A re-analysis of a caries clinical trial by survival analysis

Survival analysis of caries incidence in African-American school-aged children

OBJECTIVES

To conduct an assessment of time-dependent covariates related to dental caries of the permanent dentition among a low socioeconomic status, understudied cohort of children, incorporating time-dependent covariates through the application of extended Cox proportional hazards modeling.

METHODS

This study modeled the time to first cavitated dental caries in permanent teeth among school-aged children and assessed factors associated with this event. A cohort of 98 low socioeconomic status African-American children with mean age of 5.85 years at baseline was recruited in Uniontown, Alabama and followed prospectively for 6 years. None of these children had dental caries on permanent teeth at baseline, and oral examinations were performed annually. Caries-free survival curves were generated to describe time to event (having first decayed, filled, or missing permanent surface). Bivariate and multivariable extended Cox hazards modeling was used to assess the relationships between time-dependent and time-independent covariates and time to event.

RESULTS

Twenty-eight children (28.6 percent) had their first permanent tooth caries event during the 6-year follow-up. Multivariable results showed that greater consumption of water was associated with lower dental caries hazard, while previous primary tooth caries experience was associated with greater dental caries hazard after adjustment for frequency of consumptions of milk, added-sugar beverages, and 100 percent juice.

CONCLUSIONS

There was a global/overall significant caries protective effect of water consumption during the school-age period of child development.

Applying Modern Survival Analysis Methods to Longitudinal Dental Caries Studies

Before the 1960s, tooth-specific caries risk was reported to be highest at 2 to 4 years after eruption. We studied the tooth-specific caries risk in three contemporary age cohorts in Finland. All together, 4072 boys and girls were followed annually from age 6 to age 18+ years in three age cohorts born in the 1960s, 1970s, and 1980s. We used a survival model and Bayesian inferential methods in the statistical analyses to establish the secular changes during this period. The analysis was based on the caries risk in individual teeth as a function of tooth age instead of summary measures, such as DMFS values. Our first finding was a marked overall decrease of caries. Moreover, analyses of the 1960 and 1970 cohorts revealed that the risk in molar teeth was highest immediately after eruption; in the youngest cohort, however, the risks of individual teeth were so low that no such dependencies on tooth age could be established.

Bayesian Analysis of Clustered Interval-censored Data

The recording of multiple interval-censored failure times is common in dental research. Modeling multilevel data has been a difficult task. This paper aims to use the Bayesian approach to analyze a set of multilevel clustered interval-censored data from a clinical study to investigate the effectiveness of silver diamine fluoride and sodium fluoride varnish in arresting active dentin caries in Chinese pre-school children. The time to arrest dentin caries on a surface was measured. A three-level random-effects Weibull regression model was used. Analysis was performed with WinBUGS. Results revealed a strong positive correlation (0.596) among the caries lesions’ arrest times on different surfaces from the same child. The software WinBUGS made the above complicated estimation simple. In conclusion, the annual application of silver diamine fluoride on caries lesions, and caries removal before the application, were found to shorten the arrest time.

Efficiency Issues among Statistical Methods for Demonstrating Efficacy of Caries Prevention

Although repeated tooth-surface-specific information is commonly collected during a longitudinal caries clinical trial, traditional methods often make limited use of the repeated measures. Newer methods of analysis, such as methods based on time-to-event and methods for longitudinal or clustered data, have the potential to increase the efficiency of the statistical analysis. We compare a range of analytical methods from the traditional analysis based only on the number of caries onsets to newer methods that incorporate time at risk and surface-specific information, such as Poisson regression methods for clustered data, with respect to the efficiency of treatment comparisons. Under most circumstances, the greatest gain in efficiency associated with time-to-event methods will be due to the ability of subjects to contribute caries onsets to the analysis until they are lost from the study. Incorporating the number of surfaces at risk, the surface time at risk, and surface-specific characteristics will typically produce only a modest gain in efficiency.

