The aim of this study was to evaluate the influence of different framework materials on biomechanical behaviour of anterior two-unit cantilever resin-bonded fixed dental prostheses (RBFDPs). A three-dimensional finite element model of a two-unit cantilever RBFDP replacing a maxillary lateral incisor was created. Five framework materials were evaluated: direct fibre-reinforced composite (FRC-Z250), indirect fibre-reinforced composite (FRC-ES), gold alloy (M), glass ceramic (GC), and zirconia (ZI). Finite element analysis was performed and stress distribution was evaluated. A similar stress pattern, with stress concentrations in the connector area, was observed in RBFDPs for all materials. Maximal principal stress showed a decreasing order: ZI > M > GC > FRC-ES > FRC-Z250. The maximum displacement of RBFDPs was higher for FRC-Z250 and FRC-ES than for M, GC, and ZI. FE analysis depicted differences in location of the maximum stress at the luting cement interface between materials. For FRC-Z250 and FRC-ES, the maximum stress was located in the upper part of the proximal area of the retainer, whereas, for M, GC, and ZI, the maximum stress was located at the cervical outline of the retainer. The present study revealed differences in biomechanical behaviour between all RBFDPs. The general observation was that a RBFDP made of FRC provided a more favourable stress distribution.
Resin-bonded fixed dental prostheses (RBFDPs) have proven to be a reliable treatment alternative for the replacement of missing teeth [
The use of more extensive preparation of the abutment teeth, including palatal or lingual coverage with 180-degree wraparound, chamfer, cingulum rests, and proximal guide planes and grooves, is a way to improve the retention of RBFDPs [
The framework of RBFDPs is traditionally made of metal alloys, but their poor aesthetics and the growing awareness towards possible adverse health effects of dental alloys, such as Ni-, Cr-, Co-, Pd-, and Au-containing alloys [
The aim of the present study was to compare, by means of three-dimensional finite element analysis (3DFEA), the biomechanical behaviour of anterior two-unit cantilever RBFDPs made of various framework materials.
A FE model representing a single tooth gap in the anterior right maxilla, consisting of a central incisor, a missing lateral incisor, and a canine (Figure
3D FE model of a cantilever two-unit RBFDP: (a) abutment and adjacent tooth, (b) cement layer, and (c) RBFDP.
The geometry of the healthy standard tooth as abutment has been previously described [
Materials properties are derived from clinically used materials (reference brand between parentheses): hybrid particulate filler composite (PFC) for laboratory use (Estenia C&B; Kuraray medical Inc., Tokyo, Japan), hybrid PFC for chairside use (Filtek Z250; 3M ESPE, MN, USA), unidirectional FRC for laboratory use (Estenia C&B EG fiber; Kuraray medical Inc., Tokyo, Japan), unidirectional fibre-reinforced composite for direct and chairside use (EverStick C&B; StickTech Ltd., Turku, Finland), Au-Pd alloy (Olympia; J.F. Jelenko, Armork, NY, USA), lithium disilicate glass ceramic (IPS Empress 2; Ivoclar-Vivadent, Schaan, Liechtenstein), zirconia (InCeram Zirconia; Vita, Bad Säckingen, Germany), feldspathic porcelain (Creation; Klema, Meiningen, Austria), resin-based luting cement (Variolink 2; Ivoclar-Vivadent, Schaan, Liechtenstein), enamel, dentin, and pulp. The material properties, mostly obtained from existing literature, are summarised in Table
Elastic properties of the materials used in the finite element model.
