Variaci ´on de Posici ´on vs Tiempo

Hardness and strength testing is carried out across a range of filler/resin mixtures while particularly in hardness testing, it is often the resin surface layer which is being tested in each case (Fraunhofer and Curtis 1989); in strength testing it is probably the resin-filler interface which is being examined. The importance of a particular component in the composite material may depend on the emphasis o f the report. It is a common occurrence for workers to concentrate on a single parameter rather than on comparing a range of properties. In a study, such as on the effect of filler content on the strength characteristics of a hybrid composite (Johnson et al. 1993), the dental practitioner may be overwhelmed by the arguments on the quality o f the composite resin product where the high filler fraction is used as an analogue for high mechanical properties (Braem et al. 1989). In other studies where composite resins are being examined, the improving mechanical properties appear to be predominantly the result of an increase in degree of cure (Ferracane and Condon 1992), or the formulation of the organic part (Asmussen and Peutzfeldt 1990). Some care is required therefore as many comparisons are made of strength characteristics between variable resin/filler combinations. There are positive relationships between degree of conversion and hardness (Ferracane 1985), and between volume fraction of filler and strength (Wilson et al. 1980). Little in the literature has attempted to correlate degree o f conversion, filler concentration and the mechanical properties in highly filled dental composites (Chung 1990). It seems possible that it is the combination of the

filler concentration, the nature of the bonding between the filler particles and the matrix which plays an important role in determining some of the properties of the dental composite resins (Wu and McKinney 1982; Soderholm 1985).

4.S.4.2 Testing environment

Laboratory evaluations (investigations) of the physical parameters are uninhibited of many of the variable and operational difficulties which exert such a potent influence on the outcome of studies undertaken clinically (Jacobsen 1988). They can result therefore, in findings which may fail to reveal how the material tested may behave and perform in clinical service. However, because of the precision, objectivity, universal acceptance and widespread reporting of laboratory testing, they correctly form the basis for most standards in dental materials (Wilson 1990). The relationship between physical tests, long-term clinical behaviour and performance of materials remains largely obscure (Int. Ass. for Dental Research 1988). To date no single physical test or series of tests is able to reliably predict the typically complex long-term clinical behaviour and performance of a generic group of materials of similar composition and construction, let alone expectations for specific materials in long te r m ^ service (Wilson 1990).

Despite these limitations, physical parameters of materials from laboratory studies, in terms of reliably predicting the extent to which a material will succeed clinically, will continue to be invaluable for assessing standards, comparative purposes, and importantly contributing to knowledge and understanding of materials' behaviour.

The continuing problem with these laboratory investigations is in identifying the

in vitro tests that will facilitate accurate predictions of the behaviour and

performance of new and experimental materials in clinical practice^ 4.5.4.S Hardness and strength testing regimens.

The object of most studies is to reproduce some or as many clinical conditions as possible. Although some studies have tested composite materials immediately after manufacture (Fraunhofer 1971; Ferracane 1985), most studies appear to test samples after short periods of storage/conditioning. This may be after 24 hours to 1 week dry (Yearn 1985; Hansen and Asmussen 1992B; Johnson et al. 1993) or after being immersed in distilled water (Zidan et al. 1980; Fraunhofer and Curtis 1989; Ban and Anusavice 1990; Kildal and Ruyter 1997) at 37°C. However, it is perhaps important

to relate studies more directly to some of the known properties of these composite materials.

It has been demonstrated that polymerisation continues in Bis-GMA and UDMA resins after the original activation, with increasing hardness values (Watkins 1975; Fraunhofer 1971; Young et al. 1978; Watts et al. 1986). However, the experimental testing periods in some studies may not be helpful in view of the other physical properties of these materials. This is illustrated by some studies on microfilled resins where it has been suggested that polymerisation is virtually complete after 24 hours (Swartz et al. 1982). Longer term studies suggest that polymeric materials continue to increase in hardness long after their initial setting, 1 day to 1 week (Fan 1985; Watts et al. 1986). Further, increased hardness with a more rapid and complete cure is observed in the dry state at elevated temperatures of 37°C compared to 20°C and has been described by Wu (1983). The reaction rate depends upon the segmental mobility in the resin phase, which is thermally activated at increased temperatures (Watts et al. 1986). Whilst some have suggested that the correlation between conversion and temperature variation is not strong (Ferracane 1985), higher temperatures appear to be one of the important factors which affect the degree of conversion (Kildal and Ruyter 1994). Others conclude that the surface of dental composites may be significantly affected by temperature (Hansen 1983; Watts et al.

