- Email: [email protected]
Radiopacity of glass ionomer dental materials
Radiopacity of glass ionomer dental materials And+ P. Prhost, DDS, h4S,a Denis Forest, DDS, MSD, FICD,b Richard and Pierre DeGrandmont, DA4D.d Montrial, QuCbec, Canada
Tqpay,
&fS,c
UNIVERSITI? DE MONTRtiAL The radiopacity of glass ionomer dental materials is quite variable. The use of a poorly radiopaque material as a base under other restorative materials can mislead the dentist to a diagnosis of recurrent decay. This study investigates the radiopacity of these materials and proposes a minimal radiopacity under which a material should not be used as a base or liner. All base, liner, and core formulations of glass ionomer under investigation were more radiopaque than dentin. All restorative and luting formulations of glass ionomer under investigation were less radiopaque than dentin and therefore should be avoided as bases or liners. (ORAL SURC ORAL MED ORAL PATHOL 1990;70:231-5)
W
hen the glassionomer cementsappearedin 1971, Wilson and Kent’ recommended their possible application asa cavity liner. More recently, many authors26 have demonstrated considerable interest in the use of these cements as a protective “liner” or “base” for reconstructive purposes.This positive attitude toward these cements can be explained mainly by the adhesive qualities of these cements, which can reinforce dental structures, and their relative pulpal neutrality or biocompatibility. 7,8 In addition, ionomer cements produce a cariostatic effect associated with their capacity to enrich the surrounding dental tissues with fluoride.9-13These cements also have other advantageous properties’ 4 working and manipulation time, ease of manipulation, compressive resistance, high modulus of elasticity, bacteriostatic properties, resistance to microleakage, and possible adhesion to certain restorative materials. On the other hand, clinical experience has shown that an important problem can be associated with their use; their radiopacity may be insufficient to use
aAssociateProfessor,Section of Operative Dentistry, Universitk de Montrtal, Montrial, Qutbec, Canada. bProfessorand Head, Section of Radiology, UniversitB de Montr&al, Montrkal, Qutbec, Canada. CBiostatistician, Universitt de Montrtal, Montrkal, Qukbec, Canada. dCIinica1 Instructor, Universitt de Montrkal, Montrkal, Qubbec, Canada. 7116120722
Fig. 1. Tooth 29: compositeresin restorationlined with radiolucent glass ionomer material.
them as a base or liner material. In fact, the radiopacity of these materials is not homogeneous.14This radiopacity is especially important considering that it is often lessradiopaque than dentin and the materials under which they are used (Fig. 1). The lack of radiopacity places the clinician in a situation where radiologic interpretation can be misleading: (1) Is the radiolucent area under the restorative material recurrent decay? (2) Is it a void? (3) Or is it simply a radiolucent base or liner material? Use of this material is especially problematic considering that the usual base materials like zinc oxide, calcium hydroxide, and zinc phosphate have always been radiopaque. 231
232
Prbvost
et al.
ORAL SURC ORAL
MAD ORAL PATH~L August
Table
I. Materials under evaluation
Dispersalloy II* Zinc phosphatez
Zonalint Ful-Fil§
Dyea@ P-3011 3M Glass Ionomer
Dycal VLC§ 3M Glass Ionomer
Fuji Type IIT Fuji Linerq Ketac Bond (Yellow)**
Fuji Type I Cemq Fuji Miracle Mixq Ketac Gem** Ketac Silver**
Ketac
Ceram
(Gray)11
Fil**
Ceram Lint+ Ceram Chemt: Shofu Type Il$$ Shofu
Base$$’
ChemFil II Express@ ChemFil II Junior§§ CavalitellI[ Dentin
(Yellow)ll
Corett
Ceram Filft ShofuType I Cem$$ Shofu Liner*4 ChemFil II$$ ChemFil II Senior§§ .4qua Cem§§ Enamel Pulp
‘Johnson & Johnson Dental Prod., East Windsor, N.J. tAssociated Dental Products Ltd., Purton, Swindon. Wilts, U.K. +S.S. White, Holmdel, N.J. §L.D. Caulk Company, Milford, Del. 1)3M Dental Products Co., St. Paul, Minn. ‘I[G-C International Corp., Scottsdale, Aria. **ESPE-Premier. Norristown, Pa. t3P.S.P. Dental Co. Ltd., Belvedere, Kent. L.K. $$Shofu Dental Corp., Menlo Park, Calif. §§De Trey Division, Densply Ltd., Addlestonc, Weybtidge, Surrey, U.K. I(jIKerr/Sybron Mfg. Company, Romulus, Mich.
