Investigating dental structure response to air abrasion: a finite element analysis

Air abrasion particles, propelled by a compressed air stream, remove material from the tooth’s surface. The air abrasion parameter plays an important role in removing the strains or plaque from the teeth. The research outcomes shed light on the stress distribution within dental structures using the finite element approach. Enamel, as the hardest and outermost layer of the tooth, consistently bears the highest stress levels during air abrasion procedures, regardless of whether the impact pressure is set at 80 or 100 psi. While enamel takes the initial force, it gradually transfers these forces to the dentin layer beneath, a denser but slightly less hard tissue. For abrasive particles falling within the 40 μm to 100 μm size range, an impact pressure of 80 psi is found to strike an optimal balance between effective material removal and minimizing damage to dental structures. However, when working with larger particles exceeding 100 μm, increasing the impact pressure to 100 psi becomes preferable to maintain efficiency and precision. The results of this research provide valuable guidance for enhancing dental procedures with a strong focus on patient safety and the maintenance of dental health. It underscores the importance of thoughtfully adjusting parameters like particle size and impact pressure to attain favourable treatment results while prioritizing the health and comfort of patients.


Introduction
Microabrasion or Air abrasion is a dentistry method used for a variety of objectives, the most common of which is to remove tooth decay and prepare teeth for restorative operations.It is a less intrusive alternative to regular dental drills.The air abrasion utilises a handpiece that resembles a miniature sandblaster.The sandblaster is attached to a source of compressed air and has little abrasive particles within.During the dental procedure, the dentist sprays compressed air with abrasive particles on the defective tooth that needs the treatment.The stream of the particles needs to be regulated for the different dental applications.Using air abrasion, dentists can remove tooth decay (cavities), eliminate old dental fillings, and prepare teeth for restorative materials.
The air abrasive particles are responsible for removing material from the tooth's surface [1].These fine abrasive particles are propelled by a compressed air stream and directed onto the tooth's surface for material removal.The abrasive particles such as Aluminium Oxide (Al 2 O 3 ), Sodium Bicarbonate (NaHCO 3 ), Silica, Calcium Phosphate-Based Abrasives, Garnet and Diamond dust are used in air abrasion procedures [2].Turp et al [3], studied the effect of particle size and deposition duration in air-particle abrasion on the surface properties and microstructure of zirconia.The authors compared the effect of the Al 2 O 3 and SiO 2 .For the study, they used the Al 2 O 3 of particle sizes 110 μm and 250 μm.They conclude that the increase in particle size does not affect the disposition of zirconium material but there is an increase in surface roughness.Milly et al [4], studied the air abrasion operating conditions and compared the abrasive particle effects.Their research objective is to refine its application as a minimally invasive operative procedure.For the study, the authors used Al 2 O 3 and bioactive glass (BAG)).The 10%, 50%, and 90% percentiles of the particle size distribution for Alumina powder were 23 μm, 37 μm, and 51 μm, respectively, while the corresponding values for BAG powder were 23 μm, 56 μm, and 82 μm used for the study.They employed both a dynamic cutting protocol and high vacuum suction to reduce the surface choking of particles.The authors concluded that the BAG powder has more cutting efficiency and is controllable with an encouraging role in minimally invasive operative dentistry.Paolinelis et al [5], the significance of dentine hardness as a predictor of removal effectiveness during alumina air abrasion.The authors conducted experiments on freshly removed molars with non-cavitated carious lesions and utilized Knoop Hardness Numbers (KHN) to measure dentine hardness at various intervals.For air abrasion, 27 μm Alumina particles were utilized, with consistent powder flow rates, pressure, distance, and nozzle angle.The authors calculated dentine removal rates using surface analysis software and non-contact surface profilometry.They concluded that air abrasion systems utilizing alumina particles were found to remove healthy dentine more efficiently than carious dentine.Halpern et al [6], study was to determine the best way to prepare enamel surfaces for keeping orthodontic brackets in place when exposed to shear forces.The study was carried out with, no air abrasion, and Al 2 O 3 particle sizes of 25 μm, 50 μm and 100 μm.Their result shows that the particles 50 μm and 100 μm improved retention compared to no air abrasion.The authors concluded that careful selection of the enamel surface preparation method can influence the stability of orthodontic brackets.The width and depth of fissures increase more when using air polishing because sodium bicarbonate particles are larger than aluminium oxide particles [7,8].The use of an aluminium oxide jet increased the depth of fissures.This is mainly due to the deeper penetration of smaller particles into the dental structure [9].Air polishing using bicarbonates is a safe and efficient method for removing plaque and subgingival biofilm, even in areas around titanium surfaces [10].Bühler et al [11], analysed the root surface roughness of the human molar induced by air polishing powder containing sodium bicarbonate and the glycine.For the study, the authors used the NaHCO 3 and Glycine of particle size 62 μm, and 49 μm respectively.The air pressure was considered 1.8 bar and the standoff distance (SoD) was maintained at 2 mm and 5 mm with angles of 45 and 90 degrees.The authors compared the effect of particle impact and concluded that NaHCO 3 is more efficient in creating the alteration in human teeth.Biazussi et al [12], evaluated the effect of NaHCO 3 and Amino acid Glycine on the implant teeth by studying its surface roughness during air abrasion.The researchers concluded that NaHCO 3 results in a higher surface roughness value compared to Glycine.The size of the abrasive particles is a decisive factor in determining the effectiveness and precision of the dental procedure.Smaller particles are typically used for better and more delicate work, while larger particles are employed for more determined material removal.The maximum particle size was found to be 250 μm [13].It's crucial to remember, though, that extrinsic stains and biofilm are the main uses for sodium bicarbonate air polishing.Herr et al [14] evaluated the effect of air polish on cementum using the standard tip.For the in-vitro study, the authors used NaHCO 3 (85 μm).Their result shows that NaHCO 3 produces greater volume loss and penetration depth on the cementum surface.The air abrasion parameter plays an important role in removing the strains or plaque from the teeth.Gebro et al [1] adopted the angle of impact of 60 and 80 degrees for posterior and occlusal 90 degrees for optimal results during the air abrasion.The author also suggested maintaining the SoD of 4 to 5 mm from the tooth surface for better output during the dental procedure.For the present analysis, simulation performed at an impact angle of 80 degrees and SoD of 4 mm.The main aim of this research is to analyse and understand the stresses that are distributed within the dental structures and the surrounding bone using the finite element method.The findings of the current research can have a significant impact on improving dental treatments and patient outcomes.

