized na study. The primary outcome is Number of Participants With Revision Surgery.

2. Clinical Question

3. Trial Design

The registry describes a completed, randomized, parallel-group trial comparing total hip arthroplasty with hemi-arthroplasty in adults with displaced femoral neck fractures. Its official title describes it as a multi-centre trial. The listed enrollment is 1495. The registry lists 718 participants in the total hip arthroplasty arm and 723 in the hemi-arthroplasty arm for the primary outcome. These are the arm denominators posted with that outcome; the record does not explain any difference between the enrollment figure and the outcome denominators.

The trial start is listed as 2009-03, and primary completion as 2019-05-30. The status is completed. Masking is listed as none, so the registry does not describe a blinded comparison. The interventions are classified as devices, and the registry lists the trial phase as not applicable.

The primary outcome is assessed 2 years after the original surgery. The registry also names secondary domains including hip function and pain, functional mobility problems, health-related quality of life measured with SF-12 and EQ-5D, and complications. Those domains address experiences and events beyond the primary endpoint, but the level of detail and posted results vary across them. The registry record does not report the randomization procedure, allocation concealment, follow-up procedures, or the original statistical analysis plan. Those design features should not be inferred from the randomized designation alone.

4. Participant Flow and Accounting

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

Overall Study

5. Baseline Characteristics

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

6. Primary Endpoint

The registry lists the primary outcome as Number of Participants With Revision Surgery, measured over Measured 2 years after original surgery.

The primary endpoint is the number of participants with revision surgery, described in the registry as any unplanned secondary procedure within 2 years of the initial joint replacement surgery. It is therefore a participant-level binary outcome: each participant either had at least one qualifying procedure during the stated period or did not. The endpoint does not count how many procedures an individual participant underwent.

A secondary procedure is relevant to this comparison because it captures a subsequent intervention after the original replacement. Unlike a measure of pain or function, it records a clinical event that can lead to another procedure. The registry uses the title revision surgery, while its description defines the outcome more broadly as any unplanned secondary procedure. Those labels should be read together rather than assuming that the endpoint includes only revision of a specific implant component.

The registry records this outcome 2 years after the original surgery. It does not report the reasons for each procedure, whether procedures were planned or urgent beyond the stated unplanned criterion, or how events were independently verified. Those details would help clarify which events qualify and how consistently the outcome was measured. The posted result should consequently be interpreted as the comparison for the registry's stated participant-level endpoint, not as a detailed account of procedure type or cause.

7. Results Posted in the Registry

• Hemi-arthroplasty: 8.3% (60/723) versus 7.9% (57/718) on control; risk difference 0.4 percentage points (95% CI -2.5 to 3.2), risk ratio 1.05 (0.74–1.48); unadjusted two-proportion z-test p = 0.802.

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.

Number of Participants With Functional Mobility Problems

• Endpoint type: Binary: the number of participants with the event, compared as proportions.
• Test: Unadjusted two-proportion z-test, with a Wald 95% confidence interval for the risk difference.
• Result:
  • Hemiarthroplasty: 28.5% (206/723) versus 29.2% (210/718) on control; risk difference -0.8 percentage points (95% CI -5.4 to 3.9), risk ratio 0.97 (0.83–1.15); unadjusted two-proportion z-test p = 0.752.
• Interpretation:
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between Hemiarthroplasty and the control. The interval gives the range of effects that remain compatible with the data.

The registry names secondary domains that extend beyond the primary endpoint: hip function and pain, functional mobility problems, health-related quality of life measured with SF-12 and EQ-5D, and complications. Together, these outcomes cover aspects of function, patient-reported health, and clinical events. The available record includes a quantitative comparison for functional mobility, but results for the other named domains are not reported in the registry record in the information available for this analysis. It is therefore not possible to describe a consistent pattern across all secondary endpoints.

The mobility comparison addresses a different question from the primary endpoint. A participant's functional mobility problems are not the same event as an unplanned secondary procedure, and results on one outcome do not determine results on another. The posted record also includes an equivalence analysis for functional mobility, with an odds ratio of 0.72 and a 99% confidence interval from 0.38 to 1.36. The registry record does not report the equivalence margin, so that result alone cannot establish equivalence. The primary endpoint remains a separate comparison on a risk-difference scale.