1986).

For a period after the initial cure therefore, the observed increase of the surface micro hardness with time means that an appreciable concentration of free radicals persists in the system after cessation of light irradiation. Evidently free radicals persist in some cases for up to 1 month (Watts et al. 1986). The variation in speed of the post irradiation cure may be due to temperature but also appears to be due to differences in the measurement method. Watts et al. (1986) used a Knoop hardness measurement in a study of hybrid posterior composite resins, whilst Hansen (1983), who records more rapid and greater hardness values after 1 hour, studied "after polymerisation" of microfme composite resins using a Wallace hardness instrument. It may also be due to a possible reduction in segmental mobility in the resin phase of heavily filled hybrid composites compared to the previous studies on microfilled systems (Watts et al. 1986). Again, because the values for hardness are arrived at

differently, comparisons are not directly possible in terms of the extent of cure achieved by the end of the light irradiation period

Since the materials in this study are to be used in posterior teeth, load bearing areas, it is of interest to examine the mechanical properties after immersion in water and for periods of time after water uptake has reached an equilibrium state (Braden and Davy 1985). For comparison purposes the materials should be tested dry and then over a period of 3-6 months immersion in distilled water at 37+/-l°C as demonstrated by Osyaed and Ruyter (1986), and since composite materials are affected by water sorption and the contact time with aqueous media (Hansen 1983; Watts et al. 1986).

Whilst water is a poor solvent for dental composites, it is expected to have a softening effect on the composites by penetration o f the matrix, followed by a leaching out of unreacted monomer, degradation and leaching of filler components (Wu 1882; Soderholm 1983; Soderholm et al. 1984; Osyaed and Ruyter 1986). Over longer periods of immersion it is possible that the silane coupling agent may break down (Soderholm 1981). It has also been found that post cure heat treatment reduces the effect of water and improves their resistance to softening in biological fluids and O.IN NaOH solution. There is a decrease in surface hardness recorded following immersion in water for normally cured and post cure heat treated composites (Mante et al. 1993).

The strength characteristics of composites are affected by a variety of factors, some of which have been described, but a major influence will be determined by the filler and resin components and the nature of the bond between them (Wu and McKinney 1982). While it is difficult to determine which factor will be most critical for a given property (Chung 1990), it is important to demonstrate how some changes to the resin and filler will effect the properties of the composite.

Generally, the surface of samples stored in water are softer compared to dry samples (Osyaed and Ruyter 1986), and since hardness is a surface property, the effects of water sorption should affect the material surface much more rapidly than the bulk (Watts et al. 1986). The mechanical properties are likely to be affected over longer time periods as water uptake does not attain an equilibrium level within the composite for as many as 30 days (Fan et al. 1985). It appears that longer term studies are important in the evaluation of these materials since preliminary work in a

number of reports questions the maintenance of the properties o f the post heat treated materials, after ageing in water over longer periods (De Gee et al. 1990; Peutzfeldt and Asmussen 1991 A; Ferracane and Condon 1992).

CHAPTER 5 : CAVITY DESIGN

Cavity design for composite resin inlays with particular reference to their use in posterior teeth

Previous studies dedicated to examining the cement lute thickness of resin inlay systems, where the overall accuracy of fit is important, often use mesio-occlusal- distal preparations where no particular dimensions are given other than the detail that “MOD cavities of standardised, non-retentive design were prepared” (Ariyaratnam et al. 1990). Frequently, when cavity dimensions are mentioned, no comment is made about the taper (Qualtrough et al. 1993).

It is quite commonly reported that there is competition between the polymerisation contraction forces, and the restraining forces within the dental cavity (Watts and Hindi 1999), while others have stated that the degree of stress development can be controlled to some extent by the cavity design (Carvalho et al.

1996) or alternatively the ‘C' factor (Feilzer et al. 1988). However, unless some assessment is made of the cavity designs currently used, and their ideas incorporated into a study on the accuracy of fit of these restorations, any reasonable interpretation of how closely an inlay fits into its prepared cavity is of limited value.