Model generation
The computer-aided drafting (CAD) model of the dental incisor is developed using the Cone Beam Tomography (CBCT) images.The DICOM images are processed in the 3D SLICER, an open-source image computing software.The images are uniform intervals of 1mm.The CAD model is generated and converted to the .stpextension.The converted file is imported to ANSYS Design Modeller for further editing.The different dental sections such as dentin, pulp, and jaw bone section are created in the ANSYS.Figure 1 represents the dental front incisor model with the section.The enamel is the outermost, hard and protective layer of the teeth.The tissue of the enamel is the hardest in the human body.Dentin is located beneath the enamel and it makes up the bulk of the tooth and provides support and protection to the innermost pulp.The pulp is the innermost part of the tooth.It contains nerves blood vessels and connective tissues.The cortical and cancellous bones of the jawbone section were analysed.The cortical bone is the hard, dense outer layer of the jaw bone.It provides the strength and rigidity to the jaw.The cancellous bone is also known as spongy bone and is a less dense inner portion of the jaw bone.The material properties of each dental section are represented in table 1.The ANSYS Workbench R2 2023-explicit dynamics model is adopted for air abrasion impact simulation.Figure 2 shows the schematic representation of the air abrasion parameters.The sodium bicarbonate (NaHCO 3) particle impact analysis was carried out to investigate the stress distribution and its effect on the dental structure.Many researchers have carried out the clinical investigation of dental air abrasion parameters investigations.For the finite element simulation, the standoff distance (SoD) and angle of impact (AoI) are kept constant that is SoD; 4 mm and AoI; 80 degrees.The particle size and the impact pressure are varied and shown in table 2.

