178
J. Dent. 1992;
Effects of polymerization composite restorations R. L. Sakaguchi,
contraction
20:
178-l
82
in
M. C. R. B. Peters,* S. R. Nelson, W. H. Douglas and H. W. Poort*
Biomarerials Research Center, Department of Oral Science, *TRIKON, University of Nijmegen, The Netherlands
University
of Minnesota
School of Dentistry
and
ABSTRACT Post-gel polymerization contraction of resin composite induces contraction stresses at the composite-tooth bond and in surrounding tooth structure. Strain gauges have been shown to be an effective method for measuring linear post-gel polymerization contraction of composites. A new model was developed in which the composite sample was bonded to and circumscribed by an acrylic ring. The model simulates a composite restoration surrounded by dentine. A strain gauge measured the deformation of the ring while a second strain gauge simultaneously recorded the dimensional change of the sample. Stresses placed on the acrylic ring as a result of polymerization contraction of the composite were calculated, based on the strains on the ring and the ring’s material properties. Four composites (Heliomolar. Vivadent. Tonawanda. NY. USA: Herculite XR. Kerr Manufacturing Co., Romulus, MI, USA; P-SO. 3M Co., St Paul, MN, USA; Silux Plus 3M Co.) were evaluated for polymerization contraction strain and stress on the surrounding acrylic ring during polymerization. At the end of the 60 s light application, Heliomolar demonstrated significantly lower post-gel contraction (0.12 per cent.P < 0.05) when compared to the other materials. When the strain reached an equilibrium at the end of 14 min Heliomolar continued to demonstrate lower post-gel contraction, however this was not statistically significant at P < 0.05. When the contraction stress on the surrounding acrylic ring was considered. P-50 rapidly developed and produced the largest stress values (1.7 MPa) at the end of the light application while Heliomolar produced the lowest stress values (0.3 MPa). These values. however, were not significantly different when evaluated statistically. The model appears to be a reasonable predictor of the results of polymerization contraction of composite resins. The ability to bond the composite to the acrylic through dentine bonding agents enhances its usefulness. and the simple geometry allows for relatively straightforward calculation of stress-strain relationships. KEY WORDS:
Composites,
J. Dent. 1992; 1991)
20: 178-I
Polymerization,
Stress
82 (Received 3 July 199 1; reviewed 23 September
1991;
Correspondence should be addressed to: Dr R. L. Sakaguchi, School of Dentistry, Science Tower, 515 Delaware Street, SE Minneapolis, MN 55455, USA.
INTRODUCTION Resin composites exhibit the inherent problem of contraction during the polymerization process. The total volumetric contraction can be divided into two components: the pre-gel and post-gel phase. During pregel polymerization, the composite is able to flow which relieves stress within the structure (Davidson and deGee, 1984). However, at the gel point and beyond. the material develops a stiffness reflected in the modulus of elasticity. As the material continues to polymerize, the contraction places stresses within the composite-tooth bond which in turn is distributed into the surrounding tooth structure. After gelation, flow is unable to compensate for contrac@ 1992 Butterworth-Heinemann 0300-5712/92/030178-05
Ltd.
accepted 16 October
Malcolm
Moos Health
tion stresses. Therefore, post-gel polymerization results in clinically significant stresses in the composite-tooth bond and surrounding tooth structure (Davidson et al.. 1984; Feilzer et al.. 1987). Stresses resulting from post-gel polymerization contraction may produce defects in the composite-tooth bond. This leads to bond failure and microleakage with associated postoperative sensitivity (Eick and Welch. 1986). If the composite-tooth bond is able to withstand the deformation, stresses induced by the contracting composite can cause deformation of the surrounding tooth structure (Donley et al., 1987: Morin et al., 1988; Sheth ef al., 1988). The resulting coronal deformation may result in postoperative sensitivity and microcracks in the cervical
Sakaguchi
enamel (Bowen et al.. 1983), which predisposes the tooth to fracture. Careful composite placement can minimize the effects of the polymerization contraction (Lutz et al.. 1986). however coronal deformation is still evident. Stress relaxation through composite hydration is evident in mature composites (Feilzeretal.. 1990) but this occurs over a long period of time, well after post-gel polymerization is complete. Therefore, the tooth/restoration complex is in a pre-stressed state even before occlusal stresses result in further coronal deformation (Sakaguchi et al., 1991). A method for measuring post-gel polymerization contraction in resin composites using electrical resistance strain gauges has been described previously (Sakaguchi and Douglas. 1989: Sakaguchi et al., 1991). The model described in this paper utilizes this technology and extends it to the determination of the important contraction stresses following composite shrinkage. The simple geometry of the model enables the calculation of contraction stress by measuring contraction strain of the surrounding acrylic in real time, during the curing process of the composite and beyond. Stresses derived from the model can be used as an estimate of the effects of polymerization contraction in clinical applications.
