Selection of specimens
A variety of tooth specimens were examined in this study and were grouped into three;
Extracted teeth, as a reference to test the comparison between typodontic teeth and these natural teeth.
Commercially available typodont teeth.
Typodont teeth developed in this work using a 3D printing approach of composite materials.
Several extracted non-decayed mandibular first molars were selected for the study in order to recreate them by 3D printing. The extracted teeth were selected from a human tissue bank; with ethical approval obtained from the Queen Mary Research Ethics Committee (QMREC2008/57). Six commercial artificial mandibular first molars were selected from various suppliers commonly used in UK dental schools; Acadental (USA), Frasaco (Germany), IDEA (USA), Fabrica de Sorrisos (Brazil), One Dental (Australia) and Nissin (Japan). The development of composite teeth is described below.
Samples were imaged using the MuCAT2 Time-Delay Integration (TDI) high-contrast resolution scanner developed by Davis and Elliott26. MuCAT2 uses a charge-coupled detection camera (Spectral Instruments, USA) with a 100 μm thick columnar cesium iodide scintillator (Applied Scintillation Technologies Ltd., UK)24,26,27. Natural teeth were scanned at 90 keV, 180 μA, while typodont teeth, less opaque to X-rays, were scanned at 40 keV, 405 μA (Fig. 1). The composite materials developed in the study were then imaged using the same parameters as the commercial typodont teeth. All specimens were imaged at a voxel size of 15 μm. Images were reconstructed using a modified Feldkamp cone-beam backprojection algorithm28. The reconstructed images were then modified and converted to a 3D print file format (*.stl) as described in a previous study24. A physical map of the tooth used for 3D printing typodont teeth can be found in the supplemental material as an *.stl file.
Materials commonly used in a dental clinical and research environment were selected to create a composite material. These materials included HAp, CHAp, BAG, GF, FM, and ceramic materials used in the fabrication of dentures of different hardness. Details of material production are shown in Table 1.
All powders were ground using a Gy-Ro mill (Glen Creston, UK) for 10 min, before being sieved through a stainless steel sieve
Digital light processing
A digital light processing (DLP) printer was used (Anycubic Photon, Anycubic, China) to produce the composites. Due to the experimental nature of the fabricated materials, a low cost printer was chosen. It should be noted that the use of these experimental materials did not cause any apparent detrimental effects to the printer, such as damage to the vat film layer or the build platform. Each composite was printed at 50 μm layer height, with each layer cured for 25 s. These parameters were set using Anycubic Photon Slicer printer slicing software (version 1.3.3, 2017; Anycubic, China). Overall, printing 20 individual mandibular molars took 4 h. The printability of the tooth geometry was tested by producing molar shapes rather than cylindrical specimens, as well as to ensure comparison with force measurements on native tissue. Once printed, the models were washed in 90% ethanol for 20 minutes to remove any uncured resin, then cured for 30 minutes at 60°C using Formlabs Wash and Formlabs Cures, respectively (Formlabs Inc., USA ).
Once the potential enamel and dentin analogs were identified through the force measurements, separate enamel and dentin structures were printed using the identified materials. The enamel was printed oversized (by 2%), to allow adjustment of the dentin inside. The separate structures were assembled using an uncured “enamel” resin placed inside the enamel, the dentin was then fixed to the enamel by an additional 10 s curing using a hand-held UV curing device (3 M™ Elipar™ DeepCure-L LED Curing Light, The 3 M Company, USA).
The possibility of establishing a measure of tactile perception felt by trained and qualified dentists does not seem to have been studied before. Therefore, a new technique to measure cutting forces was developed for this study. The composite material developed in this study was embedded in acrylic blocks (Kemdent Simplex Rapid, Associated Dental Products Ltd., UK) encompassing a 3D printed mold for later use. Once fixed, the samples were mounted on a 3-axis load cell (model 3A60A, Interface Force Measurements Ltd., UK). A high-speed dental handpiece (TE-95 BC Alegra Dental Air Rotor Handpiece, The W&H Group, Austria) with a cylindrical diamond bur (111-012 M, Dentsply Sirona, USA) was mounted on a vertical stage (LMS -180 Precision Linear Stage, Physik Instrumente GmBH, Germany), which in turn was mounted on a horizontal stage (Fig. 6). Both stages were controlled using an A-81 × PIglide Motion Controller (Physik Instrumente Ltd., Germany) running PIMikroMove (Version 2.10, 2015; Physik Instrumente GmBH, Germany). The handpiece was connected and powered by a portable turbine unit (GXJ Lab, China) and maintained at a constant speed of 40,000 rpm. The speed at which the dental handpiece cut into the sample was maintained at a set speed of 0.1 mm.s-1, the forces being measured in real time. Load data was recorded via a 4-channel signal amplifier (ME-Meβsysteme GmbH, Germany) connected to a computer running GSVmulti (Version 1.40, 2018; ME-Meβsysteme GmbH, Germany) which records the real-time load acting on 3D printed composites when the dental handpiece is brought into contact. The data is displayed as a force versus time graph (Fig. 7).
Experiments were conducted to measure the force acting on the 3D printed composites when the dental handpiece was brought into contact using the positioning steps. The dental handpiece was attached to the vertical platen which was then mounted on the horizontal plates to allow cutting in the X direction (mesiodistal) and Zdirection (occlusal). The handpiece was positioned 10 mm from the sample before each cut to allow sufficient time for the bur to reach operating speed before contacting the surface of the samples. The handpiece cuts in the mesiodistal direction when the horizontal stage crosses the sample and 10 mm beyond the surface of the samples. The handpiece was then lifted 10 mm upwards after cutting to ensure that no contact was made with the specimen. Completion of the cut through the tooth was followed by the horizontal step returning to the original position before the cut (10 mm from the specimen) in the X-axis as, as well as the original position in the Zairplane. The handpiece has been moved down 1mm from the previous cut and the cutting procedure of moving the bur along the X-axis to remove more dental material was repeated. For all specimens, a first cut 1mm deep was made with the dental handpiece (1mm starting at the highest point of the occlusal surface) to establish a flat surface for subsequent cuts to be compared. The first cut would record a force variation that reflected the geometry of the occlusal surface (Fig. 7, cut 1) such that cutting across the high features of the tooth resulted in high forces recorded while the lower regions resulted in forces recorded smaller. The first cut was omitted from the data analysis to allow comparison of the material’s response to the cutting process and to eliminate morphological influences. After the initial cut, the force reading plateaued (slices 2 and 3) when the bur made contact with solid enamel. In some cases, which depended on the size of the tooth, the fourth cut caused the bur to contact both enamel and dentin, causing the cutting forces to fluctuate as the bur moved between the two. materials. Subsequent cuts would then follow cuts 2 and 3 in terms of presenting a plateau of force when the bur contacts solid dentin.
Force data was collected and then processed to establish an average force acting between the 3D printed composite and the dental handpiece by taking the numerical average of all force data points collected. Specifically, points recorded between the 5th and 95th percentile were averaged to eliminate points collected during entry and exit cuts, which were strongly affected by the external geometry of the tooth. The force collection process was further repeated on the extracted teeth so that later comparison could be made with the 3D printed typodont teeth.
Data analysis was performed using Microsoft Excel (version 1909, 2019; Microsoft, USA) using a data analysis plug-in. Data were subjected to a one-way ANOVA test and, where appropriate, Tukey’s post hoc test to calculate the significance of the results, with statistical significance measured as P