Stress Distribution Area Research Articles

Effect of loading angle and fabrication materials on stress distribution with periodontal splint in compromised periodontal tissues: a finite element study

Objective: To evaluate the effect of polyetheretherketone (PEEK) periodontal splints and splints made from other materials under static loading on stress distributions in periodontal tissues, cement layer, and splints themselves. Methods: A finite element model based on cone-beam CT imaging data of a 25-year-old male patient (treated at the Department of Prosthodontics, School of Stomatology, The Fourth Military Medical University in October 2021 for a cracked maxillary molar) with a healthy and intact mandibular dentition and periodontal health was constructed. The finite element model included anterior mandible dentition, mandibular bone model without bone resorption (WBR group), a periodontally compromised mandible model (control group), and three types of periodontal splints: a PEEK periodontal splint (0.7 mm thick, Young's modulus: 4.1 MPa), a fiber-reinforced resin (FRC) splint (1.0 mm thick, Young's modulus: 37.0 MPa), and a titanium splint (1.2 mm thick, Young's modulus: 110.0 MPa). The bone resorption models fixed with different periodontal splints constituted the experimental groups (PEEK group, FRC group and titanium group). Loading of 100 N was applied on the midpoint of the incisal edge of tooth 41. The direction was set at 0°, which was parallel to the long axis of the tooth and downward. The buccal to lingual and downward angles were 30°and 60°, respectively, perpendicular to the long axis of the tooth and 90° to the lingual side. The finite element analysis software was utilized to analyze the stress distribution characteristics of the periodontal tissues, adhesive layer, and the splint itself in the anterior mandibular teeth among the different group. Results: Under the different loading simulation, in the control group, the maximal von Mises stresses of periodontal ligament and bone were 15.7-50.2 MPa and 38.8-130.3 MPa, respectively, and in the WBR group, the maximal von Mises stresses of periodontal ligament and bone were 3.6-6.4 MPa and 16.5-42.7 MPa, respectively. Under the same loading conditions, the magnitude of maximal von Mises stresses in periodontal tissues in the PEEK group was 4.6-6.2 MPa, and the magnitude of stresses in the periodontal ligament of 41 teeth in the WBR group was similar to that in the PEEK group, but higher than that in the FRC and titanium groups. The maximal von Mises stresses of each group is primarily distributed in the periodontal ligament and alveolar bone at the cervical area of the tooth. The higher the elastic modulus of the splint, the higher its own maximal von Mises stresses, and the smaller the maximal principal stresses transmitted to the adhesive layer. In the PEEK group and titanium group, the stress distribution area in the adhesive layer and the splint was near the splint connection adjacent to tooth 41. Conclusions: Periodontal splints fabricated from three types of materials, are effective in distributing stress within the periodontal tissues of the abutment teeth. Compared to FRC and titanium group, the higher PEEK splint stress value was obtained, and the smaller the stress value was transmitted to its adhesive layer.

The cross-sectional morphology of the proximal femoral diaphysis is defined by the anteversion angle.

Osteoporosis in postmenopausal women is one of the causes of femoral fractures and is prevented by the administration of bisphosphonates. Individual morphologies are considered to increase the risk of atypical fractures associated with long-term administration. To evaluate cortical bone morphology quantitatively, we established a method to measure the distance from the center point of a cross-section to the external and internal borders based on CT images. Using this method, 44 sides of a female femoral skeleton specimen were examined and areas of protrusion and thickening in the medial anterior and lateral posterior regions just below the lesser trochanter were identified. These positions strongly correlated with the anteversion angle, suggesting the involvement of the distribution of the load received from body weight defined by the angle. The finite element method was used to examine the relationships between the positions of these areas with compressive and tensile stress distribution areas in the one-legged standing condition. The medial anterior region and lateral posterior region protruded and thickened in response to compressive and tensile stress, respectively. In addition, a hierarchical relationship was observed between the anteversion angle, tensile stress distribution, protrusion, and thickening in femurs with thinning of cortical bone, indicating that morphogenesis occurs adaptively to loading. The present results demonstrate the usefulness of this method in considering the formation mechanism and function of the femoral diaphysis and suggest that bone remodeling is necessary to maintain adaptability.

