The materials used in this study are listed in Table 1.
Tooth selection and preparation A total of 40 sound extracted mandibu-lar momandibu-lars were collected from the Oral Surgery Department, Faculty of Den-tistry, Mansoura University, Mansoura, Egypt. The patients were informed that the voluntarily donated extracted teeth would be used for research purposes.
To be included in the study, the mo-lars were required to have the following crown dimensions: 9-mm buccolingual distance and 10- to 11-mm mesiodistal
distance. The collected molars were observed under magnification (×10) in a binocular stereomicroscope (LOMO SF-100 Binocular Stereo Microscope, MBC-10). Teeth with preexisting cracks, caries, or attrition were discarded. The selected molars were carefully cleaned using an ultrasonic scaler (UDS-J Ultrasonic Scaler, Ningbo Sunglow Imp
& Exp) and then debrided with pumice (Americos Industries) using a rotary brush (Merssage Brush, Shofu). The molars were disinfected with 0.2% so-dium azide solution for 48 hours.30 To prevent dehydration, they were stored in physiologic saline for a period of no more than 1 month at 37°C until the time of the test.
The molars were randomly divid-ed into two groups (20 molars each) based on the restorative material used (Tetric EvoCeram or Grandio). Next, each group was randomly subdivided into two subgroups (n =10) relative to the application of flowable composite resin at the internal cavity line angles.
Table 1 Composite resin systems used in this study
Material Scientific classification Trade name Batch no. Manufacturer
Composite resin
Restoration Nanohybrid Tetric EvoCeram HB Grandio
J13387
J14049 Ivoclar Vivadent VOCO Restoration liner Nanohybrid Tetric EvoFlow
Grandio Flow
K15010 Ivoclar Vivadent VOCO
Bonding system
Filled, light-curing single component bonding agent for enamel and
dentin in conjunction with the acid etch technique Etching gel containing
37% phosphoric acid
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Each specimen received a nonreten-tive MOD slot preparation with the fol-lowing criteria: occlusogingival depth of 4 ± 0.3 mm without an axial wall and a buccolingual diameter of 3 ± 0.3 mm.
The remaining buccal and lingual cav-ity walls were measured using an elec-tronic digital caliper (MAX-CAL) after preparation for verification of the buc-colingual diameter. The preparation was carried out using a no. 59 carbide bur (Komet Dental).
Cuspal deflection test
A 3-cm polyvinyl chloride tube was filled with acrylic resin (Rapid Repair, Dent-sply) in the dough stage. The molar’s roots were positioned at the tube center and parallel to its long axis, leaving the crown and 2 mm of the root below the cementoenamel junction uncovered to accommodate the leads of the strain gauge. A dental surveyor was used to verify that the tooth was parallel to the tube. After setting of the acrylic resin, the parallelism of the cavity buccal and lingual walls was confirmed by the same surveyor.
All prepared cavities were etched using Total Etch for 15 seconds accord-ing to the manufacturer’s instructions.
Two precision strain gauges (KFG-2N-120-C1-11L1M2R, Kyowa Electronic Instruments) were attached to the buc-cal and lingual surfaces of each unre-stored specimen and bonded with epoxy adhesive resin (Strain Gauge Cement, Kyowa Electronic Instruments) to the middle third of the cavity’s exter-nal buccal and lingual walls (Fig 1). The leads of the strain gauge indicator were
connected to the gauge so that the gauge constituted one-half of a Wheat-stone bridge, with the other half internal to the strain gauge indicator. One layer of the Excite universal bonding system was applied and cured using a light-curing unit (Bluephase C8, Ivoclar Vivadent) at 800 mW/cm2 for 20 sec-onds. The light intensity was checked by radiometer (Bluephase Meter, Ivoclar Vivadent) prior to performing the test to confirm that the light inten-sity was not less than 700 mW/cm2.
The teeth were restored with resin-based composite materials in shade A2 according to the following groups:
• Group A: Flowable composite resin (Tetric EvoFlow) was applied at the internal cavity line angles with a small-gauge needle and cured for 20 seconds. Tetric EvoCeram was inserted horizontally in increments.
Each increment was approximately 2-mm thick and cured for 60 sec-onds: 20 seconds from the occlusal aspect, 20 seconds from the mesial aspect, and 20 seconds from the distal aspect.
• Group B: This group was similar to group A, except it did not receive flowable composite resin prior to in-sertion of Tetric EvoCeram.
• Group C: Flowable composite resin (Grandio Flow) was applied at the internal cavity line angles with a small-gauge needle and cured for 20 seconds. Grandio was inserted horizontally in increments. Each in-crement was approximately 2-mm thick and cured for 60 seconds: 20 seconds from the occlusal aspect,
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20 seconds from the mesial aspect, and 20 seconds from the distal as-pect.
• Group D: This group was similar to group C, except it did not receive flowable composite resin prior to in-sertion of Grandio.
During polymerization, the strain gauge recorded the changes in volt-age signals, which were then amplified by the recording system. The ampli-fied signals were transferred using an analog-to–digital converter (AD) card to a specially designed computer pro-gram (SIProg) for analysis. The results appeared as a curve between the time (seconds) and strain values. These readings for the Wheatstone bridge are directly proportional to the internal cuspal deflection of the buccal and lingual cusps of tested specimens.31
The present study is based on the data range, ie, the difference between the maximum and minimum strain values.
However, the data curves will be ana-lyzed in detail in future studies.
Strain measurement system The strain measurement system con-sisted of a three-channel amplifier circuit (Metrology Lab, Faculty of En-gineering, University of Mansoura), an AD card, and a personal computer with the Strain Indicator Program (SIProg), which was designed for this study by Dr Abouelatta (Fig 2).
SIProg was fully written in house us-ing Matlab packages. The main inter-face is shown in Fig 3. It consists of a title bar, menu bar, measuring infor-mation panel, processing panel, option panel, and display area. To calibrate
Fig 1 (above) Buccal view of the strain gauges bonded to the middle third of the external cavity wall.
Fig 2a to 2c (right) The strain measurement system.
Personal computer Strain indicator program AD card Tooth fixation 3-channel amplifier Curing unit
a
b c
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the strain measuring system, specific microstrain (μ
ε
) values were applied on a sample tooth. The strain meas-urements were recorded using P-3500 Strain Indicators and SB-10 Switch and Balance Units (Vishay Measurements Group). The same values were record-ed by the system in millivolts. A graph representing the relation between the strain measurement and correspond-ing voltage was plotted uscorrespond-ing an Excel spreadsheet (Microsoft).Data analysis
Ten specimens were tested for each group. SAS version 6.12 for Windows (SAS Institute) was used for all statis-tical analyses. The mean cuspal de-flection values (µm/m) and standard deviations were calculated and sub-jected to normality and homogeneity of variance tests. If they passed the tests, they were subjected to parametric sta-tistical analysis (independent sample t test). All tests were two-sided analy-ses, and differences were considered
statistically significant when P < .05 with a confidence level of 95%.