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Time to Survival for the Restoration of the Shortened Lower Dental Arch
1 School of Dental Sciences and Correspondence: * corresponding author, j.m.thomason{at}ncl.ac.uk
Removable partial dentures may adversely affect remaining tissues and have a low prevalence of use. This randomized controlled trial was designed to compare the time to survival of cantilever resin-bonded fixed partial dentures and conventional removable partial dentures to restore shortened lower dental arches. We randomly allocated 25 male and 35 female patients (median age, 67 years) to fixed or removable partial denture groups of 30 persons, matched for age and sex. Survival of the prostheses was assessed, based on listed criteria, at each review or when problems arose. Although the removable partial denture group required rather more maintenance visits, the difference in survival rates was not statistically significant (hazard ratio = 0.59, with 95% CI 0.27, 1.29). In the absence of significant differences in five-year survival, the reported advantages of fixed partial dentures, including reduced maintenance frequency, offer positive support for the use of resin-bonded fixed partial dentures.
Key Words: denture • partial fixed • resin-bonded • removable • randomized clinical trial • survival
The severely shortened lower dental arch has traditionally been restored by means of a bilateral free-end saddle removable partial denture. These prostheses are used to restore appearance and masticatory function, and may also act to stabilize an opposing complete upper denture. Longitudinal studies have reported an increased incidence of caries and periodontal breakdown when removable partial dentures are worn (Carlsson et al., 1965; Lappalainen et al., 1987; Tuominen et al., 1988; Budtz-Jorgensen and Isidor, 1990; Jepson et al., 2001). In elderly persons, this may be a particular problem, as demonstrated by growing evidence for the association between removable partial dentures and root-surface caries (Wright et al., 1992; Drake and Beck, 1993; Locker, 1996; Steele et al., 2001). Lower bilateral free-end saddle removable partial dentures have a low prevalence of use, with between 30 and 50% of individuals reporting that they never or only occasionally wear their prosthesis (Tomlin and Osborne, 1961; Wetherell and Smales, 1980; Kapur et al., 1994; Jepson et al., 1995). While this may partly offset the biological cost associated with their provision, it clearly represents a considerable waste of time and resources. Given the trend for increasing numbers of older adults with natural teeth (Kelly, 2000), research into alternatives to removable partial dentures is timely. The use of fixed partial dentures, rather than conventional removable partial dentures, to restore part of the lower dentition offers the advantage of less bulk, a more normal contour as well as a lower caries prevalence (Budtz-Jorgensen and Isidor, 1990; Jepson et al., 2001), and improved comfort and acceptance (Jepson et al., 2003). Cantilever resin-bonded fixed partial dentures are simple, non-destructive, and cost-effective fixed restorations (Creugers and Kayser, 1992; Hussey and Linden, 1996). This randomized controlled trial compared the effectiveness of bilateral cantilever resin-bonded fixed partial dentures and conventional removable partial dentures in the restoration of shortened lower dental arches. Primary outcome measures were survival, the influence on dietary selection, and nutrient intake, as reported previously (Moynihan et al., 2000). The a priori hypothesis was that restoration with cantilever resin-bonded fixed partial dentures was as effective as that with the use of removable partial dentures. This paper reports the time to survival for the restoration of lower shortened arches with bilateral cantilever resin-bonded fixed partial dentures and conventional removable partial dentures at 5 yrs.
Protocol Adult participants were drawn from Dental Hospital patients awaiting the provision of a lower bilateral free-end saddle removable partial dentures. Inclusion and exclusion criteria are listed in Table 1  . Each participant received preparatory treatment, consisting of: a course of oral hygiene instruction, including the use of interdental cleaners, and scaling; periodontal therapy as necessary; the restoration of caries-affected teeth; and replacement of defective restorations. Participants were required to achieve plaque and gingival indices of > 20%, and to demonstrate the effective use of floss or interdental brushes to enter the trial, which had received prior approval from the relevant Ethics Committee. Written consent was obtained from all participants.
