University of Groningen

Implant-supported removable partial dentures in the mandibleLouwerse, Charlotte

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CHAPTER 1

GENERAL INTRODUCTION

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Introduction

Partial edentulism is seen more frequently with increasing age. It is more prevalent in the mandible compared to the maxilla.1 For inter-professional communication many classifications to describe different partial edentate situations have been proposed. The most widely used is the one originally proposed by Edward Kennedy in 1928: the Ken-nedy classification.2 It was modified by Applegate some decades later.3 This classifica-tion basically divides partial edentulism into four classes, depending on the position and the extent of the edentulous breaches. It allows quick visualization of the dental situation (Figure 1):

Kennedy class I: bilateral edentulous areas at the end of the dental arch: bilateral free-end saddle;

Kennedy class II: a unilateral edentulous area at the end of the dental arch: unilateral free-end saddle;

Kennedy class III: a single, unilateral interrupted posterior dental arch. The edentu-lous gap is bounded by natural teeth: unilateral bounded posterior saddle;

Kennedy class IV: a single, bilateral interrupted dental arch crossing the midline in the anterior: single anterior bounded saddle.

A combination of classes is possible for which the Applegate rules apply.

Figure 1. Kennedy classification

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Free-ending situations with missing molars and/or premolars as classified in a Kennedy class I or II are seen more often in elderly patients.1 This thesis focusses on some aspects of the Kennedy class I situation, in particular on the merits of implant-support to a re-movable partial denture in a bilaterally free-ending situation in the mandible. However, there are other restorative approaches to handle this predicament as well. These are described and discussed in general terms underneath as well in light of the knowledge available at the initiation stages of this PhD project.

Dealing with the Kennedy class I situation

Generally speaking there are two treatment philosophies to deal with the Kennedy class I situation. One could choose to maintain the partially edentulous state. This is also described as the ‘shortened dental arch’ concept (Figure 2). On the other hand functional, occluding units can be added, either by fixed or by removable restorative means. Options that can be distinguished are a cantilever bridge, an implant-supported crown or bridge or a tooth-implant-supported bridge, a removable partial denture or an implant-supported removable partial denture.

1. The shortened dental arch concept The shortened dental arch (SDA) treatment philosophy builds on the idea that to main-tain adequate objective and subjective oral function, not all missing (pre-) molar teeth need to be replaced.4 This idea has been substantiated by a large number of studies, initially with short term results, but later also documenting consequences in the longer run 5-10 as well as by several systematic reviews.11,12 From an extensive review of the literature focusing on subsets of oral function like mas-ticatory efficiency and ability, appearance, psychological ability and comfort, social abili-ty and comfort, occlusal support and dental arch stability, as well as other functions like tactile perception, phonetics and taste it is concluded that a minimum of 20 teeth with 9-10 pairs of contacting units (including anterior teeth) is associated with adequate oral function.13 Restorative and non-restorative approaches do not differ in effectiveness, although a fixed substitution of teeth is considered better than a removable one.14 In a more recent systematic review comparing prosthodontically- treated and untreated shortened dental arches and including only papers of high methodological quality, the merits of the SDA concept are underscored again.15 All former findings rationalise the highly ambitious goal that was set by the World Health Organisation in 1992 to be achieved for the year 2000: individuals worldwide should maintain a dentition of at least 20 teeth throughout life, which they presumed would be associated with adequa-te oral function.16 An example of a mandibular shortened dental arch is given in figure 2

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2. Fixed Partial Denture (FPD): the cantilever bridge With a distally cantilevered FPD a single premolar can be added to the mutilated den-tition. A cantilever bridge, whether conventional or adhesive, has a good prognosis if properly executed.17 When this option is considered, careful treatment planning is particularly important. The abutment teeth need to be strong and in a healthy, stable biomechanical, endodontic and periodontal state. Functional forces should be control-led and the connector should be strong and rigid.18,19 If these prerequisites are met it can result in a stable occlusal stop for the mandible (Figure 3).In general, survival rates of cantilever FPD’s (82 % after 10 years) are lower and compli-cation rates are higher than those of end-butted FPD’s.20,21 On the other hand, compa-rable survival rates to FPD’s with two end abutments are also reported.22 Randomized clinical trials comparing the two are quite rare. In one well designed split mouth study, replacing second premolars by means of a cantilever FPD, did not lead to a compromi-sed survival of the abutment teeth compared to the non-restored contra-lateral teeth after 5 years (94 % versus 92 %).23

Figure 2. Shortened dental arch in the man-

dible

Figure 3. Fixed Partial Denture (FDP) with two

abutment teeth and a cantilevered pontic.

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3. Fixed Partial Denture (FPD): implant-supported crown or bridge Fixed implant-supported and tooth-implant-supported restorations can in theory re-place each and every missing posterior tooth (Figure 4). They have a good prognosis with reported 10-year survival rates in the order of 93 %.24,25 Unfortunately, such tre-atment is expensive and local anatomical conditions sometimes render treatment com-prising of multiple implant crowns or a FPD not feasible. Limited vertical bone height would jeopardize the mandibular alveolar nerve when implant surgery is contemplated. With implants becoming increasingly shorter, the former can be overcome. However, their clinical performance has not been properly documented to date. Clinical trials are running.

