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An Eight-year Follow-up to a Randomized Clinical Trial of Aftercare and Cost-analysis with Three Types of Mandibular Implant-retained Overdentures

¹ Free University, Department of Oral Function, Academic Centre for Dentistry Amsterdam, Dental School, Amsterdam, The Netherlands; and
² Amphia Teaching Hospital, Department of Special Dental Care and Maxillofacial Prosthodontics, Breda, The Netherlands

³ corresponding author, Hogeweg 5, NL-3212 LG Simonshaven, The Netherlands, geertstoker{at}wxs.nl

	ABSTRACT

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Mandibular implant overdentures increase satisfaction and the quality of life of edentulous individuals. Long-term aftercare and costs may depend on the type of overdentures. One hundred and ten individuals received one of 3 types of implant-retained overdentures, randomly assigned, and were evaluated with respect to aftercare and costs. The follow-up time was 8 years, with only seven drop-outs. No significant differences (Kruskal-Wallis test) were observed for direct costs of aftercare (p = 0.94). The initial costs constituted 75% of the total costs and were significantly higher in the group with a bar on 4 implants, compared with the group with a bar on 2 implants and the group with ball attachments on 2 implants (p = 0.018). The last group needed a significantly higher number of prosthodontist-patient aftercare contacts, mostly for re-adjustment of the retentive system. It can be concluded that an overdenture with a bar on 2 implants might be the most efficient in the long term.

KEY WORDS: randomized controlled clinical trial • edentulism • cost-analysis • aftercare • implant-retained overdentures

	INTRODUCTION

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The success of implant-retained mandibular overdentures in terms of stability, function, speech, and patient satisfaction has been shown in many studies (Feine et al., 2002; Attard and Zarb, 2004; Naert et al., 2004; Timmerman et al., 2004), and some of these have demonstrated the benefits of a mandibular overdenture supported on implants over those of a conventional complete denture (e.g., Doundoulakis et al., 2003; Meijer et al., 2003). In a consensus conference at McGill University (Montreal, Canada, May, 2002), mandibular implant overdenture treatment was suggested as the minimum standard of care for the edentulous individual (Feine et al., 2002, Feine and Carlsson, 2003). For general implementation, however, the costs of treatment and aftercare are important and provide vital information to patients, health authorities, and third-party payers.

More than a decade ago, the first studies on the costs and cost-effectiveness of dental implants were published (Jacobson et al., 1990; Jonsson and Karlsson, 1990). During the past 3 yrs, more studies have been published evaluating the costs not only of the initial stage of the treatment, but also of care up to 1 yr after treatment (Walton, 2003; Takanashi et al., 2004). In that first year, only 3–4% of the total cost was related to the cost of aftercare. Studies with the emphasis on cost-effectiveness of mandibular conventional dentures vs. implant-retained overdentures were reported by Van der Wijk et al.(1998) and Heydecke et al.(2005). The latter study had an original approach, with the use of a panel of experts for the estimation of cost of aftercare. A limitation of the above-mentioned studies was the lack of long-term observations on cost of aftercare or the use of fees charged by clinicians instead of cost to society. The use of projections and assumptions instead of actual data weakens prognoses.

Treatment of edentulous individuals with implant-retained overdentures is becoming more and more common in the Netherlands. The discussion about treatment strategies (2 vs. 4 implants and ball vs. bar attachments) to reduce cost with preservation of function is current, and a favorite topic from an economic point of view.

This study on cost comparison of aftercare of mandibular implant-retained overdentures developed as part of a randomized controlled clinical trial (RCT), called the "Breda-Implant-Overdenture-Study" (BIOS), involves 110 completely edentulous individuals with denture problems, who received one of 3 different types of overdentures, assigned randomly. Earlier outcomes of this trial showed significant differences with respect to initial patient satisfaction and costs, as well as long-term patient satisfaction, and have been published previously (Wismeijer, 1996; Wismeijer et al., 1997, 1999; Timmerman et al., 2004). Thus, it can be expected that there are also differences in the eight-year follow-up costs. The aim of this study was to compare the direct costs of aftercare of the 3 groups through 8 yrs after delivery of the prostheses. Therefore, the statistical null hypothesis was that there are no differences in cost among the 3 groups.

	MATERIALS & METHODS

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Study Design
One hundred and ten edentulous individuals with atrophic mandibles and persistent problems with their conventional complete dentures were referred by their dentists to the Department of Oral and Maxillofacial Surgery and/or the Department of Special Dental Care and Maxillofacial Prosthodontics of the Amphia Teaching Hospital in Breda, in the period 1991–1993. They were treated with one-stage ITI dental implants (Straumann, Switzerland) and overdentures. Persons who met all inclusion and exclusion criteria were asked to participate in this RCT. They were informed about the 3 different treatment strategies, as well as the possible benefits and risks of the treatment. The study was approved by the medical ethical board of the Hospital.