A METHOD OF SIMULATING SEAT LOAD FOR NUMERICAL ANALYSIS OF WOOD CHAIR STRUCTURE

This study aimed to investigate the characteristic values of the human-seat interface in a normal sitting posture, and to numerically mode the load on the chair seat for the structural design of chairs. The stress distributions and the characteristic values of seat were measured under normal sitting posture by using a human body pressure distribution measurement system considering the effects of gender and body mass index (BMI). The stress distribution on the seat was then numerically modeled using three modeling methods. The observed results and the numerical analysisresults were compared. The results showed that an inverted U-shaped pressure distribution was observed in normal sitting posture. The stress was concentrated on the ischial tuberosity with a maximum value of 0.066 MPa. The ratio of the load on the seat to the gravity of the human body weight was about 65.3%. The numerical model established using the body pressure mapping method was superior to those of the uniform load method and the standard loading pad method in terms of stress distribution, maximum stress, and contact area.

FSI investigation on the discharge valve lift in a R32 rotary compressor and its verification

R32 rotary compressor is commonly applied in the household air conditioner. The valve motion is significant to the performance and reliability of rotary compressor. Valve lift is an important factor which directly affects the valve motion. A suitable valve lift can both reduce power consumption and fracture risk of valve plate. Based on a compressor prototype, this paper developed a FSI model to simulate the working process of discharge valve. The simulation results show that a large valve lift means a large discharge effective flow area and a low indicated power. But both the velocity of valve plate impacting the retainer and the impact stress of valve plate are relatively large, which will lead to the fracture in the valve plate. The simulated initial impact location on the head of valve plate is near the oblique incision due to the uneven pressure distribution in the discharge port and the torsional movement of valve plate. The maximum impact stress distribution areas on the valve plate obtained from simulation results are matched with the fracture areas of two valve plate samples, which proves that the simulation results are accurate enough to guide the optimization design of discharge valve in the rotary compressor.

Investigating the influence of graphene coating thickness on Al0.3CoCrFeNi high-entropy alloy using molecular dynamics simulation

Graphene was proven to be an excellent surface-strengthening material. However, there needs to be more literature explaining its strengthening mechanism at the atomic level. This study systematically investigated the surface strengthening mechanism of graphene coating on the Al0.3CoCrFeNi high-entropy alloy (HEA) and the influence of loading-unloading cycles on the formation of internal defects in the material through molecular dynamics (MD) simulations at the atomic level. Research results show that the internal stress concentration in HEA without graphene coating leads to a more considerable residual depth after unloading. With the addition of a graphene coating, the material's stress area increases, effectively alleviating stress concentration and reducing residual depth. Furthermore, the lengths of Lomer-Cottrell dislocation locks and Hirth dislocation locks inside the material increase, enhancing the material's deformation resistance. The more layers of graphene, the larger the stress distribution area and the greater the hardness. Under the same indentation force, the number of HCP structures and amorphous structures inside the material is inversely proportional to the thickness of the graphene layers. In addition, after multiple loading and unloading cycles, the hardness of HEA/GP materials basically remains unchanged, while internal defects increase and residual depth increases.

Designed wrinkles for optical encryption and flexible integrated circuit carrier board

Patterns on polymers usually have different mechanical properties as those of the substrates, causing deformation or distortion and even detachment of the patterns from the polymer substrates. Herein, we present a wrinkling strategy, which utilizes photolithography to define the area of stress distribution by light-induced physical crosslinking of polymers and controls diffusion of residual solvent to redistribute the stress and then offers the same material for patterns as substrate by thermal polymerization, providing uniform wrinkles without worrying about force relaxation. The strategy allows the recording and hiding of up to eight switchable images in one place that can be read by the naked eye without crosstalk, applying the wrinkled polymer for optical anti-counterfeiting. The wrinkled polyimide film was also utilized to act as a substrate for the creation of fine copper circuit by a full-additive process. It generates flexible integrated circuit (IC) carrier board with copper wire density of 400% higher than that of the state-of-the-art in industry while fulfilling the standards for industrialization.

Biomechanical effects of clear aligner with different shape design at extraction space area during anterior teeth retraction.