It thus seemsimportant to establish standards of radiopacity for the evaluation of materials used in restorative dentistry. It is also important to determine the radiopacity of glass ionomer dental materials so that the clinician can appreciate the type of restorative materials used when radiographically evaluating the possibility of recurrent dental caries. MATERIAL Samples
AND METHODS
Thirty-two restorative materials and specimensof dentin, enamel, and pulp were tested for radiopacity (Table I). The restorative materials were prepared according to the manufacturer’s instructions. They were inserted in a brass cylinder, 5 mm in diameter by 3 mm in thickness, placed on a glass slab; this thicknesscould represent the buccolingual thickness of the material in a clinical situation. A second glass slab was placed on top of the filled mold and a weight of 15 kg placed for the remainder of the material’s setting time. As for the photopolymerized materials, they were placed under the 15 kg load until the sum of the working time and the time under the load equaled 10 minutes. They were then exposedto light curing for two periods of 40 secondson each side of the mold through the glass slabs. The amalgam was condensed and carved by scraping the mold surface with a glassslab. The sampleswere extracted from the
1990
mold when their setting time was completed according to manufacturer’s instructions. The sampleswere identified and immersed in a varnish (P.S.P. Varnish, P.S.P. Dental Co. Ltd, Belvedere, Kent, United Kingdom) for 5 secondsbecause some of them, like glass ionomer cements, are sensitive to syneresis and imbibition. The samples were kept in a humidor at 37” C until their radiographic exposure. Five samples of each material were prepared according to this method. In addition, for comparison purposes, five impacted molars, recently extracted and kept in water, were cut mesiodistally in their center to obtain a slice 3 mm in thickness. Radiographic
evaluation
The samples were identified and placed on a 10 by 12 inch film (Kodak R-P XOmat, Eastman Kodak Company, Rochester, New York). The film was maintained in a cardboard cassettewithout any reinforcing screen (Fuji EC Cassette A, Fuji Medical System USA Inc., Stanford, Connecticut). The samples were exposedaccording to a standard technique. The exposure factors were determined to produce a mean radiodensity of 1.0 under 6 mm of aluminum (70 kVp, 200 mA, l/20 second, FFD 60”). Four consecutive exposures were performed. Thereafter, the films were developedaccording to the manufacturer’s instructions in an automatic developer. The radiodensity of each material was established, along with the background radiodensity of the film (the radiodensity above each sample), with a photometer (densitometer) (X-Rite 301, densitometer, Michigan) by means of a recording head diameter of 3 mm (accuracy: -+0.02). The mean radiodensity of each sample was obtained from all four films as well as their background radiodensity. Statistical
methods
The statistical analysis wasdesignedto estimate the proportion of variance that belongs to the materials (l), to the sampleswithin the materials (2), and to the radiographs within the samplesand the materials (3). This kind of analysis is referred to as variance components analysis. For this analysis, the radiologic density is considered as the responsevariable measured on a series of samples.The samplescome from a number of different materials. We use the subscript ‘9” to refer to the material and the subscript “j” to refer to the sample. There are “ni” samplesin the “i”th material and “m” materials in total, with a total number of samples“n.” The basic model is written: Yij = a, + ai + eij (1) where a, is an overall constant term, the ai is the con-
Radiopacity
Volume 70 Number 2