Long-Term Effect of Erythritol on Dental Caries Development during Childhood: A Posttreatment Survival Analysis

Objective: To assess the effect of daily consumption of erythritol, xylitol, and sorbitol candies on caries development in mixed dentition during a 3-year intervention and 3 years after the intervention. Methods: 485 Estonian first- and second-grade primary school children participated. Children were randomly allocated to an erythritol, xylitol, or sorbitol (control) group. Polyol-containing candies were administered on school days with a daily polyol consumption of 3 × 2.5 g. Yearly, caries development was assessed by calibrated dentists using the ICDAS criteria. Six years after initiation of the study and 3 years after cessation of daily polyol consumption, 420 participants were re-examined to identify potential long-term effects of polyol consumption. Survival curves were generated at the end of the intervention period and 3 years after intervention. The model included age of the subjects, schools, tooth surface ages and years of surface exposure to intervention. ICDAS scoring system-based events included enamel/dentin caries development, dentin caries development, increase in caries score, and dentist intervention. Results: At the end of the intervention, time to enamel/dentin caries development, dentin caries development, increase in caries score, and dentist intervention were significantly longer in the erythritol group as compared to the sorbitol group. Except for increase in caries score, all effects persisted 3 years after cessation of daily polyol consumption. Conclusions: A caries-preventive effect of 3-year erythritol consumption as compared to sorbitol was established in children with mixed dentition. The effect persisted up to 3 years after the end of the intervention.

Statistical power of multilevel modelling in dental caries clinical trials: a simulation study

Outcome data from dental caries clinical trials have a naturally hierarchical structure, with surfaces clustered within teeth, clustered within individuals. Data are often aggregated into the DMF index for each individual, losing tooth- and surface-specific information. If these data are to be analysed by tooth or surface, allowing exploration of effects of interventions on different teeth and surfaces, appropriate methods must be used to adjust for the clustered nature of the data. Multilevel modelling allows analysis of clustered data using individual observations without aggregating data, and has been little used in the field of dental caries. A simulation study was conducted to investigate the performance of multilevel modelling methods and standard caries increment analysis. Data sets were simulated from a three-level binomial distribution based on analysis of a caries clinical trial in Scottish adolescents, with varying sample sizes, treatment effects and random tooth level effects based on trials reported in Cochrane reviews of topical fluoride, and analysed to compare the power of multilevel models and traditional analysis. 40,500 data sets were simulated. Analysis showed that estimated power for the traditional caries increment method was similar to that for multilevel modelling, with more variation in smaller data sets. Multilevel modelling may not allow significant reductions in the number of participants required in a caries clinical trial, compared to the use of traditional analyses, but investigators interested in exploring the effect of their intervention in more detail may wish to consider the application of multilevel modelling to their clinical trial data.

Statistical methodology in oral and dental research: pitfalls and recommendations

OBJECTIVES

This study describes the pitfalls for commonly used statistical techniques in dental research and gives some recommendations for avoiding them. It also explores the potential of some of the newer statistical techniques for dental research.

METHODS

Each of the commonly used techniques e.g. descriptive statistics, correlation and regression, hypothesis tests (parametric and non-parametric) and survival analysis are explored with examples and recommendations for their use are provided. Common sources of error including those of study design, insufficient information, ignoring the impact of clustering and underuse of confidence intervals are outlined. The potential of statistical techniques such as multivariate survival models, generalized estimating equations and multilevel models are also explored.

CONCLUSIONS

Reviews of published dental research repeatedly identify statistical errors in the design, analysis and conclusions of the study. Educating researchers on common pitfalls and giving recommendations for avoiding them may help researchers to eliminate statistical errors. Developments in statistical methodology should be routinely monitored to ensure the most appropriate statistical methods are used in dental research.

Review of 36,537 patient records for tooth health and longevity of dental restorations

To develop an automatic system for utilizing electronic dental records, a data mining system to extract the diagnostic and treatment codes from the records for an intermediate file and automatic drawing of Kaplan-Meier-type survival curves was first created. Then this intermediate file was analyzed with SAS software for the scientific determination of Kaplan-Meier survival of tooth/surface-specific healthy time and survival of restorations in each permanent tooth, health center, and age cohort and also combined. All patients born in 1985, 1990 or 1995 in 28 health centers in Finland were analyzed. Patients classified as caries-active were those who had caries in any first permanent molar under the age of 8 years, while resistant patients did not have caries in these teeth before 10 years. In the younger age cohorts, a shortening of survival of caries-free teeth was seen. The shortest caries-free survival was seen in mandibular and maxillary molars in the youngest age cohort. Occlusal surfaces of molars determined their caries onsets and proximal caries occurred equally in molars, incisors and premolars, whereas canines or mandibular incisors did not have caries in these age cohorts. Caries-prone subjects had the shortest survival in all their teeth. The median longevity of all restorations was 11.7 years, with great variation between health centers and teeth. Because of the great variation between individual teeth, the tooth-specific approach seems appropriate in both caries epidemiology and material sciences.