This study aimed to investigate the biomechanical effects of clear aligner (CA) with different shape designs at extraction space (CAES) area during space closing. A finite-element method (FEM) model of mandibular dentition, periodontal ligaments, attachments, and corresponding CA was established. The connecting rod design of CAES was modelled for the control group. Eight test groups with different heights of CAES from -4 mm to +4 mm were designed. Tooth displacement tendencies were calculated. The maximum principal stress in PDLs, teeth, and CAs was analysed. Both global coordinate system and local coordinate system were also used to evaluate individual tooth movements. Across all groups, stresses concentrated on the lingual outer surface of CAESs. For the lowered CAES groups, both the stress value and the stress distribution area at CAESs were increased. The lowered CAES groups showed reduced movement in anterior teeth and less tipping tendency of the canines. The shape of CAES has a biomechanical impact on anterior teeth movement and should be considered in aligner design. The results suggest that increasing the height of CAES can enhance anterior teeth retraction, while lowered CAES may facilitate controlled root movement. Changes in the shape of CAES represent a potential direction for biomechanical improvement of clear aligner in extraction cases and are worth exploring.

Impact Velocity-Dependent Patterns and Mechanisms of Spalling Behavior in Single Crystal Nickel

To reveal the impact velocity (U<sub>p</sub>) effect on the spalling and fracture behavior of single crystal nickel, a non-equilibrium molecular dynamics approach is performed to investigate the free surface velocity curve, radial distribution function, atomic crystal structures, dislocations, and void evolution process. The results show that the critical U<sub>p</sub> for spalling behavior in single crystal nickel is 1.5 km/s, the spallation mechanism is classical spallation damage (U<sub>p</sub>≤1.5 km/s) and micro-spallation damage (U<sub>p</sub>＞1.5 km/s). The number and distribution area, and stress distribution area under micro-spallation damage much higher than those under classical spallation damage. Analyzed the influence of impact velocity on the classical spalling damage behavior (U<sub>p</sub> ≤ 1.5 km/s) and obtained the corresponding spalling strength, an accident of spalling strength occurs at the U<sub>p</sub> of 1.3 km/s. The spalling strength of single crystal nickel is influenced by the combined effects of stacking faults, phase transformation, and dislocation mechanisms. The nucleation and emission of dislocations increase lead to a decrease in the spalling strength. When U<sub>p</sub> <1.3 km/s, spalling damage is primarily influenced by stacking faults. When U<sub>p</sub> =1.3 km/s, spalling strength is mainly affected by the competition between stacking faults and phase transformation. When U<sub>p</sub> >1.3 km/s, spalling strength is predominantly influenced by the body-centered cubic (BCC) phase transformation mechanism (transformation path: FCC → BCT → BCC). This study reveals the impact velocitydependent patterns, mechanisms, and effects on spalling damage and fracture, providing a theoretical basis for the protective application of nickel-based materials under extreme impact conditions.

The influences of ply interference on ballistic impact response and energy absorption behavior of multilayered woven fabrics

This paper presents a detailed experimental and numerical analysis aiming to investigate the ballistic impact response and energy absorption behavior of multilayered woven fabrics with different ply interference. Different levels of ply interference are provided by controlling the influencing factors such as angle-plying, stitching the constituent plies and spacing the adjacent plies. The three aforementioned designs were evaluated for their energy absorption capacity against ballistic impact with the impact velocities ranging from 300∼400 m/s. The energy absorption mechanism was studied through the combination of high-speed camera and numerical simulations in terms of fabric deformation, and evolution of energy absorption for each ply at different impact velocity. It was found that stitching the aligned systems showed the most pronounced beneficial effect on energy absorption, showing approximate 50 % ∼ 110 % increase than the quasi-isotropic system with a mismatched lay-up angle of 22.5º.This is probably because that thread stitching enables the panel to be more efficient in spreading energy to a more global extent, involving a more defined transverse deformation and a larger area of stress distribution, which is beneficial for energy absorption.

