Thermal Stress Analysis of Maxillary Dentures with Different Reinforcement Materials Under Occlusal Load Using Finite Element Method

Abstract

The purpose of this study was to determine the effect of fiber reinforcement materials on the magnitude of stresses in a critical part of the maxillary denture base under thermal and occlusal load. Thermal stress analyses of the models were carried out using the finite element method. The models consisted of bone, soft tissue, interface gap, and maxillary dentures with and without reinforcements. A concentrated occlusal load of 230 N was applied bilaterally on the molar teeth. A 36 °C reference and 0 °C, 36 °C, and 70 °C variable ambient temperatures were applied to the models. CrCo, unidirectional and woven carbon/epoxy, unidirectional and woven glass/epoxy, and unidirectional and woven Kevlar/epoxy were used as reinforcing materials in the maxillary denture base made of PMMA (polymethyl methacrylate). Stress distributions on the maxillary denture’s midline and lateral line direction were evaluated. Maximum stresses in the incisal notch and the labial frenal notch of the maxillary denture were determined. Failure analysis of reinforcement materials used in maxillary dentures was carried out using the Tsai-Wu index criterion. The results obtained show that the thermal properties of reinforcement materials should be considered as an important criterion in their selection.

1. Introduction

Teeth health loss is one of the biggest problems for human health and impairs patients’ appearance, masticatory ability, speech etc. These functional disorders affect quality of life standards for patients. To resolve these unhealthy situations, partial or complete removable dentures are frequently utilized [1,2].

Since the early 1940s, acrylic resin based on PMMA (polymethyl methacrylate) has been one of the materials routinely used in prosthetic dentistry [3,4,5,6,7,8,9,10]. PMMA has been the material of choice due to its excellent appearance, stability in the oral environment, repairability, and ease of repair [4,5,7,10,11,12,13].

The mechanical weakness of the PMMA denture base material, in terms of low tensile and bending strength, remains an unresolved problem [2,4,6,7,8,14,15,16,17,18]. Midline fracture is the most common failure in maxillary and mandibular dentures [19]. Fractures of maxillary complete dentures are the most widespread denture failure [20]. To overcome these deficiencies, there are some ways that have been investigated to improve the strength of PMMA by reducing stress concentration and preventing mechanical weakness. In those investigations, researchers conducted studies on alternative materials to PMMA, chemical modification of PMMA, and the reinforcement of PMMA with fibers such as glass fiber, carbon fiber, Kevlar, and metal inserts [7,9,10,21].

In many studies, different methods such as strain gauges [22], holography [23], photoelasticity [24], and finite element analysis have been used to determine the stress concentration and distribution of dentures. Since the strain gauge method measures strains occurring on the surface of dentures, it cannot measure stresses in reinforcement materials embedded in the dentures. Both photoelasticity and holography are optical methods and not appropriate for investigating the stress distribution of dentures embedded with reinforcing materials. This is because the light beams cannot pass through the reinforcement. The finite element method (FEM) has been used for numerical stress analysis. The method is superior to experimental methods because it can perform the stress analysis of complex geometries, such as dentures embedded with reinforcement material, and evaluation of the stress results in detail. Additionally, analytical methods for stress analysis of dental implants have been developed [25,26]. However, it is quite difficult to apply analytical methods to complex implant structures such as maxillary dentures with reinforcing materials. Therefore, there is much research showing that the finite element method is the most effective method for stress analysis of denture base materials [2,8,11,27]. However, the effect of temperature changes on stresses in dentures was not considered in those studies.

Although the placement of reinforcement materials inside PMMA dentures reduces stress at critical points of maxillary dentures, the risk of damage due to thermal expansion may increase with the change in temperature. Thermal expansion coefficients of PMMA and fiber reinforcements used in maxillary dentures are different from each other. This difference may cause serious distortion and increase stress in critical areas when cold or hot food is consumed. Normally, the standard temperature in the mouth changes from 32 °C to 37 °C, but when patients consume hot/cold drinks or food, the oral cavity temperature increases by a range of 0 °C to 70 °C [28]. The effects of temperature changes on stress at critical points in maxillary dentures with reinforcement material have not been extensively studied in the literature.