MATERIALS
AND METHODS
The test configuration utilized a methylmethacrylate ring (inner diameter 6.5 mm. outer diameter 9.0 mm, height 2.0 mm) which circumscribed the composite sample (Fig I) (Peters et al.. 1991). An electrical resistance strain gauge (CEA-06-032UW-120: Measurements Group, Raleigh, NC, USA, 0.032 in. gauge length) was attached to the outer surface of the ring with cyanoacrylate bonding agents. The inner surface of the ring was treated with chloroform to condition the surface. Immediately after conditioning. a dentine bonding agent (Scotchbond 2.3M Co., St Paul, MN, USA; batch 9DK) was applied following manufacturer’s instructions. The composite sample was placed inside the ring against the surface of the dentine bonding agent. Care was taken to limit overfilling of the ring and to minimize contact of the composite with the top surface of
et al.: Polymerization
contraction
in composites
179
Table 1. Materials Composite
resin
Silux Plus* Heliomolart Herculite XR$ p-50*
Classification Microfill Posterior Hybrid Posterior
U, universal; LY, light yellow. *3M Co., St Paul, MN, USA. tvivadent, Tonawanda, NY, USA. *Kerr Manufacturing Co., Romulus,
Batch OBKI 460901 02053 OCR5D
Shade U LY U U
Ml, USA.
the ring. The experimental set-up was supported on a glass slide which had been smeared by a thin layerofpetroleum gel as a release agent. A second strain gauge of the same physical parameters was placed on top of the composite sample prior to photocuring. Surface tack of the composite resin was adequate to ensure adhesion between the strain gauge and the composite sample. The leads from both strain gauges were connected to a strain gauge conditioner (2100 Series: Measurements Group) initially balanced at zero. The gauge placed on the outer surface of the acrylic ring measured deformation of the ring along the circumference and the second gauge measured dimensional change of the composite sample during the polymerization process. The composite sample was cured for 60 s with a curing light source (Visilux 2,3M Co.. St Paul, MN, USA). A new ring was used for each trial. Measurements were recorded during the light application period and continued for a period of 14 min until an equilibrium was reached in the polymerization contraction. Four different composites were evaluated in this study (Table I). Multiple trials (n = 5) were performed for each of the four different composites. Point stresses produced in the ring as a result of the polymerization contraction of the composite sample were calculated by multiplying the strain values from the acrylic ring by the modulus of elasticity of the acrylic as provided by Hooke’s law. The following assumptions were made regarding the model: (1) axisymmetry of the model, (2) uniform cure of the composite sample, and (3) homogeneous and adequate bond between the composite and acrylic ring.
RESULTS Glass
slide
Cavity filled composite
with
Acrylic ring (9.0 mm OD 2.Omm. 6.5 mm ID)
x
Strain
Glass slide mounted mechanical stage Fig.
7.
Experimental
on
configuration.
gauge
for
ring
The typical dimensional behaviour of the composite material and surrounding acrylic ring as recorded in real time are illustrated in Fig 2. Fig. 3 illustrates strain of the composite material during the 60 s light application for the four materials. Heliomolar (HM) exhibited the lowest strain values for this period and was statistically significant at P = 0.05. At the end of the 14 min. when the polymerization contraction had reached an equilibrium, Heliomolar continued to demonstrate considerably lower strain values, however this was not statistically significant at P = 0.05 (Table II). Stress in the acrylic ring which
J. Dent.
180
1992:
20:
No. 3
Table II. Acrylic ring stress and composite
Composite
n
Silux Heliomolar Herculite XR P-50
5 Z
-
5
strain
Ring
Composite
stress (MPa 7 min)
strain (ptrain 7 min)
Ring stress (MPa 74 min)
- 492 (103) -176(76) * - 521 (139) - 581 (167)
-
0.85 0.34 0.78 1.70
(0.23) (0.15) (0.22) (0.24)
2.09 2.09 2.05 2.32
(0.29) (0.22) (0.27) (0.30)
Stress (MPa at 1 and 14 min): no significant difference at P < 0.05 (S-N-K). *Strain (ystrain at 1 min): Heliomolar significantly different from all others at P < 0.05 Strain (pstrain at 14 min): no significant difference at P < 0.05 (S-N-K).
circumscribed the composite sample for the 60 s time period is shown inFig. 4 and listed in Table II. Heliomolar demonstrated the lowest stress values while P-50 rapidly developed and maintained the largest stress values, however there were no statistically significant differences between the four materials at 60 s or 14 min (P = 0.05).
DISCUSSION The test configuration is designed to simulate a buccolingual section through a Class II, MOD composite restoration. The ability to bond the composite resin to the surface of the acrylic ring furthers the clinical simulation. Substituting an acrylic ring for tooth structure eliminates biological variables such as moisture contamination during composite placement, variability in cavity preparation, and varying physical properties of extracted teeth. The physical properties of the acrylic are well documented and enable calculation of stress from measurements of deformation. The deformation on the outer surface of the ring is linearly related to that on the inner surface of the ring. Thus. calculation of stress on the outer surface of the ring simulated stress within the dentine-composite interface. The model, however, utilizes a homogeneous material unlike dentine and enamel and does not incorporate the stiffening effect of the higher modulus enamel. The compatibility between the composites tested and the acrylic leads to a high bond strength between these two groups of materials, similar to the bond strength between enamel and composite, which is generally higher
Composite strain (ptrain 74 min) - 1136(210) - 853 (102) - 1206 (162) - 1089 (170)
(S-N-K).
than that achieved between dentine and composite. This assumption was confirmed by the strain curves which showed no sign of bond rupture. The strain of the composite sample typically exhibits a peak early in the polymerization process which represents expansion due to the polymerization exotherm as well as radiant heat from the light curing source. The gauges do not change in their sensitivity over the temperature range of the experiment since they are self-temperaturecompensating. The contribution of the thermal expansion of the gauge to the total expansion during light curing typically reaches a maximum of 600 microstrain after 5 s which levels off to 470 microstrain for the duration of the 200
r
0
.r: :m
,o
-200
.u I
-400
-6OOL
0
’
5
’
10
’
15
’
’
20
25
’
’
35
30
’
40
’
45
’
50
55
60
Time (sl Fig.
