Two-unit Adhesive Cantilevers Supported

A van Dalen, PhD

Summary, conclusion and future research

Summary

Depending on the researcher’s view on society in general or dentistry in particular, tooth loss can be regarded as a socio-political, socio-economic, habitual, genetic, esthetic, anatomical, prosthodontic, periodontal, implant or even age or drug related problem. But, the question the victims of tooth loss ask their dentists is always the same: “Please provide me with a solution as if nothing happened.” Depending on the degree of tooth loss this might lead to a certain degree of disappointment, because an imitation by definition is never as good as the original. This thesis deals with single tooth replacement through two-unit cantilevered resin-bonded fixed partial dentures, being an adhesive solution using one abutment tooth only, causing as less damage as possible to the existing dentition. For reasons of legibility in this summary the term “resin-bonded fixed partial denture”, which means an adhesively fixed bridge, is substituted by the abbreviation RBFPD.

The general introduction in Chapter 1 provides background information on the various removable and fixed partial denture solutions for single tooth replacement. The dental literature has been searched and the research and development of materials used for adhesively fixed appliances, is considered. This brings many doubts of newly developed and developing materials to the surface. Amongst the recently introduced tooth-colored materials, all-ceramic and fiber-reinforced restorative materials are promising materials and enjoy great interest from both the profession and the general public. But the clinical use of these new materials for RBFPDs is not yet fully supported by reliable clinical evidence. The choice of restorative materials, the required adhesive techniques and computer simulation techniques for restoration design and problem solving are discussed.

Chapter 2 evaluated in a literature survey the clinical performance of two-unit cantilevered, single abutment, single pontic RBFPDs by comparing them with three-unit non-cantilevered RBFPDs with two abutments and a single pontic for similar situations. The historical development of adhesive dentistry is briefly explored and the first clinical applications in this field are discussed. Originally macromechanical retention was achieved through perforated retainers, to be followed by micromechanical retention, made possible by etching the retainers. When it was discovered that this technique too did not provide enough retention for lasting restorations, the profession reverted to the long known retentive tooth preparations. Longevity of metal based adhesive restorations, comparable with fixed partial dentures came within reach. The review describes the influence on longevity of prosthesis and preparation design, surface pretreatment of the restoration, choice of luting cements, influence of the operator, gender and age of the patient and periodontal consequences. Several studies specifically on two-unit RBFPDs report different success rates, but they consider better esthetics, easy cleaning, less biological damage and hardly a chance of an undetected debonded retainer good reasons to consider application of this technique.

The purpose of the study described in Chapter 3 was to find in vitro the optimal combination of metal surface pretreatment and luting cement for two-unit cantilevered RBFPDs. To evaluate the cement behavior as such, a retentive tooth preparation has deliberately been omitted. In this study beams (7 x 15 x 0.55 mm), simulating two-unit cantilevered RBFPDs, made of a clinically frequently used cobalt chromium alloy (Bondiloy), have been used. Two different surface pretreatments have been applied, i.e. (i) sandblasting and (ii) a tribochemical silica coating (Rocatec) followed by silanization. Subsequently the beams have been luted with Panavia, Resiment, RelyX ARC or UniFix onto the flat ground buccal enamel surfaces of bovine teeth. Simulated chewing forces from different directions, called respectively peel, load and torque, have been applied on the specimens in a universal testing machine until failure occurred, and the failure loads have been recorded. The results showed that the peel failure loads, compared to load and torque, have the lowest values with each of the cements and surface pretreatments. One cement (UniFix), applied with sandblasting as pretreatment, generated a significantly higher failure load compared with the other three cements, irrespective of pretreatment. Remarkably this was also the cement with the highest percentage of cement remaining on the beams after the tests, proving the highest adherence capability to this CoCr alloy compared to the other cements. It was shown that after sandblasting, in spite of thorough cleaning afterwards, a certain content of Al₂O₃ was left behind on/in the CoCr surface, but the influence of these particles on bond strength is not tested. Chemically active monomers in the cements tested, like MDP in Panavia and 4-META in UniFix, do play a role in adhesion, but their exact role is still not completely clear.

