Discussion.
Long-term clinical and radiographic follow-up is essential for the assessment of endodontic treatment outcome. Whilst the process of periapical healing may often be diagnosed radiographically at the 1-year recall, individual cases may take up to 4 years for complete healing of periapical lesions (Drstavik 1996). Before this time, teeth with an incompletely resolved area were neither regarded as a failure nor success, because the data were not considered valid. Given a minimum postoperative period of 4 years, the presence of a radiolucent area of any size was regarded as a failure, with the exception of a locally widened periodontal ligament space (European Society of Endodontology 1994). These criteria for failure were rather stringent, indeed, much stricter than those used in many studies of similar or shorter periods of observation (reviewed by Friedman 1998). This may account for the relatively high percentage of cases (52%) that were deemed to have failed in this study.
The 378 teeth that were excluded from entry into the data set were distributed at a similar proportion amongst various tooth type as those being analyzed (Table 1), and thus were unlikely to affect the results here. Cases of extraction due to nonendodontic reasons were excluded for convenience in the analysis. On reviewing the data, majority of these ‘nonendodontic extractions’ were carried out 2-4 years postoperatively. Whilst one might include these cases as ‘censored’ at the time of extraction in the K-M analysis, their inclusion would merely ‘refine’ the initial part of the survival curve. The survival function beyond this early period was unlikely to be affected to any significant degree by the omission of these cases.
The different rates of decline in survival of the treated teeth before and after a postoperative period of about 18months suggest the presence of two overall processes leading to failure of the treatment, one rapid and one slow. Examination of the life span of individual failed cases confirmed this proposition; some 44% of all failures occurred in the first 18months (Fig. 3). Thereafter, the incidence of failures declined with time. Further investigation is justified to examine the factors contributing to failures in these different periods.
The median survival time of the root canal treatment was111months, which was remarkably similar to a previous but independent study in the same hospital that examined teeth treated between1990 and1995 (Cheung 2002), but the method used to estimate the ‘time to failure’ was somewhat different. The Cox Regression analysis was used in preference to the Log Rank Test, as the former is able to identify any confounding of covariables being examined (Lee & Go1997). Four factors were identified to be significant in affecting the survival function: preoperative periapical status, tooth type, the type of coronal restoration and the type of post used. We had not examined the influence of preoperative pulpal condition on the longevity of treatment because such information sometimes was missing in the patients’ records nor was it included as part of the computerized dental database at that time.
The cumulative probability of survival was significantly higher when there was no preoperative periapical lesion; this corroborates many previous findings based on the calculated percentage of successful cases (Heling & Tamse1970, Sjogren et al.1990, Smith et al.1993).However, there seemed to be a lack of effect for small versus large periapical lesions on the survival outcome in this study, despite reports suggesting a lower success rate with lesions larger than 5 mm in diameter (Storms1969, Matsumoto et al.1987). These two studies, however, had a mean review period of1-2 years only. Giventhat a long enough interval may be required for complete healing of large lesions (Drstavik 1996), it was possible that premature evaluation of periapical status might have produced an inaccurate result (Friedman1998). A recent study also indicated that the extent of preoperative lesion had the largest effect on treatment outcome (Chugal et al. 2001). However, the same observation was not apparent in the present study, nor supported in a previous report (Sjogren et al. 1990). Cases with widened periodontal ligament seemed to behave similarly as those with periapical radiolucent areas. As pathological lesions could exist in the periapical region, which might not be discernible on the periapical radiograph (Brynolf 1967), it was likely that periapical inflammation was present when the periodontal ligament space was seen to be widened radiographically.
Our results indicated that tooth type was a significant factor affecting the endodontic treatment outcome. It can be seen that the survival curves associated with different tooth types were in two clusters. Mandibular premolars and maxillary and mandibular anterior teeth were in one group which had a significantly longer survival time than other tooth types. Molar teeth demonstrated the worst survival outcome. From a clinical point of view, molars are more difficult to treat because of accessibility and complex root canal anatomy. Root- filled posterior teeth might fracture more readily if they had not been restored in such a way that crown-root fracture may be prevented (Hansen & Asmussen 1990, Gutmann 1992). It is also arguable that the criteria for success might have discriminated against molars because a multirooted tooth here was regarded as failed if any one of its roots had met the criteria for failure (Friedman1998).Mandibular premolars usually possess relatively simple canal morphology in the Chinese population (Walker 1987), which together with better accessibility could imply easier treatment and thus a greater chance of attaining a high ‘technical’ standard of treatment. Maxillary anterior teeth share a similar survival time probably because of the same reason.
Many teeth surveyed in this study did not receive any inter appointment medicament during treatment, which was a common practice at one time in the 1980s. When calcium hydroxide was used as the root canal medicament, a seemingly higher survival probability of root-filled teeth was attained, compared with other medicaments or no dressing when all cases were included in the analysis. Calcium hydroxide has been documented to be an effective disinfectant for the root canal system (Bystrom et al. 1985, Sjogren et al. 1991). However, knowing that no medicament would have been used in cases of single-visit treatment, the multivariate test was rerun with these cases removed - the medicament then became an insignificant covariable (Cox Regression, P = 0.102). The lack of significance of the medicament used in multiple-visit therapy might be related to its placement method. As calcium hydroxide paste was delivered into the canal with a K-file rotated in reverse by hand during those times at PPDH, the quality of ‘fill’ was probably not very dense (Sigurdsson et al. 1 992) which could affect its antimicrobial effectiveness. Other factors, e.g. the integrity of the temporary restoration that had not been studied because such information was often not entered in the patient’s record, might have a bearing on the effectiveness of calcium hydroxide.