3. Restricted polymerization
contraction P-50; +,
cycle. &, Silux Plus; -0-, +, Herculite XR.
for 60 s curing Heliomolar;
500
0 .c e ;; ,o
-500
2 -1000
Material -1500
0
I
I
I
I
I
I
I
I
I
100
200
300
400
500
600
700
800
900
-21
0
”
5
10
”
15
”
25
30
”
35
40
“1
45
50
55
c
60
Time (5)
Time (5)
Fig. 2. Strain of composite and surrounding acrylic ring resulting from polymerization contraction of composite sample.
20
Fig. 4. Stresses resulting from during 60 s curing cycle. -O--,
--It,
Heliomolar;
+,
polymerization
Silux Plus; &, Herculite XR.
contraction
P-50;
Sakaguchi
60 s curing time. When the light is turned off, expansion of the gauge returns to zero within 2 s. Therefore the dynamic readings during the curing cycle are relative measures because they include the expansion of the gauge due to the radiant heat. The contraction rate of the composite is highest for the first 30-40 s of polymerization and diminishes as the 60 s point is approached. When the light curing source is turned off, there is a corresponding contraction due to the loss of radiant heat that is no longer applied to the sample. The high contraction rate for the first 30-40 s of the polymerization reaction is clinically significant because the integrity of the composite-tooth interface is rapidly challenged during the early phases of polymerization, when the bond between the hard tissue and the composite is still maturing. The development of stress within the acrylic ring follows the same pattern as the strain of the composite sample. Fig. 4 illustrates the early tensile stress (positive sign) placed on the ring due to the expansion of the composite. This is followed by the development of compressive stresses (negative sign) which are attributed to the polymerization contraction of the composite. Heliomolar is distinguished by its lower polymerization contraction relative to the other materials tested (Table II, Fig. 3). At 60 s this was statistically significant, however at 14 min, although the contraction was still considerably less than the other materials, it was not statistically significant (P < 0.05, Student-Newman-Keuls multirange test). Each of the composites in this study contain varying concentrations of BISGMA/TEGDMA resin except Heliomolar (HM) which consists of BISGMA and urethanedimethacrylate (UEDMA), which may have a lower modulus of elasticity when cured. During the polymerization reaction UEDMA may inherently contract less or its lower modulus may in part compensate for contraction. Heliomolar contains a substantial amount of organic filler in the form of prepolymerized resin particles. The filler loading as reported by the manufacturer is 78 per cent, however when an ashing experiment was performed. the inorganic filler content was found to be 62 per cent. The discrepancy between the two filler loading quantities is presumably due to the loss of silane coupling following the ashing process procedure. The relatively lower unpolymerized resin content and the lower modulus of the resin in Heliomolar may explain the lower measured polymerization contraction and resulting stress as compared to the other materials tested. It should also be cautioned that a lower degree of curing would also produce a smaller contraction stress, but this would be accompanied by compromised physical properties. It is not surprising that P-50 rapidly developed and maintained the largest stress values given the high filler loading (85 per cent by weight filler) of this composite. Because of the high filler loading, the filler particles become physically interlocked as the resin matrix polymerizes and contracts. The bulk physical properties of the composite then approach the physical properties of the inorganic filler. The higher modulus of elasticity of
et al.: Polymerization
contraction
in composites
181
this type of material will tend to generate higher early stresses on the surrounding materials as indicated by the elevated ring stress at 1 min (Table II), assuming an adequate interfacial bond between the composite and surrounding material. On the other hand, the higher modulus of elasticity of P-50 enables the material to withstand the high occlusal stresses generated in the posterior segment. without undue flexure. All other factors being equal, a resin composite demonstrating low polymerization contraction and low induced stresses would be clinically preferred. On the basis of these two properties alone. Heliomolar appeared to possess the best combination of contraction and stress on the surrounding material. However. other factors such as wear rate, fracture resistance and resistance to microleakage may be adversely affected by low modulus of elasticity. Also, the technique for composite placement needs to be carefully considered to minimize the effects of polymerization contraction. A bulk placement technique was used in this experiment, which would not be utilized in a clinical setting. The net effect on the surrounding tooth structure can be minimized, or at least reduced through incremental placement of the composite (Lutz by all composite et al., 1986), as is recommended manufacturers.
Acknowledgements This study was supported in part by USPHS Grant T32 DE07098 from the National Institute Research, Bethesda, MD 20892.
