Study Comparing BoneMaster Hydroxyapatite Coating With Plasma-sprayed Hydroxyapatite Coating on Acetabular Cups: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Study Comparing BoneMaster Hydroxyapatite Coating With Plasma-sprayed Hydroxyapatite Coating on Acetabular Cups (NCT00859976), a randomized na study. The primary outcome is Record and Measure Bone Density Using DEXA Scans at 24 Months..

2. Clinical Question

The trial asked whether the coating on an acetabular cup is associated with differences in bone density around the cup and in clinical outcomes after total hip replacement. It compared BoneMaster hydroxyapatite coating with plasma-sprayed hydroxyapatite coating. The primary endpoint was net average bone mineral density percentage change from baseline at 2 years, measured in a DEXA scan cohort. Clinical scores were secondary outcomes.

The study’s rationale was that coating may encourage bone growth onto the implant, and that poor bony attachment is a known complication associated with early failure of total hip replacements. That rationale explains the endpoint choice; it does not establish that a difference in the measured density necessarily predicts a difference in implant failure or patient function.

3. Trial Design

This was a completed, randomized, parallel-group trial with no masking. The registry lists an enrollment of 167. The study began in 2009-03 and reached primary completion in 2012-06. It is identified as a device study and lists Zimmer Biomet as sponsor. The phase entry is NA.

Participants requiring total hip replacement were assigned to receive an acetabular cup with either BoneMaster hydroxyapatite coating or plasma-sprayed hydroxyapatite coating. The trial therefore evaluates the coatings in the setting of the same broad type of replacement procedure. The record does not report details of sequence generation, allocation concealment, or how assignment was implemented, so those features cannot be assessed from the registry entry.

The primary endpoint was measured in a DEXA scan cohort at 2 years. The registry describes bone density assessment by standard x-ray as well as DEXA in a smaller sample. It also lists the Harris Hip Score, Oxford Hip Score, and WOMAC Hip Score as secondary outcomes at 2 years. The outcome-specific participant counts differ, and the registry does not explain the reasons for those differences.

Randomization is intended to make comparison groups similar on average with respect to factors that may affect outcomes. It does not guarantee that a small analysis cohort will be balanced, nor does it remove uncertainty caused by limited outcome data. The DEXA analysis has n = 7 in each group, so its descriptive estimates are particularly dependent on the observations in that cohort. Because masking is listed as none, the record does not describe a blinded trial; it does not specify whether outcome assessment or other parts of the trial were masked independently.

The registry record does not report the trial’s sample-size justification, a detailed statistical analysis plan, interim analyses, or the analysis population definition. Planning illustrations based on observed effects are not a substitute for the original design assumptions and do not show the power the completed trial actually had.

4. Participant Flow and Accounting

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

Overall Study

Analysis population for the primary outcome, as defined in the registry: DEXA scan cohort: Bonemaster group, n = 7; Plasma-Sprayed group, n = 7

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 Record and Measure Bone Density Using DEXA Scans at 24 Months., measured over 2 years..

The primary endpoint is net average bone mineral density percentage change from baseline at 2 years, measured by DEXA in the DEXA scan cohort. Bone mineral density quantifies the density of bone in the region assessed by the scan. In this study, the relevant question is whether the groups differed in the change in bone density around the acetabular cup after implantation.

A change-from-baseline endpoint uses each participant’s baseline measurement as a reference for the later measurement. Expressing that change as a percentage puts the change on a relative scale rather than reporting only the scan’s original measurement units. The registry describes the posted group values as means with standard deviations. Those summaries convey the average change and the spread of individual observations, but they do not show the full distribution of participant-level results.

Bone density was chosen because the trial’s stated rationale concerns bony attachment to the implant. The registry also describes standard x-ray assessment of surrounding bone density and says DEXA was used in a smaller sample. The posted primary endpoint specifically identifies the DEXA measure; it should not be conflated with the x-ray assessment or with the clinical scores.

This is a surrogate or intermediate biological measure in the context of the stated rationale: it assesses bone density, not early implant failure itself. The record does not establish how a difference in this measure would translate into implant survival, symptoms, or function. Clinical outcomes were assessed separately using hip scores.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The primary outcome is continuous: it is a percentage change in bone mineral density. The registry reports a mean and standard deviation for each group. A mean describes the group’s average, while the standard deviation describes how dispersed individual values are around that average. These summaries are useful, but with a small cohort they can be strongly influenced by a few observations. They also do not display the shape of the data or identify which observations drive the spread.