tribution of the “i”th material to the radiologic density, and the eij is the contribution of the “j”th sample in the “i”th material. This term is often referred to as the “residual” since it represents the difference between what is predicted by the rest of the model and the actual observed value. It is assumed in these models, typically, that the mean or expectedvalue of E(eij) = 0 and the variance is a constant: Var (eij) = a*. As it stands, (1) is essentially a simple regression although it does take into account the contribution from level-2 units (the ai ); the only random variation is between level-l units. Of course, in some circumstances, such a model might well be appropriate. If there were only two or three materials and we were interested in whether the radiologic density differed between them, then (l), as it stands, would be an appropriate model, and could be analyzed by means of any standard regression or analysis of variance program. More generally, however, rather than consider the materials as “fixed” levels of a factor, we prefer to treat them as a random sample of all possible materials available (population). This implies that the ai are to be thought of as having a distribution over materials, and our interest will be mainly in the parameters of this distribution, notably the mean and the variance. As we did with the eij we write: E(aJ = 0 and Var (ai) = a,,* and we can write the equation as: Yij = a, + (ai + eij)
(2)
We now have a 2-level model becausethe term in the brackets contains two random variables, one for each level. The constant term a, now includes both samples and materials means. In the same way, radiographs are included in the model as a third level and the resulting model is written as follows: Ykij = a0 + (vk + uki + t&j)
(3)
where Ykij is the radiologic density for the ‘Y’th radiograph in the “j”th sample in the “i”th material. Equation (3) contains four parameters, namely one fixed coefficient: a0 and the three random coefficients, the variances: * a“,* a2 aVr These parameters are estimated by means of iterative generalized least squares procedure (IGLS) according to Goldstein. l5 Hence the analysis is mainly concerned with the estimation of the variance at each level of the model: materials, samples, and radiographs. To establish the clinical reliability of the radiologic
of glass ionomer dental materials
233
Table II. Results from the analysis of variance of the samples: Radiographic data Source
Variance
Standard error
Materials Samples Films (radiographs)
0.91674 (99.1%) 0.00357 (0.4%) 0.00446 (0.5%)
0.21950 0.00056 0.00027
Ill. Results from the analysis of variance between and within evaluators
Table
Source (1) Between-evaluators difference (systematic difference) (2) Within-evaluator difference (systematic difference) (3) Interaction evaluators/materials/ evaluations (random difference)
Variance 0.00108 0.0008 I 0.01066
density readings, two sampleswere chosenon one radiograph; one sample was of moderately low radiodensity, the other of moderately high radiodensity. Nine dentists visually estimated the radiologic density of the two sampleswith a manual photometer (Kodak densitometer), each reading was made twice by each dentist. An analysis of variance’(j was conducted on the readings [evaluators (9), materials (2), evaluations (2)]. RESULTS
The results from the analysis of variance of the samples’ radiographic density data are reported in Table II. As expected, most of the variation occurred at the materials level (99.1%). Though variation for the samplesand radiographs was small, their standard error was far below their estimates, clearly indicating that they cannot be ignored. The results from the analysis of variance between and within evaluators are reported in Table III. The analysis of variance indicates the origins of variation in radiopacity. Three sources of variation were identified: (1) the difference between evaluators, (2) the difference between repeated evaluations of the evaluators, (3) the random difference from the interaction evaluators/materials/evaluations. The variance between evaluators and the variance between the two evaluations of the same evaluator (within evaluator) were treated as systematic differences.In other words, we wanted to seewhether some evaluators systematically gave higher or lower rating (reading); in the sameway for the two readings of the sameevaluator, we wanted to seewhether the second
234
Prb~ost et al.
ORAL SURG ORAL MED ORAL PATHOL
August 1990 2 3
4 5 6
7 0 9
IO II 12 13
I4 15 16 17
I8 19 20 21 22
Dispersaltoy II Kstsc Silver !X Fuji Mrrscla Mix Zonslin Zinc Phosphate 3M (Yellow) Shofu Lin Ful-Fil 3M (Grey) KetacBond (Yellow) P-30 Cavsllte 0ycal GCFuj
(a) Powder/liquid (b) Powder/liquid
Fig.
suming a normal distribution) can be perceived as different by 97.5% of the clinicians.
