Stress generation in mandibular anterior teeth restored with different types of post-and-core at various levels of ferrule.

STATEMENT OF PROBLEM: Pertinent evidence regarding the mechanical integrity of mandibular anterior teeth restored with a post-and-core is limited. PURPOSE: The purpose of this finite element analysis study was to compare the impact of the post type (glass fiber post-and-resin core or cast post-and-core) along with the ferrule effect on the stress fields generated in endodontically treated mandibular lateral incisors and canines. MATERIAL AND METHODS: Three-dimensional models of the segmented mandible were developed. Mandibular incisors and canines with or without a 2-mm circular ferrule and restored with a cast post-and-core or glass fiber post-and-resin core were simulated and subjected to linear elastic static analysis. The principal stress values were calculated. von Mises equivalent stresses were used to evaluate the stress. RESULTS: Maximum principal stresses in dentin were highest in incisors, with a ferrule. Stress parameters in composite resin core in both incisors and canines were critically close to the tensile failure limit of the core material. Cast post-and-cores cemented in incisors without a ferrule accumulated the highest stresses, exceeding the tensile failure limit of resin-modified glass ionomer cement. CONCLUSIONS: Tooth preparation with a ferrule led to a remarkable rise in stress in the dentin of mandibular incisors but favored the mechanical integrity of the restoration.

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 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).

Effect of severely reduced bone support on the stress field developed within the connectors of three types of cross-arch fixed partial dentures.

STATEMENT OF PROBLEM: Increased technical failure has been reported for cross-arch fixed partial dentures (FPDs), particularly for cantilever units with minimum osseous support. PURPOSE: The purpose of this study was to investigate the effect of severely reduced bone support on the stress field developed within the connectors for 3 types of cross-arch FPDs. MATERIAL AND METHODS: Six digital parametric models were developed, derived from a 3-dimensional (3-D) initial model. The latter simulated a human mandible, dentate bilaterally to second premolars, with normal height of alveolar bone. In the initial model, the teeth were prepared and splinted with a cross-arch FPD bilaterally with one of the following: no cantilever, 1-unit cantilever, or 2-unit cantilever. All structures were obtained from computed tomography (CT)-scan images or constructed in 3-D computer-aided design (CAD) and reverse engineering environments. A 3-D finite element analysis (3-D FEA) was performed. RESULTS: The reduction of the alveolar bone in the no-cantilever FPD model caused a greater increase of stress in the region of connectors among the splinted teeth in comparison to the 1- and 2-unit cantilever FPD model. The stress state within the connectors of the cantilever segment remained constant. The connectors proximal to the retaining abutment demonstrated the highest stress values, independent of the osseous support. The stress values in the region of the same connectors in the 2-unit cantilever restoration were almost double, compared to the 1-unit cantilever model. Decreasing the osseous level in the 1- and 2-unit cantilever FPD models caused milder stress distribution and magnitude in the region of connectors among the splinted teeth. CONCLUSIONS: The stress field developed within the connectors of the cantilever segment is independent of the osseous level. The stress field within the connectors among splinted teeth depends on the osseous support and both the presence and length of the cantilever segment. Furthermore, this study indicates that adding a cantilever segment has a positive effect on the stress behavior of the splinted teeth connectors in situations of reduced osseous support.

Gellan gum-hydroxyapatite composite spongy-like hydrogels for bone tissue engineering.

Osteoinductive biomaterials represent a promising approach to advance bone grafting. Despite promising, the combination of sustained biodegradability, mechanical strength, and biocompatibility in a unique biomaterial that can also support cell performance and bone formation in vivo is demanding. Herein, we developed gellan gum (GG)-hydroxyapatite (HAp) spongy-like hydrogels to mimic the organic (GG) and inorganic (HAp) phases of the bone. HAp was successfully introduced within the GG polymeric networks, as determined by FTIR and XRD, without compromising the thermostability of the biomaterials, as showed by TGA. The developed biomaterials showed sustained degradation, high swelling, pore sizes between 200 and 300 µm, high porosity (>90%) and interconnectivity (<60%) that was inversely proportional to the total polymeric amount and to CaCl2 crosslinker. CaCl2 and HAp reinforced the mechanical properties of the biomaterials from a storage modulus of 40 KPa to 70-80 KPa. This study also showed that HAp and CaCl2 favored the bioactivity and that cells were able to adhere and spread within the biomaterials up to 21 days of culture. Overall, the possibility to tailor spongy-like hydrogels properties by including calcium as a crosslinker and by varying the amount of HAp will further contribute to understand how these features influence bone cells performance in vitro and bone formation in vivo. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 479-490, 2018.

The effects of implant length and diameter prior to and after osseointegration: a 2-D finite element analysis.