|
Poisson’s ratio | Shear modulus |
Reference | |
---|---|---|---|---|
Enamel | 80.0 | 0.30 | — | [ |
Dentin | 17.6 | 0.25 | — | [ |
Pulp | 0.002 | 0.45 | — | [ |
Resin luting cement | 8.3 | 0.24 | — | [ |
Chairside PFC | 11.5 | 0.31 | — | [ |
Laboratory PFC | 22.0 | 0.27 | — | [ |
Chairside FRC | a | |||
Longitudinal ( |
46.0 | 0.39 | 16.5 | |
Transverse ( |
7.0 | 0.29 | 2.7 | |
Laboratory FRC | [ | |||
Longitudinal ( |
39.0 | 0.35 | 14.0 | |
Transverse ( |
12.0 | 0.11 | 5.4 | |
Lithium disilicate glass ceramic | 96.0 | 0.25 | — | [ |
Zirconia | 205 | 0.22 | — | [ |
Au-Pd alloy | 103 | 0.33 | — | [ |
a: data obtained by StickTech Ltd. (Turku, Finland).
Five different groups with various framework materials were evaluated: FRC-Z250: a FRC-FDP made of continuous unidirectional E-glass FRC framework (Figure FRC-ES: a FRC-FDP made of continuous unidirectional E-glass FRC framework veneered with hybrid PFC for laboratory use; M: a metal-ceramic FDP made of type 3 Au-Pd alloy framework veneered with feldspathic porcelain; GC: an all-ceramic FDP made of lithium disilicate glass ceramic framework veneered with feldspathic porcelain; ZI: an all-ceramic FDP made of zirconia framework and veneered with feldspathic porcelain.
3D FE model of a two-unit cantilever FRC RBFDP: position of the FRC framework in relation to the FDP and the abutment teeth is shown. Double arrowed black line represents the fibre direction.
A FRC framework was designed with thickness of 0.6 mm and a height of 3.0 mm [
In order to avoid quantitative differences in stress value, all solid models were derived from a single mapping mesh pattern that generated 103,861 twenty-node brick element (Solid 95 in ANSYS) and 154,784 nodes. Loading and boundary conditions are depicted in Figure
Loading and boundary conditions of a 3D FE model representing two-unit cantilever RBFDPs.
Principal stress distribution within two-unit cantilever RBFDPs of various framework materials.
Differences in maximum principal stress were observed (Table
Maximum and minimum principal stress (MPa) and displacement (mm) for two-unit cantilever RBFDPs of various framework materials.
FDP | Cement-retainer interface | Cement layer | Abutment tooth | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Max. | Min. | Disp. | Max. | Min. | Disp. | Max. | Min. | Disp. | Max. | Min. | Disp. | |
FRC-Z250 | 156.9 | −56.2 | 0.048 | 17.5 | −5.3 | 0.010 | 31.3 | −7.1 | 0.010 | 34.9 | −7.6 | 0.010 |
FRC-ES | 177.1 | −67.2 | 0.035 | 23.9 | −9.7 | 0.010 | 27.3 | −7.1 | 0.010 | 30.9 | −9.8 | 0.010 |
GC | 178.4 | −116.3 | 0.019 | 32.7 | −42.5 | 0.009 | 23.7 | −4.1 | 0.009 | 31.4 | −4.8 | 0.009 |
ZI | 239.6 | −154.3 | 0.017 | 60.8 | −75.3 | 0.009 | 27.5 | −3.3 | 0.009 | 31.7 | −7.2 | 0.009 |
M | 197.1 | −149.9 | 0.019 | 36.1 | −45.8 | 0.009 | 24.5 | −3.7 | 0.009 | 31.9 | −5.0 | 0.009 |
Principal stress distribution at the cement-retainer interface for two-unit cantilever RBFDPs of various framework materials.
Differences in maximal principal stress were also observed (Table
Principal stress distribution within the cement layer for two-unit cantilever RBFDPs of various framework materials.
FEA revealed (Table
Principal stress distribution at the abutment tooth for two-unit cantilever RBFDPs of various framework materials.
On the abutment tooth only slight differences in maximal principal stress were observed (Table
Differences in maximum displacement were observed in the pontic part of the RBFDP between the different materials. Higher displacement of the RBFDP was encountered with FRC-Z250 and FRC-ES and then with M, GC, and ZI. Although, the maximum displacement at the cement-retainer interface, cement layer, and abutment tooth revealed the same trend as those for RBFDPs, a difference of 0.001 mm between highest and lowest value could not be regarded as clinically relevant.