0.204 0.265 0.27 0.501
DISCUSSION
0.658 1.023 1.067
I. 137 1.165 1.867
I.956 2.073 2.132 2. I41 2.312 2.357 2.409 2.45 2.49 2.727 2.87 2.907
ratio I:3 ratio I:2
2. Observed values of mean
density of materials.
rating was systematically higher or lower than the first one. In addition to these possible systematic differences,random variation occurred during the reading. That is, two readings of the same ratio usually differed for the sameevaluator or between evaluators inasmuch as these differences were not systematic, they were just random variation; this random variation is estimated by the interactions (evaluators/materials/evaluations) illustrated in Table III. To find out which degree of radiopacities can be discriminated by most of the clinicians, the standard deviation of the readings must be used taking into account both different readings between clinicians and within clinicians. Hence, the sum of the components of the variance yielded the total variance (0.00108 + 0.00081 + 0.01066 = 0.01255) from which the standard deviation can be obtained (Vm = 0.112). Two standard deviations (0.2) represent, given a normal distribution, 97.5% of the population. The result (0.2) established the minimal magnitude of variance for which two radiographic densities (as-
The radiopacity of dental restorative materials is variable. It should be understood that the interpretation of the radiodensity of a dental restorative material must be compared with the radiodensity of dentin, enamel, pulp, and other restorative materials when present. Variation in the thickness of a material is lessimportant than its molecular structure, but still may significantly influence the radiodensity. This is especially true with materials of low radiopacityl’ and when such materials are superimposed on enamel.” Almost half of the materials evaluated were less radiopaque than dentin; four were more radiopaque than dentin and less radiopaque than enamel; the others were more radiopaque than enamel. The materials that were less radiopaque than dentin should not be considered as base and lining materials since they could be radiologically mistaken for decalcified or carious dentin. These are the restorative and luting formulations (Fig. 2). With the exception of Aqua Cem and Ketac Cem, all others were more radiopaque than dentin by more than 0.2 in the present study. Although the clinical interest of a base material, easily differentiated from dentin, can be appreciated, this study was unable to proposea minimal radiologic density for which a material would appear more radiopaque than dentin in a clinical situation. But it appearsreasonableto usea base(or lining) material that has a radiodensity equal to or greater than the radiodensity of dentin to assure that, radiographically, a baseor liner material would not be mistaken for carious dentin, as is noted in Fig. 1. CONCLUSION
This study showsthat if a restorative material is less radiopaque than dentin by 0.2, it should be avoided as a base or liner.
REFERENCES
1. Wilson AD, Kent BE. The glass ionomer, a new translucent dental filling material. Appl Chem Biotech 1971;21:313. 2. Yakushiji M, Kinumatsu T, Fuchino T, Machida Y. Effects of
3. 4. 5. 6.
glass ionomer cements on the dental pulp and its efficacy as a base material. Bull Tokyo Dent Co111979;20:47-59. Farah JW, Clark AE, h&hem M, Thomas PA. Effect of cement base thickness on MOD amalgam restorations. JDR 1983;62:109-11. Hanst MT. The amalgam prep’s completed-what’s next? Arkansas Dent J 1985;56:25-8. Seed WD, Looper SW. Shear bond strength of a composite resin to an etched glass ionomer. Dent Mater 1985;1:127-8. McLean JW, Powis DR, Presser HJ, Wilson AD. The use of
Volume 70 Number 2 glass-ionomer cements in bonding composite resins to dentine. Br Dent J 1985;158:410-4. 7. Wilson AD, ProsserHJ. Biocompatibility of the glass ionomer cement. J Dent Assoc S Afr 1982;37:872-9. 8. Cooper IR. The response of the human dental pulp to glass ionomer cements. Int Endod J 1980;13:76-88. 9. Wesenberg G, Einar H. The in vitro effect of a glass ionomer cement on dentine and enamel walls. J Oral Rehabil 1980;7:3542. 10. Retief DH, Bradley EL, Denton JC, Switzer P. Enamel and cementum fluoride uptake from a glass ionomer cement. Caries Res 1984;18:250-‘7. 11. Swartz ML. Phillins RW. Clark HE. Lona-term F releasefrom glass ionomer cements. JDR 1984;63:158-60. 12. Hamilton IR. Effects of fluoride on enzymatic regulation of bacterial carbohydrate metabolism. Caries Res 1977;l l(supp 11):262-91. 13. Harper DS, Loesche WJ. Inhibition of acid production from oral bacteria by fluoroapatite-derived fluoride. JDR 1986; 65:30-3. 14. Warren JA. Glass ionomer: its emerging role as an intermediary dental base. J Am Dent Assoc 1986;57:21-4.
