Determination of enamel and coronal dentin microhardness of upper fist premolars in age aspect
Olena Bolgova, M.D., Ph.D., Volodymyr Mavrych, M.D., Ph.D., DSc, Volodymyr Vozniy, M.D., Ph.D.
Dr. Olena Bolgova, MD, PhD, Associative Professor of Gross Anatomy, Department of Anatomy, St. Matthew’s University, School of Medicine, Cayman Islands, email@example.com
Dr. Volodymyr Mavrych, MD, PhD, DSc Professor of Gross Anatomy, Department of Anatomy, St. Matthew’s University, School of Medicine, Cayman Islands, firstname.lastname@example.org
Dr. Volodymyr Vozniy, MD, PhD, orthodontist, Department of Anatomy, Lugansk State Medical University, private dentistry hospital “Moy Doctor”, Lugansk, Ukraine, email@example.com
The anatomic and functional reconstruction of the tooth is the main goal of preserving and restoring odontology. Knowledge of the physical properties of dental tissues is important in that it allows for the comprehension of the effect of several materials and substances on dental surfaces. The Vickers microhardness of enamel and coronal dentin of 18 first upper premolars of different age individuals was evaluated. Enamel was impressed in two regions: zone 1 – buccal site in the middle 1/3 of the crown; zone 2 – tip of the tubercle. Coronal dentin was impressed in three regions: zone 3 – tip of the tubercle; zone 4 – buccal site in the middle 1/3 of the crown; zone 5 – close to the pulp cavity. Thus there were 90 indentations in general. Enamel microhardness was higher to that of coronal dentin. However, there was no statistically significant difference between enamel microhardness for the 1st and 2nd zones. Coronal dentin microhardness revealed statistically significant differences for the 3rd and 5th zones in past middle age group and zones 3 and 4, 4 and 5 for young individuals. Enamel and dentin microhardness increases with the age of the tooth, but without statistically significant difference.
Dentin microstructure and its properties are principal determinants of nearly all procedures in restorative dentistry. Since many years, patients' demands for more aesthetic restorations have caused an increase in the use of tooth colored restorative materials and adhesive restorations have become an acceptable part of routine dental restorative treatment. Dental procedures are most often based upon mechanical and chemical interventions on dental surfaces. Microhardness is a physical property that allows for the evaluation of the effects that chemical and/or physical agents have on dental surfaces. It is defined as the resistance to local deformation  and its tests are based on the induced permanent surface deformation that remains after removal of the load . Hardness measurements can be correlated with other mechanical properties such as fracture resistance , modulus of elasticity, and yield strength [4, 7]. A strong relationship exists between microhardness of dentin and the respective bond strength . So, microhardness provides a first step toward predicting the behavior of dentin/restoration interfaces .
The hardness of dentin and enamel has been studied by a number of investigators using several methods. Early studies by Hodge  evaluated various procedures to measure the hardness of tooth tissue. The tests varied from scratching, indentation, elastic impact, cutting, and permanent deformation. Most of these tests, however, destroyed the tissue, thus distorting the gradient factors of measurement. Burg  and Richter  reported conflicting differences of enamel hardness, carious and non-carious dentin in pregnant and non-pregnant women. Investigative studies by Gustafson  showed that two similar indentation tests on ground tooth tissues revealed that differences in hardness may be due to specific structural characteristics of each tooth tissue, e.g. sclerosis, age, or caries. He reported that tufts and spindles are lower in density (hardness) than adjacent enamel. One study  stated that no trends could be detected between tooth types and that no microhardness differences existed from the dentin enamel junction (DEJ) to the outer enamel surface.
Thus, literature is not clear and due to the lack of uniformity presented by the authors as to a standard of microhardness of both the enamel and coronal dentin, it seems to us that this physical property needs further studies.
