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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 Correspondence: 3 corresponding author, Hogeweg 5, NL-3212 LG Simonshaven, The Netherlands, geertstoker{at}wxs.nl
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
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.
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
Cost-analysis 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
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
Details of the aftercare provided are stated in Table 2  . Statistical analysis revealed no significant differences among the 3 groups for the recorded types of aftercare. More than 50% of the patients needed no extra treatment after receiving their overdenture, other than regular check-ups. A minority of the patients demanded extra treatment.
The mean total costs of aftercare with the 3 different types of overdentures, starting at 3 mos after insertion of the overdenture, showed no significant differences among the 3 groups (Table 3  ). The initial costs (costs up to 3 mos after insertion of the overdenture) were calculated by Wismeijer (1996) and re-calculated in 2000 prices according to the Harmonized Indices of Consumer Prices (Statistics Netherlands) over the yrs 1993 to 2000 (2IBA,  2413.03; 2ISB, 2602.27; and 4ITB, 3564.08). The mean total costs of treatment for the 3 groups after 8 yrs were: (2IBA) 3410.46; (2ISB) 3563.48; and (4ITB) 4548.40. After 8 yrs of aftercare, the initial costs were still the major part of the costs: 71–78% of the total costs.
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. 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, 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.
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 for publication February 17, 2005. Revision received September 4, 2006. Accepted for publication November 2, 2006.
Journal of Dental Research, Vol. 86, No. 3,
276-280 (2007)
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