Research of Dental
References Bowen R. L., Nemoto K. and Rapson J. E. (1983) Adhesive bonding of various materials to hard tooth tissues: forces developing in composite materials during hardening. J. Am. Dent Assoc. 106, 475-477. Davidson C. L. and de&e A. J. (1984) Relaxation of polymerization contraction stresses by flow in dental composites.J. Dent. Res. 63, 146-148. Davidson C. L., deGee A. J. and Feilzer A. (1984) The competition between the composite-dentin bond strength and the polymerization contraction stress. J. Dent. Res. 63, 1396-l 399. Donley K. J., Jensen M. E., Reinhardt J. et al. (1987) Posterior composite polymerization shrinkage in primary teeth: an in viva comparison of three restorative techniques. Pediarr. Dent. 9, 22-25. Eick J. D. and Welch F. H. (1986) Polymerization shrinkage of posterior min and its possible influence on postoperative sensitivity. Quintessence In?. 17, 103-l 11. Feilzer A. J.. deGee A. J. and Davidson C. L. (1987) Setting stress in composite resin in relation to configuration of the restoration. J. Dent. Res. 66, 1636-1639. Feilzer A. J.. deGee A. J. and Davidson C. L. (1990) Relaxation of polymerization contraction shear stress by hygroscopic expansion. J. Dent. Res. 69, 36-39. Lutz F.. Krejci 1. and Oldenburg T. R. (1986) Elimination of polymerization stresses at the margins of posterior composite resin restorations: a new restorative technique. Quintessence ht. 17, 777-784.
182
J. Dent.
1992; 20: No. 3
Morin D. L., Douglas W. H., Cross M. et al. (1988) Biophysical stress analysis of restored teeth: experimental strain measurement. Dent Mater. 4,41-48. Peters M. C. R. B., Sakaguchi R. L.. Nelson S. R. et al. (1991) Polymerization contraction stresses in composite resins. J. Dent. Res. 70, 295 (abstr. 239). Sakaguchi R. L. and Douglas W. H. (1989) Strain gauge measurement of polymerization shrinkage. J. Dent Res. 68, 977 (abstr. 885).
Sakaguchi R. L.. Sasik C. T., Bunczak M. A. et al. (1991) Strain gauge method for measuring polymerization contraction of composite resins. J. Dent. 19, 312-316. Sheth J. J.. Fuller J. L. and Jensen M. E. (1988) Cuspal deformation and fracture resistance of teeth with dentin adhesives and composites. J. Prosthet Dent 60, 560-569.
Book Reviews Collagen and Dental Matrices. J. P. Gage, M. J. 0. Francis and J. T. Triffitt. Pp. 136. 1989. Oxford, Butterworth-Heinemann. Hardback, f30.00 I approached this text with a great deal of enthusiasm. Here, at last, is the book which combines the basic biology of collagen with its role in the dental tissues. My first misgivings began to appear when I read the authors’ declared intention to provide a reference book for undergraduate, postgraduate and all workers in basic dental research by putting together ‘all currently known information on collagen in the oral environment’. An awesome task indeed. Unfortunately, this well-intentioned Jack-of-all-trades is in grave danger of mastering none. The text provides a worthy account of the role of collagen in the dental tissues and the chapters dealing specifically with the biology of collagen per se are comprehensive level. and informative, but only at the undergraduate Sadly, many of the concepts (e.g. the role of the periodontal ligament in tooth eruption) have been oversimplified to the point where their worth is questionable at any higher level of study. In addition, it is difficult for me to accept that a book with no references in the text can be of use to postgraduate workers in the field. The lists provided at the end of each chapter for ‘further reading’ are perfunctory and are not a substitute. Any appeal to undergraduates must inevitably suffer when its unprepossessing layout (pages of closely printed text often unrelieved by diagrams) is compared with the latest textbooks and cell biology and biochemistry which use colour and ‘Scientific American’-type illustrations. The line drawings included here, particularly those of tissue histology, are at best poor and occasionally useless. In its favour, the book is clearly and concisely written and includes a section on ‘Clinical relevance’ at the end of each chapter. It would not be out of place on the shelves of medical and dental libraries, but for undergraduate reference only. J. Kirkham
Atlas of Oral Diagnostic Imaging. T. Higashi, J. K. C. Shiba and H. Ikutu. Pp. 168. 1990. Tokyo, lshiyaku EuroAmerica Inc (distributors: Gazelle Books, Lancaster). Hardback, f34.95. The new Atlas of Oral Diagnostic Imaging gives a refreshingly different view to the study of dental radiology. From a clinical point of view, the subject has to do with interpretation and thus a good textbook should be replete with good radiographs. This is just such a book and the majority of the radiographs have adjacent, good explanatory diagrams. Thus, there is no long text to read but instead, many brief summary tables giving the features, and differences, of the lesions and the radiographic findings. A few also relate the histology to the radiographic findings. While the book contains valuable information on, for instance, the prevalence of carcinoma of gingiva at initial visit to GDPs (it is very high in Japan) and the major factors in diagnostic imaging of maxillary sinus disorders, I would have liked to have seen some discussion of the factors that affect the radiographic appearance of caries. Most of the radiographs are of the conventional type but, where required, there is a good sprinkling of MRI, CT and some radioisotope scans. Some chapters could have been more comprehensive, for example, that on anomalies where some discussion on the variation in number of roots and root canals would have been helpful. There are other chapters where unrelated subjects appear to have been clumped together at random; facial clefts should be dealt with separately, from caries, while ghosts images in panoramic radiography should be discussed with those arising from the anatomy, and not in a chapter which also includes soft-tissue calcifications and accidentally swallowed foreign bodies. These criticisms apart this is a good book that will appeal to both the student and practitioner who want to get the basic facts without too much to read. N. J. Serman