A static fracture strength test, during which a FDP is vertically loaded till failure, is the most common way to evaluate the mechanical behaviour of FDPs in laboratory conditions [
In the present study, the FE model was loaded by applying a stress of 90 MPa in a 45° angle to the incisal edge of the pontic tooth. An applied stress of 90 MPa to a 5.5 mm² incisal area corresponds to a load of 495 N. The applied load is significantly higher than previously reported maximum anterior mastication loads of 108–382 N [
Roots, periodontal ligament, and bone, which are responsible for physiologic tooth mobility, were not included in the FE model. Under clinical conditions, a part of the loading is transferred via the roots and the periodontal ligament into the bone. The lack of physiologic tooth mobility in the present FE model negatively influences the outcome of the FEA, in such a way the principal stress values are overestimated. The effect of tooth mobility was illustrated by Rosentritt et al., who found higher fracture strengths for anterior cantilever RBFDPs when luted to abutment teeth with high mobility [
The present FEA revealed differences in biomechanical behaviour between RBFDPs made of different framework materials. Although the location of the stress concentration, observed at the FDP level, was identical for all framework materials, the values differed. The differences in displacement and principal stress can be explained by the differences in elastic modulus between framework materials. RBFDPs made of materials with higher stiffness suffered less displacement, but higher principal stress, than those made of less stiff materials, which can be illustrated by comparison of zirconia and chairside FRC. Zirconia exhibits an elastic modulus of 205 GPa and showed 0.017 mm displacement and 239.6 MPa maximum principal stress in comparison to 0.048 mm and 156.9 MPa by the chairside FRC with an elastic modulus between 11 GPa (chairside hybrid composite) and 46 GPa (FRC). The highest maximum principal stress was located at the occlusal embrasure of the connector. It has to be noticed that the connector in our FE model was designed with a sharp embrasure and that stresses in this location can be significantly decreased by changing the connector design [
A similar situation with regard to stress values was found at the level of cement-retainer interface. Far more interesting were the differences in location between FRC on one hand and M, GC, and ZI on the other hand (Figure
At the level of the cement layer there was only a slight difference in maximum principal stress values, but as expected the differences in location, as seen at the cement-retainer interface, between FRC on one hand and M, GC, and ZI on the other hand (Figure
The difference in maximum principal stress value between different framework materials was even lower at the level of the abutment tooth. However, the location of the stress concentration, as depicted in Figure
Based on the results of this study the predominant failure mode of two-unit cantilever RBFDPs for each framework material might be predicted. Although acceptable bond strength to resin luting cements can be achieved by glass ceramics, their low strength could make them susceptible to connector fracture and therefore probably less suitable for the fabrication of anterior two-unit cantilever RBFDPs. On the contrary, the only clinical study published on cantilever glass ceramic RBFDPs reported 100% survival after 6-year concluding [
Although FRC RBFDPs seem to be more promising as they exhibit good bond strength to resin luting cement, connector fracture seems to be the failure mode to be expected. Clinical [
Zirconia and metal RBFDPs are suspected to fail most likely because of debonding. A multitude of clinical research on cantilever metal RBFDPs corroborates this prediction [
Within the limitations of this study, 3DFEA revealed differences in biomechanical behaviour between RBFDPs made of different framework materials. The general observation was that a RBFDP made of FRC provided a more evenly distributed stress pattern from loading area towards abutment tooth. Maximum principal stress was located at the occlusal embrasure of the connector for all framework materials: highest value was found for ZI, while the lowest was found for FRC-Z250. Advanced stress analyses suggest a possible difference in predominant failure mode: connector fracture for FRC- and glass ceramic-based RBFDPs and debonding for metal- and zirconia-based RBFDPs. A stress concentration was found at the contact area with the adjacent tooth, indicating that the applied load is partially transferred to the adjacent tooth.
The authors declare that there is no conflict of interests regarding the publication of this paper.