Technical complication of a zirconia multiple-unit FDP supported by titanium base abutments - case report on a bonding failure and treatment alternative
João Pitta, Dr Med Dent, MASa, Ana Todorovic, DDS, PhDb, Vincent Fehmer, MDTa, Malin Strasding, Dr Med Denta, Irena Sailer, Prof Dr Med Denta
a Division of Fixed Prosthodontics and Biomaterials, University Clinics of Dental Medicine, University of Geneva, Switzerland
b Division of Prosthodontics, School of Dental Medicine, University of Belgrade, Serbia
Corresponding author:
Med dent. MAS, João Pitta
Division of Fixed Prosthodontics and Biomaterials
University Clinics of Dental Medicine, University of Geneva 1 Rue Michel-Servet, 1211 Genève 4. SWITZERLAND Tel: +41 22 379 40 50 / Fax +4122 379 40 52
Email: joao.pitta@unige.ch
Submitted June 2, 2020; accepted November 15, 2020
Abstract
Monolithic zirconia implant-supported restorations connected to titanium bases or titanium inserts are increasing in popularity due to their application in a full digital workflow. These prefabricated abutments are connected to the all-ceramic superstructure by adhesive cementation. Although limited clinical data on the outcomes
of this type of restoration are available, a few laboratory studies have shown possible debonding issues. This case report presents a bonding failure of a fixed dental prosthesis supported by titanium bases after short clinical use. A treatment alternative is also proposed using the available digital dental technology. Int J Prosthodont 2021. doi:
10.11607/ijp.7194
Introduction
Implant-supported fixed dental prostheses (FDPs) are a well-documented and predictable treatment option to restore edentulous gaps with high survival rates after 5 years (95.4%) (1). However, a significant number of technical complications is still reported, such as chippings and/or fractures of the veneering ceramic (2).
A possible solution to reduce the incidence of chippings might be the use of monolithic full-contour restorations. This is now possible with the recent introduction of more translucent and colored types of zirconia (3). Technical development in the digital CAD/CAM technology opened the possibility for the implementation of monolithic implant-supported restorations, connected to prefabricated or individualized abutments (4-6). The combination of high strength monolithic materials connected to titanium base abutments (ti-bases) or titanium inserts, is increasing in popularity mostly due to an efficient fabrication time and economic reasons (7). Moreover, its use has been suggested especially in posterior regions where stress forces are higher and esthetics is less important (8). Different designs of ti-bases are currently available, for single- as well as for multiple-unit implant restorations. Ti-bases for multiple-unit FDPs are usually designed with conical shapes and more convergent walls, a smaller bonding surface and a without anti-rotational indexation. This specific design could result in more technical complications such as debonding or screw loosening (9). Currently, no
clinical data is available reporting on the bonding outcomes of these types of restorations, although few laboratory studies have raised this question showing debonding events after artificial aging (10, 11). The purpose of this case report was to present a bonding failure of an FDP restoration on ti-bases after a short clinical use, and to provide an alternative treatment option using the available digital dental technologies.
Clinical report
A 67-year-old male patient was referred to the Center of Dental Medicine at the University of Geneva to improve esthetic and function of his dentition. After complete diagnostics and treatment planning, a hygienic phase was performed in order to treat the existing periodontal conditions. When the periodontal treatment goals were achieved, a full-mouth rehabilitation was performed. Following the treatment plan, the posterior maxillary segments were firstly restored with implant-supported multiple-unit FDP in order to provide posterior support in a new vertical dimension of occlusion. Full- contour monolithic zirconia buccally-veneered was selected as restoration material. The restorations were extra-orally cemented on ti-bases (for single and multiple-unit FDPs) using a definitive resin-cement recommended for this purpose (Multilink Hybrid Abutment H0, Ivoclar Vivadent) (12). The remaining existing posterior teeth were restored with lithium-disilicate adhesively cemented onlays. After restoring the anterior maxillary segment with lithium-disilicate adhesively-cemented veneers, the patient requested an improvement on the color match and volume in position 24 on the restoration in the 2nd quadrant, which was delivered approximately 3 months before (Fig.1).
As this restoration was screw-retained, it was decided to retrieve it in order to apply some veneering ceramic in the laboratory. The FDP screw-retained bridge was
removed and a gap was inspected between the ti-base and the margin of the crown in implant position 26. By inserting a probe in this gap, a complete loss of retention of the ti-base to the restoration was noticed (Fig 2), with most of the cement remaining on the abutment surface.
After informing the patient of the occurred situation, a new optical impression (Trios 3, 3Shape) of the implants was taken in order to fabricate a new restoration using different types of abutments (Fig. 3). The former FDP bridge was recemented on the ti- base (Multilink Hybrid Abutment H0), and it was used as interim restoration during the fabrication of the new one. Once the optical impression was sent online to the dental laboratory, the FDP was designed (Dental Designer 18, 3Shape) in a full contour applying a split-file approach based on two individually designed titanium abutments (Dedicam, Camlog Biotechnologies GmbH). Thereafter, the design software led to the model-builder software (Model-Builder, 3Shape) in which the master model was designed and the digital selected analogs were positioned. Finalizing the workflow, five STL (Standard Tessellation Language) files were exported from the software, the maxillary- and mandibular master model along with the FDP and the two individual abutments (Fig. 4).
In a first step, the two individual titanium abutments STL-files were sent online to the respective centralized milling center of the implant manufacturer for production (Dedicam, Camlog Biotechnologies GmbH). Following the digital workflow, and prior to the manufacturing of the full-contour zirconia FDP, the same STL file was used to manufacture a printed prototype. It was produced alongside the two master models and made of a printable resin (SHERAPRINT Model Plus UV, SHERA Werkstoff- Technologie GmbH & Co) with a digital light processing printer (DLP) (Rapidshape,
Straumann AG) with a resolution HD 1920 × 1080, native pixel of 34 μm and a layer size of 50 μm (Fig. 5).