J. Dent. 1992;
Effects of polymerization composite restorations R. L. Sakaguchi,
contraction
20:
178-l
82
in
M. C. R. B. Peters,* S. R. Nelson, W. H. Douglas and H. W. Poort*
Biomarerials Research Center, Department of Oral Science, *TRIKON, University of Nijmegen, The Netherlands
University
of Minnesota
School of Dentistry
and
ABSTRACT Post-gel polymerization contraction of resin composite induces contraction stresses at the composite-tooth bond and in surrounding tooth structure. Strain gauges have been shown to be an effective method for measuring linear post-gel polymerization contraction of composites. A new model was developed in which the composite sample was bonded to and circumscribed by an acrylic ring. The model simulates a composite restoration surrounded by dentine. A strain gauge measured the deformation of the ring while a second strain gauge simultaneously recorded the dimensional change of the sample. Stresses placed on the acrylic ring as a result of polymerization contraction of the composite were calculated, based on the strains on the ring and the ring’s material properties. Four composites (Heliomolar. Vivadent. Tonawanda. NY. USA: Herculite XR. Kerr Manufacturing Co., Romulus, MI, USA; P-SO. 3M Co., St Paul, MN, USA; Silux Plus 3M Co.) were evaluated for polymerization contraction strain and stress on the surrounding acrylic ring during polymerization. At the end of the 60 s light application, Heliomolar demonstrated significantly lower post-gel contraction (0.12 per cent.P < 0.05) when compared to the other materials. When the strain reached an equilibrium at the end of 14 min Heliomolar continued to demonstrate lower post-gel contraction, however this was not statistically significant at P < 0.05. When the contraction stress on the surrounding acrylic ring was considered. P-50 rapidly developed and produced the largest stress values (1.7 MPa) at the end of the light application while Heliomolar produced the lowest stress values (0.3 MPa). These values. however, were not significantly different when evaluated statistically. The model appears to be a reasonable predictor of the results of polymerization contraction of composite resins. The ability to bond the composite to the acrylic through dentine bonding agents enhances its usefulness. and the simple geometry allows for relatively straightforward calculation of stress-strain relationships. KEY WORDS:
Composites,
J. Dent. 1992; 1991)
20: 178-I
Polymerization,
Stress
82 (Received 3 July 199 1; reviewed 23 September
1991;
Correspondence should be addressed to: Dr R. L. Sakaguchi, School of Dentistry, Science Tower, 515 Delaware Street, SE Minneapolis, MN 55455, USA.
INTRODUCTION Resin composites exhibit the inherent problem of contraction during the polymerization process. The total volumetric contraction can be divided into two components: the pre-gel and post-gel phase. During pregel polymerization, the composite is able to flow which relieves stress within the structure (Davidson and deGee, 1984). However, at the gel point and beyond. the material develops a stiffness reflected in the modulus of elasticity. As the material continues to polymerize, the contraction places stresses within the composite-tooth bond which in turn is distributed into the surrounding tooth structure. After gelation, flow is unable to compensate for contrac@ 1992 Butterworth-Heinemann 0300-5712/92/030178-05
Ltd.
accepted 16 October
Malcolm
Moos Health
tion stresses. Therefore, post-gel polymerization results in clinically significant stresses in the composite-tooth bond and surrounding tooth structure (Davidson et al.. 1984; Feilzer et al.. 1987). Stresses resulting from post-gel polymerization contraction may produce defects in the composite-tooth bond. This leads to bond failure and microleakage with associated postoperative sensitivity (Eick and Welch. 1986). If the composite-tooth bond is able to withstand the deformation, stresses induced by the contracting composite can cause deformation of the surrounding tooth structure (Donley et al., 1987: Morin et al., 1988; Sheth ef al., 1988). The resulting coronal deformation may result in postoperative sensitivity and microcracks in the cervical
Sakaguchi
enamel (Bowen et al.. 1983), which predisposes the tooth to fracture. Careful composite placement can minimize the effects of the polymerization contraction (Lutz et al.. 1986). however coronal deformation is still evident. Stress relaxation through composite hydration is evident in mature composites (Feilzeretal.. 1990) but this occurs over a long period of time, well after post-gel polymerization is complete. Therefore, the tooth/restoration complex is in a pre-stressed state even before occlusal stresses result in further coronal deformation (Sakaguchi et al., 1991). A method for measuring post-gel polymerization contraction in resin composites using electrical resistance strain gauges has been described previously (Sakaguchi and Douglas. 1989: Sakaguchi et al., 1991). The model described in this paper utilizes this technology and extends it to the determination of the important contraction stresses following composite shrinkage. The simple geometry of the model enables the calculation of contraction stress by measuring contraction strain of the surrounding acrylic in real time, during the curing process of the composite and beyond. Stresses derived from the model can be used as an estimate of the effects of polymerization contraction in clinical applications.