Dropping impact experiment of crisp pears and contact pressure analysis

ABSTRACT Efforts have been made to streamline the abstract as follows. If further changes are made we are concerned that the integrity of the content may be compromised. We hope you will understand. This study aimed to investigate the impact damage experienced by Pucheng crisp pears during various stages, including harvest, transportation, and processing. Drop impact tests were conducted on pears, considering drop height and collision contact material as factors influencing damage. The bruise area was used as an indicator for damage evaluation. Additionally, the pressure-sensitive film technique was employed to measure the contact stress distribution characteristics of Pucheng crisp pears after the drop impact. The relationship between contact stress distribution and damage area was examined, and the stress area range close to the damage area was determined. The results revealed that the damage area of pears increased linearly with the increasing drop height. Among the four contact materials, foam board exhibited the best cushioning properties, with a maximum safe drop height of 20 cm. The pressure distribution tended to follow a normal distribution, with a pressure range of 0.2-0.3 MPa covering the largest area, corresponding to the majority of the bruise area on the pears. Furthermore, the pressure area and average pressure were found to be related to the severity of bruising. When pears were dropped onto steel, rubber plate and corrugated board, for the former two the stress area exceeding 0.2 MPa closely approximated the damage area, with an average relative error of 7.2%. For the latter, the closest range was above 0.3 MPa, with an average relative error of 3.6%. However, when dropped onto foam board, the average relative error was 75%. This research provides valuable insights for designing packaging tools to minimize damage and serves as a theoretical basis for predicting and assessing pear bruise area using finite element analysis.

Simulation-based Fatigue Life Analysis of Polyimide Film in Afterburner Fuel Distributor

Aiming at the problem of premature failure of organic material polyimide film in the afterburner fuel distributor of a turbofan engine, the material parameters of polyimide film and the temperature and the fluctuation amplitude of the film during use were obtained based on previous investigation. The finite element simulation model of polyimide film was established in ANSYS software to study the Mises stress distribution and weak area distribution of the film when the film fails in the form of fracture. Based on the fatigue life calculation theory of probabilistic fracture mechanics, the fatigue life cycle times and their variation rules of the film under different reliability were obtained. The results show that there is stress concentration in the film, and the stress con-centration and weak area of the film occur at the outer edge of the film clamping. The fatigue life cycle times of the film under different reliability are calculated, and the fatigue life cycle times decrease with the increase of reliability.

Stability Analysis of Wind Turbine Blades Based on Different Structural Models

In order to better simulate the actual working conditions of wind turbines more realistically, this paper adopts the two-way fluid–structure coupling method to study the NREL 5 MW wind turbine, considering the blade coupling deformation and equivalent stress and strain distribution of the blades with different internal structures under different working conditions. The results show that the maximum equivalent stress and strain distribution of the beam–structure wind turbine blade was near the leading edge of the blade. The maximum equivalent stress and strain distribution of the shell structure wind turbine blade was near the leading edge of the blade root, and the dangerous area is obvious but smaller than that of the beam-type wind turbine. The coupled deformation of a wind turbine model with a shell structure blade with a web is significantly reduced, and the equivalent stress and strain distribution of the skin is similar to that of the shell structure, but the numerical value and the maximum equivalent stress distribution area are significantly smaller. From the comparison of the three, the shell structure blade with a web is the best.

Influence of Blasting Disturbance on the Dynamic Stress Distribution and Fracture Area of Rock Tunnels

In order to study the dynamic stress distribution and the fracture area of rock around the tunnel under different orientations of blasting disturbance, AUTODYN finite difference method software was used to conduct numerical simulation research. Gauge monitoring points were set around the numerical model of the tunnel to conduct real-time monitoring of the stress distribution, displacement and fracture area of the tunnel. Based on the analysis of the stress wave propagation law, the following conclusions are obtained: (1) under the condition of the same blasting loads, the stress and displacement of the tunnel is relatively small when the blasting disturbance source is located above the roof, i.e., the stress state of the tunnel is relatively stable and the fracture area around the tunnel is minimal; (2) from the uniaxial stress around the tunnel and the tunnel peripheral displacement, it can be seen that the displacement caused by horizontal direction stress of the tunnel is the largest, and the deformation is mainly concentrated above the floor and at the shoulder, while the vertical wall part has almost no deformation; (3) for brittle materials such as rock, the arch-shaped stress-bearing surface is more likely to disperse stress, while the straight wall and flat floor of the tunnel cannot well disperse stress, resulting in uneven stress on the stress-bearing surface, uncoordinated deformation and ultimately, failure.

