Volume contraction in photocured dental resins: The shrinkage-conversion relationship revisited
Introduction
Dental resin composites comprise a blend of hard, inorganic particles bound together by a soft, resin matrix. During polymerization, the conversion of the monomer molecules of the matrix into a polymer network is accompanied by a closer packing of the molecules, which leads to bulk contraction [1], [2], [3].
To enhance the marginal integrity of composite resin restorations, bonding agents are used to withstand the polymerization contraction forces. The insertion of adhesive resin composites into cavity preparations leads to a competition between contraction forces and the strength of bonds to tooth structure [4], [5], [6]. This competition may lead to three different kinds of problems. Cohesive fractures due to the internal stresses can occur either in the tooth structure, or in the composite [7], [8], [9]. The resulting internal microcracks or the debonding at the filler particle/resin interface may accelerate damaging reactions such as wear [10]. A third possibility is the occurrence of adhesive fractures due to stress at the tooth-restoration interface that may lead to marginal gap formation, marginal discoloration, post-operative sensitivity, and secondary caries [5], [6].
Viscous flow and polymerization shrinkage [5], [6], [11], [12], [13], [14], [15], [16], [17] of the resin composites are both considered as significant determinants of gap formation. As a result of the time-dependent character of visco-elastic behavior, slower rates of polymerization reaction have been shown to be associated with lower contraction stress without compromising the final conversion [18], [19].
The clinical consequences of polymerization shrinkage, and mainly imperfections in marginal adaptation and the appearance of recurrent caries, constitute the main reasons for premature replacement of resin composite restorations [11], [20], [21], [22], [23]. This explains why it is regarded as the main limitation of present-day resin composites and why its elimination or minimization is one of the most important research tasks in this field.
During the polymerization of methacrylate-based resins, the viscous liquid gradually transforms into a rigid material by radical polymerization involving the double bonds C=C of methacrylate groups. The extent of transformation of double to simple bonds (monomers in polymer) is called ‘degree of conversion’. This polymerization involves a volume shrinkage which has three origins: a ‘chemical’ contraction (the most important), a ‘thermal’ contraction and a ‘post-contraction’.
The chemical contraction is attributed to a change in inter-atomic spacing between molecules. Before polymerization, monomer molecules are about 4 Angströms apart and linked by secondary cohesion forces, the so-called van der Waals forces. During polymerization, the latter are replaced by single covalent bonds about 1,5 Angströms length [24], [25].
Thermal contraction occurs during the cooling as the curing reaction is exothermic and overheats the resin, which contracts when returning to room temperature. This contraction is less important but it can create internal stresses.
During chemical reaction, the vitrification of the system induces a ‘freezing’ of the radicals in the cross-linked structure, stopping further chemical reaction. A so-called ‘post-contraction’ occurs up to about 24 h after polymerization [26].
Depending on the materials, the magnitude of the total volumetric ‘free’ curing contraction, mentioned in the literature, varies from more or less 5.3 vol % (pure Bis-GMA) [27] to 12.0 vol % (Bis-GMA/TEGDMA 20/80 w/w) for unfilled resins [3]. As polymerization shrinkage arises mainly from the chemical reaction itself, palliative solutions are to be searched for. This study focuses only on the chemical reaction and its related contraction in neat resins.
Shrinkage and conversion are closely related manifestations of the same process. In many studies, the rate of contraction is related to the degree of conversion. The general trend is that the volumetric shrinkage increases with an increasing degree of conversion [3], [11], [18], [19], [28], [29], [30].
It is a common belief that, when resin composites are cured, a high degree of conversion is to be aimed for. Indeed, several studies have shown that, for a given monomer composition, a significant correlation is observed between the degree of conversion and nearly every physical and biological property of the polymer. In Bis-GMA based dental restorative resins, residual methacrylate groups appear clearly linked to the reduction of hardness [31], [32], [33], [34], wear resistance [35], strength [31], [34], color stability, fracture toughness and resistance to abrasive wear [35]. The degradative reactions are also responsible of formation and release of by-products that could be sufficient to induce allergic reactions or may affect the compatibility of the resin with oral tissues [25], [36].