Secondary results generally carry less confirmatory weight when many outcomes or analyses are considered without a clearly reported plan for how to handle multiplicity. Repeated testing increases the chance that some results may appear notable by chance. Selective availability of results can also make an overall pattern difficult to assess. The record lists 5 posted secondary outcomes and 9 analyses, but does not report a multiplicity adjustment or identify a hierarchy for interpreting them. Accordingly, secondary findings can help describe the breadth of outcomes assessed, but they should not be treated as independent confirmation of the primary comparison. The registry record does not provide enough results to support a broader claim about the overall balance of secondary outcomes.

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

• Hip Function and Pain
• Number of Participants With Functional Mobility Problems
• Health-Related Quality of Life (SF-12)
• Health-Related Quality of Life (EQ-5D)
• Complications, Including Mortality, Dislocation, Infection, Femoral Fracture, Deep Venous Thrombosis, and Prosthesis Loosening

9. Statistical Interpretation

10. Statistical Methods Explained

The primary comparison is presented as a risk difference, a measure of the absolute difference between the proportions of participants with an event in the two arms. Here, the comparison is hemi-arthroplasty against total hip arthroplasty. A value of zero would correspond to equal observed proportions; a positive value means the event proportion is higher in the first-named comparison arm, and a negative value means it is lower. The estimate is 0.004, with a 95% confidence interval from -0.025 to 0.032.

What does the confidence interval describe?

A confidence interval represents uncertainty in an estimated effect under the assumptions of the statistical method. It is not a guarantee that the true effect lies inside the interval, nor does it give the probability that one treatment is better. For the posted primary estimate, the interval crosses zero. It is compatible with a modest difference in either direction, so the point estimate alone should not be treated as a precise description of the underlying effect. The interval also cannot identify which values would be clinically important; the registry record does not state a threshold for that judgment.

How should the p-value be read?

The unadjusted p-value is 0.802. In general, a p-value describes how unusual data at least as incompatible with a specified null hypothesis would be if that hypothesis and the model assumptions were true. It is not the probability that the null hypothesis is true, and it does not measure the size or importance of an effect. A large p-value does not prove that the groups are the same. Considered with the estimate and interval, it indicates that the posted unadjusted analysis does not provide strong evidence against a zero risk difference.

What do unadjusted estimates leave out?

The estimates are simple, unadjusted comparisons from posted summary statistics. An unadjusted analysis compares the outcome summaries without accounting for other participant characteristics or design factors in a model. Adjustment can sometimes improve precision or address imbalances, but whether it is appropriate depends on the prespecified analysis plan and data. The registry record does not report an adjusted primary analysis, covariates, or the statistical model used for the primary endpoint. The recalculated summary comparison therefore should not be mistaken for a complete reconstruction of the trial's analysis.

Why distinguish risk difference from relative measures?

The risk difference describes an absolute contrast in event proportions. Relative measures, such as a risk ratio or odds ratio, express a comparison on a different scale and can convey a different sense of effect size. The primary posted comparison is a risk difference, so interpretation should remain anchored to that absolute scale. The registry also contains an odds-ratio result for a functional-mobility analysis, but that is a distinct outcome and should not be substituted for the primary estimate. In general, odds ratios are not interchangeable with risk differences, particularly when event proportions are not very small.

What can randomization support?

Random allocation is intended to make comparison groups similar, on average, with respect to factors that could affect outcomes. This supports a causal interpretation when allocation, follow-up, outcome assessment, and analysis are handled appropriately. The registry labels the study randomized, but the allocation method, concealment, adherence, loss to follow-up, and analysis population are not reported in the registry record. Randomization alone does not answer those questions, and the posted summary cannot establish that every assumption for an unbiased treatment comparison was met.

How does the endpoint shape interpretation?

The endpoint records whether a participant had any unplanned secondary procedure during the specified follow-up. It does not distinguish one procedure from multiple procedures for the same participant, and it does not identify the indication for an event. This is a useful distinction when reading a count-of-participants outcome: the event count is not a procedure count, and the risk difference is not a contrast in total procedural burden. General statistical interpretation should stay aligned with the outcome definition actually recorded.

11. Confidence Intervals and Precision

The primary estimate is a risk difference of 0.004, with a 95% confidence interval from -0.025 to 0.032. The interval spans zero, so the posted estimate is compatible with no difference as well as with a difference in either direction. Its width signals that the point estimate is not precise enough to rule out a range of possible effects. The registry record does not state which differences would be considered clinically important, so statistical uncertainty cannot be translated into a judgment about clinical importance from these figures alone.