In our study, the finite element method was used to evaluate thermal stress analysis of maxillary dentures with reinforcement material. Three different ambient temperatures of 0 °C, 36 °C, and 70 °C and a 230 N occlusal load under the suitable boundary conditions were applied to different unreinforced and reinforced models of maxillary dentures. The latter models were reinforced with CrCo, unidirectional carbon/epoxy, woven carbon/epoxy, unidirectional glass/epoxy, woven glass/epoxy, unidirectional Kevlar/epoxy, and woven Kevlar/epoxy. Stress distributions on the maxillary denture’s midline and lateral line direction were evaluated. Maximum stresses in the incisal notch and the labial frenal notch of the maxillary denture were determined. Failure analysis of reinforcement materials used in maxillary dentures was carried out using the Tsai-Wu index criterion.

2. Materials and Methods

All the finite element models were analyzed using the finite element method on the ANSYS Release 19.1 program. The model consisted of bone, soft tissue, interface gap, and maxillary denture with and without reinforcement, as shown in Figure 1. Teeth in the maxillary denture were not modeled in detail because only the stresses at the critical points of the denture were examined. During solid modeling, the surface coordinates of the maxillary denture base were taken from a real maxillary denture. The ‘key points’ were created from the coordinates to form the surface of the maxillary denture. These key points were merged by the ‘line’ command. Within the merged lines, the maxillary denture’s upper surface area was created. The area obtained was transformed into a volumetric dimension. Unnecessary parts of the volume were removed and created volumes were bonded. The created model was solidified as a symmetrical half of the maxillary denture to reduce analysis time by reducing the number of elements. Material properties were entered into all individual volumes. Solid models were meshed with SOLID 186 elements in a controlled manner, which allowed for better geometric homogeneity of the elements and better discretization of the model by performing a mesh convergence study. The mesh consisted of a total of 39,170 nodes and 26,499 tetrahedral elements, as shown in Figure 2. SOLID 186 is a rigid element that contains 20 high-dimensional 3-dimensional nodes and exhibits three-dimensional deforming behavior. The element is defined by 20 nodes with three degrees of freedom per node. SOLID 186 is a type of element that exhibits plasticity, hyperplasticity, creep, stress cure, high impact, and high deformation characteristics (Figure 3) [29].

Considering the boundary conditions of the skull and semi-model, boundary conditions were applied to the upper surface of the maxillary denture in the x, y, and z directions, and only in the x direction for the side surface shown in Figure 4, as the symmetrical half of the maxillary denture was modeled.

The concentrated occlusal force of 230 N was applied, as shown in Figure 5 [2,8,30]. Thermal loadings were considered as a steady state to simplify the problem. The initial temperature was taken as 36 °C, which is the human body temperature. Ambient temperatures of 36 °C, 0 °C, and 70 °C were applied to all the models. The effects of temperature variation on the mechanical properties of the materials were ignored to simplify the stress analysis.

The properties of the reinforcement materials for a volume fraction of 55% were calculated approximately with the help of the literature [31]. Epoxy was considered a matrix material. The mechanical and thermal properties of each element on the model and seven different reinforcing elements are listed separately in

Table 1. Mechanical properties of the elements on the model [8].

Elements	Young’s Modulus (MPa)	Poisson’s Ratio	Thermal Expansion Coefficient (Strain/K)
Bone	13.700	0.3	-
Mucosal Tissue	2.8	0.4	-
Interface Gap	0.07	0.3	-
Maxillary Denture	3200	0.36	70

and

Table 2. Mechanical properties of reinforcement elements.

Elements	E1 (GPa)	E2 (GPa)	Thermal Expansion Coefficient α1 (Strain/K)	Thermal Expansion Coefficient α2 (Strain/K)
CrCo	210	210	14.4	14.4
UD Carbon/Epoxy	135	10	−0.3	28
UD Glass/Epoxy	40	8	6	35
UD Kevlar/Epoxy	75	6	4	40
Woven Carbon/Epoxy	70	70	2.1	2.1
Woven Glass/Epoxy	25	25	11.6	11.6
Woven Kevlar/Epoxy	30	30	7.4	7.4

.

The reinforcing materials were completely embedded into the maxillary denture. Seven different maxillary denture reinforcements made of CrCo, unidirectional and woven carbon/epoxy, unidirectional and woven glass/epoxy, and unidirectional and woven Kevlar/epoxy were modeled as volumes into the maxillary denture. Because of the irregularity of the model, the volume of reinforcement materials was meshed using SOLID 186 elements to simplify the reinforcement part, instead of modeling it as a multilayer composite.

