- Research article
- Open Access
- Open Peer Review
An economic evaluation of maxillary implant overdentures based on six vs. four implants
© Listl et al.; licensee BioMed Central Ltd. 2014
- Received: 27 June 2014
- Accepted: 13 August 2014
- Published: 18 August 2014
The purpose of the present study was to assess the value for money achieved by bar-retained implant overdentures based on six implants compared with four implants as treatment alternatives for the edentulous maxilla.
A Markov decision tree model was constructed and populated with parameter estimates for implant and denture failure as well as patient-centred health outcomes as available from recent literature. The decision scenario was modelled within a ten year time horizon and relied on cost reimbursement regulations of the German health care system. The cost-effectiveness threshold was identified above which the six-implant solution is preferable over the four-implant solution. Uncertainties regarding input parameters were incorporated via one-way and probabilistic sensitivity analysis based on Monte-Carlo simulation.
Within a base case scenario of average treatment complexity, the cost-effectiveness threshold was identified to be 17,564 € per year of denture satisfaction gained above of which the alternative with six implants is preferable over treatment including four implants. Sensitivity analysis yielded that, depending on the specification of model input parameters such as patients’ denture satisfaction, the respective cost-effectiveness threshold varies substantially.
The results of the present study suggest that bar-retained maxillary overdentures based on six implants provide better patient satisfaction than bar-retained overdentures based on four implants but are considerably more expensive. Final judgements about value for money require more comprehensive clinical evidence including patient-centred health outcomes.
- Cost-effectiveness analysis
Implant-retained overdentures have become an important treatment option of modern dentistry. Such treatment presents the prospect of high levels of oral health related quality of life and is particularly important in times of population aging as edentulousness rates continue to be relevantly high. For mandibular implant-based overdentures, current consensus is that patients’ satisfaction and quality of life is significantly greater for implant-supported overdentures than for conventional dentures and that a two-implant mandibular overdenture should be the minimum treatment standard for most patients. Not least, the availability of evidence already facilitated an assessment of the cost-effectiveness of implant-retained mandibular overdentures.
Yet comparably little evidence and consensus seem to exist with respect to implant-based overdentures for treatment of the edentulous maxilla. Maxillary overdentures have however been considered a relevant treatment alternative, particularly when retention and stability of conventional dentures is dissatisfactory. It was suggested that a number of four implants would be the minimum to support a maxillary overdenture and six implants would provide additional clinical advantages. In a relevant meta-analysis, most evidence on the clinical performance of maxillary overdentures was identified to originate from studies examining either six or four implants connected with a bar. Recently, moreover, some evidence became available about patient-reported outcomes of maxillary implant-supported overdentures[7–9]. It yet remains unclear whether the value gained by six instead of four implants within bar-retained implant overdentures outweighs the potentially higher costs.
To our knowledge, the cost-effectiveness of maxillary overdentures based on six or four implants has never been investigated before. Therefore, the purpose of the present study was to assess, on basis of currently available evidence, the value for money achieved by bar-retained implant overdentures with six implants compared with four implants as treatment alternatives for the edentulous maxilla.
Model input parameters
Implant and denture failure
A literature search via PubMed/Medline (April 9th 2014) found six meta-analyses for the search terms "maxillary overdenture", "upper jaw overdenture", "implant overdenture, maxilla", "maxillary over-denture", "upper jaw over-denture", and "implant over-denture, maxilla"[5, 6, 12–15]. From these six meta-analyses, five did not facilitate comparisons of various numbers of implants for maxillary overdentures and were thus excluded from the present analysis[5, 12–15]. The remaining meta-analysis by Slot et al. was the only one which provided comparisons of various numbers of implants and respective information on implant and denture failure. The information from this study was therefore included for the purpose of our investigation.