Radiopacity
of glass ionomer dental materials
235
15. Goldstein H. Multilevel mixed linear model analysis using iterative generalized least squares. Biometrika 1986;73:43-56. 16. Buschang PH, Tanguay R. Estimating variance components with BMDP for analyses of technical reliability. Computer Methods and Programs in Biomedicine 1985;21:119-22. 17. Abreu MJN, Tavares D, Vieira DF. Radiopacity of restorative materials. Oper Dent 1977;2:3-16. 18. Stanford CM, Fan PL, Schoenfeld CM, Knoeppel R, Stanford JW. Radiopacity of light-cured posterior composite resins. J Am Dent Assoc 1987;115:722-4. Reprint requests to:
Dr. Andre P. Prevost Section of Operative Dentistry Faculd de Medecine Dentaire Universitt de Montreal C.P. 6128, Succ. “A” Montreal, Quebec Canada H3C 357
Tqpay,
&fS,c
UNIVERSITI? DE MONTRtiAL The radiopacity of glass ionomer dental materials is quite variable. The use of a poorly radiopaque material as a base under other restorative materials can mislead the dentist to a diagnosis of recurrent decay. This study investigates the radiopacity of these materials and proposes a minimal radiopacity under which a material should not be used as a base or liner. All base, liner, and core formulations of glass ionomer under investigation were more radiopaque than dentin. All restorative and luting formulations of glass ionomer under investigation were less radiopaque than dentin and therefore should be avoided as bases or liners. (ORAL SURC ORAL MED ORAL PATHOL 1990;70:231-5)
W
hen the glassionomer cementsappearedin 1971, Wilson and Kent’ recommended their possible application asa cavity liner. More recently, many authors26 have demonstrated considerable interest in the use of these cements as a protective “liner” or “base” for reconstructive purposes.This positive attitude toward these cements can be explained mainly by the adhesive qualities of these cements, which can reinforce dental structures, and their relative pulpal neutrality or biocompatibility. 7,8 In addition, ionomer cements produce a cariostatic effect associated with their capacity to enrich the surrounding dental tissues with fluoride.9-13These cements also have other advantageous properties’ 4 working and manipulation time, ease of manipulation, compressive resistance, high modulus of elasticity, bacteriostatic properties, resistance to microleakage, and possible adhesion to certain restorative materials. On the other hand, clinical experience has shown that an important problem can be associated with their use; their radiopacity may be insufficient to use
aAssociateProfessor,Section of Operative Dentistry, Universitk de Montrtal, Montrial, Qutbec, Canada. bProfessorand Head, Section of Radiology, UniversitB de Montr&al, Montrkal, Qutbec, Canada. CBiostatistician, Universitt de Montrtal, Montrkal, Qukbec, Canada. dCIinica1 Instructor, Universitt de Montrkal, Montrkal, Qubbec, Canada. 7116120722
Fig. 1. Tooth 29: compositeresin restorationlined with radiolucent glass ionomer material.
them as a base or liner material. In fact, the radiopacity of these materials is not homogeneous.14This radiopacity is especially important considering that it is often lessradiopaque than dentin and the materials under which they are used (Fig. 1). The lack of radiopacity places the clinician in a situation where radiologic interpretation can be misleading: (1) Is the radiolucent area under the restorative material recurrent decay? (2) Is it a void? (3) Or is it simply a radiolucent base or liner material? Use of this material is especially problematic considering that the usual base materials like zinc oxide, calcium hydroxide, and zinc phosphate have always been radiopaque. 231
232
Prbvost
et al.