The survival of root-filled teeth was enhanced when a crown restoration had been provided. On the other hand, studies that had attempted to examine the relationship between the type of coronal restoration and the endodontic treatment outcome failed to demonstrate a statistically significant association (Safavi et al. 1 987, Friedman et al. 1995). The difference may be related to the criteria for failure used in the present study. As all extractions, except for documented reasons not related to the root canal treatment, were regarded as failures of the primary treatment here, this would include cases of nontraumatic tooth fracture that had been extracted over the postoperative period. It has been reported that teeth with no subsequent restoration or restored only with amalgam or resin composite were significantly more likely to undergo extraction than teeth that had received a crown (Lazarski et al. 2001). Thus, not only does a crown restoration provide a coronal seal to prevent bacterial recontamination of the root canal system (Saunders & Saunders 1994), but it may also serve to protect the tooth from the risk of fracture.
The placement of a cast post and core had an adverse effect on the survival function of the root-filled teeth. The theoretical disruption of the apical seal (of the cold laterally compacted gutta-percha root canal filling) was unlikely to be the cause because teeth with prefabricated posts were not so affected. Most cast posts constructed in the 1980s were tapered, and the teaching then did not require the placement of crown margin at a distance beyond the tooth-core junction to provide a ‘ferrule effect’. Without this design, the post-crown assembly could fail at a much lower load and with an unpredictable pattern (Sorensen & Engelman 1990). Such nontraumatic tooth fractures had been considered as a failure because the ultimate goal of treatment, i.e. to maintain the tooth as a functional unit of the dentition, was not achieved.
The use of an integrated amalgam postcore, or coronal- radicular amalgam core (Nayyar et al. 1 980) was associated with a favourable survival outcome, possibly because a crown was of ten placed as the final restoration for these teeth and thus some form of protection was present. The prefabricated posts used were almost exclusively parallel-sided and were placed mostly in posterior teeth (anterior : posterior @ 1: 10). About 80% of them were subsequently restored with a crown. It appears that the favourable survival outcome for these two types of posts is related to the final restoration - teeth with crown restorations survived significantly longer than those with plastic restorations only.
The great majority of the 224 teeth that did not receive any form of posts here were restored with a plastic intracoronal restoration with or without bonding, and they had a significantly lower survival probability than the groups with amalgam postcores and prefabricated posts. Again, it can be noticed that the survival pattern of the different postcore groups mirrored that of the restoration type. It seems that even with the multivariate analysis used in this study, the confounding of the two variables, type of restoration and type of post could not be separated. Clinically, these two variables are closely related to each other because a crown restoration is likely to be planned with the decision for a post and core. The relatively poor survival outcome for teeth with a cast post may be related to a lack of ‘ferrule’ protection (due to the prevailing practice then), and iatrogenesis during posthole preparation.
To our surprise, the apical extent and the ‘quality’ of the root canal filling were not significant covariables affecting the survival function. The assessment of quality here was determined from radiographs alone and was scored on basis of the homogeneity of its appearance. Such evaluation has major limitations, as any buccal or lingual voids to the root filling could have been masked and hence not detected. The same may also be masked by root canal sealers with high radiopacity. The presence of voids at the apical third of the canal has been reported to associate with a lower survival probability in another survey at the same hospital (Cheung 2002). A similar result, however, is not reproduced in the present study, although there was a tendency for a longer survival for cases that did not show any void radiographically. The difference may be related to the substantial number of cases that had received no medicament in the present study. These ‘empty canals’ may have harboured residual bacteria which survived the root canal preparation process (Bystrom& Sundqvist1981), because hypochlorite irrigant was not used at that time. The effect of the location of voids could have been masked by the general presence of microorganisms. Thus, a knowledge of the prevailing treatment technique is essential for studies on treatment outcome.
The apical extent of root fillings was found to be difficult to determine at times, especially in maxillary posterior teeth which apices tended to overlap with the image of anatomical structures on the radiograph. It has been argued that the apical extent of root canal fillings bears little, if any, correlation to endodontic failure because the main determinant appears to be the presence of microorganisms (Lin et al. 1991). The lack of influence by the apical extent of the root canal filling was also reported in a study where treatment was carried out under controlled asepsis (Bystrom et al. 1987). Once microorganisms are effectively removed from the root canal system, periapical healing can usually be expected. In contrary, the apical level of obturation was shown to have a significant influence on the outcome of treatment for roots with necrotic pulps and periapical lesions in a hospital environment where treatment was carried out by dental students (Sjogren et al. 1990). Unfortunately, the preoperative pulpal status of the treated teeth was not available in our database, and thus a direct comparison with the corresponding group of teeth is not possible.
The influence of the number of treatment visits on the survival time was found to be insignificant after the Cox Regression analysis. The teaching at PPDH has always been biased towards multiple-visit therapy. The small number of cases in the single-visit group may have diminished the power of the statistical tool to substantiate a difference. In theory, once effective chemomechanical preparation is carried out and a high level of root canal disinfection is achieved with the use of calcium hydroxide, there seems little justification to extend the treatment to a third or more visit. Additional appointments can pose a risk for coronal leakage and bacterial recontamination of the root canal system due to fracture, dislodgement or disintegration of the temporary restoration.

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