Methods to improve a periodontally involved terminal abutment of a cantilever fixed partial denture--a finite element stress analysis

A comparative finite elemental analysis of glass abutment supported and unsupported cantilever fixed partial denture

OBJECTIVE

The purpose of this study was to investigate and compare the load distribution and displacement of cantilever prostheses with and without glass abutment by three dimensional finite element analysis. Micro-computed tomography was used to study the relationship between the glass abutment and the ridge.

METHODS

The external surface of the maxilla was scanned, and a simplified finite element model was constructed. The ZX-27 glass abutment and the maxillary first and second premolars were created and modified. The solid model of the three-unit cantilever fixed partial denture was scanned, and the fitting surface was modified with reference to the created abutments using the 3D CAD system. The finite element analysis was completed in ANSYS. The fit and total gap volume between the glass abutment and dental model were determined by Skyscan 1173 high-energy spiral micro-CT scan.

RESULTS

The results of the finite element analysis in this study showed that the cantilever prosthesis supported by the glass abutment demonstrated significantly less stress on the terminal abutment and overall deformation of the prosthesis under vertical and oblique load. Micro-computed tomography determined a gap volume of 6.74162 mm(3).

SIGNIFICANCE

By contacting the mucosa, glass abutments transfer some amount of masticatory load to the residual alveolar ridge, thereby preventing damage to the periodontal microstructures of the terminal abutment. The passive contact of the glass abutment with the mucosa not only preserves the health of the mucosa covering the ridge but also permits easy cleaning. It is possible to increase the success rate of cantilever FPDs by supporting the cantilevered pontic with glass abutments.

The effect of type of restoration on the stress field developed in terminal abutments with severely reduced periodontal support and coronal structure

STATEMENT OF PROBLEM

Periodontally compromised teeth (PCT) that serve as terminal abutments (TAs) are often challenging depending on the post-and-core treatment, the type of partial fixed dental prosthesis (PFDP), and the periodontal support.

PURPOSE

The purpose of this study was to investigate the biomechanical impact of 3 types of PFDP supported by cast post-and-cores on PCT serving as terminal abutments.

MATERIAL AND METHODS

A 3-dimensional (3D) model of a human mandible was fabricated by using computed tomography (CT) images and parameterized in a computer-aided design (CAD) environment as follows: Right premolar preparation geometries were designed. The second premolar was assembled with 7-mm or 10-mm cast post-and-core models. Both premolar-models were designed to support single, splinted, or 1-unit cantilever splinted crowns. In each situation, their periodontium geometries were designed to be reduced by 10%, 50%, and 70%. All models were imported into a 3D finite element analysis (FEA) environment and loaded; von Mises stress values and distribution patterns were evaluated.

RESULTS

Insertion of the post primarily affected the apical areas of both the root and post; the type of PFDP and periodontal support mainly affected stress distribution. In patients with a normal periodontium, splinting the teeth did not contribute to their stress relief. By extending the post length, a stressful area close to the apex of the post was developed. Splinting mitigated the stress field of the coronal part of the 50% PCT (up to 98.9%); the 30% PCT experienced a substantial decrease (up to 215.9%) in stress in the radical part as well. The increase in the length of the post produced negligible stress-related differences in the apical part of the 50% PCT (0.2% to 2.6%). The use of the 7-mm post effectively relieved the radical part of the splinted 30% PCT. The magnitude of the stress on the radical part of post-restored PCT was considerably increased in the presence of a cantilever.

CONCLUSIONS

Splinted crowns supported by a 7-mm cast post-and-core are a favorable biomechanical approach for the restoration of PCT with severe loss of coronal structure. The use of a cantilever greatly aggravates the biomechanical response, especially of post-restored PCT.

Effect of proximal contact strength on the three-dimensional displacements of implant-supported cantilever fixed partial dentures under axial loading

OBJECTIVE

This study investigated the effect of proximal contact strength on the three-dimensional displacements of cantilever fixed partial denture (CFPD) under vertically concentrated loading with digital laser speckle (DLS) technique.

