Survival analysis with clustered observations of orthodontic brackets

Abstract

Failures in the bonding of dental bracket are a big concern for orthodontists. Clinical experience suggests that some patients are more prone than others to experience failures. Therefore, it can be expected that in statistical analysis of orthodontic bracket failures, the usual assumption of independence between the observations is violated. An approach to overcome this problem is to apply the frailty model, in which the association between failure times is modelled with a random-effect term (i.e. frailty). We postulated that brackets of the same subject share the same frailty, that is, a latent common group effect, due to some unknown or unobserved covariates.The aim of this study was to investigate possible risk factors related to bracket failure using Cox proportional hazards model with a shared frailty term and to compare the results with those obtained using a basic Cox proportional hazards model.Survival data for 1677 brackets were obtained from a cohort of 54 females (mean age +/- SD: 13.3+/-4.8 yrs) and 46 males (mean age +/-SD: 13.1+/-3.8 yrs) over a eight-year period. Age, gender, vertical craniofacial morphology and anatomical location of brackets were entered into Cox models as covariates. The findings indicated that bracket failure was significantly affected by tooth position within the dental arch, with the highest failure risk in maxillary posterior region. Age, gender and vertical craniofacial morphology did not affect bracket failure. A Cox proportional hazards model with a shared frailty term represents a useful approach for modelling orthodontic bracket failures.

Randomized controlled clinical trial of the 24-months survival of composite resin restorations after one-step incomplete and complete excavation on primary teeth

“…The second point relates to the statistical analysis, but this was methodologically controlled by accounting for the correlation of observations within each child using Weibull Cox regression models with shared gamma frailty. 23 Regardless of the outcome observed for the performance of composite restorations, the clinical and radiographic follow-up of children from the present study, considering the pulp vitality, showed that PCR had a high success rate (92%) without significant differences from other conventional treatments over 24 months. 11 This result indicates the favorable biological response of the pulp tissue and the arrestment of caries progression.…”

“…Analyses accounted for the correlation of observations within each child using Weibull Cox regression models with shared gamma frailty. 23 Age, sex, tooth type (first or second molar), type of cavity (occlusal and occlusoproximal) and treatment were included in the analyses as independent variables. Univariable models were fitted, and variables demonstrating p values <0.20 were entered in a multivariable model.…”

Assessment of preparation time and 1-year Invisalign aligner attachment survival using flowable and packable composites:

“…Some previous studies on bracket bonding failure reported similar findings, showing that sex, arch, and the occurrence of tooth extraction did not affect bracket failure. [17][18][19] However, some studies showed that bracket failure was significantly affected by the presence of deep bite and tooth type. 19,20 Attachment damage in the current study was not affected by the presence of deep bite, which might have been due to the advantage of using computer-aided design for avoiding occlusal interference.…”

Outcomes of One-Step Incomplete and Complete Excavation in Primary Teeth: A 24-Month Randomized Controlled Trial

“…Analyses accounted for the correlation of observations within each child using Weibull Cox regression models with shared gamma frailty [Petracci et al, 2009]. Age, sex, tooth position (first or second molar), number of restored surfaces, operator and treatment were included in the analyses as independent variables.…”