A MULTILEVEL ANALYSIS OF FACTORS AFFECTING THE LONGEVITY OF FIXED PARTIAL DENTURES, RETAINERS AND ABUTMENTS

There is a methodological problem in analysis of data belonging to dif ferent hierarchical levels, e.g. patient, arch, and tooth. A method fo r multilevel modelling (MLM) that resolves this problem is available. This method which is presented in this article, was used to reanalyse previously published long-term treatment results concerning the longev ity of fixed partial dentures, retainers and abutments. The main resul ts were consistent, indicating that no great faults were committed whe n using the conventional logistic regression method. There were, howev er, differences when calculating the combined risks for various situat ions. The conventional method was found to over-and underestimate prob abilities of survival in different cases, compared to the MLM method. Especially when the most negative factors were combined, the conventio nal method had a tendency to underestimate the risk. For example, if a n abutment had the combination 'endodontically treated'; 'FPD placed b y 'another dentist''; 'distal abutment'; and 'marginal bone loss >50%' ; the probability of the tooth remaining after 18-23 years was only 20 % according to the MLM method but 35% according to the conventional me thod. It was concluded that MLM is the method of choice for many situa tions in dental research where data belonging to different levels are to be analysed.