METHODS

Fresh mandible of beagle dog was used to establish the implant-supported CFPD for specimen. DLS technique was employed for measuring the three-dimensional displacement of the prosthesis under vertically concentrated loading ranging from 200 to 3000 g. The effect of the contact tightness on the displacement of CFPD was investigated by means of changing the contact tightness.

RESULTS

When an axial concentrated loading was exerted on the pontic of the implant-supported CFPD, the displacement of the CFPD was the greatest. The displacement of the prosthesis decreased with the increase of contact strength. When the contact strength was 0, 0.95, and 3.25 N, the displacement of the buccolingual direction was smaller than that of the mesiodistal direction but greater than that of the occlusogingival direction. When the force on the contact area was 6.50 N, the mesiodistal displacement of the prosthesis was the biggest while the buccolingual displacement was the smallest.

CONCLUSIONS

The implant-supported CFPD is an effective therapy for fully or partially edentulous patients. The restoration of the contact area and the selection of the appropriate contact strength can reduce the displacement of the CFPD, and get a better stress distribution. The most appropriate force value is 3.25 N in this study.

Comparison of fracture-load values of cantilevered FDPs

OBJECTIVE

Evaluation of the effect of different framework designs and of fatigue on the fracture-load values of cantilevered fixed dental prostheses (FDPs). The load values were compared with those for lithium disilicate ceramic and metal-ceramic FDPs.

MATERIALS AND METHODS

Fifty cantilevered FDPs were manufactured using a zirconia framework veneered with a feldspathic ceramic. Ten FDPs were made from a lithium disilicate ceramic and 10 were designed as metal-ceramic FDPs. All FDPs were anchored by two premolar crowns to replace a missing premolar. Twenty of the 50 zirconia FDPs were regarded as the control groups; these were divided into two groups-with and without fatigue. The other 30 zirconia FDPs were divided into three test groups with different framework designs. The load to fracture was measured and fracture sites were identified. The Kruskal-Wallis test and the Mann-Whitney U-test were used for statistical analysis.

RESULTS

Most of the all-ceramic FDPs fractured within the distal wall of the terminal crown abutment. The mean fracture-load ranged between 346-493 N for the FDPs with the 0.7 mm framework. Mean values for the three zirconia test groups ranged from 529-590 N. Reinforcement of the framework resulted in significantly higher fracture-loads than for the control group. Values for lithium disilicate restorations were significantly lower than those for the test groups and the values for the metal-ceramic group were significantly higher than those for the test groups.

CONCLUSION

Although reinforcement of the distal crown core might enhance the fracture resistance of all-ceramic cantilever FPDs, they cannot yet be unreservedly recommended for clinical use.

Effect of varying the vertical dimension of connectors of cantilever cross-arch fixed dental prostheses in patients with severely reduced osseous support: a three-dimensional finite element analysis

STATEMENT OF PROBLEM

Inadequate dimensioning of the connectors in a cantilever cross-arch fixed dental prosthesis (FDP) in perioprosthetic patients jeopardizes the prognosis of the restoration.

PURPOSE

The purpose of this study was to investigate the effect of increasing the vertical dimension (VD) on the maximum stress developed within the connectors during the static loading of a cross-arch FDP extended as a 1- and 2-unit cantilever.

MATERIAL AND METHODS

Six digital models were developed, derived from a 3-dimensional (3-D) initial model. In the initial model, the teeth were prepared for metal ceramic restorations and splinted with a cross-arch FDP, extended as a 1- or 2-unit cantilever. The VDs of the connectors proximal to the retaining abutment were 3, 4, or 5 mm. A 3-D finite element analysis (FEA) was performed.

RESULTS

The VD increase, from 3 to 4 mm and from 3 to 5 mm, of the connector distal to the retaining abutment, for each FDP, presented a maximum stress value decrease of approximately 25% and 48%, respectively. The similar VD increase of the connector mesial to the retaining abutment, for each FDP, resulted in relatively smaller stress changes. For the 2-unit cantilever restoration, the stress decreases were approximately 9% and 15%, respectively, whereas in the 1-unit cantilever restoration, the decrease was about 10% for the 4-mm connector. Further increase of the VD to 5 mm did not relieve the peak stress. The highest stress value was measured on the 3-mm connector distal to the retaining abutment in the 2-unit cantilever restoration. Despite the VD increase, the connectors proximal to the retaining abutment still developed the highest stress values of all the connectors for every model.

