Dental Materials Journal 2013; 32(4): 643 647 Newly developed hardness testing system, Cariotester : Measurement principles and development of a program for measuring Knoop hardness of carious dentin Akihiko SHIMIZU 1, Syozi NAKASHIMA 2, Toru NIKAIDO 2, Toyotaro SUGAWARA 3, Takatsugu YAMAMOTO 3 and Yasuko MOMOI 3 1 Department of Dentistry and Oral Surgery, Hyogo College of Medicine, Mukogawa-cho, Nishinomiya, Hyogo 663-8501, Japan 2 Cariology and Operative Dentistry, Department of Oral Health Sciences, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan 3 Department of Operative Dentistry, Tsurumi University School of Dental Medicine, 2-1-3 Tsurumi, Tsurumi-ku, Yokohama, Kanagawa 230-8501, Japan Corresponding author, Akihiko SHIMIZU; E-mail: shimizu@diary.ocn.ne.jp We previously discovered that when a cone-shaped indenter coated with paint was pressed into an object, the paint disappeared in accordance with the depth of the indentation. Based on this fact, we developed the Cariotester, a portable system for measuring the Knoop hardness (KHN) of carious dentin. The Cariotester is composed of a handpiece with an indenter, a microscope, and a computer. In this system, the painted indenter is forced into the material with a 150-gf load, and the indentation depth (CT depth) is obtained from the paint disappearance. The CT depth by the Cariotester and the KHN by a microhardness tester were determined at 14 dentin regions. From the data, a program was created to convert the CT depth of the carious dentin into the KHN. As a result, if the CT depth is measured with this system, the KHN of carious dentin can be displayed in real time. Keywords: Carious dentin, Knoop hardness, Cariotester INTRODUCTION Clinically, dentin color and/or hardness have been used as a guide to indicate proper removal of a carious lesion. However, dentin color is not a reliable guide for the determination of carious and noncarious dentin 1), and discolored dentin should not be removed in many cases 2). In contrast, the hardness and the texture of cavity floor dentin serve as indicators of caries penetration 2). The hardness of the dentin as tested by a sharp dental explorer or excavator is now the most common clinical guide 1,2), but the hardness determined by the tactile sense varies greatly according to individual experiences and thus lacks objectivity. The preparation of teeth to receive restorations involves the proper removal of the carious lesion. To achieve evidence-based caries treatment, it is necessary to objectively and quantitatively evaluate the hardness of carious dentin. Knoop hardness has been employed extensively in testing the hardness of both enamel and dentin 3). However, measuring the hardness of a tooth with a Knoop hardness tester inevitably requires specimens of the extracted tooth, and so this method is unusable in clinical practice. Until now, no hardness tester could be put to clinical use. This fact motivated us to develop a new hardness tester that can be used in clinical practice. In 2009 we discovered a new phenomenon that showed strong potential for measuring hardness. This article introduces the measurement principles, composition, construction, and usage of the Cariotester SUK-971 (SaneiME Corp., Yokohama, Japan), the new carious dentin hardness testing system we developed utilizing this phenomenon, and describes a dedicated program we developed to display the hardness of carious dentin as the Knoop hardness number (KHN). MATERIALS AND METHODS Principles of the new method for measuring hardness Hardness may be defined as the resistance to permanent indentation 3). In other words, a material is considered hard if it strongly resists indentation by a hard material such as diamond 4). Hardness is generally obtained by measuring the depth or width of the indentation produced by an indenter. For example, Rockwell hardness numbers are obtained by making an indentation with a ball or cone indenter and measuring the depth of the indentation 3,4). The KHN is obtained by measuring the long diagonal of an indentation from a rhombic-based pyramid-shaped indenter 3,4). We discovered that when a cone-shaped indenter coated with paint was pressed into an object, the paint disappeared in accordance with the depth of the indentation produced in the object. This fact suggests that the distance of the paint disappearance could be used as a substitute for the depth of