4. Removable Partial DentureConventional distal extension base removable partial dentures (RPD’s) are a restorative option replacing all missing teeth but they generally yield poor patients’ acceptance. Adding occlusal units has aesthetic and functional benefits.26 However, the effect on patients’ health related quality of life and oral comfort seems unwarranted, at least not to a level that outweighs that of not replacing missing molars. This was demonstrated in a rare randomized clinical trial comparing RPD’s with the shortened dental arch treat-ment concept.27 Approximately 10, 25 and 50 % of distal extension base RPD’s are no longer in service after respectively 1, 5 and 10 years.28

Not surprisingly, ‘pain when wearing a removable partial denture’ is significantly rela-ted to patients’ appreciation of treatment.29 Because of the discrepancy between the compressibility of the alveolar mucosa on the one hand and the limited freedom of mo-vement of a natural tooth in its alveolus on the other hand, discomfort in free-ending RPD’s can be expected. Under such conditions occlusal forces are also detrimental for

Figure 4. 3-unit implant-supported Fixed Par-

tial Denture (FDP)

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the abutment teeth. To reduce or more evenly distribute the load on abutment teeth and on the residual alveolar ridge several solutions have been proposed, among which are reduction in the number of artificial teeth and in their occlusal surface, lengthening of the saddle, meticulous attention to occlusion and articulation and several variations in retainer design.30 An example of a distal extention base Removable Partial Denture (RDP) in the mandible is given in figure 5.

5. Implant-Supported Removable Partial Denture (ISRPD)By placing posterior implants, a Kennedy class I situation is basically transformed into a more favourable situation with transfer of forces from the mucosa toward the im-plant(s) and the abutment teeth (Figure 6). The first report suggesting the potential merits of implants to support an RPD dates from 1991.31 Since then several studies have pointed out that providing implant-support to an RPD is of benefit to the patient.32-36 The use of unaesthetic clasps can often be avoided with implant support.37

Figure 6. Implant-supported Removable Pati-

al Denture (ISRPD)

Figure 5. Distal extention base Removable Partial Denture (RDP) in the mandible

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Outline of this thesis

Taking into account all advantages and disadvantages of alternative treatment options it seems that an ISRPD is a viable treatment option for patients with a Kennedy class I situation. However, the provided evidence for ISRPD’s is obtained from a rather hetero-geneous collection of studies. Populations often include a variety of intraoral conditions and retention concepts. Furthermore, evidence is often based on case reports or studies of a retrospective nature with few subjects or finite element methods. Consequently, better controlled and randomized clinical trials to validate the outcomes of ISRPD’s are needed.38,39 Besides the function of the implants, outcome parameters should entail functional and patient-based outcome measures.When competing treatment options are available, their documented or presumed effec-tiveness and their (additional) costs should be considered and critically assessed when favouring one treatment over the other. A cost-effectiveness analysis can provide in-sight into whether the more costly treatment option offers sufficient added value to the patient to justify its application and it can aid dentists and patients to make treatment decisions in the most cost-effective way.40,41 No information comparing the anticipated benefits of RPD’s and ISRPD’s and relating these to the expected extra costs is available.

The position for the implant that offers the optimal support is also not elucidated in the literature. With the cuspid or the first bicuspid as most distal tooth, the position of the implant in the edentulous zone is more or less optional and at the discretion of the prosthodontist or surgeon. Little evidence is available with respect to functional and cli-nical outcomes on which to base the decision. Theoretical models indicate that a more posterior position, i.e. at the position of the first or second molar, reduces the pressure to soft tissues and alveolar bone the most, whereas an implant positioned directly distal to the remaining dentition reduces the stress on the abutment teeth.42 When planning implants for the lateral parts of the mandible, clinical inspection and a panoramic radiograph suffice in cases with ample bone volume. A cone beam compu-ted tomography (CBCT) may provide additional value in challenging cases because it provides data to produce slices in three dimensions of the anticipated implant location, enhancing the decision whether or not implant placement is possible and safe, i.e. re-ducing the risk of jeopardizing the alveolar nerve during implant surgery.43,44 The ALARA principle raises the question under which conditions the additional information gained from CBCT outweighs the extra biological and financial risks and costs when evaluating implant sites pre-operatively.

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The general aim of the research described in this thesis was to assess the treatment outcome of implant-supported removable partial dentures in the mandible. More spe-cifically:- to assess, in a retrospective study, the performance, biological and technical compli-cations, of ISRPD’s in mandibular Kennedy class I situations with implants placed in the anterior or posterior position (chapter 2);

- to assess patients’ appreciation of implant support to removable partial dentures in patients with a bilateral free-ending situation in the mandible and to determine the most favourable implant position: premolar or molar region (chapter 3);

- to assess the functional benefits of implant support to removable partial dentures in patients with a bilateral free-ending situation in the mandible, to assess clinical and radiographical performance of the implants and to determine the most favourable im-plant position with respect to these aspects: premolar or molar region (chapter 4);

- to conduct a cost-effectiveness analysis comparing conventional removable partial denture and implant-supported removable partial denture treatment in the mandible (chapter 5);

- to compare two imaging modalities (panoramic radiograph and cone-beam computed tomography) in pre-operative implant planning in the severely resorbed mandible and the influence on the observers assessments (chapter 6).

The findings are discussed in light of the current literature in chapter 7, leading to re-commendations for clinical practice and future research.

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