What does the reported Wilcoxon test assess?

The reported primary analysis uses the Wilcoxon (Mann-Whitney) method. This is a rank-based test: rather than directly testing a difference between arithmetic means, it compares the ordering of observations between groups. Its interpretation as a test of medians requires additional assumptions about the shapes of the group distributions. When spreads differ considerably, a rank-based result should not be casually translated into a statement about mean differences or medians alone.

The reported p-value is 0.457. A p-value describes how compatible the observed data are with a specified null comparison under the test’s assumptions. It is not the probability that the null hypothesis is true, and it does not measure the size or clinical importance of a difference. A result that does not provide strong evidence against the null is not proof of equivalence. Equivalence requires a prespecified margin and an analysis designed to assess whether the uncertainty excludes differences larger than that margin; such a margin is not reported in the registry record.

Why is a confidence interval useful?

A confidence interval gives a range of estimates compatible with the data and the method used to construct it. It helps readers assess precision and the range of effects that remain plausible, rather than relying only on a test result. The registry record does not report a confidence interval for the primary between-group contrast. The group standard deviations do not replace that interval, because they describe within-group variation rather than uncertainty in a between-group estimate.

For the secondary clinical scores, the posted re-calculations are simple, unadjusted comparisons from the summary statistics. Their confidence intervals describe uncertainty around the estimated mean differences under the calculation method, and the associated unadjusted p-values do not account for testing multiple outcomes. These calculations are not identified as the sponsor’s own analyses. Where least-squares means are reported, the recalculation uses the available summary information and may not reproduce any adjusted analysis that could have been used in the original trial.

What does over-dispersion mean in this setting?

In general, over-dispersion means that the data vary more than a simple model would expect. For a continuous outcome, unusually large spread can make a mean less stable and can widen uncertainty in a comparison. The BoneMaster group’s reported primary standard deviation is markedly larger than the plasma-sprayed group’s, so the primary averages need to be read alongside the variability. A small cohort with uneven spread can make both the estimate and the assumptions behind a test important. The registry does not report diagnostics or participant-level data that would allow a fuller assessment of distribution shape or influential observations.

For clinical score comparisons, confidence intervals spanning the no-difference value indicate that the summary data are compatible with effects in either direction as well as no difference under the calculation method. Such intervals should not be treated as proof that effects are absent. The estimates remain subject to sampling uncertainty, and the participant counts vary among outcomes.

What do the planning illustrations mean?

The planning illustrations use the observed effect as a starting point and show participants per arm for 80% power and participants per arm for 90% power under stated assumptions. Those assumptions are a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation; the effects to detect are stated fractions of the observed effect. These are hypothetical planning calculations, not the trial’s original sample-size justification, and not observed power. The registry record does not report the original design assumptions or a prospective power calculation. Planning from an observed estimate is also uncertain because that estimate may be imprecise, especially when outcome counts are limited.

Recalculated estimates are unadjusted and derived from posted summary statistics. They can help show the scale and uncertainty of comparisons, but they do not replace the sponsor’s reported primary test or recover details absent from the record, such as covariate adjustment, handling of missing outcomes, or the original analysis plan. No new inference about clinical importance can be made without a contextually justified threshold for meaningful change.

10. Confidence Intervals for Each Arm

No two-arm comparison is shown for this trial, but each arm's mean can be given with a t-based 95% confidence interval computed from the posted mean, spread and number of participants.

R code and output

arm <- c("Plasma-sprayed Shell", "BoneMaster Coated Shell")
n   <- c(7, 7)
m   <- c(-11, 5.7)
sp  <- c(4, 57.3)
sd  <- sp      # posted standard deviations
for (i in seq_along(n)) {
  se <- sd[i] / sqrt(n[i]); h <- qt(0.975, n[i] - 1) * se
  cat(sprintf("CARM|%s|%d|%.3f|%.3f|%.3f|%.3f\n", arm[i], n[i], m[i], sd[i], m[i] - h, m[i] + h))
}

CARM  Plasma-sprayed Shell  7  -11.000  4.000  -14.699  -7.301
CARM  BoneMaster Coated Shell  7  5.700  57.300  -47.294  58.694

11. Analyses Reported by the Sponsor

The registry reports the sponsor’s primary comparison using a Wilcoxon (Mann-Whitney) test, with p = 0.457. It does not report a sponsor-estimated between-group mean difference or a confidence interval for the primary endpoint. The displayed primary group means and standard deviations are descriptive summaries; they are not the same statistic as the rank-based test result.