the indentation. In the Cariotester system, therefore, the paint disappearance distance was measured to estimate hardness instead of measuring the depth of the indentation. In the Rockwell method, the depth of the indentation is measured by a sensitive dial micrometer 3). In the Cariotester system, in contrast, the method for detecting the indentation depth is completely different from anything reported to date and can be considered as a novel method of hardness measurement. In any case, as the Cariotester evaluates the depth of the indentation, the measurement principle Received Dec 2, 2012: Accepted May 27, 2013 doi:10.4012/dmj.2012-312 JOI JST.JSTAGE/dmj/2012-312
644 Dent Mater J 2013; 32(4): 643 647 of the Cariotester method can essentially be considered similar to the Rockwell method. Cariotester system composition, construction, and functions The Cariotester system is composed of a handpiece with a removable arm, a digital microscope, and a laptop computer (Fig. 1(a)). The cordless handpiece has a total length of 165 mm and weight of 55 g (including a CR2 battery). Figure 1(b) is a view of the internal structure of the handpiece. One end of an aluminum leaf spring (65 12 2 mm) is fastened to a metal block. On the other end of the leaf spring, a metal block with a socket for inserting the arm is fixed, and a tactile switch is mounted on the top of the block. As the load of the indenter increases, the leaf spring bends upward. A control screw is adjusted so that the switch turns on and a buzzer sounds when the indentation load reaches exactly 150 gf (1.45 N). A tungsten carbide rod (12 mm long, 1 mm diameter) is attached to the end of the arm. The tip of the rod is a cone-shaped indenter having a cone angle of 50 degrees and an apex that is rounded with a radius of 18 μm (Fig. 2(a)). A specially designed cylindrical stand is mounted on the X-Y stage of the microscope. When observing the indenter, the indenter rod is placed in a V-shaped groove on the top of the stand and fixed in place with an elastic wire. The indenter is magnified with the digital microscope and then observed on a laptop computer (approximately 360 total magnification when viewed on a 15 inch screen). Procedure for measuring the indentation depth using the Cariotester system The procedure for measuring the indentation depth using the Cariotester is as follows. (1) Before measuring, apply white poster color paint (Posca, Mitsubishi Pencil, Tokyo, Japan) to the tip of the indenter (Fig. 2(a)). (2) Hold the handpiece in the fingers and press the indenter of the handpiece into the object at a right angle (Fig. 2(b)), taking 1 second to reach the load of 150 gf, then withdraw the indenter. (3) Pull out the arm Fig. 1 Cariotester system. (a) Handpiece (H), digital microscope (M) and laptop computer (C) (b) Schematic illustration of internal structure of the handpiece Fig. 2 Procedure for measuring the depth of the indentation. (a) The indenter coated with white poster color paint (b) Press the indenter into the carious dentin with a 150-gf load (c) Measure the distance from the indenter tip to the paint disappearance line (CT depth)
Dent Mater J 2013; 32(4): 643 647 645 from the metal block. (4) Place the indenter rod in the V-shaped groove in the microscope stand and fix it in place with the elastic wire. (5) Measure the distance from the indenter tip to the paint disappearance line (CT depth, Fig. 2(c)), when the inclination of the line is not more than 15 degrees. If the inclination is more than 15 degrees, re-indentation will be recommended. In the case of the mobile tooth, an operator should support the tooth with his fingers on the opposite side of the cavity being tested, in order to measure on equal terms with non-mobile tooth. Development of a program to convert indentation depth into Knoop hardness Four molars with moderate caries were selected from among extracted human teeth preserved in a 10% formalin solution. The use of human teeth was approved by the Ethics Committee of Hyogo College of Medicine (Permit Number 586). The occlusal enamel was removed to expose the dentin caries. Each tooth was ground longitudinally with waterproof abrasive papers, consecutively #120, #320 and #1000, under running water to expose the center of the carious lesion (Fig. 3). The superficial heavily softened carious tissue was removed by using a spoon excavator (CD excavator #1, YDM