Advances in biomaterials from 1957 to 1997

Functional dendritic compounds: potential prospective candidates for dental restorative materials and in situ re-mineralization of human tooth enamel

This review provides a snapshot of recent progress in the synthesis and application of dendritic compounds as potential prospective candidates for dental restorative materials andin siture-mineralization of human tooth enamel.

Resin composite blocks via high-pressure high-temperature polymerization

OBJECTIVES

The aim of this study was to thermo-polymerize under high pressure four commercially available dental resin composites to obtain and characterize composite blocks suitable for CAD/CAM procedures.

METHODS

Gradia (GC, Japan), Vita VM LC (Vita Zahnfabrik, Germany), Grandio (VOCO, Germany), and EsthetX (Dentsply, Germany), were selected for this study. Paradigm (3 M ESPE, USA), a CAD/CAM composite block, was included for comparison. Composite blocks were obtained through polymerization at high-temperature high-pressure (HT/HP). Samples for mechanical/physical characterizations were cut from Paradigm and HT/HP composite blocks while control samples were obtained by photo-polymerizing (PP) the materials in molds. Flexural strength (σ(f)), fracture toughness (K(IC)), hardness, and density (ρ) were determined and compared by pairwise t-tests (α=0.05). Fractured surfaces were characterized under a scanning electron microscope.

RESULTS

The results have shown that HT/HP polymerization resulted in a significant (p<0.05) increase in σ(f), hardness, and ρ for all composites investigated. Even if K(IC) of all materials was increased by HT/HP polymerization, significant increases were detected only for Gradia and EsthetX. The Weibull modulus of HT/HP polymerized composites was higher than that of PP counterparts. HT/HP materials had higher σ(f), Weibull modulus, and K(IC) compared to Paradigm. The most significant SEM observation of fractured K(IC) specimens from all the materials tested was the presence of fewer and smaller voids in HT/HP polymerized composites.

SIGNIFICANCE

The results of this study suggest that HT/HP polymerization could be used to obtain dental resin composite blocks with superior mechanical properties, suitable for CAD/CAM processing.

Micro-indentation fracture behavior of human enamel

OBJECTIVE

The purpose of this study was to determine the crack resistance behavior (K(R)) of human enamel in relation to its microstructure.

METHODS

Human molar teeth were precision cut, polished and tested using Vickers micro-indentation at different loads ranging from 0.98 to 9.8 N. Five indentation load levels were considered, 20 indentation cracks for each load level were introduced on the surface of the test specimen (10 indentations per tooth) and their variability was evaluated using Weibull statistics and an empirical model. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to analyze the crack morphology and propagation mechanisms involved.

RESULTS

The results showed that enamel exhibited increasing cracking resistance (K(R)) with increasing load. It was found that the crack propagation mainly depended on the location and the microstructure it encountered. SEM showed the formation of crack bridges and crack deflection near the indentation crack tip. The crack mode was of Palmqvist type even at larger loads of 9.8 N. This was mainly attributed to the large process zone created by the interwoven lamellar rod like microstructure exhibited by the enamel surface.

SIGNIFICANCE

This study shows that there are still considerable prospects for improving dental ceramics and for mimicking the enamel structure developed by nature.

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 toughness of human dentin

The purpose of this study was to determine the fracture toughness (K(IC)) of human dentin and to test the null hypothesis that K(IC) is not affected by the orientation of dentinal tubules relative to the plane of crack propagation. Triangular prisms (4 x 4 x 4 x 8 mm) were obtained from human molars and tested using the notchless triangular prism (NTP) specimen K(IC) test. Dentin prisms were prepared so that the plane of crack propagation would have three different orientations relative to the orientation of dentinal tubules: perpendicular (PE), parallel aligned (PAA), and parallel transverse (PAT). The prepared specimens were secured in the specimen holder and loaded in tension until fracture or crack arrest. The maximum load recorded was used to calculate K(IC). There was no significant difference between the K(IC) of PAA specimens (1.97 +/- 0.17 MPa/m(1/2)) and PAT (2.02 +/- 0.18 MPa/m(1/2)). The K(IC) for the PE specimens (1.13 +/- 0.36 MPa/m(1/2)) was significantly lower. The SEM images of the fractured surfaces showed distinct differences that were correlated with the determined K(IC) values. The fractured surfaces of PAA and PAT specimens were rougher compared to PE specimens. Both the hyper mineralized peritubular dentin and the orientation of collagen fibrils surrounding the tubules could be responsible for the significant differences in K(IC). The results of this study identified a significant anisotropy of dentin with respect to its K(IC).