The study design has been described extensively in earlier publications (Wismeijer, 1996; Timmerman et al., 2004). The determination of the sample size was based on a long-term follow-up with expected drop-outs over time. Patients were randomly assigned to one of the 3 treatment groups. One group received an implant-retained overdenture on 2 implants with ball attachments (2IBA) and Dalla Bona matrices (Cendres et Métaux, Switzerland). The second group received an implant-retained overdenture on 2 implants with a single egg-shaped Dolder bar (2ISB) (CMST53012P20, Cendres et Métaux). The final group received an implant-retained overdenture on 4 implants with a triple bar (4ITB). Two or 4 titanium implants were installed in the symphysial area of the mandible. The chair-time of all the visits for each patient was recorded in minutes. Besides chair-time, the number of scheduled as well as unscheduled visits and the types of provided aftercare were recorded. With respect to the scheduled visits, the patients received a "check-up" appointment once every 2 yrs for the oral and maxillofacial surgeon, once a yr for the prosthodontist, and at least twice a yr for the oral hygienist.

Aftercare
Aftercare in total was defined as all care and maintenance provided during the evaluation period, including check-ups. These consisted of surgical, prosthetic, and/or oral hygiene measures taken to keep the peri-implant tissues healthy and to ensure optimal denture function. The individual chair-time and costs of providing the aftercare by the oral and maxillofacial surgeon, the prosthodontist, and the oral hygienist were recorded, as were the costs of the dental technician. Care given up to 3 mos after the insertion of the overdenture was recorded as part of the initial treatment, and not as aftercare.

Cost-analysis
When costs are analyzed, direct and indirect costs can be differentiated. In this study, the calculation of the total direct costs of aftercare was based on chair-time in minutes, multiplied by the hourly rate of the Center of Special Dental Care, and the costs of the dental technician. It included check-ups and all kinds of surgical and prosthetic treatment in the aftercare period. In the Dutch hospital system, all cost-components of surgical and prosthetic treatment, and of oral hygiene measures—i.e., hourly rate for all providers, materials and equipment used, disposables, and costs of auxiliary assistants—together determine the hourly rate of the Center of Special Dental Care, where all patients were treated. Each year, this hourly rate was re-calculated on the basis of all resources used and approximate resource-based micro-costing techniques. Multiplication of treatment time by the hourly rate of the Center gives a fair approximation of the true cost of treatment. All cost-components, including costs of the dental technician, were based on individual patient data. All amounts in this study were re-calculated in Euros () in 2000 values.

Indirect costs include loss of the patients’ productive working hours or spare time, the use of medication, travel time, and environmental costs. It is difficult and time-consuming to calculate all these indirect costs. Takanashi et al.(2004) attempted to calculate them. Despite all calculations, many factors concerning indirect costs were still estimated in that study. Since the patients in this study were randomized over the 3 groups, and aftercare circumstances did not differ in the 3 treatment types, we hypothesized that the indirect costs might be equal for the 3 groups and did not influence the absolute differences among them. For that reason, indirect costs have not been taken into account.

Statistical Analysis
The individual data from all patients—such as time, number of visits, and provided aftercare—were analyzed by SPSS (SPSS Inc., Chicago, IL, USA), and levels of statistical significance were set at p < 0.05. Although cost was a continuous variable, the costs of aftercare per patient were not expected to be normally distributed. Thus, a non-parametric approach to the analysis was chosen, and the Kruskall-Wallis test for k-related samples was performed. Drop-outs unrelated to the treatment were considered as missing completely at random.

	RESULTS

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At the eight-year follow-up, 103 (94%) of the 110 patients were still visiting the hospital for aftercare and participated in the evaluation. Three patients had died; four had dropped out of the study for reasons of hospitalization and relocation. There was no correlation between these dropouts and the intervention of the treatment. At the start of the study, the ages of the patients ranged from 39 to 87 yrs (mean = 59.0); 30 patients were male, and 73 female.

Testing all data for normality with the Kolmogorov-Smirnov test proved our assumption that the costs of aftercare were not normally distributed (p < 10^–30), so non-parametric tests were used.

The mean frequencies for check-ups and treatment times of aftercare are given in Table 1. Mutual differences of the 3 groups were tested with the Kruskall-Wallis test for k-related samples. The 3 groups showed no mutually significant differences in the total number of check-ups (prosthodontist, annual with and without simple treatment; oral hygienist; and oral surgeon). When we broke the prosthodontic checkups into ’without simple treatment’ and ’with simple treatment’ categories, we found significant differences among the 3 groups (check-ups without simple treatment, p < 0.0001; check-ups with simple treatment, p = 0.018). This means that there was an increased demand for aftercare in group 2IBA for simple readjustments, such as re-activating the matrices. No significant differences among the groups were found in the mean total treatment time (Table 1) and costs (Table 3) attributable to the prosthodontist (p = 0.96), the oral hygienist (p = 0.89), and the oral surgeon (p = 0.40). There were also no significant differences among the 3 groups for the costs for the dental technician (p = 0.27).