Corp., Tokyo, Japan). The CT depth (μm) was determined by the Cariotester system with indentations made in the cavity floor dentin, as close as possible to the edge of the longitudinal section of the tooth and a 150-gf load applied with the Cariotester handpiece (Fig. 3). Next, the KHN was determined by a microhardness tester (MHT-1, Matsuzawa Seiki Kogyo Corp., Osaka, Japan) by fixing the specimen to the stage of the tester. The KHN indentations were made in the longitudinal section at 25 μm from the cavity floor with a 25-gf load for 15 s (Fig. 3). The data sets for CT depth and KHN were obtained at approximately equivalent positions. Then, a small part of remaining carious dentin was removed with the excavator, and further data sets for CT depth and KHN were obtained. In this way we obtained 14 pairs of corresponding data sets from the dentin positions located at various depths of carious dentin. The mean number of indentations in one dentin position was 3.9 for the Cariotester system (54 data points in 14 positions), and it was 3.6 for the hardness tester (50 data points in 14 positions). The data pairs were input into a computer and a fitted exponential curve was calculated to create a program for converting Cariotester CT depth into KHN. Table 1 RESULTS Table 1 shows a summary of the CT depth from the Cariotester indenter and the KHN from the microhardness tester for the 14 pairs of data sets obtained from dentin positions located at various depths of carious dentin. Figure 4 shows a scatter plot based on the data in Table 1 and a graph of the fitted exponential curve. The KHN was calculated from the relationship KHN = 2926D 1.438 R 2 =0.925 where D is the CT depth in µm and R 2 is the coefficient of determination. We created a program to convert the CT depth into the KHN and installed the program on a laptop computer. With this setup, if an CT depth (μm) measured by the Cariotester system and Knoop hardness number (KHN) determined by the microhardness tester for the 14 pair of dentin positions located at various depth of carious dentin Position no. CT depth KHN Fig. 3 Schematic illustration showing the locations of Cariotester indentations in cavity floor and Knoop indentations in longitudinal section. 1 48.9 (7.8) 9.7 (0.3) 2 49.0 (3.6) 10.0 (3.3) 3 37.8 (3.5) 14.1 (1.2) 4 30.5 (0.7) 18.9 (2.3) 5 33.5 (0.1) 21.6 (1.1) 6 30.2 (0.6) 23.7 (2.4) 7 30.2 (1.8) 24.9 (4.8) 8 22.9 (0.9) 26.8 (1.8) 9 24.3 (1.4) 27.2 (5.8) 10 23.4 (2.9) 32.8 (7.2) 11 28.8 (3.2) 34.1 (4.9) 12 20.0 (1.3) 40.6 (7.0) 13 15.5 (0.7) 55.1 (0.2) 14 14.5 (0.7) 57.9 (4.2)
646 Dent Mater J 2013; 32(4): 643 647 Fig. 4 Fig. 5 A scatter plot based on the data in Table 1 and a graph of the fitted exponential curve. The CT depth measured on the computer screen (L). The KHN is displayed concurrently (K). indentation is made in carious dentin with a 150-gf load by the Cariotester handpiece and the CT depth is measured on the computer, the KHN is displayed concurrently (Fig. 5). DISCUSSION Generally speaking, it is sufficient to measure the depth of the indentation for a quantitative evaluation of the hardness of an object, but most investigators have been using the KHN for the hardness of teeth for many years 5-10). The Cariotester system therefore needed to display not only the indentation depth but also the KHN for matching with previous studies. We measured the CT depth (μm) by using the Cariotester and the KHN by using a microhardness tester at 14 pairs of dentin positions. We installed our program on a laptop computer to convert the CT depth into the KHN. As a result, the Cariotester system can automatically display the KHN if the CT depth is measured. The hardness and the texture of dentin at the base of the cavity serve as indicators of caries penetration 2). When carious dentin remains in a cavity wall, the tip of an explorer tends to penetrate its surface 1). In contrast, healthy dentin is very resistant to an excavator 2). Therefore, the hardness of the remaining dentin is considered a reliable clinical guide for determining proper removal 1,2,11,12). However, the hardness judged by tactile sense with the explorer or excavator can be considered to lack objectivity. For example, a dental student might find that one teacher s definition of firm and leathery is another s interpretation of rather soft 13). Evidence-based treatment of carious lesions requires judging the hardness of the remaining