MATERIALS
AND METHODS
The test configuration utilized a methylmethacrylate ring (inner diameter 6.5 mm. outer diameter 9.0 mm, height 2.0 mm) which circumscribed the composite sample (Fig I) (Peters et al.. 1991). An electrical resistance strain gauge (CEA-06-032UW-120: Measurements Group, Raleigh, NC, USA, 0.032 in. gauge length) was attached to the outer surface of the ring with cyanoacrylate bonding agents. The inner surface of the ring was treated with chloroform to condition the surface. Immediately after conditioning. a dentine bonding agent (Scotchbond 2.3M Co., St Paul, MN, USA; batch 9DK) was applied following manufacturer’s instructions. The composite sample was placed inside the ring against the surface of the dentine bonding agent. Care was taken to limit overfilling of the ring and to minimize contact of the composite with the top surface of
et al.: Polymerization
contraction
in composites
179
Table 1. Materials Composite
resin
Silux Plus* Heliomolart Herculite XR$ p-50*
Classification Microfill Posterior Hybrid Posterior
U, universal; LY, light yellow. *3M Co., St Paul, MN, USA. tvivadent, Tonawanda, NY, USA. *Kerr Manufacturing Co., Romulus,
Batch OBKI 460901 02053 OCR5D
Shade U LY U U
Ml, USA.
the ring. The experimental set-up was supported on a glass slide which had been smeared by a thin layerofpetroleum gel as a release agent. A second strain gauge of the same physical parameters was placed on top of the composite sample prior to photocuring. Surface tack of the composite resin was adequate to ensure adhesion between the strain gauge and the composite sample. The leads from both strain gauges were connected to a strain gauge conditioner (2100 Series: Measurements Group) initially balanced at zero. The gauge placed on the outer surface of the acrylic ring measured deformation of the ring along the circumference and the second gauge measured dimensional change of the composite sample during the polymerization process. The composite sample was cured for 60 s with a curing light source (Visilux 2,3M Co.. St Paul, MN, USA). A new ring was used for each trial. Measurements were recorded during the light application period and continued for a period of 14 min until an equilibrium was reached in the polymerization contraction. Four different composites were evaluated in this study (Table I). Multiple trials (n = 5) were performed for each of the four different composites. Point stresses produced in the ring as a result of the polymerization contraction of the composite sample were calculated by multiplying the strain values from the acrylic ring by the modulus of elasticity of the acrylic as provided by Hooke’s law. The following assumptions were made regarding the model: (1) axisymmetry of the model, (2) uniform cure of the composite sample, and (3) homogeneous and adequate bond between the composite and acrylic ring.
RESULTS Glass
slide
Cavity filled composite
with
Acrylic ring (9.0 mm OD 2.Omm. 6.5 mm ID)
x
Strain
Glass slide mounted mechanical stage Fig.
7.
Experimental
on
configuration.
gauge
for
ring
The typical dimensional behaviour of the composite material and surrounding acrylic ring as recorded in real time are illustrated in Fig 2. Fig. 3 illustrates strain of the composite material during the 60 s light application for the four materials. Heliomolar (HM) exhibited the lowest strain values for this period and was statistically significant at P = 0.05. At the end of the 14 min. when the polymerization contraction had reached an equilibrium, Heliomolar continued to demonstrate considerably lower strain values, however this was not statistically significant at P = 0.05 (Table II). Stress in the acrylic ring which
J. Dent.
180
1992:
20:
No. 3
Table II. Acrylic ring stress and composite
Composite
n
Silux Heliomolar Herculite XR P-50
5 Z
-
5
strain
Ring
Composite
stress (MPa 7 min)
strain (ptrain 7 min)
Ring stress (MPa 74 min)
- 492 (103) -176(76) * - 521 (139) - 581 (167)
-
0.85 0.34 0.78 1.70
(0.23) (0.15) (0.22) (0.24)
2.09 2.09 2.05 2.32
(0.29) (0.22) (0.27) (0.30)
Stress (MPa at 1 and 14 min): no significant difference at P < 0.05 (S-N-K). *Strain (ystrain at 1 min): Heliomolar significantly different from all others at P < 0.05 Strain (pstrain at 14 min): no significant difference at P < 0.05 (S-N-K).
circumscribed the composite sample for the 60 s time period is shown inFig. 4 and listed in Table II. Heliomolar demonstrated the lowest stress values while P-50 rapidly developed and maintained the largest stress values, however there were no statistically significant differences between the four materials at 60 s or 14 min (P = 0.05).