The ideal composite would exhibit an optimal degree of conversion and minimal polymerization shrinkage. These seem to be antagonistic goals, as increased monomer conversion invariably leads to large polymerization shrinkage values. But both parameters are key ones for optimizing resin composite restoration.
However, instead to the degree of conversion (in %) the volume contraction has to be directly linked to the actual decrease of vinyl bond concentration (in mole/cm3).
A linear correlation, between volume contraction and mole of converted double bonds, was first proposed in 1953 by Loshaeck [1] and recent literature [29] still refers to this early paper. In Loshaeck's study, it was determined that for every mole of C=C being converted in the C–C, there was an associated volume shrinkage of 23.0 cm3/mole. In 1987, the results of a study conducted by Patel on a series of linear polymethacrylates [2] confirmed that the change in molar volume due to polymerization is reasonably constant at about 22.5 cm3/mole. Moreover, this study, like others [29], uses polymerization shrinkage to determine the degree of conversion of a few bifunctional cured resins as used in dentistry. But, in the latter case, the correlation between volume shrinkage and degree of conversion has not been checked.
In the present study, the authors revisit and extend these results and attempt to determine more accurately the volume contraction of a series of cured dental resins and to link it to the number of actual vinyl double bonds converted using recent characterization techniques and the precision they offer.
Section snippets
Preparation of the different resins:
The monomers most often used in the resinous matrix of present-day dental resin composites are Bis-GMA and TEGDMA, (the latter used as diluent). To determine the volume contraction associated with the number of double bonds converted, a range of Bis-GMA/ TEGDMA mixtures were analyzed. Eight different mixtures were prepared from Bis-GMA/TEGDMA consisting of 0, 20, 30, 40, 50, 60, 70 and 80% weight TEGDMA.
A photoinitiation system was added to each mixture in the proportion of 1%. This system
Results
Fig. 1 General trend of the values of the DC measured at 0 h and after 24 h. Student's t-test showed a significant difference (p<0.05) between the values of DC (t=0) and DC (t≥24 h).
Table 1 shows all the data required to calculate the volume contraction with the specific masses of the uncured (dunpolymerized) and the cured (dpolymerized) samples and to calculate the number of moles (M) of C=C polymerized per gram and per cm3 with the initial number of mole of C=C bonds per gram and the conversion
Discussion
The extent of polymerization shrinkage depends, among other things, on the relative mobility, the molecular weight and functionality of the monomers. Comparing monomers of the same functionality (such as Bis-GMA and TEGDMA), polymerization shrinkage increases when initial molecular weight decreases [2], [25]. Because of favorable stereochemistry, long-chain flexible TEGDMA exhibits a relatively high degree of conversion of the methacrylate double bonds. Due to the presence in the cross-linking
Conclusion
Notwithstanding the efforts to develop new (‘no-shrink’) resins, the curing contraction, still remains, so far, inevitable and the practitioner has to ‘live with’ the problem of polymerization shrinkage and destructive shrinkage stress. Despite different clinical procedures to reduce their effects, there is nowadays no straightforward way of handling the adhesive restorative materials, which guarantees a leakproof restoration. As polymerization shrinkage arises mainly from the chemical reaction
References (42)
- et al.
Polymerization shrinkage of methacrylate esters
Biomaterials
(1987) - et al.
Polymerization contraction and conversion of light-curing Bis-GMA-based methacrylate resins
Biomaterials
(1993) - et al.
Adhesive bonding of various materials to hard tooth tissues: forces developing in composite materials during hardening
J Am Dent Assoc
(1983) - et al.
Polymerization shrinkage and polymerization shrinkage stress in polymer-based restoratives
J Dent
(1997) - et al.
Polymerization shrinkage of composite resins: comparison with tooth deformation
J Prosthet Dent
(1994) - et al.
In vitro measurement of cuspal strain and displacement in composite restored teeth
J Dent
(1997) - et al.
Clinical significance of polymerization shrinkage of composite resins
J Prosthet Dent
(1982) - et al.
Curing contraction of composites and glass-ionomer cements
J Prosthet Dent
(1988) - et al.
Polymerization shrinkage and elasticity of flowable composites and filled adhesives
Dent Mater
(1999) - et al.
Modeling of the viscoelastic behavior of dental light-activated resin composites during curing
Dent Mater
(2003)