Fatigue is a mode of failure occurring after repeated loading at which the magnitude of the load itself is not large enough to cause failure when applied once. Fatigue occurs because of crack formation and propagation caused by cyclic loadings. Chapter 4 describes a fatigue study into the behavior of simulated two-unit cantilevered RBFPDs as used in the study of Chapter 3. The aim of the study was to evaluate the influence of all possible combinations of metal surface pretreatment and luting cement on failure loads and finite fatigue limits. As surface pretreatments of the beams have been chosen (i) sandblasting, (ii) a tribochemical silica coating (Rocatec), followed by silanization and (iii) flame silicon oxide coating (Silicoater) with subsequent silanization. Panavia, RelyX ARC and UniFix were used as luting cement. The “staircase” method was used for fatigue evaluation. This method is testing specimens for a chosen maximum number of cycles, in this case one series with 10⁴ and one series with 10⁵ cycles. During the first series of tests (10⁴) UniFix combined with sandblasting showed some debondings, but with Rocatec or Silicoater pretreatments there were no UniFix debondings at all. On the contrary, the UniFix beams during the progression of each test, gradually bent under the influence of the cyclic loadings. During the second series of tests (10⁵) there were no UniFix debondings at all, which after six out of twenty intended tests was a good reason for discontinuation of the UniFix tests. Finally it could be concluded that compared to RelyX ARC, being a conventional Bis-GMA luting cement, both Panavia and UniFix did perform significantly better, probably due to the added bond enhancing monomers MDP and 4-META, respectively.

The study described in Chapter 5 explores and explains the debonding mechanisms of both two-unit cantilevered and three-unit fixed-fixed RBFPDs. The reason for debonding of three-unit fixed-fixed RBFPDs has been described in the literature as being caused by the differential movement of the abutment teeth. Jaw movements during articulation cause the teeth involved to move perpendicular to the dental arch. If the abutment teeth are situated in the (pre)molar region the directions of their articulation induced movements are mainly parallel. But when the RBFPD is situated in a curved part of the dental arch, their movements are not, causing stress in the tooth restoration interface, i.e. the cement layer. Application of a two-unit cantilevered RBFPD omits this problem simply by the fact that only one abutment tooth is involved. The aim of this study was to explain why the reported survival rates of two-unit cantilevered RBFPDs are higher than those reported of three-unit fixed-fixed RBFPDs. Furthermore, the influence of the geometry (straight or bent, depending on the location within the dental arch) has been explored and explained. When in a clinical situation one abutment tooth of a three-unit fixed-fixed RBFPD is loaded, a limited axial movement will occur, creating counteracting forces in its socket. But the other abutment will experience relatively high interfacial torque and/or peel stresses. These are the stresses this study is focussed on. The in vitro obtained failure loads have been used in finite element analysis (FEA) models to explain the differences in the debonding mechanisms of two-unit cantilevered and three-unit fixed-fixed RBFPDs and the influence of the design.

Three types of simulated RBFPDs have been used. For rationalization reasons as surface pretreatment for the Bondiloy beams only sandblasting was used and Panavia as luting cement. The results of the tests showed significantly higher bond strength values for the two-unit cantilevered (CAN) and three-unit straight fixed-fixed RBFPDs (SFF) than for the bent ones (BFF). The latter showed a typical pattern of remaining cement on the beam after failure, indicating from which point the debonding process starts and how it propagates. The FEA models confirmed these findings and explained how and where the stress buildup in the cement layer occurs. Although static failure tests have been used in this study, it is reasonable to assume that during fatigue the same debonding process takes place. The factual findings of this study revealed the interfacial stress buildup, indicating where the tooth preparation needs to be modified to offer better resistance against debonding forces.

Chapter 6 describes the final study of this thesis comparing the failure strengths of resin composite, fiber-reinforced resin composite, zirconia and CoCr two-unit cantilevered RBFPD beams (CAN).

The fiber-reinforced resin composite beams did have a layered texture; approximately one half of the thickness consisted of a commercially available ready to use photopolymerizable matrix-fiber combination (everStick), while the other half was made of resin composite (Filtek Z100 or Estenia). The fiber-reinforced resin composite beams were divided into two groups: (i) one group was luted with the fiber side on the bovine enamel, while the other group (ii) was luted with the composite side on the enamel. Then the specimens were peel tested until failure.