DISCUSSION The test configuration is designed to simulate a buccolingual section through a Class II, MOD composite restoration. The ability to bond the composite resin to the surface of the acrylic ring furthers the clinical simulation. Substituting an acrylic ring for tooth structure eliminates biological variables such as moisture contamination during composite placement, variability in cavity preparation, and varying physical properties of extracted teeth. The physical properties of the acrylic are well documented and enable calculation of stress from measurements of deformation. The deformation on the outer surface of the ring is linearly related to that on the inner surface of the ring. Thus. calculation of stress on the outer surface of the ring simulated stress within the dentine-composite interface. The model, however, utilizes a homogeneous material unlike dentine and enamel and does not incorporate the stiffening effect of the higher modulus enamel. The compatibility between the composites tested and the acrylic leads to a high bond strength between these two groups of materials, similar to the bond strength between enamel and composite, which is generally higher
Composite strain (ptrain 74 min) - 1136(210) - 853 (102) - 1206 (162) - 1089 (170)
(S-N-K).
than that achieved between dentine and composite. This assumption was confirmed by the strain curves which showed no sign of bond rupture. The strain of the composite sample typically exhibits a peak early in the polymerization process which represents expansion due to the polymerization exotherm as well as radiant heat from the light curing source. The gauges do not change in their sensitivity over the temperature range of the experiment since they are self-temperaturecompensating. The contribution of the thermal expansion of the gauge to the total expansion during light curing typically reaches a maximum of 600 microstrain after 5 s which levels off to 470 microstrain for the duration of the 200
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Fig. 2. Strain of composite and surrounding acrylic ring resulting from polymerization contraction of composite sample.
20
Fig. 4. Stresses resulting from during 60 s curing cycle. -O--,
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Heliomolar;
+,
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Silux Plus; &, Herculite XR.
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P-50;
Sakaguchi
60 s curing time. When the light is turned off, expansion of the gauge returns to zero within 2 s. Therefore the dynamic readings during the curing cycle are relative measures because they include the expansion of the gauge due to the radiant heat. The contraction rate of the composite is highest for the first 30-40 s of polymerization and diminishes as the 60 s point is approached. When the light curing source is turned off, there is a corresponding contraction due to the loss of radiant heat that is no longer applied to the sample. The high contraction rate for the first 30-40 s of the polymerization reaction is clinically significant because the integrity of the composite-tooth interface is rapidly challenged during the early phases of polymerization, when the bond between the hard tissue and the composite is still maturing. The development of stress within the acrylic ring follows the same pattern as the strain of the composite sample. Fig. 4 illustrates the early tensile stress (positive sign) placed on the ring due to the expansion of the composite. This is followed by the development of compressive stresses (negative sign) which are attributed to the polymerization contraction of the composite. Heliomolar is distinguished by its lower polymerization contraction relative to the other materials tested (Table II, Fig. 3). At 60 s this was statistically significant, however at 14 min, although the contraction was still considerably less than the other materials, it was not statistically significant (P < 0.05, Student-Newman-Keuls multirange test). Each of the composites in this study contain varying concentrations of BISGMA/TEGDMA resin except Heliomolar (HM) which consists of BISGMA and urethanedimethacrylate (UEDMA), which may have a lower modulus of elasticity when cured. During the polymerization reaction UEDMA may inherently contract less or its lower modulus may in part compensate for contraction. Heliomolar contains a substantial amount of organic filler in the form of prepolymerized resin particles. The filler loading as reported by the manufacturer is 78 per cent, however when an ashing experiment was performed. the inorganic filler content was found to be 62 per cent. The discrepancy between the two filler loading quantities is presumably due to the loss of silane coupling following the ashing process procedure. The relatively lower unpolymerized resin content and the lower modulus of the resin in Heliomolar may explain the lower measured polymerization contraction and resulting stress as compared to the other materials tested. It should also be cautioned that a lower degree of curing would also produce a smaller contraction stress, but this would be accompanied by compromised physical properties. It is not surprising that P-50 rapidly developed and maintained the largest stress values given the high filler loading (85 per cent by weight filler) of this composite. Because of the high filler loading, the filler particles become physically interlocked as the resin matrix polymerizes and contracts. The bulk physical properties of the composite then approach the physical properties of the inorganic filler. The higher modulus of elasticity of
et al.: Polymerization
contraction
in composites
181
this type of material will tend to generate higher early stresses on the surrounding materials as indicated by the elevated ring stress at 1 min (Table II), assuming an adequate interfacial bond between the composite and surrounding material. On the other hand, the higher modulus of elasticity of P-50 enables the material to withstand the high occlusal stresses generated in the posterior segment. without undue flexure. All other factors being equal, a resin composite demonstrating low polymerization contraction and low induced stresses would be clinically preferred. On the basis of these two properties alone. Heliomolar appeared to possess the best combination of contraction and stress on the surrounding material. However. other factors such as wear rate, fracture resistance and resistance to microleakage may be adversely affected by low modulus of elasticity. Also, the technique for composite placement needs to be carefully considered to minimize the effects of polymerization contraction. A bulk placement technique was used in this experiment, which would not be utilized in a clinical setting. The net effect on the surrounding tooth structure can be minimized, or at least reduced through incremental placement of the composite (Lutz by all composite et al., 1986), as is recommended manufacturers.
Acknowledgements This study was supported in part by USPHS Grant T32 DE07098 from the National Institute Research, Bethesda, MD 20892.