Meshing and boundary conditions
The generated CAD model is meshed with the 10-noded SOLID 187 higher-order 3D element.The mesh size is finalised based on the number of elements matching with previous published literature [20].An optimal meshed model cross-section is shown in figure 3(a).A maximum of 103326 elements and 21272 nodes are generated considering a mesh size of 0.05 mm.For the analysis, the jaw bone is fixed at the end surfaces as shown in figure 3(b).The abrasive particle is made to impact the incisor teeth surface and the abrasive particle is assumed to be rigid and circular in shape.For the air abrasion of NaHCO 3 particle impact, the pressure is selected as 20, 40, 80, 120 and 160 psi.The initial condition is applied as the velocity.This velocity is considered from the impact pressure applied.The pressure is converted to the velocity using equation (1).
Where ν, is the velocity (m s −1 ), ρ represents the fluid mass density (kg m −3 ), and P d signifies the dynamics pressure (Pa).
The contacts between the components of the dental surfaces are intact with each other.Therefore the bonded contact is selected between the components.
The pressure was selected based on the literature by Hegde et al [21], which described the pressure range of 40 to 160 psi and also recommended maintaining the pressure of 80 psi for etching, and 100 psi for cutting [22].For the abrasion, NaHCO 3 particle sizes selected are 40 μm, 45 μm, 74 μm, 120 μm, and 250 μm [8,18,19].The other parameters such as SoD and angle of impact (AoI) are considered as 4 mm and 80 degrees respectively [1].

Result and discussion
Explicit dynamics analysis is being conducted within the ANSYS Workbench-R2 environment.The simulation involves setting up a virtual model of the Incisor teeth, including geometry and material properties, within the ANSYS Workbench-R2 software.The primary objective of this analysis is to understand the behaviour of a single particle as it impacts a modelled set of Incisor teeth.The analysis can provide valuable information about potential damage or stresses that may occur as a result of such impacts.

Stress distribution in dental structure
Enamel is the hard outer structure of the dental structure.It is considered the hardest and most mineralized tissue dental structure.It acts as a barrier to exterior factors like bacteria, acids, and mechanical forces.Enamel bears the forces generated during chewing and biting.It is hard and durable, which allows it to withstand the mechanical forces associated with dental procedures.Figure 4(a) shows the maximum stress distribution in the enamel for parameters of 250 μm and an impact pressure of 80 psi.The maximum stress developed in the enamel is 0.7579 MPa for the impact pressure of 80 psi and particle size of 250 μm.When compared to other particle sizes 250 μm is generated the maximum stress on the enamel.Similar research findings were reported by Gerbo et al [1].The authors showed that the maximum stress was generated on the enamel.The impact pressure of 20 and 60 psi with the smaller particle size will not have any effect on the surface of the teeth.
Excessive stress during air abrasion can cause micro fractures or cracks in the dentin.Understanding stress patterns can help optimize the technique to minimize the risk of these cracks, which could weaken the tooth.During the simulation, the maximum stress of 0.252 MPa in the dentin is obtained for the impact pressure of 100 psi.But the particle size is maximum of 250 μm (figure 4(b)).
The pulp is the innermost portion of the tooth, containing nerves and blood vessels, and it is sensitive to mechanical forces.During the particle impact, the maximum stress of 0.0000712 MPa in the dentin is obtained for the impact pressure of 100 psi for the particle size of 250 μm (figure 5).Dentin is a vital component of the tooth structure, and high stress in this area can potentially lead to pain and discomfort, especially if the stress levels are sustained or repeated.
Air abrasion is primarily directed at dental hard tissues like enamel and dentin, there can be some indirect effects on both cortical and cancellous bone in specific dental procedures.Cancellous bone, on the other hand, is the spongy, less dense bone found within the interior of bones, such as in the jawbone.It is not a direct target of air abrasion procedures.Therefore, air abrasion typically doesn't directly generate stress within the cancellous bone.Cortical bone is the dense outer layer of bone that provides structural support.Similar to cancellous bone, the stress in cortical bone during air abrasion may be a result of mechanical forces or vibrations.However, it was observed from figures 6(a) and (b), that stress maximum stress of 0.028 MPa and 0.082 MPa was found in the cancellous and cortical bone respectively.This maximum stress is obtained for the particle size of 250 μm.Dental practitioners should be aware of these effects and take precautions to minimize the damage to the tissue, particularly when performing procedures near bone structures.