Precision and statistical significance are related but not identical ideas. A confidence interval describes the uncertainty around an estimate, while a p-value evaluates the data relative to a null hypothesis under a statistical model. The primary unadjusted p-value is 0.802. That value does not show that the effect is exactly zero, and it should not replace consideration of the estimate and interval. In particular, absence of a clear statistical difference is not evidence that the procedures meet an equivalence criterion.

The registry also posts a 99% confidence interval for an odds ratio of 0.72 to 1.36 in an equivalence analysis associated with functional mobility. It is a different endpoint and a different effect scale from the primary risk difference. The registry record does not report the equivalence margin or the method for that analysis. An interval crossing the null value alone cannot establish equivalence; that conclusion requires a prespecified margin and a suitable analysis.

Confidence intervals reflect sampling uncertainty under the analysis assumptions. They do not account automatically for missing outcomes, measurement error, deviations from assigned treatment, or selective reporting. The registry record does not provide enough information to assess those sources of uncertainty for the primary result. Precision should therefore be read as the uncertainty represented by the posted estimate, not as a complete measure of every limitation in the evidence.

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 planning values are stated as participants per arm, not as total events. Each row describes planning for a fraction of the observed effect: 100%, 75%, or 50%. Read across a row to see the illustrative number of participants per arm for 80% power and for 90% power under the stated assumptions. The 80% column represents a planning target with an 80% chance of detecting the specified effect if that effect and the assumptions are correct; the 90% column uses a higher target chance of detection. These are prospective planning concepts, not probabilities that the observed result is true.

The calculations assume a two-sided significance level of 0.05, equal allocation between the arms, and a normal approximation. They use the observed effect as the reference and apply the stated fraction of that effect as the effect to detect. The planning figures increase as the assumed effect becomes smaller: a subtler difference is harder to distinguish from random variation and generally requires more participants for the same power target. The values are planning illustrations, not observed power.

These calculations do not report the trial's original sample-size justification. The trial's own design assumptions, including its intended target effect and planned power, are not reported in the registry record. The displayed planning figures should therefore be treated as hypothetical illustrations based on the observed estimate, not as evidence about how the trial was designed or whether it was adequately powered for its original objectives. They also do not resolve the uncertainty in the actual result or replace interpretation of its confidence interval.

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, p₀ and p₁ are the control and treatment proportions, and p̄ is their average.

Worked calculation for this trial

1. Control proportion p₀ = 0.079; treatment proportion p₁ = 0.083; average p̄ = 0.081.
2. First term: z_1−α/2 × √[2p̄(1−p̄)] = 1.96 × 0.3863 = 0.7570.
3. Second term: z_1−β × √[p₀(1−p₀) + p₁(1−p₁)] = 0.842 × 0.3862 = 0.3251.
4. n = (0.7570 + 0.3251)² / (0.0036)² = 90,334.6, rounded up to 90,335 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
p0 <- 0.0793871866295265                          # control proportion
p1 <- 0.0829875518672199                          # treatment proportion
n_per_arm <- function(power, frac = 1) {
  q1 <- p0 + (p1 - p0) * frac               # smaller effect = fraction of the observed difference
  pbar <- (p0 + q1) / 2
  z_b <- qnorm(power)
  ceiling(round((z_a * sqrt(2 * pbar * (1 - pbar)) + z_b * sqrt(p0 * (1 - p0) + q1 * (1 - q1)))^2 / (q1 - p0)^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 (unrounded n per arm)
cat("power.prop.test:", round(power.prop.test(p1 = p0, p2 = p1, power = 0.80)$n, 1), "\n")

# Simulation: how often does the z-test reject at the 80%-power sample size?
set.seed(556842)
n    <- n_per_arm(0.80)
reps <- 20000
x0 <- rbinom(reps, n, p0); x1 <- rbinom(reps, n, p1)
ph <- (x0 + x1) / (2 * n)
z  <- ((x1 - x0) / n) / sqrt(ph * (1 - ph) * 2 / n)
cat(sprintf("Simulated power at n = %s per arm: %.3f (%d replicates)\n", format(n, big.mark = ","), mean(abs(z) > z_a, na.rm = TRUE), reps))