In this study, we proposed to measure Vickers microhardness in some regions of enamel and coronal dentin of upper first premolars extracted from individuals of different age groups. So, this study tried to evaluate variations in microhardness between different regions of the same tooth and as to distance in relation to the external surface and possible different of this property in age aspect.
Material and Methods
The microhardness of enamel and dentine was assessed using grinded surfaces of 18 upper first premolars. Eight enamel/dentin specimens were prepared from teeth of individuals in between 16-21y.o. Ten enamel/dentin specimens were prepared from teeth of individuals in between 65-71y.o. After extraction, teeth were fixed in a 10% neutral formalin solution. Each tooth was cut longitudinally, to separate the buccal and lingual halves, using a diamond lade saw. Each specimen was then embedded in a resin block. Grinding and polishing was carried out using grinding machines with cooling physiological solution. For the evaluation of microhardness, each specimen was positioned in an adjustable clamp. The clamp was locked into a position so that the exposed surface stayed parallel to the horizontal plane, immobilizing the specimen completely during measurement. The microhardness of the enamel and dentine was determined using a device of MTI-3M type on Vickers Hardness Test. The microhardness index was expressed in kg per square mm (kg/mm2). Five penetrations were carried out from each enamel/dentin specimen on the regions identified in figure 1 with indentation load of 20 grams during 15 seconds using Vickers indenter. Enamel was impressed in two regions: zone 1 – buccal site in the middle 1/3 of the crown; zone 2 – tip of the tubercle. Coronal dentin was impressed in three regions: zone 3 – tip of the tubercle; zone 4 – buccal site in the middle 1/3 of the crown; zone 5 – close to the pulp cavity. Thus there were 90 indentations in general. Data of each experimental condition were averaged and compared using two-way ANOVA followed by a Tukey test.
According to aim of our research we noticed that microhardness of enamel for upper first premolars of past middle age individuals was different for the 1st and the 2nd zones. For buccal site of the crown it was 433.4±27.5 kg/mm2 which is in 4.2% higher than for the tip of tubercle (415.9±20.7 kg/mm2). Microhardness of dentin for the same age group was decreased in direction from the tip of tubercle to the pulp. It was identified for the 3rd, 4th and 5th zones as 171.4±13.0 kg/mm2, 155.7±28.8 kg/mm2 and 123.3±7.4 kg/mm2, accordingly. The difference in between the 3rd and 4th zones was 9.2%, in between the 3rd and 5th zones – 28.1%. It is important to mention that ANOVA analysis of variance revealed statistically significant differences in dentin microhardness for the 3rd and 5th zones (t = 9.1; p<0.0001).
Next in our research we evaluated microhardness of enamel and dentin for the upper first premolars of the young individuals. Microhardness of enamel in 1st zone was 424.2±25.4 kg/mm2, which is in 10.7% higher than in 2nd zone (383.3±14.8 kg/mm2). Microhardness of dentin in the same age group was decreased in direction from the tip of tubercle to the pulp. T-test revealed statistically significant differences in dentin microhardness for different zones: for the region of tubercle and buccal surface (t = 2.6; p<0.01) and for the region of tubercle and pulp cavity (t = 5.7; p<0.0001). So, dentin microhardness was 159.7±17.0 kg/mm2 (for the 3rd zone), 139.3±18.3 kg/mm2 (for the 4th zone) and 118.4±15.1 kg/mm2 (for the 5th zone).
The comparison between microhardness of enamel and dentin for different age groups was then carried out (figure 2). Microhardness of enamel of past middle age individuals was slightly higher than in young age group in both 1st and 2nd zones (in 7.8% for the region of tubercle and in 2.1% for buccal surface). However, there was no statistically significant difference between them: t = 0.73; p=0.48 for the 1st zone, t = 0.75; p=0.46 for the 2nd zone. The same tendency was identified for dentin microhardness: it was higher in past middle age individuals’ premolars compared with the young age persons. For the 3rd zone this difference was 6.8%, for the 4th zone – 10.5%, for the 5th zone – 4.0%. In order to verify these results Tukey´s comparison test was applied, showing no statistically significant difference between the data (t = 1.60; p=0.129 for the 3rd zone, t = 1.47; p=0.161 for the 4th zone, t = 0.82; p=0.419 for the 5th zone).
