This study was approved by the Ethical Committee of Stomatological Hospital of the Air Force Military Medical University (grant number IRB-REV-2016044), and the healthy 7-12-year-old school-age children who were voluntary willing to participate in the project were recruited open to the public. The inclusion and exclusion criteria of this study were formulated according to the application precautions of LDF, and the target population that met the experimental requirements was screened. The experimental method and purpose was informed to the subjects and their guardians and the written informed consent was obtained from the guardians, and meanwhile, the subject's medical personal file was established respectively.
1.1.1 Inclusion criteria
1) Physical and mental health, no systematic, infectious or genetic diseases;
2) No history of anterior tooth trauma, surgery, repair or orthodontic treatment;
3) No obvious pigmentation, caries, crowding or malocclusion;
4) The eruption height of the test teeth meet the testing needs that the occlusal surface is at least 5mm above the gingival margin, the periodontal and oral health is good;
5) Subjects can effectively cooperate to complete the test.
1.1.2 Exclusion criteria
1) Don’t meet the inclusion criteria;
2) Suffering hematological or other systemic illnesses;
3) Use the cardiovascular drugs recently;
4) Sensitive to the testing materials;
5) Incomplete collection of the basic information or refuse to participate the test.
1.2 PBF data acquisition
In order to unify the standard, all the data collected in this study were taken under the room temperature (20℃) and the subjects were kept in a seated and calm state. A silicone rubber impression (3M，ExpressTMSTD，3M ESPE Dental Products，Minnesota, USA) was made for the subject after the maxillary anterior testing area was cleaned thoroughly. The thickness of the lip side of the impression was better to maintain about 3-5mm so that an effective retention for the probe could be formed to enhance the stability of the detection after punching. The position of drilling corresponding to the test teeth in the impression was situated in the middle of the tooth surface and distanced from the gingival margin about 3-5mm , and the diameter of the drilling hole was consistent with the probe. The subjects were asked to keep in the seated, calm and relaxed state after the LDF was preheated and calibrated, and then the silicone rubber impression was reset to the detected area in the test area (Fig 1). Test subjects were asked to maintain normal and steady breath and keep head static as far as possible in order to ensure the stability of the detected data. Generally, the time for detection was 1-3min after which the output waveform was relatively stable, and the test results were taken from a relatively stable waveband which the required time was not less than 30s and SD < 4 in the output result was regarded as a valid testing result. The PBF of the 12-22 teeth would be detected following this step in turn and the test results were recorded as mentioned above. The subjects were asked to have a quiet rest for 10 minutes after the test was completed and then the above test would be repeated and the corresponding record was recorded as the second measurement. The mean value the two tests was considered as the final results. Finally, photos of the maxillary anterior teeth and root tip films were taken. The instruments used in this study were LDF (Perimed, PF 5001, Perimed AB, Stockholm, Sweden) and LDF probe (Perimed, DP 416, Perimed AB, Stockholm, Sweden).
1.3 Statistical Analysis
The test results were statistically analyzed using SPSS22.0 software. The basic characteristics of the subjects are described in terms of age and gender. The mean, standard deviation, and reference range of the subjects’ PBF values are described based on their age, sex, and tooth position. Multi-factor analysis of variance was implemented, mainly including age, gender, and tooth position; the test level α was set to 0.05.