The abutments and the FDP prototype were tried-in. Besides aesthetic evaluation of tooth-shape and volume, margin adaptation, cleanability and occlusion could also be checked and verified (Fig. 6).
After approving the prototype FDP, the same STL file was used to manufacture a full contour zirconia FDP with a small buccal cut-back. The generated bridge file was then milled in a 5-axis milling unit (inLab MC X5, Dentsply Sirona) using the classical 98mm zirconia disk (Katana ML, Kuraray Noritake) in a dry milling environment.
The milled restoration was adjusted in the white stage, including the perforation of two occlusal screw-accesses, before it was sintered to its final density. Thereafter the FDP was buccally micro-veneered (Creation ZI-CT, Willi Geller) and superficially characterized applying stains (Ivocolor, Ivoclar Vivadent) according to the custom shade developed with the patient. The occlusal surfaces were only high gloss-polished (StarGloss polisher for ceramic, Edenta Dental), and no glaze was applied. Once the full contour zirconia FDP was finalized, the internal surfaces were sandblasted with 2.0 bar and 50-μm aluminum oxide cleaned with alcohol and rinsed with water.
Previous to cementation, the abutments were sandblasted with 50-μm alumina- oxide (2.0 bar, 20 seconds), cleaned in an alcohol ultrasonic bath for 5 minutes, and rinsed with water before being screw-retained on the implants on the corresponding position with 20Ncm torque (as recommended by the manufacturer). The screw-access was then closed with Teflon-tape + light-curing temporary restorative material (Telio, Ivoclar Vivadent), and the zirconia FDP was intra-orally cemented onto the abutments (Ketac CEM, 3M ESPE). After removing the cement excess, the occlusal access-holes on the zirconia were conditioned with a universal adhesive (Scotchbond Universal, 3M
ESPE), closed with resin-composite material (Tetric Evoceram, Ivoclar Vivadent), and stained using a resin color modifier (Kolor+Plus, Kerr) (Fig. 8). A final control was done one week after the cementation of the zirconia-FDP (Fig. 9).
At 1-year follow-up the restoration showed to be clinically and radiographically stable (Fig. 10), with no biological or mechanical complications detected. The patient presented to be comfortable and able to properly clean.
Discussion (1-2 pages maximum)
This case report presented a bonding failure of a ti-base supported-FDP and revealed possible limitations of this concept. An alternative treatment option was proposed in the way to overcome those limitations.
In this clinical case no loss of retention was clinically noticed. The reason might be the stability and retention still given by the second ti-base. Even though the debonding could only be detected because the bridge was un-screwed from the implants, more serious complications could have developed over time. The presence of a gap at the interface between abutment and crown may have led to the accumulation of bacteria, and consequent deleterious effects on the peri-implant complex stability, as it is suggested for micro-gaps at implant-abutment connections (13).
Several reasons might explain the bonding failure between the ti-base and the zirconia restoration. The short height of the ti-base specifically designed for multiple- unit FDPs (4.0 mm of bonding height), and the more convergent walls could have contributed to this finding (14, 15). These characteristics combined with a deep implant position resulted in a severely increased crown-abutment ratio. Short titanium base and high crown superstructure result in a longer lever arm, and greater bending moment during function (10, 16). Moreover, the use of a cantilever FDP may additionally
increase the biomechanical risk. Although cantilever implant restorations have shown to be a valid treatment option (17), the presence of non-axial or rotational occlusal forces can result in minor technical complications such as loss of retention, abutment or screw loosening, or chipping of the veneering ceramic (18).
Despite the fact that an extra-oral cementation with an MDP-containing primer and a resin-cement was performed, possible error at the cementation procedures cannot be excluded. The fact that the majority of the cement remained on the titanium base might indicate an adhesive failure between the resin cement and zirconia.
To overcome these limitations, customized titanium abutments were selected instead. Despite the increase in cost, the use of this type of abutment is well documented in the literature (19). The customization allowed for an individual emergence profile and a good use of available space with the overall prosthetic height. In this way, the bonding area was increased, while the crown-abutment ration was also improved. Due to fabrication process limitations, both implant abutments could only be designed with an implant internal anti-rotation feature, characteristic of single-implant abutments. In order to assure a perfect fit of the abutments to the implants, as the full parallelism of the implants was not verified, an intra-oral cementation was preferred. The location of the cement margins only slightly subgingival and the use of a semi-permanent cement (20) allowed a controlled removal of the cement excesses, which is important to avoid biological peri-implant complications (21, 22).
While clinical data on zirconia restorations supported by ti-bases is urgently necessary, case selection and a careful risk evaluation must be done previous to clinical application. In high risk situations, combining different mechanical risk factors, customized titanium abutments might be an alternative solution applying a digital workflow.
Acknowledgements
The authors would like to thank to CDT Mr. Cem Piskin for the collaboration and fabrication of the restorations. The authors report no conflicts of interest.
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FIGURES
Fig 1. Initial situation after cementation of anterior veneers, when the patient showed intention of improving the aesthetic appearance of the second quadrant.
Fig 2. Debonded titanium base.
Fig. 3 – Optical impression.
Fig 4. Design of the customized titanium abutments and FDP.
Fig 5. (a) Customized titanium abutments and (b) prototype on a 3D-printed models.
Fig 6. (a) Customized titanium abutments and (b) prototype try-in.
Fig 7. Finalized zirconia restoration and customized titanium abutments.
Fig 8. Intra-oral cementation.
Fig 9. Final (a) buccal and (b) smile view.
Fig 10. Follow-up after 1 year: buccal intra-oral view and (b) radiographic control.