This study evaluated the existence of variation in enamel and dentin microhardness between different regions and in relation to different age groups. Furthermore, we compared our results to those obtained by other authors that worked with Vickers or Knoop microhardness.
Enamel microhardness did not present statistically significant differences between the surfaces studied. In enamel, the greatest mean value of microhardness (433.4±27.5 kg/mm2) occurred in the 1st zone of upper first premolars of past middle age individuals. This region was identified in buccal side of the middle 1/3 of the crown, 0.05 to 0.1mm from the external surface. The smallest mean value (383.3±14.8 kg/mm2) occurred in the 2nd zone of upper first premolars of young individuals. This region was identified in tip of the tubercle. These values seem to be in agreement to the variation obtained by Craig and Peyton , that was of 272 to 440 KHN (Knoop Hardness Number), and also to that found in the study by Gaspersic , in which a variation of 277,1 and 424,3 VHN (Vickers Hardness Number) in enamel microhardness was found. Cirano, Romito and Todescan  reported, that the greatest mean value of enamel microhardness (395.92 VHN) occurred in the region 0.05 to 0.1mm from the external surface and the smallest mean value (255.02 VHN) occurred on the free surfaces, 0.05 to 0.1mm from the dentinoenamel junction. Fukuda et al.  presented a mean value of 264 VHN for enamel of erupted third molar microhardness. This value is apparently low, when compared to our study and to Craig and Peyton  and Gaspersic  and Cirano, Romito and Todescan . Fukuda et al.  justified these low values by the fact that the teeth analyzed belonged to young patients, who were at the most, 20 years old. However, in literature we cannot find studies that show a clear influence of age on enamel and dentin tissue microhardness.
In coronal dentin, the largest value of microhardness was found in the 3rd zone (171.4±13.0 kg/mm2) of upper first premolars of past middle age individuals. The 5th zone of upper first premolars of young individuals presented the lowest value (118.4±15.1 kg/mm2). T-test revealed statistically significant differences in dentin microhardness between zones 3rd and 5th in past middle age group and zones 3rd and 4th, 3rd and 5th in young group. Our study is in agreement with that of Craig et al. , Rautiola and Craig , Shannon and Keuper , Banerjee et al.  and Cirano, Romito and Todescan  who showed that coronal dentin, which is not close to the pulp, presented the largest microhardness. However, in our study, the value found for all three zones was lower to that found in these aforementioned studies. Craig et al. , Rautiola and Craig , Shannon and Keuper  and Kinney et al.  reported that the smallest values of microhardness of the coronal dentin are located close to the dentinoenamel junction and the pulp. Caldwell et al.  did not find any pronounced differences in microhardness among different teeth belonging to the same person, or different surfaces of the same tooth, or deciduous teeth, or erupted and non-erupted teeth, and teeth belonging to individuals of different age groups. On the other hand, Leforestier et al.  reported that shear bond strength and microhardness increases with the age of the tooth.
Enamel and dentin microhardness increases with the age of the tooth, but without statistically significant difference. Within the boundaries of the scope of this research, we can conclude that enamel microhardness was remarkably superior to that of coronal dentin. Coronal dentin microhardness in tip of tubercle and middle 1/3 of the crown was statistically higher to that of coronal dentin located close to pulp.
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Fig. 1. Distribution of penetration regions on enamel/dentin specimens.
Fig. 2. Microhardness of different zones of enamel/dentin specimens for individuals of different age groups: axis Y – microhardness kg/mm2; axis X – different penetration zones (1 and 2 for enamel; 3, 4, and 5 for dentin).