View this table: [in this window] [in a new window]   Table 3. Mean Total Costs of Aftercare in Euros at 2000 Values ( = 1.08$)

	DISCUSSION

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It is unique that, in a RCT, such a large group of patients can been evaluated for over 8 yrs. That 94% of the patients still participated in this follow-up could be achieved only by a well-designed protocol and the efforts of the professionals in the team. Well-maintained oral hygiene by the patients, and prosthetic and professional oral hygiene aftercare are important factors for the long-term success of the implant overdenture treatment. The small number of drop-outs was due to factors unrelated to the intervention of the treatment. It may be assumed that those missing data can be considered as missing completely at random, and thus "ignorable" for the purposes of the study (Little and Rubin, 1987). Thus, we are confident that the already small number of drop-outs did not confound the results.

The calculated costs in this study are the real-time direct costs associated with initial treatment and aftercare. These costs varied per patient, hospital, treatment provider, type and brand of implant system, and country. A few studies (Van der Wijk et al., 1998; Takanashi et al., 2004) presented the real direct costs by calculating all components. In the latter study, conventional dentures were compared with implant-retained overdentures on 2 implants with ball attachments. This treatment was identical to our group 2IBA. Only the initial cost can be compared: CAN$ 2258 vs. 2413. They found, in one-year follow-up, that the cost of aftercare was 3–4% of the total cost. Following the definitions of our study, with the aftercare period starting 3 mos after delivery of the overdenture, and assuming that the costs of aftercare were stable over the years, then this percentage approximates the 22–29% after 8 yrs found in our study. Other studies (Walton et al., 1996; MacEntee and Walton, 1998) estimated the direct costs on fees charged by clinicians. Data based on these charges are less appropriate in an economic analysis, because they often deviate from true costs (Finkler, 1982). Watson et al.(2002b) estimated professional time by multiplying the numbers of aftercare events by the respective time allocations. Comparison of direct costs of treatment in this study with those of other studies was almost impossible. Recorded treatment time would probably have been a helpful variable.

The patients with the ball attachments needed to visit the prosthodontist more often between scheduled check-ups to have the retentive system re-activated. In one extreme case, a patient visited the prosthodontist more than 30 times. Chaffee et al.(2002) reported 194 non-scheduled visits of 327 returns for their group with ball attachments. More studies reported that ball attachments needed more aftercare, regardless of the implant system used (Naert et al., 1997; Davis and Packer, 2000; Walton, 2003). Abutment design and the choice of material used for the retentive part of the matrix influence the friction grip and thus the need for aftercare and the lifetime of the implant (Watson et al., 2002a). Changes in abutment design by the manufacturer over time can lead to other conclusions.

When one examines the total cost of the treatment over more than 8 yrs, the initial costs account for the majority of the total costs and the differences among the 3 groups. Installing 4 implants and manufacturing an overdenture with the triple bar in group 4ITB required more time than installing 2 implants. The costs of aftercare, however, seemed to be independent of the 3 types of overdentures for (almost) the life span of the overdenture. The choice of 4 implants interconnected with bars resulted in 28% more costs than with the use of 2 implants with a single bar (4ITB, 4548 vs. 2ISB, 3563). The difference between a single bar and 2 ball attachments was only 4.5% (2ISB, 3563 vs. 2IBA, 3410).

In many cost-effectiveness studies, the costs of aftercare were extrapolated with data over a period of 1 yr or less, or questionnaires were completed by a panel of experts (Heydecke et al., 2005). This makes the usual sensitivity analysis less reliable. Perhaps the long-term data and the results of this study can help to improve the input for future cost-effectiveness studies.

It is difficult to decide which type of overdenture was the most favorable. It depended not only upon costs, but also upon patient satisfaction, function, and clinical results. A recently published portion of this study, on patient satisfaction, showed that, after 8 yrs, the levels of satisfaction and social functioning were still high, and that patient satisfaction concerning retention and stability of the mandibular implant-retained overdenture had decreased significantly in the 2 implants-ball attachment group (Timmerman et al., 2004). The clinical results will be published in the near future.

Taking the results on patient satisfaction into account, together with the aspects of cost, it can be concluded that an overdenture on 2 implants interconnected by a single bar might be the first treatment of choice, with high cost-effectiveness and efficacy and proven stability for a long-term period.

	ACKNOWLEDGMENTS

 
This project was supported by a grant from the ITI Foundation for the Promotion of Oral Implantology, Switzerland. This grant did not, in any way, create a conflict of interest in the conduct of this study.

Received February 17, 2005; Last revision September 4, 2006; Accepted November 2, 2006

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