Annual failure rates and health outcome parameters used in the analysis (base case scenario)
Mode 0.018 (min-max 0.014-0.023)
Mode 0.0275 (min-max 0.022-0.0345)
Mode 0.037 (min-max 0.030-0.046)
Mode 0.0465 (min-max 0.038-0.0575)
Mode 0.056 (min-max 0.046-0.069)
Mode 0.0655 (min-max 0.054-0.0805)
Mode 0.026 (min-max 0.015-0.044)
Mode 0.0305 (min-max 0.0185-0.0565)
Mode 0.035(min-max 0.022-0.069)
Mode 0.0395 (min-max 0.0255-0.0815)
Mode 0.044 (min-max 0.029-0.094)
Mode 0.0485 (min-max 0.0325-0.1065)
Mode 0.05 (min-max 0.01-0.10)
Patient satisfaction [1: full satisfaction; 0: no satisfaction]
Repair period adjustment factor [1: full satisfaction; 0: no satisfaction]
Mode 0.9 (min-max 0.8-0.99)
As relevant health outcome parameter, we populated the Markov decision tree model (Figure 1) with information about patient satisfaction with overdentures as available from recent literature. For denture satisfaction in relation to six and four implants, we relied on overall patient satisfaction with implant-retained maxillary overdentures as reported by Slot et al.. Note that this study reports outcomes for implants located in the posterior maxilla. This seems justifiable given that a further study reported similar outcomes for the anterior maxilla and confirmed that patient satisfaction with dentures does not differ significantly between four and six implants. For patients’ satisfaction in relation to two or no implants, we relied on recently reported findings from Zembic & Wismeijer on general satisfaction with fitted conventional and two-implant-retained maxillary dentures. Note that the results from Slot et al. and from Zembic & Wismeijer were reported on different scales and on basis of different sample sizes[7, 8]. Therefore, the respective parameter values were included as point estimates and rescaled such that a value of 1 always indicates perfect satisfaction and zero indicates total dissatisfaction. In order to model denture satisfaction associated with five, three, and one implant(s), the respective values were linearly interpolated. Moreover, compromised patient satisfaction due to temporary denture non-functionality throughout repair periods was incorporated via a triangularly distributed satisfaction adjustment parameter. Patient satisfaction parameters incorporated in the present study are summarized in Table 1.
Cost parameters used in the analysis [in €]
Dentist labor costs
Six-implant over-denture (new)
Four-implant over-denture (new)
Denture repair after implant failure
Denture repair without implant loss
Material and lab cost
Six-implant over-denture (new)
Four-implant over-denture (new)
Denture repair after implant loss
Denture repair without implant loss
Measuring ‘value for money’
Alternative patient satisfaction scenarios used throughout sensitivity analysis [1: full satisfaction; 0: no satisfaction]
Alternative scenario A [constant satisfaction decline]
Alternative scenario B [proportionally increasing satisfaction decline]
Cost-effectiveness thresholds for preferability of six vs. four implants [in € per year of patient satisfaction]
Cost factor 1.0
Sensitivity analysis (one-way)
Alternative scenario A (see Table 3)
Alternative scenario B (see Table 3)
Annual failure rate of denture w/o implants
Failure rate = 0.00
Failure rate = 0.06
Failure rate = 0.12
Repair period satisfaction adjustment factor
Adjustment factor = 0.0
Adjustment factor = 0.2
Adjustment factor = 0.4
Adjustment factor = 0.6
Adjustment factor = 0.8
Adjustment factor = 1.0
This study is, to our knowledge, the first economic evaluation of maxillary implant overdentures so far. Given resource scarcity within and outside dental care, results from such a health economic evaluation may be highly relevant not only to patients but also to health insurers and other health care decision makers who need to decide how resources are best spent in order to increase population wellbeing. The question to address is whether relying on bar-retained maxillary overdentures based on six instead of four implants represents good value for money.
The present study found that, within a ten-year time horizon, bar-retained maxillary overdentures based on six implants provide better patient satisfaction than overdentures based on four implants but at considerably higher treatment expenses. For a base case scenario of average treatment complexity, the cost-effectiveness threshold was identified to be about 17,564 € per year of denture satisfaction gained above which the alternative with six implants is preferable over treatment including four implants. However, sensitivity analysis revealed that cost-effectiveness thresholds depend considerably on the distribution of patients’ denture satisfaction as relating to the number of implants and on the extent of satisfaction throughout denture repair periods. Plausibly, the preferability of the six-implant treatment alternative increased with more pronounced satisfaction margins in comparison with the four-implant solution. In addition, because the probability of denture and implant failure increases with decreasing number of implants, the preferability of the six-implant treatment alternative also increased with more amply constrained satisfaction throughout repair periods. In order to pinpoint the value for money of maxillary overdentures based on six instead of four implants more precisely, future research should thus specifically intend to provide more detailed insights into patient satisfaction.
