G. S. P. Cheung & T. K. Chan
Faculty of Dentistry, The University of Hong Kong, Prince Philip Dental Hospital, Hong Kong.

The literature is replete with studies on the outcome of nonsurgical root canal treatment, which has been reported to have a success rate of 53-95% over various periods of observation (Friedman 1998). These success rates were calculated as a percentage of those teeth examined at recall where it was assumed that the cases survived up to the longest follow-up, although the mean observation period was usually shorter. A number of factors have been evaluated using simple statistical tests for their association with the percentage of successful cases. For instance, the presence of a pre-existing periapical radiolucent lesion is generally agreed to result in a lower rate of success (Sjogren et al. 1990, Smith et al. 1993), whereas vital pulpectomy appeared to enjoy a higher success rate compared with root canal treatment of infected pulps (Heling & Tamse1970, Smith et al.1993). These conclusions were based on the calculation of binomial proportions of successful cases, and had not taken into account any confounding of factors being evaluated.

The presence of microorganisms has been implicated to be a major cause of failed root canal treatment (Bystrom et al.1987, Molander et al.1998, Sundqvist et al. 1998, Cheung &Ho 2001). Whereas early failures usually reflect the inadequacy of the treatment in controlling the root canal infection, the late failures may reflect the influence of other factors such as coronal microbial ingress over time (Saunders & Saunders 1994, Drstavik 1996, Hayes & Dummer1997). Although the presence of cultivable bacteria in root canals prior to filling appears to result in a lower success rate (Engstrom et al. 1 964, Sjogren et al. 1997), bacteriological sampling of root canals is not without limitations (Dahle. n & Haapasalo 1998), and hence its use has not been required as routine procedure prior to completion of nonsurgical treatment (Molander et al.1996).To assess the outcome of endodontic treatment, long-term radiographic review is essential to supplement clinical findings. It has been suggested that root canal treatment should be followed for up to 4 years (European Society of Endodontology 1994). Unfortunately, not many studies have incorporated a review period of more than 4 years (reviewed by Friedman 1998). As the rate at which failures occur may not be constant over a certain period of observation that itself may also be a variable for each subject of the study, a different approach to evaluate treatment outcome is necessary. A life-table or survival analysis, which examines the occurrence of critical events such as development of failure throughout the observation period, is preferred (Mitchell & Walls 1991, Weiger et al.1998). Such analysis allows the assessment of the survival function over time and the prediction of longevity of cases related to treatment modality, as well as the effects of any co variables (Anderson & Zarb 2000).
The purpose of this study was to evaluate the long-term survival of primary root canal treatment performed in a dental teaching hospital more than 10 years ago, and to identify factors that might affect the survival.