A model for the analysis of caries occurrence in primary molar tooth surfaces

Recently methods of caries quantification in the primary dentition have moved away from summary 'whole mouth' measures at the individual level to methods based on generalised linear modelling (GLM) approaches or survival analysis approaches. However, GLM approaches based on logistic transformation fail to take into account the time-dependent process of tooth/surface survival to caries. There may also be practical difficulties associated with casting parametric survival-based approaches in a complex multilevel hierarchy and the selection of an optimal survival distribution, while non-parametric survival methods are not generally suitable for the assessment of supplementary information recorded on study participants. In the current investigation, a hybrid semi-parametric approach comprising elements of survival-based and GLM methodologies suitable for modelling of caries occurrence within fixed time periods is assessed, using an illustrative multilevel data set of caries occurrence in primary molars from a cohort study, with clustering of data assumed to occur at surface and tooth levels. Inferences of parameter significance were found to be consistent with previous parametric survival-based analyses of the same data set, with gender, socio-economic status, fluoridation status, tooth location, surface type and fluoridation status-surface type interaction significantly associated with caries occurrence. The appropriateness of the hierarchical structure facilitated by the hybrid approach was also confirmed. Hence the hybrid approach is proposed as a more appropriate alternative to primary caries modelling than non-parametric survival methods or other GLM-based models, and as a practical alternative to more rigorous survival-based methods unlikely to be fully accessible to most researchers.

Virus and antibody dynamics in acute west nile virus infection

BACKGROUND

The dynamics of the early stages of West Nile virus (WNV) infection can be assessed by follow-up studies of viremic blood donors.

METHODS

A total of 245 donors with WNV viremia were followed up weekly for 4 weeks and then monthly for up to 6 additional months or until seroconversion. Plasma samples were tested for WNV RNA by transcription-mediated amplification (TMA) and for WNV-specific IgM and IgG antibodies. RNA persistence was investigated by 6 replicate TMA tests; samples that were viremic for >40 days were tested for WNV-neutralizing activity. Follow up of 35 additional viremic donors for up to 404 days was conducted to evaluate persistence of WNV-specific antibody.

RESULTS

The median time from RNA detection to IgM seroconversion was 3.9 days; to IgG seroconversion, 7.7 days; to RNA negativity by single-replicate TMA, 13.2 days; and to RNA negativity by 6-replicate TMA, 6.1 additional days after results of single-replicate TMA are negative. For 4 donors in whom RNA persisted for >40 days after the index donation, all samples obtained after this threshold were also positive for WNV IgG and neutralizing activity. The mean times to IgM and IgA negativity were 156 and 220 days, respectively.

CONCLUSIONS

IgM and IgG develop rapidly after viremia and before RNA levels become undetectable, which occurred a mean of 13.2 days after the index donation among donors in this study. WNV RNA detection by replicate TMA rarely persists for >40 days after the index donation and is accompanied by WNV-specific neutralizing antibody, consistent with an absence of WNV transmission via transfusion of seropositive blood components.

A practice-based study of the sealant treatment effectiveness in Finns

OBJECTIVES

The effectiveness of fissure sealing in preventing dental caries in a subject-specific fashion was studied in order to forge a link between the controlled trials and knowledge creation in clinical practice.

METHODS

The subjects were divided retrospectively into three categories according to the sealant treatment status of their first permanent molars at the first examination after the eruption and the survival of first molars in each group were estimated using the Kaplan-Meier method.

RESULTS

Sealing of all first molars in caries-prone subjects was an effective caries preventive method. The highest survival of first molars was observed in the unsealed subjects in the health center focusing on sealing risk-subjects only. The first molars of partly sealed subjects had the lowest probability of survival in both health centers.

CONCLUSION

Sealing all four permanent first molars rather than some of them in high caries risk subjects and leaving unsealed the first molars of caries resistant subjects enable to decrease unnecessary sealant treatment by focusing it especially to those benefiting it.

Chlorhexidine and Preservation of Sound Tooth Structure in Older Adults

The Trial to Enhance Elderly Teeth Health (TEETH) was designed to test the impact of regular rinsing with a 0.12% chlorhexidine (CHX) solution on tooth loss, and the causes of tooth loss (caries, periodontal disease and trauma) were also investigated. This paper reports on the effectiveness of a 0.12% CHX solution for controlling caries using a tooth surface (coronal and root) survival analysis. A total of 1,101 low income elders in Seattle (United States) and Vancouver (Canada), aged 60-75 years, were recruited for a double-blind clinical trial and assigned to either a CHX (n = 550) or a placebo (n = 551) mouth rinse. Subjects alternated between daily rinsing for 1 month, followed by weekly rinsing for 5 months. All sound coronal and root surfaces at baseline were followed annually for up to 5 years. At each follow-up examination, those tooth surfaces with caries, restored, or extracted were scored as 'carious'. The hazard ratio associated with CHX for a sound surface to become filled, decayed, or extracted was 0.87 for coronal surfaces (95% confidence interval: 0.71-1.14, p = 0.20) and 0.91 for root surfaces (95% confidence interval: 0.73-1.14, p = 0.41). These findings suggest that regular rinsing with CHX does not have a substantial effect on the preservation of sound tooth structure in older adults.

