To analyze the influence of cavity geometry and lining materials in MOD composite restorations by characterizing the polymerization shrinkage and cusp deflection.
Eighty intact molars with similar sizes were collected and randomly divided into eight groups. MOD cavities with various widths and depths were prepared on these teeth: Group I, 2 ( W ) × 2 ( D ) mm; Group II, 4 ( W ) × 2 ( D ) mm; Groups III, IIIf, IIIg, 2 ( W ) × 4 ( D ) mm; and Groups IV, IVf, IVg, 4 ( W ) × 4 ( D ) mm. In Groups IIIf and IVf, flowable composite liner was placed prior to composite restoration, while glass-ionomer liner was used in Groups IIIg and IVg. Deformations of restorations resulted from composite shrinkage were recorded for 30 min following light irradiation using a digital-image-correlation (DIC) method to subpixel level. The displacements at the boundaries of the restorations were analyzed using one-way ANOVA and the post hoc test at a 5% significance level. The correlation between the geometric factors and the displacements was also analyzed.
The inward displacements on free surfaces were greater than those on the bonded surfaces. Groups with flowable composite linings showed greater amount of displacements on free and bonded surfaces compared to the unlined and glass ionomer lining groups. The correlation analysis showed that the free surface shrinkage was related with the cavity width and C-factor, while cusp deflections were correlated with the cavity depth and the cusp compliance.
The DIC technique measures composite shrinkage on different boundaries of restorations to facilitate the investigation of polymerization kinetics. Using flowable composite lining and increased cusp depth may aggravate the cusp flexure.
Although resin-based materials have been widely used in contemporary restorative dentistry, the polymerization shrinkage is still a great drawback of light-cured dental composites. The shrinkage of the resin-based materials depends on their plastic flow prior to gel point, and the post-gel contraction and stiffness . Current composites display 2–5% volumetric shrinkage , depending on the composition of composite materials and light curing protocols. The advanced adhesion technique establishes promising resin–tooth bonding, but the polymerization shrinkage associating with the intimate bonding and the confinement of the surrounding cavity arise the contraction stresses. The destructive effects of transient and residual contraction stress may cause post-operative problems such as sensitivity and microleakage, cusp deformation, and bonded enamel microcracks . Fundamental information about the kinetics of contraction stress is valuable to improve the quality and longevity of the composite restorations.
Previous model systems have measured polymerization contraction stress from 2 to 13 MPa . The internal stress state of a composite restoration is related to parameters including the shrinkage rate of the restorative materials, the cavity geometry and boundary condition. The configuration factor (C-factor), proposed by Feilzer et al. , defines boundary conditions as the ratio of bonded to unbonded surfaces of a cavity preparation. Unbonded areas are considered to facilitate the flow and plastic deformation during composite polymerization in the pre-gel stage, thus reducing the final stress values. Related investigations have validated that cavity configuration as an influential factor imparting composite shrinkage kinetics using finite element analysis and mechanical testing . Restorations with a low C-factor are likely to overcome contraction stresses, whereas C-factor exceeding 2.3 caused bond failure . Some researchers claimed that microleakage was related to a restoration’s volume rather than its C-factor . However, recent research found that more geometric factors should be considered in the biomechanics and shrinkage-stress state in restorations of composite-restored teeth except the C-factor. Hood proposed that the cusps after cavity preparation behave as cantilever beams to bend under occlusal loads. A similar concept was applied in the investigation by Lee et al. to predict the cusp deflection using the remaining cusp geometry (the ratio of the length cubed to the thickness cubed of the remaining cusp). An inverse relationship was established between the flexure strain and cusp stiffness.
The “sandwich technique”, proposed by McLean and Wilson, uses the low modulus lining material beneath the resin composite restoration . Up to date, materials including resinous liners, conventional glass ionomer cements (GICs) and resin-modified glass-ionomer (RMGI) cements have been utilized as liners to provide the stress-buffering capacity for reducing the contraction stress and improving marginal sealing . Alomari et al. reported that the low elastic-modulus liners, either GIC or flowable composite, may decrease the amount of cusp deformation. Although these investigations proved the reduced marginal leakage or cusp deformation in teeth with liners, their results cannot be directly contributed to the strain capacity of liners. The finite element analysis by Ausiello et al. demonstrated that a compliant adhesive layer partially absorbed the composite deformation and limited the intensity of the contraction stress. Contrarily, Hofmann et al. found that composite resin increased cuspal stability of endodontically treated teeth, whereas the stiffen effect by conventional GIC bases was very limited . Figueiredo Reis et al. found that the interfacial bond strength was not improved using a low-viscosity composite liner. From these reports, the beneficial effect of using sandwich technique to absorb polymerization shrinkage stress is controversial.