Study on bonding performance and load transfer model between polyurethane anchor bolts and silty soil

The bonding performance between the anchor anchorage body and soil is an important index to evaluate the anchoring performance of anchor bolts. In this work, the vertical polyurethane grouting anchor bolt is taken as the research object, and the bonding performance between polyurethane anchorage body and silty soil was studied by 13 sets of field pull-out tests, and the distribution law of the axial force and the bonding stress of the polyurethane anchor bolts along the anchoring depth, as well as the influence of the diameter (D = 60 mm, 75 mm, 90 mm, 100 mm, 120 mm), density (ρ = 0.2 g/cm3, 0.3 g/cm3, 0.4 g/cm3, 0.5 g/cm3, 0.6 g/cm3) and length (L = 2.0 m, 2.5 m, 3.0 m, 4.0 m, 5.0 m) of the polyurethane anchorage body on the bonding strength between the polyurethane anchorage body and silty soil were analyzed in detail, and finally the load transfer equation of polyurethane anchorage body under vertical pull-out load was obtained by theoretical derivation. The results show that the error between the calculated results of the load transfer equation and the experimental results is less than 12%, which further indicates that the load transfer equation derived in this paper can reasonably describe the distribution of axial force and bonding stress along the anchorage depth of the polyurethane anchor bolts. The axial force of polyurethane anchor bolt is the largest at the top of the bolt body, and decreases along the anchoring depth, and the axial force at the bottom is far less than that at the top. Polyurethane anchor bolt has significant bonding stress concentration phenomenon, with the increase of load the peak bonding stress and bonding stress distribution area increases, and shifts to the bottom of the anchorage body. The borehole diameter, anchorage body density, and anchorage body length all have important effects on the bonding strength: the bonding strength increases with the increase of anchorage body density, and the increase is more obvious in the case of small density, and the increase of bonding strength becomes smaller after the density exceeds 0.4 g/cm3, the bonding strength decreases with the increase of anchorage body length and borehole diameter.

Effects of a Novel Stereoscopic Flow-Diverting Stent on Intracranial Aneurysm Hemodynamics

Objective: To preliminarily explore the effects of a novel stereoscopic flow-diverting stent on the hemodynamics and pathophysiology of intracranial aneurysm. Methods: Three-dimensional digital subtraction angiography (3D-DSA) and transcranial Doppler ultrasound (TCD) findings and blood examination-related parameters were collected from eligible patients for clinical research. Then a model of aneurysm with its bearing artery was established, based on which a novel stereoscopic flow-diverting stent model was constructed, and hemodynamic data of fluid models with and without novel stereoscopic flow-diverting stent implantation were analyzed. Results: The novel stereoscopic flow-diverting stent could effectively decrease blood flow into the aneurysm and notably reduce blood flow velocity in the aneurysm. The time-averaged wall shear stress (TAWSS) in the aneurysm was distinctly attenuated, and the high shear stress distribution area was also markedly diminished. Conclusion: The novel stereoscopic flow-diverting stent plays an effective isolating role, which is conducive to inducing thrombosis in aneurysm, thus protecting aneurysm vessels. Furthermore, the novel stereoscopic flow-diverting stent can evidently alter the hemodynamic environment in the aneurysm, notably reduce blood flow into the aneurysm, attenuate the mechanical compression of pulsatile blood flow on the vascular wall, and slow down the blood flow velocity in the aneurysm, thus contributing to stabilizing aneurysm. Improving the hemodynamic environment guarantees arterial revascularization and stable thrombosis, thereby effectively controlling the illness.

Prediction of comprehensive mechanical properties of thermoplastic vulcanizate (TPV) with various composition ratios based on a micromechanical method

In the field of thermoplastic vulcanizate (TPV), experimental methods cannot quantify the relationship between the internal structure and performance of TPV, and are not conducive to the accurate design of TPV structure and performance, which is one of the problems to be solved in this field. In this study, a simple and effective two-dimensional micromechanical model was established based on the real microstructure of TPV by using the micromechanical method and the mechanical properties of TPV with different ethylene propylene diene monomer (EPDM) mass fractions were studied. The results show that with the increase of EPDM content, the maximum stress distribution area of TPV would change, the elastic modulus of TPV would gradually decrease, while the maximum stress of polypropylene (PP) phase would first decrease and then increase and strain corresponding to elastic–plastic change would also increase. The resilience of TPV increases with the increase of EPDM content and decreases with the increase of strain load. When the EPDM content is higher than 70%, the “S” bending deformation would occur at the thinnest part of PP matrix ligament.

