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RESEARCH REPORT |
1 Clinic for Reconstructive Dentistry and TMJ Disorders, University of Basel, Hebelstr. 3, CH-4056 Basel, Switzerland; and
2 Institute for Clinical Epidemiology, Basel University Hospital, Hebelstr. 10, 3rd Floor, CH-4031 Basel
* corresponding author, N.Zitzmann{at}unibas.ch
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: cost-effectiveness • economic evaluation • dental prosthesis • implant-supported denture • complete denture patient satisfaction
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Little is known about the long-term cost-effectiveness of complete dentures and implant prostheses in edentulous patients (Heydecke et al., 2005). In most studies, the estimation of costs and/or effects has been restricted to the follow-up period of the clinical trial (Jönsson and Karlsson, 1990; Tang et al., 1997; Palmqvist et al., 2004; Takanashi et al., 2004). However, most of the costs for prosthetic restorations usually accrue during the first year, whereas improved dental outcomes usually last during the functional period of the denture, which may be much longer (Attard and Zarb, 2004). It is therefore decisive to choose a time-horizon that reflects the life-expectancy of the prosthetic restoration, to avoid a bias against the cost-effectiveness of the intervention. Therefore, we conducted a cost-effectiveness analysis comparing the 3 treatment strategies—implant-supported overdentures, implant-retained overdentures, and complete dentures—in edentulous patients by combining clinical trial data with a follow-up of 3 yrs, with mathematical modeling to project long-term costs and effects over a time period of up to ten years.
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MATERIALS & METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Effects
At baseline, patients were asked to complete a questionnaire assessing several functional and psychological parameters related to their present complete denture in the mandible and maxilla (Zitzmann et al., 2005). The questionnaire was administered again after 6 mos and 3 yrs. Perceived chewing ability, measured on a Visual Analogue Scale with bounds between 0 (worst possible state) and 1 (best possible state), was used as the main parameter in this economic evaluation, reflecting the patient’s dental health state preference. We used the concept of Quality-adjusted Prosthesis Years to estimate the effectiveness of the intervention (Jacobson et al., 1992; Sendi et al., 1997). Quality-adjusted Prosthesis Years are calculated by adjustment of the duration of a dental health state by the patient’s preference for that state.
The importance of adjustment for baseline utility in economic evaluations has been emphasized elsewhere; hence the difference in the patient’s dental health state preference before and after posthetic treatment was used to calculate Quality-adjusted Prosthesis Years (Manca et al., 2005). Patients’ preferences recorded at 6 and 36 mos were used to estimate Quality-adjusted Prosthesis Years in years 1 and 3, respectively. For year 2, each patient’s dental health state preference was determined by linear interpolation, and this value was then used for the estimation of the Quality-adjusted Prosthesis Years during the second year. Since the time horizon of the clinical trial was 3 yrs, but dental restorations may have a much longer life-expectancy, we projected Quality-adjusted Prosthesis Years experienced after year three up to 10 yrs by assuming that the patient’s dental health state preference observed at month 36 would be stable over the projected time horizon.
Costs
Initial dental health care costs were calculated on an individual basis according to the official national dental tariff structure and were comprised of implant material, surgical and prosthodontic treatment, as well as laboratory fees (Schweizerische Zahnärzte-Gesellschaft, 1994). In addition, treatment time and health care resource consumption were recorded for maintenance dental care in scheduled or unscheduled visits during the following 3 yrs. The patient’s time spent for traveling was assumed to be 30 min twice per visit. We estimated time costs (treatment time and traveling time measured in 30-minute units) of patients by assuming an opportunity cost of 25 Swiss Francs (CHF) per hour (wage rate of an unskilled worker). All costs were expressed in year 2000 Swiss Francs (CHF 100 = US$61 on July 1st, 2000, www.oanda.com).