A seven-year survival analysis of caries onset in primary second molars and permanent first molars in different caries risk groups determined at age two years

OBJECTIVE

To investigate in a group of children (n=183) the effect of possible risk factors registered at the age of 2 years on caries development in 7 years of follow-up, and to study associations between risk groups.

MATERIAL AND METHODS

Consumption of candies, use of a nursing bottle at night, use of fluorides, toothbrushing, pacifier sucking, and prolonged breastfeeding (>or=12 months) were recorded at the age of 2 years. The timing of caries onset in different groups was compared by applying a survival analysis method--the survival curves produced separately for selected teeth in different risk groups.

RESULTS

The survival curves of caries onset for both primary and permanent molars were consistently lower for children who consumed candies more than once a week, did not brush their teeth daily, were given a nursing bottle at night or a pacifier at age 2 years. The multivariate survival analysis confirmed that consumption of candies and lack of daily toothbrushing were the factors that had the major impact on caries onset in both primary and permanent molars. Prolonged pacifier sucking (>or=2 years) was related only with short duration of breastfeeding. Children with prolonged use of a nursing bottle at night also consumed candies more than once a week, did not brush their teeth regularly, and did not use fluoride tablets.

CONCLUSIONS

Consumption of candies and inadequate oral hygiene at age 2 years are important long-term risk factors for caries development in both primary and permanent molars.

Multilevel modelling of clustered grouped survival data using Cox regression model: an application to ART dental restorations

In some controlled clinical trials in dental research, multiple failure time data from the same patient are frequently observed that result in clustered multiple failure time. Moreover, the treatments are often delivered by more than one operator and thus the multiple failure times are clustered according to a multilevel structure when the operator effects are assumed to be random. In practice, it is often too expensive or even impossible to monitor the study subjects continuously, but they are examined periodically at some regular pre-scheduled visits. Hence, discrete or grouped clustered failure time data are collected. The aim of this paper is to illustrate the use of the Monte Carlo Markov chain (MCMC) approach and non-informative prior in a Bayesian framework to mimic the maximum likelihood (ML) estimation in a frequentist approach in multilevel modelling of clustered grouped survival data. A three-level model with additive variance components model for the random effects is considered in this paper. Both the grouped proportional hazards model and the dynamic logistic regression model are used. The approximate intra-cluster correlation of the log failure times can be estimated when the grouped proportional hazards model is used. The statistical package WinBUGS is adopted to estimate the parameter of interest based on the MCMC method. The models and method are applied to a data set obtained from a prospective clinical study on a cohort of Chinese school children that atraumatic restorative treatment (ART) restorations were placed on permanent teeth with carious lesions. Altogether 284 ART restorations were placed by five dentists and clinical status of the ART restorations was evaluated annually for 6 years after placement, thus clustered grouped failure times of the restorations were recorded. Results based on the grouped proportional hazards model revealed that clustering effect among the log failure times of the different restorations from the same child was fairly strong (corr(child)=0.55) but the effects attributed to the dentists could be regarded as negligible (corr(dentist)=0.03). Gender and the location of the restoration were found to have no effects on the failure times and no difference in failure times was found between small restorations placed on molars and non-molars. Large restorations placed on molars were found to have shorter failure times compared to small restorations. The estimates of the baseline parameters were increasing indicating increasing hazard rates from interval 1 to 6. Results based on the logistic regression models were similar. In conclusion, the use of the MCMC approach and non-informative prior in a Bayesian framework to mimic the ML estimation in a frequentist approach in multilevel modelling of clustered grouped survival data can be easily applied with the use of the software WinBUGS.

Multivariate survival analysis for the identification of factors associated with cavity formation in permanent first molars

This investigation used a multivariate survival model, which allowed for dependent caries data with possible censoring, to analyse the impact of timing of tooth emergence, gender and several reported oral hygiene and dietary habits on the incidence of cavity formation in permanent first molars (PFM) in young children. A 6-yr prospective oral health screening project in Flanders, Belgium, provided clinical and questionnaire data on 4,468 children. The results revealed that PFM in children who reported frequent brushing, who had no visible caries experience in the primary dentition, and who presented without occlusal plaque on the PFM had the best survival estimates. Girls had a higher incidence than boys of cavity formation in mandibular molars. The multiple survival analysis confirmed the major impact of the caries status of the deciduous dentition and self-reported oral hygiene habits on the incidence of cavities in permanent first molars.