Previous investigations measured polymerization shrinkage utilizing devices including the dilatometry, strain gage, or infrared linometer . Using the “bonded disk” method, the shrinkage kinetics of the composite materials can be characterized along the light-irradiation process . These experimental approaches precisely measured the free strain and defined the polymerization kinetic. However, the measurement of the shrinkage or contraction stress in a meaningful context such as a real cavity is clinically important. A previous investigation has confirmed that shrinkage strains changed with the boundary conditions . The constraints in a dental preparation differ from those of a metal or Teflon mold due to the varying adherence and compliance of bonded substrates. Recently, electronic speckle pattern interferometry was applied to assess the influence of restorative materials on tooth deformation . The interferometry described a comprehensive 3D nature of the cuspal deformations, while polymerization shrinkage data in bonded restoration was found to be determined by a function of free shrinkage and modulus of composite materials. However, this technique requires expensive instruments and the processing of fringe patterns is laborious.
Digital image correlation (DIC, or texture correlation), first developed in 1980s , is a non-contacting optical technique for full-field strain measurement in planar and three dimensional displacement analyses. Displacements and displacement gradients (strains) of a specific object can be measured by applying microscopic measurements under a digital vision and computation algorithm. This methodology has been applied to assess the mechanical properties of solid materials in the engineering field using a simple system without the laborious interpretation of interferometric fringes. Recently, DIC was successfully applied to the full-field shrinkage strain of dental composites . In these investigations, DIC facilitated non-contact measuring and yielded promising results of material’s dimensional changes. Our previous work measured the composite shrinkage within a simulated cavity in a metal model and human teeth using the DIC method . These experimental results did not only demonstrate the spatial and temporal relationship of displacement in a dental restoration, but also provide a validation of computational models to examine the polymerization consequence.
Although numerous studies have investigated the composite polymerization kinetics via various approaches, the assessments of shrinkage pattern in real tooth preparations is still lacking. The objective of the current study was to establish a new DIC technique measuring the polymerization shrinkage and deformation of a bonded composite restoration, and to determine the effects of surrounding constrain and the liner application on the resultant shrinkage.
Materials and methods
Rationale of DIC method
DIC method analyzes an object’s displacement based on the comparison of two similar speckled images. Using two characterized images acquired at different states, one prior to deformation (reference image) and one after deformation (deformed image), the displacement of a regular grid of points can be defined using a complex algorithm . The experimental method first records the reference and deformed images using a CCD camera through an optical microscope, then the images are stored in a computer for analyses. The object is characterized by a set of neighboring points by means of different gray levels of light intensity which is assumed to remain adjacent after deformation. In a planar analysis, the algorithm based on the correlation analysis or other statistical functions detects the subtle differences of the objects position and finds out the local displacements between two images . In a system measuring in-plane displacements u and <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='v’>vv
, an object O in the reference image must be matched with the object O ′ in the corresponding deformed image using a correlation operation ( Fig. 1 ). If the displacement is sufficiently small, the coordinates of Point Q 2 in subimage O ′ can be approximated by first-order displacement gradient denoted as:
x q 2 = x p 1 + u + 1 + ∂ u ∂ x Δ x + ∂ u ∂ y Δ y ,
y q 2 = y p 1 + v + ∂ v ∂ x Δ x + 1 + ∂ v ∂ y Δ y .
where ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-4-Frame class=MathJax style="POSITION: relative" data-mathml='xp1,yp1′>xp1,yp1xp1,yp1
x p 1 , y p 1
) is the coordinate of Point P 1 , ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-5-Frame class=MathJax style="POSITION: relative" data-mathml='xq2,yq2′>xq2,yq2xq2,yq2
x q 2 , y q 2
) is the coordinate of Point Q 2 . (Δ x , Δ y ) are the position differences between the reference point and the nearby point before deformation. ( u , <SPAN role=presentation tabIndex=0 id=MathJax-Element-6-Frame class=MathJax style="POSITION: relative" data-mathml='v’>vv
) represent the real displacement, while the other items are the displacement derivatives of Point P 1 which represent the deformation.