The primary result does not have a separate recalculated comparison listed. The registry does include simple, unadjusted re-calculations for the secondary clinical scores. Those calculations use posted group summaries and should be understood as illustrations of unadjusted comparisons, not as a replacement for an analysis specified or conducted by the sponsor. No sponsor-reported analyses are listed for those secondary scores in the record.

The reported primary p-value and descriptive mean summaries answer related but different questions. A rank-based test evaluates the relative ordering of observations, while a difference in means describes a difference between group averages. The registry’s primary report does not provide enough analysis detail to determine whether the sponsor also conducted other analyses that were not posted. The record does not report a discrepancy between a sponsor result and a recalculation.

12. Multiplicity, Missing Data and Analysis Populations

The registry lists three secondary clinical scores in addition to the primary DEXA endpoint. It does not report whether the analyses were arranged in a prespecified hierarchy, whether a family-wise error procedure was used, or whether any other multiplicity strategy was planned. No adjustment for multiple testing is reported for the secondary re-calculations. As a general statistical matter, examining multiple outcomes can raise the chance that at least one result appears notable by chance, so individual unadjusted results warrant restraint.

The record also does not report a detailed missing-data plan. It does not state whether missing outcome values were imputed, whether any sensitivity analyses were conducted, or how participants without an outcome measurement were handled. The posted outcome counts vary: the DEXA cohort has n = 7 in each group, while the clinical-score counts differ by measure. The record does not explain these differences, so their causes should not be inferred.

The analysis population is not defined in the registry record. It does not say whether the primary or secondary comparisons follow intention-to-treat, per-protocol, or another population definition. The randomized allocation is relevant to the design, but without an analysis-population description and information on missing observations, readers cannot establish from the posted record how fully the analyses preserve the randomized comparison.

The record does not report whether interim analyses were conducted or whether any stopping rules were specified. It also does not provide a full statistical analysis plan, including details of covariate adjustment, outcome transformations, or the handling of departures from model assumptions. These omissions do not show that such procedures were absent; they mean that they are not reported in the registry record. Accordingly, conclusions should be limited to the outcomes and analyses actually posted.

13. 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 serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: death (2.4%), dislocation (1.2%).

The registry reports serious adverse events affecting 1 participant in the plasma-sprayed group, with 82 at risk, and 2 participants in the BoneMaster group, with 85 at risk. For other adverse events, it reports 5 affected participants in the plasma-sprayed group and 10 in the BoneMaster group. These are the counts recorded for the listed groups; the entry does not give event-level descriptions, timing, severity beyond the serious/other classification, or a comparative statistical analysis.

The registry lists Death at 2.4% and Dislocation at 1.2% among the top serious-event terms. The deaths field for the group summaries has no reported value, so these term listings should not be treated as a complete mortality analysis. The record does not explain how the term percentages were derived or whether participants could contribute more than one event. Counts and percentages alone do not establish that a difference between groups is due to the coating.

Safety comparisons are difficult to interpret without fuller definitions, follow-up information, and uncertainty estimates. The registry record does not report a formal safety analysis plan or a statistical comparison of adverse-event rates. These figures are descriptive reporting, not evidence by themselves that one coating is safer or less safe.

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

The primary result describes bone-density change in a small DEXA cohort, not the full range of outcomes relevant to total hip replacement. The reported group means differ in direction, but the primary test is rank-based, the reported spread is uneven, and no confidence interval for the primary between-group contrast is available. Those features leave uncertainty about the size and precision of any group difference.

The p-value of 0.457 is not evidence that the coatings are equivalent. A non-significant test does not demonstrate that any difference is absent or small enough to be clinically unimportant. Demonstrating equivalence would require a defined margin and an analysis suited to that question; neither is reported in the registry record. Equally, the group mean pattern alone is not sufficient to establish a dependable or clinically meaningful difference.

The endpoint is bone mineral density, selected because the study rationale concerns bony attachment. It does not directly measure early implant failure, implant survival, pain, or function. The registry does list clinical scores, but their unadjusted analyses have their own uncertainty and do not turn the DEXA measure into a direct measure of clinical benefit. No conclusion about treatment benefit or lack of benefit follows from these posted results alone.