ORAL SURC ORAL
MAD ORAL PATH~L August
Table
I. Materials under evaluation
Dispersalloy II* Zinc phosphatez
Zonalint Ful-Fil§
Dyea@ P-3011 3M Glass Ionomer
Dycal VLC§ 3M Glass Ionomer
Fuji Type IIT Fuji Linerq Ketac Bond (Yellow)**
Fuji Type I Cemq Fuji Miracle Mixq Ketac Gem** Ketac Silver**
Ketac
Ceram
(Gray)11
Fil**
Ceram Lint+ Ceram Chemt: Shofu Type Il$$ Shofu
Base$$’
ChemFil II Express@ ChemFil II Junior§§ CavalitellI[ Dentin
(Yellow)ll
Corett
Ceram Filft ShofuType I Cem$$ Shofu Liner*4 ChemFil II$$ ChemFil II Senior§§ .4qua Cem§§ Enamel Pulp
‘Johnson & Johnson Dental Prod., East Windsor, N.J. tAssociated Dental Products Ltd., Purton, Swindon. Wilts, U.K. +S.S. White, Holmdel, N.J. §L.D. Caulk Company, Milford, Del. 1)3M Dental Products Co., St. Paul, Minn. ‘I[G-C International Corp., Scottsdale, Aria. **ESPE-Premier. Norristown, Pa. t3P.S.P. Dental Co. Ltd., Belvedere, Kent. L.K. $$Shofu Dental Corp., Menlo Park, Calif. §§De Trey Division, Densply Ltd., Addlestonc, Weybtidge, Surrey, U.K. I(jIKerr/Sybron Mfg. Company, Romulus, Mich.
It thus seemsimportant to establish standards of radiopacity for the evaluation of materials used in restorative dentistry. It is also important to determine the radiopacity of glass ionomer dental materials so that the clinician can appreciate the type of restorative materials used when radiographically evaluating the possibility of recurrent dental caries. MATERIAL Samples
AND METHODS
Thirty-two restorative materials and specimensof dentin, enamel, and pulp were tested for radiopacity (Table I). The restorative materials were prepared according to the manufacturer’s instructions. They were inserted in a brass cylinder, 5 mm in diameter by 3 mm in thickness, placed on a glass slab; this thicknesscould represent the buccolingual thickness of the material in a clinical situation. A second glass slab was placed on top of the filled mold and a weight of 15 kg placed for the remainder of the material’s setting time. As for the photopolymerized materials, they were placed under the 15 kg load until the sum of the working time and the time under the load equaled 10 minutes. They were then exposedto light curing for two periods of 40 secondson each side of the mold through the glass slabs. The amalgam was condensed and carved by scraping the mold surface with a glassslab. The sampleswere extracted from the
1990
mold when their setting time was completed according to manufacturer’s instructions. The sampleswere identified and immersed in a varnish (P.S.P. Varnish, P.S.P. Dental Co. Ltd, Belvedere, Kent, United Kingdom) for 5 secondsbecause some of them, like glass ionomer cements, are sensitive to syneresis and imbibition. The samples were kept in a humidor at 37” C until their radiographic exposure. Five samples of each material were prepared according to this method. In addition, for comparison purposes, five impacted molars, recently extracted and kept in water, were cut mesiodistally in their center to obtain a slice 3 mm in thickness. Radiographic
evaluation
The samples were identified and placed on a 10 by 12 inch film (Kodak R-P XOmat, Eastman Kodak Company, Rochester, New York). The film was maintained in a cardboard cassettewithout any reinforcing screen (Fuji EC Cassette A, Fuji Medical System USA Inc., Stanford, Connecticut). The samples were exposedaccording to a standard technique. The exposure factors were determined to produce a mean radiodensity of 1.0 under 6 mm of aluminum (70 kVp, 200 mA, l/20 second, FFD 60”). Four consecutive exposures were performed. Thereafter, the films were developedaccording to the manufacturer’s instructions in an automatic developer. The radiodensity of each material was established, along with the background radiodensity of the film (the radiodensity above each sample), with a photometer (densitometer) (X-Rite 301, densitometer, Michigan) by means of a recording head diameter of 3 mm (accuracy: -+0.02). The mean radiodensity of each sample was obtained from all four films as well as their background radiodensity. Statistical
methods
The statistical analysis wasdesignedto estimate the proportion of variance that belongs to the materials (l), to the sampleswithin the materials (2), and to the radiographs within the samplesand the materials (3). This kind of analysis is referred to as variance components analysis. For this analysis, the radiologic density is considered as the responsevariable measured on a series of samples.The samplescome from a number of different materials. We use the subscript ‘9” to refer to the material and the subscript “j” to refer to the sample. There are “ni” samplesin the “i”th material and “m” materials in total, with a total number of samples“n.” The basic model is written: Yij = a, + ai + eij (1) where a, is an overall constant term, the ai is the con-