Effect of particles and impact pressure on the teeth structure
Lower pressure settings are often used for delicate procedures and higher pressure settings are employed for more aggressive removal of dental materials.The choice of abrasive particle size has a significant impact on the amount of tooth structure removed.Finer abrasive particles typically remove less tooth structure per impact.Coarser particles remove more material per impact and are used for more aggressive treatments [23].
Figure 7 shows the effects of the stress distribution on the teeth structure.The particle size of 250 μm has the maximum effect on the dental structure.It can be concluded from figure 7, that as the particle size increases the stress also increases.When considering the impact pressure, the maximum stress is obtained on the enamel for the impact pressure of 80 psi.It was observed that the impact pressure of 80 to 100 psi had the maximum effect on the dental structure.Coskun et al [24] shows that the dental structure experiences the highest stress when subjected to an impact pressure of 75 psi, coupled with abrasive particles measuring 110 μm in size.

Optimum parameters concerning stress distribution
Figure 8 illustrates the distribution of stress within the dental structures concerning various particle sizes used in the dental procedure.The observations from figure 8 indicate the following significant findings; maximum stresses in the enamel and lowest in the pulp (figures 8(a)-(e)).The highest levels of stress are localized at the enamel for the particle size of 250 μm (figures 8(e)), which is the outermost layer of the tooth.This is a noteworthy observation because enamel is the hardest and most mineralized tissue in the human body.In contrast, the study found that the lowest stress levels were observed in the pulp region of the tooth.The dental pulp is the innermost part of the tooth and contains nerves and blood vessels.Between the outer enamel and the inner pulp of a tooth, there is a layer known as dentin.Dentin is a hard tissue that lies beneath the enamel and surrounds the pulp.It serves as a cushioning layer and acts as a buffer between the hard enamel and the sensitive pulp tissue.In dental biomechanics, dentin plays a critical role in transmitting forces and stress.When external forces, such as impact pressure or biting forces, are applied to the enamel, dentin absorbs some of that stress and helps distribute it evenly across the tooth.This function is essential for protecting the dental pulp from excessive stress and damage.
Analysing the stresses in cortical and cancellous bone during air abrasion is an essential aspect of dental biomechanics and safety.While air abrasion primarily targets dental hard tissues like enamel and dentin, it can have some indirect effects on the surrounding bone structures.Cortical bone is the dense, hard outer layer of bone.It is highly resistant to stress and is less susceptible to damage from air abrasion compared to cancellous bone.Cancellous bone can absorb and transmit stresses more readily than cortical bone.High-pressure air abrasion near cancellous bone can potentially lead to localized stress concentrations The research findings indicate that the highest stress levels in the dental region were consistently observed on the enamel for both impact pressures of 80 psi and 100 psi.Enamel, being the hardest and outermost layer of the tooth, is the first to bear and distribute the impact forces of the particles.The data indicates that a maximum stress level of 0.757 MPa (figure 8(e)) was observed at an impact pressure of 80 psi.This information provides a quantitative measure of the stress experienced by the enamel under these conditions.While enamel bears the initial impact, it gradually transfers the forces to the dentin layer beneath it.Dentin, being a denser but slightly less hard tissue, acts as a cushioning layer, absorbing and distributing forces deeper into the tooth structure.A stress level of 0.252 MPa (figure 8(e)) in the dentin is significant and suggests that this level of impact pressure can have a notable effect on the dentin structure.Dentin is a vital component of the tooth, and it plays a crucial role in transmitting and distributing forces within the tooth.For abrasive particles falling within the 40 μm to 100 μm size range, it appears that an impact pressure of 80 psi is most effective in achieving the desired outcome.This combination likely provides the right balance between effective material removal and minimizing the risk of excessive stress or damage to dental structures.When using abrasive particles larger than 100 μm, the observation suggests that increasing the impact pressure to 100 psi is preferable.Larger particles may require higher pressure to achieve efficient material removal while maintaining control and precision during the dental procedure.
The dental pulp is well-protected by the surrounding enamel and dentin layers, it can withstand low stress levels without experiencing adverse consequences.This reaffirms the natural cushioning and insulating properties of dental tissues, which help preserve the health and comfort of the dental pulp.The stress observed on the dental pulp during the impact of 100 psi.It's noteworthy that the maximum stress level on the dental pulp, which is 7.12E-05 MPa for the particle size of 250 μm, is relatively low and is not expected to have any significant adverse effects on the dental structure or the dental pulp itself.The results of the study reveal that the cortical bone in the jaw underwent a peak stress level measuring 0.082 MPa, as illustrated in figure 8(e).This maximum stress was observed when using abrasive particles with a size of 250 μm, and the impact pressure applied in this context was 80 psi.It was observed that when the particle size falls within the range of 40 μm to 120 μm, the maximum effect of stress on the jawbone is observed when using an impact pressure of 100 psi.Conversely, when the particle size exceeds 120 μm, it is advisable to reduce the impact pressure to 80 psi.The research results highlight that the cancellous bone registered a maximum stress level of 0.028 MPa figure 8(e).This stress level was observed when using abrasive particles with a size of 250 μm and an impact pressure of 160 psi.It is observed that as the particle size increases, the stress in the cancellous bone also increases.This suggests that larger abrasive particles may have a more significant impact on the cancellous bone structure.The observation that clinicians generally do not use air abrasion on the cancellous bone aligns with standard dental practice.The optimum recommended settings from the current research investigation are shown in table 3. Air abrasion is primarily directed at dental hard tissues like enamel and dentin.Cancellous bone, being the spongy, less dense bone within the interior of bones, is not typically a direct target in dental procedures.These findings emphasize the importance of tailoring the parameters of dental procedures, such as particle size and pressure, to the specific clinical situation and the type of treatment being performed.It's important to note that the exact parameters may vary depending on the specific clinical scenario, the type of dental equipment being used, and the patient's individual needs.Dental practitioners rely on their training and experience to make informed decisions regarding these parameters to achieve the best results for their patients.
In this work, wear is not considered as one of the output parameter.Further, wear can be estimated to know the effect of different abrasive particles with different parameters [25,26].