Efficiency of an Improved Grouted Corrugated Duct (GCD) Connection Design for Precast Concrete Bridge Pier: Numerical and Parametric Study

In this study, finite element analysis (FEA) has been conducted for an improved grouted corrugated duct (GCD) connection design with a reserved recess in bridge footing. This study aims to understand the damage progression mechanism and to evaluate the contribution of each component in the improved GCD connection design. Numerical model based on the experimental results are first created, validated and calibrated. It is found that the confining effect (support and friction force) provided by recess sidewall keeps the connection in good integrity. It also prevents early deformation and early development of transverse cracks along the connection interface, which further avoids the damage concentration at connection joint, transfers the plastic hinge region. Parametric study is then carried out by considering different recess depths, cushion thicknesses, recess diameters, and mortar strengths. The effect of recess details on mechanical behavior is thus studied. Recess depth can be designed as 6–20% of the column section size to ensure a higher upper limit of overall strength and ductility, and it also influences the stress distribution area of the joint local. The stiffness and strength of recess control the local damage, while has limited impact on the overall performance. In addition, preliminary suggestions on the GCD design of recess depth, thickness of mortar cushion, recess diameter, the strength of mortar are proposed.

Research on the control of overburden deformation by filling ratio of cementing filling method with continuous mining and continuous backfilling.

Underground resource exploitation has seriously damaged the surface ecological environment and underground water system. As an effective control measure, the filling mining process has greatly reduced the surface subsidence. As a branch of the filling mining process, the continuous mining and continuous backfilling (CMCB) method solves the contradiction of mining and filling in this process. The control index of filling ratio of even number lane (FRE) was presented to investigate the technical advantages of the CMCB method. The numerical analysis model was used to investigate the laws such as deformation characteristics of the surrounding rock, stress distribution, and plastic area distribution characteristics of backfill under four typical cases. As a consequence, the FRE effect law on overburden deformation and the roof control function of the backfill was disclosed, and overburden rock deformation control solutions were provided. According to the results, the overburden deformation varies dramatically when the FRE decreases, and it rises greatly when the even-numbered lane backfill (ELB) is not contacted with the roof. The contacting condition and filling condition of the odd numbered lane backfill (OLB) are connected to the distribution of stress and plastic zone. The backfill transmits the rock beam load by building a composite support system with the roof and floor rock layers, and it accomplishes the backfill's roof control function by combining the primary and secondary load-bearing and synergistic load-bearing connections between the backfills. Measures such as differential FRE, differential strength, non-uniformity of filling lane, and synergistic bearing of temporary support and backfill may help to decrease deformations and internal cracks in the surrounding rock. This measure has been successfully implemented in the field, serving as an experience for the application of the CMCB method.

The Theoretical Value for the Tip Radius of Cracks and Notches

In this paper, using Creager and Paris's blunt elliptical hole stress distribution area equation, it is applied to crack and circular hole shaped defects using the theoretical radius value, which equalizes the maximum stress at the defect tip in terms of value to fracture toughness. By providing value equality, critical fracture stresses of all defect dimensions and tensile strength of the material were determined with a single mechanical test data. Compared with the predictions of other methodologies, it was determined that the obtained data gave results closer to the experimental values.

The effects of erupting mandibular 3 molar on the dental arch: A FEM study

Role ofErupting Mandibular 3 molars as a cause of incisor crowding in the lower arch continues to be controversial. The relation between 3 molar and dental crowding has not been established. To determine the area of stress distribution and tooth displacement during eruption of mandibular 3 molar in mesioangular and horizontal impaction. Three dimensional finite element models were generated based on computed tomography scan data. From computed tomographic scans of a human mandible two additional finite element models were generated: a mandibular model with mesioangular impaction and second model with horizontal impaction. To investigate the stress distribution to the mandible, and tooth displacement eruptive force of 10 grams was applied.High concentration of stresses was seen at the inferior border of the mandible and lowest concentration of stress was present at symphysis for both the models. The tooth displacement for both mesio-angular and horizontal impaction caused mesial tilting of all the teeth i.e. from 2 molar to central incisor; with greatest tilting of 2 molar. Along the vertical plane, there was extrusion seen with 2 molar with mesio-angular impaction and extrusion in 2 molar and 1 molar with horizontal impaction And all the teeth exhibited buccal flaring in case of both impaction. There is significant level of tooth displacement from 2 molar towards the incisors. These changes occurring along the dental arch must be taken into consideration while planning for prophylactic extraction of third molars.