dentin after excavation and evaluating whether carious dentin is properly removed. The use of caries-detecting dye is recommended as a guide to distinguish demineralized infected dentin from demineralized non-infected dentin (affected dentin) 11,14,15). One problem with the dye technique is that, even if the carious lesion is removed in accordance with the indication of the dye, no information is obtained about the hardness of the remaining dentin. Another problem is that almost no highly reliable clinical data is obtained about the rehardening of the affected dentin remaining at the base of the cavity. The Cariotester may be useful for solving these problems mentioned above. A clear relationship exists between the mineral volume percent and the square root of the KHN for partially demineralized enamel 16). In dentin, however, the relationship between microhardness and mineral content is complex 17). Dentin is composed of 68% inorganic hydroxyapatite complex, 22% organic component, and 10% water 18), and the microhardness of dentin reflects the average of a heterogeneous substrate 10) such as inter- and peri-tubular dentin, and tubules. It is suggested that in the process of remineralization of carious dentin, a time lag might occur between the rate of mineral uptake and the hardness increase 10,12). In any case, it is important to be aware of the hardness of the dentin in caries treatment. If clinicians used the Cariotester to measure the hardness of the remaining dentin following the removal of a carious lesion, they could consider the most appropriate restorative materials and methods. As the first chair-side hardness testing system, the Cariotester can provide objective data to scientifically support minimal intervention (MI) treatment. The features of the Cariotester are as follows. It is not necessary to polish the surface of the object being tested; that is, the measurement of hardness can be carried out on surfaces removed with a spoon excavator. The Cariotester can be used not only in vitro but also in vivo in the same way. The measurement procedure is simple and the time required for a single measurement is short (approximately 10 s). The clinician can quantitatively monitor the changes in the hardness of the root surface caries resulting from fluoride application or oral hygiene instruction. The clinician can objectively identify the remineralization during a stepwise excavation procedure in the case of deep caries. In addition, the Cariotester
Dent Mater J 2013; 32(4): 643 647 647 can be a useful tool for education. Dental students and recent graduates just beginning dental practice will benefit by using the Cariotester system, because the system can provide an objective guide to the hardness of the remaining dentin. They can more precisely understand the stages of caries removal and avoid the risk of excessive or random removal. It is generally accepted that all soft, carious dentin should be removed prior to restoration 19). However, the hardness level of dentin left in the cavity after excavation has not been specified. On the other hand, the need for removal of all carious dentin has been questioned 19-21). Hardness measurements can be visualized in hardnessdepth curves 6). In this curve the hardness increases from a highly softened outer lesion to a partially softened inner lesion, and then reaches its summit at the softening front whose hardness is almost on a level with the healthy dentin 6). Generally, carious dentin is divided into demineralized infected dentin that should be removed and demineralized noninfected dentin that should not be removed 11,13,14,19). However, the hardness level dividing these two softened dentin types has not been made clear. The Cariotester will be helpful in revealing the hardness levels that need removing, or leaving in the excavation procedure. As seen in Fig. 4, the indentation depths measured with the Cariotester indenter are approximately 20 μm for 40 KHN dentin and 24 μm for 30 KHN dentin. If a healthy dentin layer of more than 100 μm thickness remains after excavation, the Cariotester probing can be considered non-destructive to the pulpal wall. Hardness of dentin measured at 100 μm from the pulpal wall was 38±9.1 KHN 6). We can, therefore, use the Cariotester safely not only in moderate caries but also in the case of deep caries lesions. 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