Using survival methodologies in demonstrating caries efficacy

Exploiting recent advances in statistical methods, particularly for correlated intra-subject data, could increase the efficiency of caries clinical trials. Methods of analysis using the tooth surface as the unit should be investigated. Whole-mouth measures such as the DMFS increment ignore the variation in the number of surfaces at risk between subjects and within a subject over time. The use of "survival time" for each surface as the outcome measure--i.e., the time from the start of the trial to when a surface is recorded as decayed or filled--is proposed. Data from caries clinical trials could be described as clustered survival data, where clustering of tooth surfaces exists such that survival times within the same cluster or subject are correlated. Advances in the analysis of clustered survival data, such as the use of marginal models with robust variance estimators, have recently been exploited in the analysis of caries clinical trials. The analysis produced results similar to those achieved by conventional DMFS-based analysis. The results using survival analysis are easily interpreted-for example, the median survival time of tooth surfaces in female subjects using a toothpaste with a higher level of fluoride (1500 ppm F) is 1.07 times the median survival time of surfaces in female subjects using toothpaste with less fluoride (1000 ppm F). Further research is required to investigate if survival analysis is a more sensitive method of analysis, i.e., whether causative factors can be identified with fewer subjects than with the conventional method of analysis.

The classic caries clinical trial: constraints and opportunities

The history of clinical trials would include events in 1747 on board the Salisbury, a British Navy vessel at sea with 12 seamen critically ill with scurvy. Involving these 12 sailors in a study, an officer on board by the name of Lind evaluated six potential treatments for scurvy, and rapidly reached the conclusion that daily consumption of citrus fruits returned the men fit for duty in approximately six days (Bull, 1959). The concept of experimental randomization was first developed by Sir R.A. Fisher (1925, 1926), and the method was introduced to medical research via a study of tuberculosis treatment by Amberson and co-workers (1931), who randomized 24 TB patients into two groups, one to receive the experimental therapy, the other serving as the control. Amberson et al. also incorporated the concept of blinding into their study. Sir Austin Bradford Hill codified and built on the principles of scientific experimentation developed by Fisher, and introduced the use of random numbers in the allocation of patients in the British Medical Research Council (1948) study of the effect of streptomycin in the treatment of tuberculosis (Daniels and Hill, 1952; Hill, 1952). The first applications of clinical trial methodology for testing interventions on dental, oral, and maxillofacial diseases and conditions are more difficult to determine. For dental caries prevention, however, Chilton and Fertig (1958) and Slack and Martin (1964) were certainly among the early caries clinical trial pioneers. As clinical trials have come into the mainstream of clinical research in medicine and dentistry, a great deal of developmental work has focused on their methodological enhancement. The most successful of these efforts have come from fruitful, ongoing collaborations among clinician investigators, biostatisticians, data management specialists, biomedical ethicists, and others with an academic interest in clinical trial design and utilization. During the past 25 years, the emergence of systematic reviews and the evidence-based medicine (EBM) movement have also contributed significantly to the increasing reliance on randomized clinical trial outcomes for the advancement of better clinical practice (Richards et al., 1997; Straus and Sackett, 1998; www.cochrane.org/cochrane/ccbroch.htm#BDL, 2002).

Application of survival analysis to carious lesion transitions in intervention trials

OBJECTIVE

To demonstrate the usefulness of a survival-time regression model for the analysis of data from two 3-year trials of the caries-preventive effect of sugar-substituted chewing gums and fluoride toothpaste, carried out among 892 Lithuanian children.

METHODS

A caries onset was defined as a transition from sound to carious and a caries recovery was defined as a transition from carious to sound. The time at risk for each type of transition was calculated. Using an exponential survival-time regression model, the hazard ratios for the covariates experimental group (control, sugar substitute, fluoride), age, gender, surface type and posteruptive surface age was estimated. This analysis was repeated using two alternative definitions of the caries transitions.

RESULTS

The analyses confirmed that caries rates are higher in occlusal surfaces, and that posteruptive surface age influences caries rates. Moreover, it also confirmed that fluoride affects the outcome of ongoing caries activity more than the initiation of caries.

CONCLUSIONS

Survival-time analysis of caries transitions allows for the extraction of much more information from caries trials than does the traditional DMF-based analysis, and traditional DMF incremental values may easily be derived from the models.