A sample size of 60 persons, 30 in each intervention group, was projected as having an 80% chance of detecting a 30% difference in five-year survival at the 5% level. All new prostheses were constructed to established clinical protocols, and were fitted over the period June, 1995, to July, 1997. Cantilever resin-bonded fixed partial dentures restored 1 occlusal unit, up to but not beyond the second premolar, by means of single pontics cantilevered from single abutments whenever possible. Bilateral lower free-end saddle removable partial dentures were constructed on cast metal frameworks that incorporated rests, retainers, and a rigid connector. The design of the connector was clinically determined and reflected factors such as the presence of anterior modification spaces, depth of the anterior lingual sulcus, and the position of lingual frenal attachments. Altered cast procedures were used as required. The number of posterior occlusal units on the removable partial denture reflected the number of opposing posterior occlusal units.
Assignment
Patients were reviewed 3 mos and 1 yr after insertion of the new lower prosthesis, and thereafter at yearly intervals. Additional review and maintenance appointments were scheduled as clinically required. The survival of prostheses was assessed at each review, or when problems arose, according to the criteria listed in Table 2
Comparative analyses for time to survival between and within groups were performed with a Cox proportional hazards regression model. Results are presented in the form of a 95% confidence interval for the hazard ratio (for example, the risk of a fixed partial denture failing divided by the risk of a removable partial denture failing). Other survival analyses were made by the Kaplan-Meier procedure.
Participant Flow and Follow-up The profile of this randomized clinical trial (Fig. ) summarizes patient numbers, randomization, and withdrawals. There were 25 male and 35 female participants (median age, 67 yrs; minimum, 39, and maximum, 81 yrs). The median number of remaining lower teeth was 7 (minimum 3 and maximum 8, SD 1.21). For 51 participants, lower prostheses opposed a complete upper denture, for seven, an upper removable partial denture, and for two, upper natural teeth only. There were no significant differences between groups at baseline for any parameter describing dental (by chi-square analysis) or periodontal status (t test). Fifty-four posterior resin-bonded fixed partial dentures were placed in the fixed-partial-denture group, in which four participants received only a unilateral fixed partial denture. Equal numbers of lower canine and premolar teeth were used as abutments. Twenty-six of the 30 lower removable partial dentures used plate connectors.
Analysis Failure was initially described by participant and not by prosthesis. That is, the failure of a denture or one of the usually bilateral fixed partial dentures was considered a failure of treatment for that participant. Eleven fixed partial dentures and 15 removable partial dentures failed (Table 2  ). The most common cause of failure within the fixed-partial-denture group was repeated de-bond, with the greater proportion of terminal events occurring within the first 2 yrs. For the removable-partial-denture group, non-use of the removable partial denture and the loss of teeth that made the removable partial denture unusable were almost equal causes of failure. Characteristically, non-use of the removable partial denture occurred within the first year, and the loss of teeth during year 5 of denture use. The analysis of time to survival describes the relative performance of removable-partial-denture and fixed-partial-denture groups at the five-year time-point, and includes data from persons who withdrew from the trial. Censored data—that is, data for those for whom a terminal event had not occurred, or who had withdrawn from the study—were entered for 19 and 15 participants in the fixed-partial-denture and removable-partial-denture groups, respectively. Fixed partial dentures had a slightly lower hazard rate (and were thus more likely to survive) than removable partial dentures, but the difference was not statistically significant (hazard ratio = 0.59 with 95% CI 0.27, 1.29). In nine participants with bilateral fixed partial dentures, failure of only one fixed partial denture occurred, and the other fixed partial denture continued to function satisfactorily. Within-group survival analyses indicated cumulative prosthesis survival rates of approximately 25% and 70% at 5 yrs for the removable-partial-denture and fixed-partial-denture groups, respectively. The use of either a canine or premolar tooth as the abutment had no significant effect on the time to failure of fixed partial dentures (hazard ratio = 0.86 with 95% CI 0.40, 3.01).