CONCLUSIONS

The connector with the highest risk of failure is the 3-mm connector distal to the retaining abutment of the 2-unit cantilever restoration. Increasing the vertical dimension is beneficial for the connector distal to the retaining abutment, while the resultant stress changes are not substantial for the connectors mesial to the retaining abutment. (J Prosthet Dent 2010;103:91-100).

Fracture behaviour of zirconia ceramic cantilever fixed dental prostheses in vitro

OBJECTIVE

Evaluation of the fracture resistance of all-ceramic cantilever fixed dental prostheses (FDPs) manufactured from zirconia frameworks and veneered with a press ceramic.

MATERIAL AND METHODS

Two mandibular premolars were prepared either with a box inlay cavity or with a full crown chamfer preparation and then duplicated. 40-three-unit cantilever FDPs replacing one premolar, with a group size of eight for each design, were manufactured. In group i-i the cantilever FDPs were retained by two inlays, in group i-c by an inlay-crown combination, and in group c-c by two crowns. The frameworks in groups i-c-R and c-c-R were reinforced by an additional shoulder on the oral side of the zirconia frameworks. All FDPs were subjected to thermal cycling (TC) and 600,000 cycles of mechanical loading (ML) with 50 N. The load to fracture was measured and fracture sites were evaluated.

RESULTS

The mean fracture values ranged from 172 N to 792 N. Fracture-strength values were significantly lower for the i-i retained FDPs than for the i-c and c-c combinations. There was no significant effect of the reinforcing shoulder in groups i-c-R and c-c-R. For FDPs with a crown on the terminal tooth, fractures were usually within the distal wall of the distal crown.

CONCLUSIONS

Inlay-inlay retained cantilever FDPs cannot withstand the mastication forces expected. Fracture load values for inlay-crown and crown-crown-retained FDPs encourage further clinical investigation. The mode of fracture indicates that reinforcement of the distal crown wall might enhance fracture resistance.

A comparison of 2D and 3D finite element analysis of a restored tooth

The finite element method is widely used in dental research. The decision to use two-dimensional (2D) or three-dimensional (3D) modelling is dependent on many interrelated factors. The purpose of the present study was to compare and contrast 2D and 3D finite element analysis (FEA) in investigating the mechanical behaviour of a maxillary premolar restored with a full crown under similar conditions of axial and lateral occlusal loading. The 2D analysis required modelling both a buccolingual and mesiodistal section of the restored premolar and for comparison sections of a 3D model were examined. Differences in the results for displacement and maximum principal stress distribution within the component structures and interfaces of the 2D and 3D models were, in general, attributable to differences in geometry represented in the models. Maximum principal stresses tended to be greater under lateral rather than axial occlusal loading. It was concluded that 2D FEA may find application in investigating key aspects of the mechanical behaviour of a dental restoration in a single tooth unit, but that in certain situations combinations of 2D and 3D FEA may offer the best understanding of the biomechanical behaviour of complex dental structures. Sophisticated FE models are required to better understand the mechanical behaviour of restored tooth units.

Finite element analysis of fixed partial denture replacement

The purpose of this study was to investigate, by means of the finite element method the mechanical behaviour of three designs of fixed partial denture (FPD) for the replacement of the maxillary first premolar in shortened dental arch therapy. Two-dimensional, linear, static finite element analyses were carried out to investigate the biomechanics of the FPDs and their supporting structures under different scenarios of occlusal loading. Displacement and stress distribution for each design of FPD were examined, with particular attention being paid to the stress variations along the retainer-abutment--and the periodontal ligament-bone interfaces. The results indicated that displacement and maximum principal stresses in the fixed-fixed, three-unit FPD were substantially less than those in the two-unit cantilever FPDs. Of the two cantilever FPDs investigated, the distal cantilever design was found to suffer less displacement and stresses than the mesial cantilever design under similar conditions of loading. The highest values for maximum principal stress in the cantilever FPDs were found within the connector between the pontic and the retainer, and within the periodontal ligament and adjacent bone on the aspect of the retainer away from the pontic.