Rating the value for money to society in the context of maxillary implant overdentures is further complicated by non-availability of relevant reference values, that is willingness-to-pay (WTP) per year of denture satisfaction gained. WTP can generally be defined as the maximum a person would be willing to pay for a good or a service, in this case for one year of satisfaction with an implant-retained maxillary overdenture. Several methods exist to measure consumer WTP. These methods can be distinguished according to whether they measure consumers’ hypothetical or actual WTP, and whether they measure consumer willingness to pay directly or indirectly. Methods of measuring WTP include the sealed bid auction, the Vickrey auction, conjoint analysis, and contingency valuation. WTP investigations of oral health care are relatively rare and, so far, have focused mainly on community water fluoridation, orthognathic treatment, anaesthetic gel, and treatment of dentine hypersensitivity. However, it seems reasonable to contemplate more generic WTP reference values which are already used by policy makers. Notably, a threshold range of £20,000 to £30,000 per quality adjusted life year (QALY) is assumed to be adopted by national health care decision makers in the United Kingdom, corresponding to a threshold range of about 24,000 € to 36,000 € (exchange rate as of April 10th 2014). Given that societal WTP per year of denture satisfaction may plausibly be expected to be considerably lower than societal WTP per QALY, it thus seems unlikely that a cost-effectiveness threshold of 17,564 € per year of denture satisfaction gained would imply good value for money. Nevertheless, depending on personal preferences and wealth, some patients may still favor the six-implant alternative to the four-implant solution in spite of substantially higher costs.
The present study was based on currently available evidence on clinical and patient-centered outcomes of implant- and bar-retained maxillary overdentures. We are aware that a more extensive literature search using other search engines such as EMBASE or the COCHRANE library may yield further evidence. Nevertheless, the included evidence seems highly representative of the relevant literature and can thus be considered sufficient for the purpose of the present study, i.e. to provide an economic perspective on the value for money gained through maxillary overdentures based on six as compared to four implants. The literature in this field appears to be relatively sparse and implies considerable uncertainty regarding the input parameters of our decision analytic model. In particular, the implant and denture survival rates underlying our model are based on clinical evidence with an average follow-up time of only one year and are limited to information about six and four implants only. In addition, few studies exist which provide information about patients’ satisfaction for the relevant clinical scenarios. Given that our results varied substantially with respect to simulated alterations of model input parameters, more comprehensive clinical evidence is needed in order to achieve higher accuracy within health economic evaluation. In the future, this may also better enable to model more complex clinical scenarios such as different patterns of implant loss or potential re-implantation after implant loss. In the absence of reliable evidence, we had to assume that implants are not replaced once lost but this may not fully capture existing treatment options. In view of considerable uncertainty already implied by currently available clinical evidence, we also refrained from applying discount rates to future costs and health outcomes because, generally, our results may better be understood as providing guidance for future research priorities rather than very accurate calculations of value for money to the last decimal point. Nevertheless, the present study established a suitable and widely applicable methodological framework for the economic evaluation of maxillary implant overdentures which can be applied in future calculations as well.
The results of the present study suggest that bar-retained maxillary overdentures based on six implants provide better patient satisfaction than bar-retained overdentures based on four implants but are considerably more expensive. Making final judgments about value for money however requires more comprehensive clinical evidence including patient-centered health outcomes. Future clinical research should specifically examine long-term implant and denture survival as well as patient-centered outcomes of different alternatives for implant-retained overdenture treatment of the edentulous maxilla.
We gratefully acknowledge financial support by Deutsche Forschungsgemeinschaft (DFG) and Ruprecht-Karls-Universität Heidelberg within the funding programme Open Access Publishing. We would like to thank Mrs. Gisela Grimm and Mr. Jochen Karl from the Heidelberg University Department of Prosthodontics dental lab for simulating tenders for material and laboratory cost.
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