Output from running the code

100% of observed effect: n per arm = 90,335 (80% power), 120,932 (90% power)
 75% of observed effect: n per arm = 159,784 (80% power), 213,905 (90% power)
 50% of observed effect: n per arm = 357,687 (80% power), 478,840 (90% power)
power.prop.test: 90334.6 
Simulated power at n = 90,335 per arm: 0.801 (20000 replicates)

The simulation draws many pairs of event counts at the planned sample size and counts how often the two-proportion z-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 record does not report a statistical analysis plan describing how the primary and secondary outcomes were prioritized or how multiple comparisons were handled. It lists 5 posted secondary outcomes and 9 analyses, but does not identify a confirmatory hierarchy, an adjustment for multiplicity, or which analyses were planned before data were examined. When several outcomes and analyses are evaluated, the chance of at least one apparently notable result arising by chance can increase. That is a general statistical concern, not evidence that any particular result in this trial is false.

The record also does not report how missing outcome data were handled, how many participants lacked the primary outcome, or whether missingness differed between arms. Missing outcomes can affect an estimate if the likelihood of missing data is related to participant characteristics or outcomes. Without details of follow-up completeness and the missing-data approach, the posted arm summaries cannot show whether the primary result is sensitive to missingness.

The analysis population is not reported in the registry record. It is therefore unclear from the posted information whether analyses followed intention-to-treat, per-protocol, or another population definition, and how crossovers or departures from assigned procedures were handled. The randomized designation is important, but does not by itself establish which participants contributed to each analysis or how deviations were treated.

Other details that are not reported include the original sample-size assumptions, interim analyses, covariate adjustment, and the exact statistical models used for the primary and secondary outcomes. These omissions limit assessment of whether the analyses followed prespecified decisions and how robust the estimates are to alternative reasonable methods. They do not invalidate the posted results; they define what can and cannot be evaluated from the registry record. Interpretation should distinguish the direct summary comparison from any claims about prespecification, complete follow-up, or multiplicity control, none of which is documented.

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.

Most frequent serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: other serious adverse event (28.0%), hip fracture-related serious adverse event (8.2%), cardiac serious adverse event (7.1%), respiratory serious adverse event (5.8%), neurologic serious adverse event (3.9%).

The registry lists serious adverse events affecting 300 participants among 718 in the total hip arthroplasty arm and 265 among 723 in the hemi-arthroplasty arm. It separately lists other adverse events affecting 132 among 718 and 118 among 723, respectively. Deaths are recorded as 103 in the total hip arthroplasty arm and 95 in the hemi-arthroplasty arm. These are posted counts; they should not be added together because the record does not establish that the categories are mutually exclusive or describe how overlap was handled.

The registry lists the most frequent serious adverse-event terms as other serious adverse event, 28.0%; hip fracture-related serious adverse event, 8.2%; cardiac serious adverse event, 7.1%; respiratory serious adverse event, 5.8%; neurologic serious adverse event, 3.9%; and renal serious adverse event, 3.2%. The record does not specify the denominators or whether these percentages are pooled or arm-specific, so they should not be interpreted as comparative risks between procedures.

These figures are descriptive safety summaries, not adjusted comparisons. The registry record does not provide event-level timing, severity grading beyond the serious classification, attribution, or a statistical comparison for the listed counts. They should not be read as proof of a difference or similarity in safety between arms.

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

The primary result describes the observed comparison in participants with an unplanned secondary procedure within 2 years of the original surgery. The posted unadjusted estimate is near zero, its confidence interval includes zero, and the p-value does not indicate a clear statistical difference. Taken together, these statistics do not establish a distinct difference between the randomized groups for this endpoint in the posted analysis.

That conclusion is narrower than saying the procedures are the same. The confidence interval includes effects in both directions, and the registry record does not define a threshold for clinical equivalence or provide an equivalence margin for the primary endpoint. A finding that does not meet a conventional threshold for statistical significance cannot, by itself, establish equivalence, non-inferiority, or identical outcomes. Those claims require an appropriate design, prespecified margins where relevant, and analyses aligned with the claim.

The result also does not answer every question about the procedures. The primary endpoint counts participants with any qualifying unplanned secondary procedure, not all procedures or their reasons. It does not directly measure pain, hip function, mobility, quality of life, or each complication. Those are separate outcomes, and the registry record does not report complete results for all named secondary domains. The posted safety counts are descriptive and do not establish comparative safety.