The experimentally obtained load data were applied in the FEA model and the interfacial tensile stresses were made visible and calculated. Both the Z100 and Estenia resin composite beams did not debond from the enamel surface, but fractured along the enamel pontic transition line. The fiber luted type debonded from the enamel surface leaving the fibers exposed on the beam. So the matrix delaminated from the fibers. The composite luted type on the other hand did not debond, but delaminated within the beam structure on the fiber composite interface.

Conclusion

This thesis has focussed on two-unit cantilevered RBFPDs for single tooth replacement. Compared to removable and all other fixed solutions the two-unit cantilevered RBFPD is a cost friendly and truly minimally invasive restoration with satisfying esthetics that allows easy maintaining by the patient. The abutment tooth, however, needs to be sound in a sense of hardly or not being restored at all and preferably not periodontally compromised. The occlusion and articulation have to allow for this restoration to be applied, without extensive tooth preparation, preferably avoiding loading the pontic with detrimental forces during mastication. In conclusion, it is clear that not every single tooth loss can be substituted with a two-unit cantilevered RBFPD.

From clinical experience with metal based RBFPDs we already knew that the resin luting cement alone is not able to withstand chewing forces to guarantee long- term success. However, we found remarkable differences between the various resin luting cements involved, indicating that the choice of cement, even with a retentive preparation, is important. This kind of preparation covers only a small part of the abutment tooth, but definitely needs to supply sufficient retention, which needs to be confirmed during try-in. With a decreasing extent of the preparation, the importance of the adhesive properties of the resin luting cement increases.

The consecutive chapters of this thesis have provided us with an insight into reported clinical behavior, different performances of various types of resin luting cement and into the factual in vitro failure mechanism of two-unit cantilevered and three-unit non-cantilevered RBFPDs. The reported clinical behavior created the challenge of this thesis, more specific to research the failure mechanism and its consequences for future applications. The laboratory and FEA results have identified the failure mechanisms and potentially weak spots in the preparations. Especially understanding the failure mechanism of three-unit non-cantilevered RBFPDs in a curved part of the dental arch, contributes to a better clinical understanding. The different nature of debonding forces on two-unit cantilevered and three-unit non-cantilevered RBFPDs has clearly been demonstrated. In case of solitary tooth replacement the clinician is able to add another alternative to his array of restorative solutions.
The differences in behavior of the tested resin luting cements are remarkable. The adhesion enhancing monomers, both in Panavia and UniFix, do clearly make sense. For the general practitioner this means that the choice for a specific resin luting cement is an important factor regarding success or failure of the restoration. This choice should be wel-considered.

Future research

In this research the weak spots as far as debonding is concerned, have been identified both for the two-unit cantilevered and for the three-unit non-cantilevered RBFPD. The weak spot for two-unit cantilevered RBFPDs lies within close distance of the enamel-pontic transition line. So increase of resistance has to be found in this area. With the three-unit non-cantilevered RBFPD it is a different matter, because of the differential movements of the abutment teeth under loading by mastication forces. It is therefore reasonable to assume that a different approach compared to the two-unit cantilevered counterpart is required. The reliable retentive preparation with its 180-degree wrap-around, grooves and occlusal support, known from clinical experience, has made this clear.

As long as there are continuing longevity and fracture strength problems, relating to both fiber-reinforced and all-ceramic restorations, there will be a place for the metal based counterparts. With the obtained knowledge of the debonding process, further research should be aimed at designing the ideal preparation for two-unit cantilevered RBFPDs in various clinical situations, taking into account the specific properties of the intended restorative material. With growing concern about possible immunologic and/or toxic reactions from dental restorative materials, the profession needs to be cautious with resin composites, because they are potentially able to cause soft tissue and systemic responses. All-ceramic restorations however are supposed to be biocompatible.

Dentists feel reserved by the prospect of their patients receiving a restoration on one abutment only, while they already had bad experiences with similar restorations, supported by two or even more abutments. Using a single abutment tooth apparently contrasts with the dentist’s wish to supply his patients with well-thought out and solid solutions. Whether we like it or not, we have a mission to fulfill to convince the profession of the value and advantages of the two-unit cantilevered RBFPD. If this mission succeeds, a bright future for the two-unit cantilevered RBFPDs may lie ahead.