Cost-effectiveness
The incremental cost-effectiveness ratio represents the difference in costs (i.e., incremental costs) divided by the difference in effects (i.e., incremental effects) between two strategies, and was calculated both undiscounted and discounted (with an annual rate of 3% to address time preference) over a time horizon of 3, 5, and 10 yrs from the patient’s perspective. For these projections, we assumed that the maintenance and time costs observed in the third year would be stable over the modeled time horizon.
Statistical Analysis
Differences in costs and effects among the three groups were analyzed by the Kruskal-Wallis Rank Sum Test. A p-value < 0.05 was considered as statistically significant. We used the software package S-Plus 6.2 Professional (Insightful Corp., Seattle, WA, USA) for all statistical analyses.
To assess the uncertainty around the point estimate of the incremental cost-effectiveness ratio, we computed the bivariate distributions of mean total costs and Quality-adjusted Prosthesis Years for each of the 3 treatment strategies, using 5000 bootstrap samples, and summarized them in terms of three-way cost-effectiveness acceptability curves (Briggs et al., 2002). The cost-effectiveness ratio above which an intervention is deemed cost-ineffective corresponds to the decision-maker’s maximum willingness to pay per Quality-adjusted Prosthesis Year gained (Glick et al., 2001).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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). Follow-up was complete for all patients but one, who selected implant-retained overdentures and who died 34 mos after the prosthetic treatment.
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), but did not reach statistical significance (p = 0.112). Patients in the implant-supported overdentures group had the highest average dental health state preference after treatment, but this was not statistically significant (p = 0.472 after 6 mos and p = 0.086 after 3 yrs). The average gain in Quality-adjusted Prosthesis Years adjusted for pre-treatment dental health status was 0.68 for complete dentures, 1.46 for implant-retained overdentures, and 1.57 for implant-supported overdenture patients (p = 0.0478, boxplots in Fig. 1A). The between-group comparisons yielded significant differences between complete dentures and implant-supported overdentures (p = 0.0193), but no significant differences between complete dentures and implant-retained overdentures (p = 0.0678), or between implant-retained and implant-supported overdentures (p = 0.725).
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and boxplots in Fig. 1B). Initial prosthetic treatment and time costs represented the largest part of the costs in each group, whereas maintenance costs and time costs for follow-up controls and treatments were much lower (Table 1).
The average incremental cost-effectiveness ratios for the 3 treatment options are shown in Table 2. In the base-case analysis with a follow-up period of 3 yrs and a discount rate of 3%, implant-retained overdentures patients had to pay an additional CHF 9100 per Quality-adjusted Prosthesis Year gained, as compared with complete denture patients, and the average incremental cost-effectiveness ratio for implant-supported overdentures vs. implant-retained overdentures was CHF 81,482 per Quality-adjusted Prosthesis Year gained. Discounting had a moderate impact on the cost-effectiveness ratios (Table 2). With a longer time horizon of 5 and 10 yrs, implant treatment became more attractive, with lower cost-effectiveness ratios (Table 2). Adjustment of the cost-effectiveness ratio for potential confounders, including age, sex, and education, did not alter the results of the analyses in a net benefit regression model (Hoch et al., 2002); these co-variates were therefore omitted from the analysis to avoid a loss of power.
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). The threshold ratio at which implant-retained overdentures became preferable (i.e., more likely to be cost-effective) to complete dentures is the same as in Table 2, where mean values were used. However, the stochastic three-way comparison shows that implant-supported overdenture treatment becomes the strategy with highest likelihood of being cost-effective at a threshold ratio of CHF 19,800 (base-case) and CHF 7100 (10-year horizon) per Quality-adjusted Prosthesis Year gained (Figs. 2A, 2B).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Implant treatment leads to greater improvements in dental health outcomes, but also requires substantially higher initial costs. These were almost 3 and 6 times higher for 2 and 4 implants, compared with conventional treatment with complete dentures, as was shown in a previous study (Zitzmann et al., 2005). The proportion of maintenance and time costs in relation to the initial costs also decreased with the number of implants used and represented 46% of the initial costs in complete denture patients, 28% in implant-retained overdentures patients, and only 13% in implant-supported overdentures patients.