Radiopacity
Volume 70 Number 2
tribution of the “i”th material to the radiologic density, and the eij is the contribution of the “j”th sample in the “i”th material. This term is often referred to as the “residual” since it represents the difference between what is predicted by the rest of the model and the actual observed value. It is assumed in these models, typically, that the mean or expectedvalue of E(eij) = 0 and the variance is a constant: Var (eij) = a*. As it stands, (1) is essentially a simple regression although it does take into account the contribution from level-2 units (the ai ); the only random variation is between level-l units. Of course, in some circumstances, such a model might well be appropriate. If there were only two or three materials and we were interested in whether the radiologic density differed between them, then (l), as it stands, would be an appropriate model, and could be analyzed by means of any standard regression or analysis of variance program. More generally, however, rather than consider the materials as “fixed” levels of a factor, we prefer to treat them as a random sample of all possible materials available (population). This implies that the ai are to be thought of as having a distribution over materials, and our interest will be mainly in the parameters of this distribution, notably the mean and the variance. As we did with the eij we write: E(aJ = 0 and Var (ai) = a,,* and we can write the equation as: Yij = a, + (ai + eij)
(2)
We now have a 2-level model becausethe term in the brackets contains two random variables, one for each level. The constant term a, now includes both samples and materials means. In the same way, radiographs are included in the model as a third level and the resulting model is written as follows: Ykij = a0 + (vk + uki + t&j)
(3)
where Ykij is the radiologic density for the ‘Y’th radiograph in the “j”th sample in the “i”th material. Equation (3) contains four parameters, namely one fixed coefficient: a0 and the three random coefficients, the variances: * a“,* a2 aVr These parameters are estimated by means of iterative generalized least squares procedure (IGLS) according to Goldstein. l5 Hence the analysis is mainly concerned with the estimation of the variance at each level of the model: materials, samples, and radiographs. To establish the clinical reliability of the radiologic
of glass ionomer dental materials
233
Table II. Results from the analysis of variance of the samples: Radiographic data Source
Variance
Standard error
Materials Samples Films (radiographs)
0.91674 (99.1%) 0.00357 (0.4%) 0.00446 (0.5%)
0.21950 0.00056 0.00027
Ill. Results from the analysis of variance between and within evaluators
Table
Source (1) Between-evaluators difference (systematic difference) (2) Within-evaluator difference (systematic difference) (3) Interaction evaluators/materials/ evaluations (random difference)
Variance 0.00108 0.0008 I 0.01066
density readings, two sampleswere chosenon one radiograph; one sample was of moderately low radiodensity, the other of moderately high radiodensity. Nine dentists visually estimated the radiologic density of the two sampleswith a manual photometer (Kodak densitometer), each reading was made twice by each dentist. An analysis of variance’(j was conducted on the readings [evaluators (9), materials (2), evaluations (2)]. RESULTS
The results from the analysis of variance of the samples’ radiographic density data are reported in Table II. As expected, most of the variation occurred at the materials level (99.1%). Though variation for the samplesand radiographs was small, their standard error was far below their estimates, clearly indicating that they cannot be ignored. The results from the analysis of variance between and within evaluators are reported in Table III. The analysis of variance indicates the origins of variation in radiopacity. Three sources of variation were identified: (1) the difference between evaluators, (2) the difference between repeated evaluations of the evaluators, (3) the random difference from the interaction evaluators/materials/evaluations. The variance between evaluators and the variance between the two evaluations of the same evaluator (within evaluator) were treated as systematic differences.In other words, we wanted to seewhether some evaluators systematically gave higher or lower rating (reading); in the sameway for the two readings of the sameevaluator, we wanted to seewhether the second
234
Prb~ost et al.