The randomized design is an important feature for a comparative trial, yet the primary endpoint comes from the small DEXA cohort. The record does not explain selection into that cohort, missing outcome data, or analysis populations. Those unanswered questions limit how confidently the DEXA findings can be generalized to all participants enrolled or to patients outside the study. The most defensible reading is descriptive: the registry reports these group summaries and a rank-based test, with substantial uncertainty and important limits on interpretation.

15. Why This Trial Matters Statistically

This trial illustrates why the analysis method and the endpoint must be read together. The primary outcome is a continuous biological measure, but the reported inferential test is rank-based. The group means describe average change, whereas the Wilcoxon (Mann-Whitney) test uses ranks. These summaries and the test can complement each other, but they should not be presented as if they estimate the same quantity.

The primary cohort also makes precision central to interpretation. With n = 7 in each group and a much larger reported standard deviation in the BoneMaster group, the means are descriptive rather than definitive. The absence of a confidence interval for the primary contrast makes it harder to gauge the range of plausible between-group effects. A p-value alone cannot supply that missing precision information.

The registry also demonstrates the distinction between a biological rationale and a clinical endpoint. Bone density is relevant to the proposed attachment mechanism, but the result does not directly quantify implant failure or patient function. The secondary scores address clinical status but are unadjusted and have varying participant counts. This separation is useful when judging whether a result supports a mechanism, a clinical outcome, or both.

Finally, planning illustrations derived from observed effects are not evidence about the completed trial’s power. They can inform future design discussions under stated assumptions, but the original sample-size rationale is not reported. The case underscores the value of transparent analysis plans, outcome-specific participant counts, missing-data descriptions, and confidence intervals for clinically relevant contrasts.

16. 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 primary DEXA analysis is based on n = 7 in each group, a limited basis for estimating a group difference. The BoneMaster group has a substantially larger reported standard deviation than the plasma-sprayed group, making the mean particularly important to interpret cautiously. The record provides no participant-level data, distributional plots, or primary confidence interval, so the influence of individual observations and the precision of the contrast cannot be assessed from the posted summaries.

The registry does not describe how participants entered the DEXA cohort, why outcome-specific counts differ, or how missing measurements were handled. It does not define the analysis population or report a detailed statistical analysis plan. Although allocation is randomized, these omissions make it difficult to assess whether the analyzed cohort preserves the intended randomized comparison or whether missingness could affect the result.

The primary test is rank-based, while the reported primary summaries are means and standard deviations. Without more detail about the analysis rationale and distributional characteristics, readers should avoid interpreting the test as a direct test of the difference between means. The record also does not report a prespecified equivalence margin, so a non-significant test cannot support an equivalence conclusion.

The clinical scores are secondary, unadjusted comparisons, and the record does not report a multiplicity plan. Their participant counts vary, and reasons for that variation are not given. Safety reporting is also limited: the record gives affected counts and top serious-event terms but no detailed event descriptions or inferential comparison. Finally, the posted endpoints do not directly report early implant failure or implant survival, despite those outcomes being part of the stated rationale.

17. Related Tutorials

18. Related Statistical Calculators

19. Sources

• ClinicalTrials.gov: Study Comparing BoneMaster Hydroxyapatite Coating With Plasma-sprayed Hydroxyapatite Coating on Acetabular Cups — NCT00859976.
• Registry record last updated 2019-03-05; status last verified 2019-02.
• Results first posted 2019-02-08.
• 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.

20. Record Summary

NCT00859976 is a completed, randomized, parallel-group device trial comparing BoneMaster hydroxyapatite-coated acetabular cups with plasma-sprayed hydroxyapatite-coated cups in patients requiring total hip replacement. The registry lists an enrollment of 167, no masking, a start date of 2009-03, and primary completion in 2012-06.

The primary endpoint is net average BMD percentage change from baseline at 2 years in a DEXA scan cohort, with n = 7 in each group. The registry reports means and standard deviations and a Wilcoxon (Mann-Whitney) test with p = 0.457. No primary confidence interval is reported. Secondary outcomes are the Harris Hip Score, Oxford Hip Score, and WOMAC Hip Score at 2 years; their unadjusted re-calculations have uncertainty and should be interpreted cautiously.

The record does not report the original sample-size justification, detailed missing-data methods, analysis-population definition, or multiplicity strategy. Safety figures are descriptive and lack a posted comparative analysis. The primary result concerns bone density and does not directly establish an effect on implant failure, survival, or clinical function.