Conclusion
The choice of pressure settings and abrasive particle size in dental procedures plays a critical role in achieving desired outcomes.Lower pressure settings are suitable for delicate procedures, while higher pressures are reserved for more aggressive material removal.The analysis of stresses in cortical and cancellous bone during air abrasion is a critical facet of dental biomechanics and patient safety.While air abrasion is primarily focused on dental hard tissues, it can indirectly impact the surrounding bone structures.Cortical bone, with its dense and robust composition, exhibits a high-stress resistance and is less susceptible to damage from air abrasion compared to cancellous bone.The research findings have shed light on the stress distribution within dental structures.Enamel, as the hardest and outermost layer of the tooth, consistently bears the highest stress levels during air abrasion procedures, regardless of whether the impact pressure is set at 80 or 100 psi.While enamel takes the initial force, it gradually transfers these forces to the dentin layer beneath, which is denser but slightly Table 3. Recommended settings for the optimum output during air abrasion.

Dental Structure
Optimum output settings for air abrasion Enamel If the particle size is > 45 μm maintain impact pressure at 80 psi If the particle size is <45 μm maintain the impact pressure at 100 psi Dentin If the particle size is > 120 μm maintain the impact pressure at 100 psi If the particle size is < 120 μm maintain impact pressure at 80 psi Pulp The effect produced is negligible Cortical bone If the particle size is > 120 μm maintain the impact pressure at 100 psi If the particle size is < 120 μm maintain impact pressure at 80 psi Cancellous bone All particle sizes lead to an effect, The maximum effect will be observed at 160 psi softer tissue.For abrasive particles falling within the 40 μm to 100 μm size range, an impact pressure of 80 psi is found to strike an optimal balance between effective material removal and minimizing damage to dental structures.However, when working with larger particles exceeding 100 μm, increasing the impact pressure to 100 psi becomes preferable to maintain efficiency and precision.The results of this research provide valuable guidance for enhancing dental procedures with a strong focus on patient safety and the maintenance of dental health.It underscores the importance of thoughtfully adjusting parameters like particle size and impact pressure to attain favorable treatment results while prioritizing the health and comfort of patients.

Figure 5 .
Figure 5. Stress distribution in the pulp.

Figure 7 .
Figure 7. Effect of impact pressure and particle size.