A two-dimensional finite element analysis was used to evaluate the effects of implant length and diameter on the stress distribution of a single-implant supported crown and the strain distribution of its surrounding bone prior to and after the phase of osseointegration. The effect of length was investigated using implants with a diameter of 3.75 mm and lengths of 8 mm, 10 mm, 12 mm, and 14 mm. The effect of diameter was investigated using implants with a length of 10 mm and diameters of 3 mm, 3.75 mm, 4.5 mm, and 5mm. The phase prior to osseointegration was simulated by assuming a coefficient of friction for the interface between the implant and the surrounding bone, while the phase after osseointegration was simulated by assuming a fixed bond on the interface between the implant and the surrounding bone. The FEA results indicated a tendency towards stress reduction on the implant, both prior to and after osseointegration, when the length was increased. However, the calculated stresses on the implant were lower after the phase of osseointegration. Although no specific correlation could be seen regarding the influence of implant diameter, the calculated stresses on the implant were again lower after the phase of osseointegration. For all these cases, the maximum stress concentration occurred at the abutment-implant interface. As far as bone tissue was concerned, there was a tendency towards strain reduction, before and after osseointegration, when the length of the implant was increased from 10 mm up to 14 mm. This tendency was not manifested for the range of 8 to 10 mm. The effect of implant diameter on bone tissue was not clear. It appears that implants of a diameter more than 5 mm are not preferable for immediate loading. Finally, it seems that cortical bone is not influenced by the phase of osseointegration, while trabecular bone is highly affected.

An experimental bioactive dental ceramic for metal-ceramic restorations: Textural characteristics and investigation of the mechanical properties.

The aim of this study was the evaluation of the textural characteristics of an experimental sol-gel derived feldspathic dental ceramic, which has already been proven bioactive and the investigation of its flexural strength through Weibull Statistical Analysis. The null hypothesis was that the flexural strength of the experimental and the commercial dental ceramic would be of the same order, resulting in a dental ceramic with apatite forming ability and adequate mechanical integrity. Although the flexural strength of the experimental ceramics was not statistically significant different compared to the commercial one, the amount of blind pores due to processing was greater. The textural characteristics of the experimental ceramic were in accordance with the standard low porosity levels reported for dental ceramics used for fixed prosthetic restorations. Feldspathic dental ceramics with typical textural characteristics and advanced mechanical properties as well as enhanced apatite forming ability can be synthesized through the sol-gel method.

Efficiency of different decalcification protocols for nasal osseous structures in a rat experimental model of allergic rhinitis, and their effects on epithelial histology: an attempt at standardization.

INTRODUCTION: Decalcification of osseous specimens is required for histological analysis; this however may cause tissue damage. In rodent models of allergic rhinitis (AR), epithelial histologic assessment necessitates prior decalcification of the nasal osseous structures. However, respiratory epithelium is highly susceptible to damage, and rat nasal architecture is elaborate and its sectioning is challenging. Nevertheless, decalcification is not standardized in experimental AR. We therefore undertook this task, in order to reduce experimental bias. METHODS: Six-to-eight week-old Wistar rats underwent an AR protocol. Subsequently, nasal structures were decalcified in the following mediums: (i) formic acid 10% for 5 and 20 days; (ii) formic acid 15% for 5 and 15 days; (iii) Morse Solution for 5 and 20 days and (iv) EDTA for 20 and 40 days. Decalcification efficiency/speed was evaluated via radiographic analysis. Furthermore, specimens were stained with hematoxylin and eosin and assessed for preservation of epithelial features. RESULTS: Specimens were appropriately decalcified in 5 days in the formic acid-based mediums and in 20 days in EDTA with minimal epithelial damage. EDTA for 40 days had no unacceptable adverse effects; conversely, 15 and/or 20 days in acid-based agents provided no extra benefit for decalcification and were detrimental to the epithelium. CONCLUSIONS: EDTA treatment for 20 days is appropriate for decalcification of nasal structures in rat models of allergic rhinitis; further incubation preserves epithelial integrity but is not required. When urgency is a factor, formic-acid-based decalcification for 5 days yields acceptable results.

Observations on an in-vivo failure of a titanium dental implant/abutment screw system: a case report.

Titanium and its alloys are widely used in prosthetic dentistry, due to their biocompatibility, excellent mechanical and anti-corrosion behavior. However, delayed fracture of dental prosthetics is frequently encountered. Mechanisms leading to fracture are not generic but are strongly related to the particular environmental (quality of biological fluids) and mechanical loading conditions (mastication habits, presence of prosthetic metallic/ceramic components) in the patients' oral cavity. In this study, a commercially pure titanium implant-screw system has failed after 15 years of operation in the oral cavity of an old female. The system was retrieved in three pieces: the upper part of the implant, part of the abutment screw, and the apical part of the implant to which a part of the screw was embedded. This is considered as a rare case, where the whole dental assembly was retrieved after fracture allowing the extensive fractographic analysis of the conjugate pieces and the establishment of a thorough in-vivo failure scenario. Scanning electron microscopy observations performed on all three retrieved parts indicated a synergistic effect of distinct mechanisms, which led to total failure under extrinsic common fatigue loading. The principal mechanism was the propagation of a main crack, which was previously initiated in the body of the implant and affected by a wedging mechanism due to Ca/P aggregates developed within the crack. Because of the strong fixation between the implant and the abutment screw, this main crack was transferred to the latter causing eventually total failure of the assembly.