Finally, the result is tied to the registry's stated 2-year timeframe and the population described as adults aged 50 and older with displaced femoral neck fractures. It does not establish what happens beyond that follow-up or in populations not described in the record. Missing-data handling, analysis populations, and the primary analysis plan are not reported in the registry record, which limits further assessment of the result's robustness. The most defensible reading is therefore a statistically inconclusive unadjusted comparison for the specified endpoint and follow-up, rather than a general verdict on the relative merits of either procedure.

17. Why This Trial Matters Statistically

This trial poses a statistically important comparison because the procedures differ in the extent of joint replacement, while the primary outcome concerns subsequent unplanned procedures. The randomized parallel design gives the comparison a structure intended to support a fair assessment between treatment groups. The primary endpoint is also clinically interpretable as a participant-level event over a defined follow-up period, although its registry description does not distinguish among reasons for procedures.

The result illustrates why an estimate, its confidence interval, and a p-value should be read together. The point estimate alone can appear to suggest little separation, but the interval shows that uncertainty remains. The p-value does not convert that uncertainty into proof that the procedures are equivalent. This is a useful distinction in clinical research: failure to demonstrate a difference and demonstration that differences are sufficiently small are not the same statistical conclusion.

The record also shows why outcome families should not be collapsed into one overall judgment. A subsequent procedure, mobility, pain, quality of life, and adverse events are related but distinct measures. A comparison on one cannot stand in for all the others. The incomplete availability of secondary results and the absence of details about multiplicity, missing data, and analysis populations constrain how broadly findings can be interpreted.

Finally, the planning illustrations based on the observed effect emphasize the relationship between effect size, precision, and study size. Smaller effects require more participants to detect under otherwise similar assumptions. Those illustrations are not a substitute for the trial's original design rationale, which is not reported in the registry record. The statistical value of the trial lies both in the randomized comparison and in recognizing the limits of what its posted summaries can establish.

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.

The registry record provides summary counts and an unadjusted primary comparison, but not the full analysis plan or a detailed report of how participants were followed and analyzed. The randomization method, allocation concealment, treatment adherence, crossovers, and analysis population are not reported in the registry record. Without those details, it is not possible to assess fully how implementation or departures from assigned treatment may have influenced the comparison.

Missing-data methods and outcome completeness are also not reported. The posted denominators accompany the primary outcome, but the record does not explain the handling of participants without outcome information or whether follow-up differed between groups. The primary endpoint's description does not specify the reasons for procedures, event adjudication, or whether multiple procedures per participant were recorded separately for other purposes. The count-of-participants result cannot describe the total procedural burden.

There is no reported primary equivalence margin, and the registry does not state which risk differences would be clinically important. The primary result therefore cannot support a claim of equivalence based only on a non-significant p-value. The record also does not report multiplicity control across the listed secondary outcomes and analyses. Results for several named secondary domains are not reported in the registry record, limiting an assessment of whether the primary finding aligns with function, pain, health-related quality of life, and complications.

The timeframe for the primary endpoint is 2 years after surgery. This supports interpretation within that period but does not establish longer-term outcomes. These limitations concern the information recorded and available for interpretation; they do not establish that the trial failed to address any particular issue in its full study conduct.

19. Related Tutorials

20. Related Statistical Calculators

21. Sources

• ClinicalTrials.gov: Comparing Total Hip Arthroplasty and Hemi-Arthroplasty on Secondary Procedures and Quality of Life in Adults With Displaced Hip Fractures — NCT00556842.
• Registry record last updated 2020-07-07; status last verified 2019-11.
• Results first posted 2020-07-07.
• 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

NCT00556842 is a completed randomized parallel-group trial comparing total hip arthroplasty with hemi-arthroplasty in adults aged 50 and older with displaced femoral neck fractures. The registry lists enrollment of 1495 and describes the primary endpoint as any unplanned secondary procedure within 2 years of the initial joint replacement surgery.

The posted unadjusted risk difference for hemi-arthroplasty compared with total hip arthroplasty is 0.004, with a 95% confidence interval from -0.025 to 0.032 and a p-value of 0.802. This does not establish a clear difference, but it also does not establish equivalence. The registry names additional outcomes in mobility, function, pain, quality of life, and complications; complete results for those domains are not reported in the registry record. Important analysis details, including missing-data handling, multiplicity procedures, and analysis populations, are also not reported.