ORAL SURG ORAL MED ORAL PATHOL
August 1990 2 3
4 5 6
7 0 9
IO II 12 13
I4 15 16 17
I8 19 20 21 22
Dispersaltoy II Kstsc Silver !X Fuji Mrrscla Mix Zonslin Zinc Phosphate 3M (Yellow) Shofu Lin Ful-Fil 3M (Grey) KetacBond (Yellow) P-30 Cavsllte 0ycal GCFuj
(a) Powder/liquid (b) Powder/liquid
Fig.
suming a normal distribution) can be perceived as different by 97.5% of the clinicians.
0.204 0.265 0.27 0.501
DISCUSSION
0.658 1.023 1.067
I. 137 1.165 1.867
I.956 2.073 2.132 2. I41 2.312 2.357 2.409 2.45 2.49 2.727 2.87 2.907
ratio I:3 ratio I:2
2. Observed values of mean
density of materials.
rating was systematically higher or lower than the first one. In addition to these possible systematic differences,random variation occurred during the reading. That is, two readings of the same ratio usually differed for the sameevaluator or between evaluators inasmuch as these differences were not systematic, they were just random variation; this random variation is estimated by the interactions (evaluators/materials/evaluations) illustrated in Table III. To find out which degree of radiopacities can be discriminated by most of the clinicians, the standard deviation of the readings must be used taking into account both different readings between clinicians and within clinicians. Hence, the sum of the components of the variance yielded the total variance (0.00108 + 0.00081 + 0.01066 = 0.01255) from which the standard deviation can be obtained (Vm = 0.112). Two standard deviations (0.2) represent, given a normal distribution, 97.5% of the population. The result (0.2) established the minimal magnitude of variance for which two radiographic densities (as-
The radiopacity of dental restorative materials is variable. It should be understood that the interpretation of the radiodensity of a dental restorative material must be compared with the radiodensity of dentin, enamel, pulp, and other restorative materials when present. Variation in the thickness of a material is lessimportant than its molecular structure, but still may significantly influence the radiodensity. This is especially true with materials of low radiopacityl’ and when such materials are superimposed on enamel.” Almost half of the materials evaluated were less radiopaque than dentin; four were more radiopaque than dentin and less radiopaque than enamel; the others were more radiopaque than enamel. The materials that were less radiopaque than dentin should not be considered as base and lining materials since they could be radiologically mistaken for decalcified or carious dentin. These are the restorative and luting formulations (Fig. 2). With the exception of Aqua Cem and Ketac Cem, all others were more radiopaque than dentin by more than 0.2 in the present study. Although the clinical interest of a base material, easily differentiated from dentin, can be appreciated, this study was unable to proposea minimal radiologic density for which a material would appear more radiopaque than dentin in a clinical situation. But it appearsreasonableto usea base(or lining) material that has a radiodensity equal to or greater than the radiodensity of dentin to assure that, radiographically, a baseor liner material would not be mistaken for carious dentin, as is noted in Fig. 1. CONCLUSION
This study showsthat if a restorative material is less radiopaque than dentin by 0.2, it should be avoided as a base or liner.
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Dr. Andre P. Prevost Section of Operative Dentistry Faculd de Medecine Dentaire Universitt de Montreal C.P. 6128, Succ. “A” Montreal, Quebec Canada H3C 357