We made 175 follow-up appointments for each group (Table 3
The advantages of randomized controlled trials are well-recognized, although there have been very few that have examined the effectiveness of either resin-bonded fixed partial dentures or removable partial dentures (Kapur et al., 1994; Creugers et al., 1998). The two alternative treatments outlined in this study have not been previously reported in terms of comparative prosthesis survival. There was a relatively high loss to follow-up of 15 (25%) participants at 5 yrs. Given the age of the study participants, the number of withdrawals overall due to illness or death is perhaps not surprising. There was a larger number of deaths in the removable-partial-denture group, and it appeared that this, rather than disappointment with their treatment allocation (Feine et al., 1998), was the main explanation for the higher number of withdrawals in this group. Blinding of an independent examiner to the treatment interventions is clearly not possible in this form of study, but it should be noted that bias in the reporting of failure was technically possible, given a study design in which the operators providing treatment also assessed its outcome, despite steps taken to avoid this form of error. The cumulative survival rate for the cantilever fixed partial dentures themselves was similar to that reported for fixed-fixed resin-bonded fixed partial dentures in other randomized controlled trials (Creugers et al., 1992; De Kanter et al., 1998), although the criteria for failure used in the 1992 study were perhaps less demanding. Restoration with cantilever fixed partial dentures, with either lower canine or premolar teeth as abutments, was equally successful. Approximately 25% of participants in the removable-partial-denture group chose not to wear their denture, and similar rates of non-use have been reported previously (Budtz-Jorgensen and Isidor, 1990; Jepson et al., 1995). In effect, these participants preferred continued function with a severely shortened lower arch, despite apparent limitations, rather than the use of a removable partial denture. The failure of the study to detect a statistically significant difference in time to failure for the two treatments may indicate that there was no real difference, or that there was an actual lack of power of the study to detect the difference. The sample size calculations used to establish the study were based on the published evidence on survival for the separate treatment alternatives, and the very limited comparative survival evidence available (Budtz-Jorgensen and Isidor, 1990). On this basis, a clinically significant difference in survival was set at 30%. Post hoc analysis indicated that the study sample size used for the current study had a 53% power to detect this difference, and it is likely that patient attrition did result in a loss of power. However, with these new data, further analysis suggests that a sample size of 107 participants in each group would be required to detect a 20% difference in survival at the 5% level. This suggests the need for multi-center studies if further effectiveness studies are established to evaluate the use of cantilever resin-boned fixed partial dentures for such persons. Although an improved survival for the fixed-partial-denture treatment group was not established, it was very clear that fixed partial dentures did not fail any more often than the prostheses in the removable-partial-denture group. Subjectively, there was little difference in the time taken for required interventions in either group. In addition, although classified as a ‘treatment failure’, nine persons with the loss of one of their two fixed partial dentures continued to function with their other fixed partial denture quite satisfactorily. The need for maintenance of fixed partial dentures was less than for removable partial dentures, with over half (16) of the fixed-partial-denture group requiring no active maintenance over the five-year period. This reflects, in part, the reduced incidence of caries and the greater comfort associated with the use of fixed partial dentures that have been shown for this series of studies (Jepson et al., 2001, 2003). In terms of an overall health gain, participants in the fixed-partial-denture group were at an advantage compared with those with removable partial dentures, for whom there was a significant risk of the further loss of teeth through caries, greater discomfort, and the increased need for maintenance. A survey of elderly adults in the UK found that approximately 25% of all dentate participants were edentulous in one arch, most often the upper (Steele et al., 1996). It is a common pattern of tooth loss in which, for those in this study, cantilevered resin-bonded fixed partial dentures offered a no less effective alternative to conventional lower removable partial dentures at a lower biological price. These findings add strength to evidence that fixed rather than removable prostheses should be used in elderly persons, for whom the need for a removable denture should be carefully considered (Steele et al., 2001; Nevalainen et al., 2004). In a randomized clinical trial comparing the efficacy of cantilever resin-bonded fixed partial dentures and removable partial dentures in restoring shortened lower dental arches of elderly persons, there was no significant difference in time to survival between the two treatments. The absence of a significant difference in five-year survival, the reported advantages of fixed partial dentures, and the greater maintenance need for those in the removable-partial-denture treatment group offer positive support for the use of resin-bonded fixed partial dentures.
This investigation was funded by the NHS Executive, Northern & Yorkshire, Research and Development Directorate, Durham, UK. The authors acknowledge the assistance of support staff within the Department of Prosthodontics, Newcastle Dental Hospital and, in particular, the invaluable contributions of Josie Davison and Jan Thompson to the conduct of this trial. Received for publication December 1, 2005. Revision received February 6, 2007. Accepted for publication February 28, 2007.
Journal of Dental Research, Vol. 86, No. 7,
646-650 (2007)
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