In contrast to these findings, Takanashi et al.(2004) observed that the total time spend for initial treatment and six-month follow-up was similar for complete dentures and implant-retained overdentures, with the additional time required for implant placement being balanced by extra visits needed for marginal adaptation after the insertion of complete dentures. In a cost comparison including direct and indirect costs for complete dentures and implant-retained overdentures over 1 yr, direct costs were almost 3 times higher for implant-retained overdentures as compared with complete dentures (Takanashi et al., 2004). In a recent Canadian cost-effectiveness analysis by the same research group (Heydecke et al., 2005), implant-retained overdentures vs. complete dentures were reported to cost $14 per Oral Health Impact Profile (OHIP-20) point. In this one-year follow-up study, the annual maintenance costs in the subsequent years were based on expert opinion ($273 for complete dentures and $395 for implant-retained overdentures), which are somewhat higher than those observed in our study in Switzerland. Assuming a life-expectancy of 18 yrs, the estimated discounted total costs and effects were $8852 and 443 OHIP-20 points for implant-retained overdentures, and $5646 and 666 OHIP-20 points for complete dentures (Heydecke et al., 2005). Whereas the costs for implant-retained overdentures are in line with those reported in our study, those for complete dentures are somewhat higher. The effects cannot be directly compared with those from our study, since we used Quality-adjusted Prosthesis Years as an outcome measure as opposed to OHIP-20 points. The advantage of a Quality-adjusted Prosthesis Year is that it provides a preference-based outcome measure commonly used in health economics, with an intuitive interpretation (i.e., one Quality-adjusted Prosthesis Year corresponds to 1 yr in the best possible state). In addition, some further methodological differences merit attention. The follow-up period in our study was 3 yrs, which allowed us to estimate actual maintenance costs and effects. Furthermore, we compared 3 treatment alternatives (i.e., complete dentures, and implant-retained and implant-supported overdenture prostheses), and hence provided an economic analysis including the most expensive option (i.e., implant-supported overdentures). Finally, we used a stochastic three-way analysis to account for uncertainty (Briggs et al., 2002). As shown in this paper, the deterministic and stochastic analyses may differ substantially with respect to the thresholds, where one treatment becomes preferable to the other. However, given that Heydecke et al.(2005) reported similar results, the modeling process validity of both research groups is supported (Sendi et al., 1999).
Our study has several limitations. First, the number of patients included in each group may have been too small for us to detect differences in effects between complete dentures and implant-retained overdentures, due to a lack of statistical power. However, a sample size calculation would have entailed a randomized controlled trial with the cost of treatment being allocated randomly to patients, which was not possible due to the substantial differences in costs per treatment alternative. Second, we projected maintenance costs and effects observed in year three over a time horizon of up to 10 yrs. Although costs and effects may vary over the modeled time period, analysis of our data suggests that these variations may not substantially affect total costs and effects. The chosen time horizon is also in line with recent evidence demonstrating a longer life-expectancy of 12 yrs for implant overdentures (Attard and Zarb, 2004). Third, we used Quality-adjusted Prosthesis Years to measure dental health effects. Quality-adjusted Prosthesis Years assume no preference between the sequence of experienced dental health states, which may be addressed by the use of Healthy Year Equivalents (HYE) (Birch et al., 1998). However, since dental health may represent only a part of the patient’s health-related quality of life, the responsiveness of HYE in dentistry may need further investigation.
In conclusion, over an assumed time horizon of 10 yrs, implant treatment becomes cost-effective with implant-retained overdentures being the treatment of choice, if the patient is willing to pay at least CHF 3800 for a Quality-adjusted Prosthesis Year gained.
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ACKNOWLEDGMENTS |
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Received September 6, 2005; Last revision April 10, 2006; Accepted April 28, 2006
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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