Research of Dental
References Bowen R. L., Nemoto K. and Rapson J. E. (1983) Adhesive bonding of various materials to hard tooth tissues: forces developing in composite materials during hardening. J. Am. Dent Assoc. 106, 475-477. Davidson C. L. and de&e A. J. (1984) Relaxation of polymerization contraction stresses by flow in dental composites.J. Dent. Res. 63, 146-148. Davidson C. L., deGee A. J. and Feilzer A. (1984) The competition between the composite-dentin bond strength and the polymerization contraction stress. J. Dent. Res. 63, 1396-l 399. Donley K. J., Jensen M. E., Reinhardt J. et al. (1987) Posterior composite polymerization shrinkage in primary teeth: an in viva comparison of three restorative techniques. Pediarr. Dent. 9, 22-25. Eick J. D. and Welch F. H. (1986) Polymerization shrinkage of posterior min and its possible influence on postoperative sensitivity. Quintessence In?. 17, 103-l 11. Feilzer A. J.. deGee A. J. and Davidson C. L. (1987) Setting stress in composite resin in relation to configuration of the restoration. J. Dent. Res. 66, 1636-1639. Feilzer A. J.. deGee A. J. and Davidson C. L. (1990) Relaxation of polymerization contraction shear stress by hygroscopic expansion. J. Dent. Res. 69, 36-39. Lutz F.. Krejci 1. and Oldenburg T. R. (1986) Elimination of polymerization stresses at the margins of posterior composite resin restorations: a new restorative technique. Quintessence ht. 17, 777-784.
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Morin D. L., Douglas W. H., Cross M. et al. (1988) Biophysical stress analysis of restored teeth: experimental strain measurement. Dent Mater. 4,41-48. Peters M. C. R. B., Sakaguchi R. L.. Nelson S. R. et al. (1991) Polymerization contraction stresses in composite resins. J. Dent. Res. 70, 295 (abstr. 239). Sakaguchi R. L. and Douglas W. H. (1989) Strain gauge measurement of polymerization shrinkage. J. Dent Res. 68, 977 (abstr. 885).
Sakaguchi R. L.. Sasik C. T., Bunczak M. A. et al. (1991) Strain gauge method for measuring polymerization contraction of composite resins. J. Dent. 19, 312-316. Sheth J. J.. Fuller J. L. and Jensen M. E. (1988) Cuspal deformation and fracture resistance of teeth with dentin adhesives and composites. J. Prosthet Dent 60, 560-569.
Book Reviews Collagen and Dental Matrices. J. P. Gage, M. J. 0. Francis and J. T. Triffitt. Pp. 136. 1989. Oxford, Butterworth-Heinemann. Hardback, f30.00 I approached this text with a great deal of enthusiasm. Here, at last, is the book which combines the basic biology of collagen with its role in the dental tissues. My first misgivings began to appear when I read the authors’ declared intention to provide a reference book for undergraduate, postgraduate and all workers in basic dental research by putting together ‘all currently known information on collagen in the oral environment’. An awesome task indeed. Unfortunately, this well-intentioned Jack-of-all-trades is in grave danger of mastering none. The text provides a worthy account of the role of collagen in the dental tissues and the chapters dealing specifically with the biology of collagen per se are comprehensive level. and informative, but only at the undergraduate Sadly, many of the concepts (e.g. the role of the periodontal ligament in tooth eruption) have been oversimplified to the point where their worth is questionable at any higher level of study. In addition, it is difficult for me to accept that a book with no references in the text can be of use to postgraduate workers in the field. The lists provided at the end of each chapter for ‘further reading’ are perfunctory and are not a substitute. Any appeal to undergraduates must inevitably suffer when its unprepossessing layout (pages of closely printed text often unrelieved by diagrams) is compared with the latest textbooks and cell biology and biochemistry which use colour and ‘Scientific American’-type illustrations. The line drawings included here, particularly those of tissue histology, are at best poor and occasionally useless. In its favour, the book is clearly and concisely written and includes a section on ‘Clinical relevance’ at the end of each chapter. It would not be out of place on the shelves of medical and dental libraries, but for undergraduate reference only. J. Kirkham
Atlas of Oral Diagnostic Imaging. T. Higashi, J. K. C. Shiba and H. Ikutu. Pp. 168. 1990. Tokyo, lshiyaku EuroAmerica Inc (distributors: Gazelle Books, Lancaster). Hardback, f34.95. The new Atlas of Oral Diagnostic Imaging gives a refreshingly different view to the study of dental radiology. From a clinical point of view, the subject has to do with interpretation and thus a good textbook should be replete with good radiographs. This is just such a book and the majority of the radiographs have adjacent, good explanatory diagrams. Thus, there is no long text to read but instead, many brief summary tables giving the features, and differences, of the lesions and the radiographic findings. A few also relate the histology to the radiographic findings. While the book contains valuable information on, for instance, the prevalence of carcinoma of gingiva at initial visit to GDPs (it is very high in Japan) and the major factors in diagnostic imaging of maxillary sinus disorders, I would have liked to have seen some discussion of the factors that affect the radiographic appearance of caries. Most of the radiographs are of the conventional type but, where required, there is a good sprinkling of MRI, CT and some radioisotope scans. Some chapters could have been more comprehensive, for example, that on anomalies where some discussion on the variation in number of roots and root canals would have been helpful. There are other chapters where unrelated subjects appear to have been clumped together at random; facial clefts should be dealt with separately, from caries, while ghosts images in panoramic radiography should be discussed with those arising from the anatomy, and not in a chapter which also includes soft-tissue calcifications and accidentally swallowed foreign bodies. These criticisms apart this is a good book that will appeal to both the student and practitioner who want to get the basic facts without too much to read. N. J. Serman
