In this study, the HH Nomad Pro 2 (Kavo Kerr, Biberach, Germany) was used to conduct fully hand-guided dental X-ray images by means of a digital dental X-ray sensor (GXS 700, size 1 and 2, Gendex, Kavo Kerr, Biberach, Germany). X-ray image quality parameters (according to the current statutes of the German Dental Association; ) of the Nomad Pro 2 were then compared to a typical WM X-ray unit (Heliodent Plus, Sirona Dental Systems, Bensheim, Germany) with the same digital dental X-ray sensors (GXS 700, size 1 and 2, Gendex, Kavo Kerr, Biberach, Germany). Quality parameters were distortion, level of detail, image size, overlay, resolution, radiation field, and technical parameters, such as distance from tube end to focus and receiver dose (see Table 1).
In order to record these X-ray image quality parameters, dental X-ray images as well as bite wing images (mandatory criteria for bite wing images are marked with an asterisk in Table 1) of the anterior teeth, premolars and molars of the left and right maxilla and mandible were conducted with a real tooth phantom (DXTTR mannequin, Densply, Ontario, USA). All shots were taken using a positioning ring with a metal rod (Fig. 1b). Images were taken holding the HH device first (Fig. 1a). Afterwards, all images were taken again using the WM device. Each image was positioned and shot individually.
All examinations were carried out and controlled by the study leader (specialist for oral and maxillofacial surgery) and a medical student in her 5th year of study.
Furthermore, three dental students (having taken dental X-ray images supervised for 2 years), three dental assistants (> 5 years of work experience) and three dentists (> 5 years of work experience) were instructed to use the Nomad Pro 2 and conduct X-ray images with the real tooth phantom. The dental students had to be in their clinical study phase, the dental assistants had to have completed their vocational training and the dentists had to have two years of experience on the job after graduation. Everyone received a 20-min introduction and training with the HH device before the X-ray images were taken. The following images had to be taken independently: Ten dental X-ray images of (1) the anterior teeth (Fig. 2), (2) ten of the premolars and (3) ten of the molars of the maxilla and mandible in each quadrant (n = 120) Additionally, ten bite wing images (4) (of molars/premolars) were taken on each side with the Nomad Pro 2 (n = 20, in total n = 140) (Table 2). They were compared and analysed using the image quality parameters described above as well (Table 1).
The real tooth phantom used in this study has been used for approximately 20 years in numerous scientific international studies and has been validated to physiologically represent the dentoalveolar anatomy [11, 12].
Only an accepted and constancy checked diagnostic monitor approved according to German standards (DIN standard 6868-157) was used for the examination and diagnosis of the generated dental film images of both the HH and the WM system.
Image receptor alignment and recording space
Sensor alignment has been identical for both WM and HH device generated images. The prescribed and approved sensor holders for the GXS 700-sensor, size 1 and 2 (Gendex, Kavo Kerr, Biberach, Germany) were used. The experimental setup is shown as an example in Fig. 1. The images were taken in the approved X-ray room, in which the remaining standard dental X-ray units are also located.
Radiation protection aspects
Throughout this study, a shared film-based X-ray dosimeter (whole-body dosimeter; radiation type: X-ray s and gamma rays; measurement: depth person dose; measuring range 0.1 mSv–1 Sv, 13 keV–1.4 MeV) was worn by all investigating personnel in order to measure released radiation (carried at chest height on an X-ray apron approved for dental radiology) for each taken image (separate dosimeters for HH and WM devices). Between active image acquisition it was stored in a radiation protected room (image release chamber for dental X-ray devices). After the acquisition sequence, the X-ray dosimeter was read according to the routine praxis of radiation dose monitoring once a month by local material testing office (the reading of the dosimeter was corrected for natural, background dose).
Ethics approval and trial registration
This study was approved by the Ethics Committee of the University of Duesseldorf (Study number: 2018-162-KFogU). Trial registration of prospective trials: Central Study Register of Duesseldorf University Hospital, Registration-ID: 2018064716. Additionally, this study was approved by the district government of North Rhine-Westphalia due to the use of X-ray radiation (approval 772/18).
A statistical power analysis was used to determine a significant sample size (G*Power open source software . Data analysis was performed using IBM SPSS statistics for Mac version 26 (SPSS Inc., Chicago, IL USA) and Microsoft Excel for Mac version 16.16.3 (Microsoft, Redmond, WA, USA). Frequency distributions for the analyses of image quality parameters were examined with the help of cross-tabs (Pearson’s Chi-square test). Due to skewed/abnormal distribution, nonparametric tests were used, additionally, to compare the image quality parameters between the HH Nomad Pro 2 and the WM Heliodent Plus (Mann–Whitney U, Kruskal–Wallis H). The level of significance was set to p ≤ 0.05. Values of p ≤ 0.01 were considered highly significant.