Flexural strength glass-ionomer

Flexural strength and fracture toughness are clinically more significant than compressive strength. The flexural strength of a glass-ionomer cement can reach 39 MPa after 24 hours (Pearson Atkinson, 1991) which is a much higher value than that attained by any dental silicate cement. 

However, these values are less than those recorded for composite resins used in dentistry. Goldman (1985) reports values of 29 to 49 MPa for anterior composite resins and Lloyd Adamson (1987) values of 76 to 125 MPa for posterior composite resins. A typical amalgam has a flexural strength of 6 MPa (Lloyd Adamson, 1987) (Table 5.16). However, the flexural strengths of some glass-ionomer cements increase with time and values as high as 59 MPa (after 3 months) and 70 MPa (after 7 days) have been reported (Pearson Atkinson, 1991). 

These low values for flexural strength and fracture toughness compared with the values for composite resins and dental amalgams make the glass-ionomer cement less suitable than these materials in high-stress situations. 

Recently, Oldfield Ellis (1991) have examined the reinforcement of glass-ionomer cement with alumina (Safil) and carbon fibres. The introduction of only small amounts of carbon fibres (5% to 7-5% by volume) into cements based on MP4 and G-338 glasses was found to increase considerably both the elastic modulus and flexural strength. There was an increase in work of fracture attributable to fibre pull-out. A modulus as high as 12-5 GPa has been attained with the addition of 12% by voliune of fibre into MP4 glass (Bailey et al, 1991). Results using alumina fibre were less promising as there was no fibre pull-out because of the brittle nature of alumina fibres which fractured under load. 

Very recently, Williams, Billington Pearson (1992) have examined the effect of reinforcement by silver or silver-tin alloy on the mechanical properties of three glass-ionomer cements. Measurements of compressive, flexural, tensile (measured by the diametral compressive procedure) and shell strength are given in Table 5.17. These results show that the effect of reinforcement varies from cement to cement but, in general, increases it. 

Pearson, G. J. Atkinson, A. S. (1991). Long-term flexural strength of glass-ionomer cements. Biomaterials, 12, 658-60. 

Prosser, H. J., Powis, D. R. Wilson, A. D. (1986). Glass-ionomer cements of improved flexural strength. Journal of Dental Research, 65,... 

The relevant International Standard for resin-modified glass-ionomer cements uses flexural strength as the criterion of acceptable mechanical performance [26], and further specifies a minimum flexural strength of 20 MPa for these materials. No distinction is made between values required for liner/base materials and full restoratives, or indeed for materials intended for any other application, such as luting, but most brands of resin-modified glass-ionomer cement comfortable exceed this minimum value [27]. 

Another recently launched material of the glass-ionomer type also contains novel components, in particular a patented diurethane monomer capable of polymerizing to give a set material of superior flexural strength and reduced brittleness. It is described as resin-modified glass-ionomer with the added description of bioactive ionic resin-based composite . 

These materials have been smdied for flexural strength and the results compared to those Irom two conventional glass-ionomer and two resin-modified glass-ionomer cements [31]. Results are shown in Table 8.6. 

Table 8.6 Flexural strength of selected glass-ionomer materials [31]...

C.H. Pameijer, F. Garcia-Godoy, B.R. Morrow, S.J. Jefferies, Flexural strength and flexural fatigue properties of resin-modified glass ionomers, J. Clin. Dent. 26 (2015) 23-27. 

ABCDEFGH I JKLOPQR Figure 2. Flexural Strength (FS) for Glass-Ionomer (Copolymer) Formulations in Study With Error Bars and Standard Deviations Obtained. 

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