A gap was modeled instead of performing a contact analysis between the soft tissue and the denture, significantly reducing the calculation time compared to contact analysis. The Young’s modulus of the gap was set at zero, but using this value alone caused abnormal large deformations between the soft tissue and the denture. Therefore, after analyzing the model with different values, the optimal value of the Young’s modulus for the gap was selected as 0.07 MPa.

Paths from the anterior to the posterior and the posterior to the lateral parts of the maxillary denture were defined. The paths were divided into three parts: A–B, B–C, C–D, and two parts: D–E, E–F, as shown in Figure 6 and Figure 7, respectively. The stress distributions along these paths were evaluated. It was determined that the greatest stress values were in the x direction, while shear stresses had very small values. Therefore, only the stresses in the x direction were evaluated.

3. Results

3.1. Stress Analysis of Maxillary Denture Under Only Temperature Change

When only the effect of temperature change on stress component SX was investigated without the occlusal singular force of 230 N, tensile stresses occurred in all the models at 0 °C, and compressive stresses occurred in all the models at 70 °C on the lower surface of the denture. When the temperature was decreased from 36 °C to 0 °C, the denture tended to shrink much more than the reinforcement material and the curvature of the denture decreased. Decreasing temperature is the reason why tensile stresses occur on the lower surface of dentures. When the temperature was increased from 36 °C to 70 °C, the denture tended to expand much more than the reinforcement material and the curvature of the denture increased with the increase in temperature, with compressive stresses occurring on the lower surface of dentures. When unreinforced dentures were investigated at all temperatures, it was seen that thermal stresses did not occur on the denture. This is due to the same thermal expansion occurring all over the denture.

When stress distributions through the lower midline of the maxillary denture at 0 °C were investigated, it was seen that the tensile stresses on maxillary dentures decreased along the A–B path and showed a gradual increase from path B–C to path C–D. In path A–B, the highest tensile stress occurred at 10.936 MPa in the CrCo-reinforced model. The highest tensile stress occurred at 15.271 MPa in the woven carbon epoxy-reinforced model in path C–D, as shown in Figure 8.

When stress distributions through the lower midline of the maxillary denture at 70 °C were investigated, it was seen that the compressive stresses on the lower surface of maxillary dentures decreased along the A–B path and increased from path B–C to path B–C. In path A–B, the highest stress occurred at 10.649 MPa in the CrCo-reinforced model. The highest stress occurred at −11.707 MPa in the CrCo-reinforced model in path C–D, as shown in Figure 9. Furthermore, thermal stresses did not occur in unreinforced dentures under 0 and 70 °C, as temperature change caused the same expansion in all parts of the maxillary denture.

When stress distributions through the lower lateral path of the maxillary denture under temperature variations were investigated, at 0 °C, the tensile stresses on maxillary dentures decreased from the D–E path to the E-F path. On the D–E path, the highest tensile stress was 16.334 MPa in the woven carbon epoxy-reinforced model, as shown in Figure 10. At 70 °C, the compressive stresses on the maxillary denture decreased from the D–E path to the E–F path. On the D-E path, the highest tensile stress was −12.443 MPa in the woven Kevlar/epoxy reinforced model, as shown in Figure 11.

3.2. Stress Distributions Under Only 230 N Load

Thermal stresses did not occur under the 230 N occlusal load only, as the reference and loading temperatures (36 °C) were the same. Along the midline, the tensile stresses decreased along the A–C path for all models. The highest stress was 27.362 MPa in the unreinforced model. High stresses occurred in the incisal notch. The results showed that the reinforcement materials considerably reduced the stresses in the incisal notch. The lowest stress was 5.25 MPa in the CrCo-reinforced model, as the CrCo reinforcement material has a high Young’s modulus, which restrains deformations of the denture under the force. For all the models, tensile stresses increased along the C–D line, and the maximum stress was 19.122 MPa in the unreinforced model, as shown in Figure 12.

On the lateral line, for all models, the tensile stresses decreased from the D–E path to the E–F path because of the decreasing bending moment along the path. On the D–E path, the highest tensile stress was 22.765 MPa in the unreinforced model, as shown in Figure 13.

3.3. Stress Distributions Under Both Temperature Change and Occlusal Force of 230 N

For all models, the tensile stresses decreased from the A–B part to the B–C part under a temperature of 0 °C and an occlusal force of 230 N. The tensile stresses stayed stable along the B–C line and increased towards the end of the C–D part. In the A–B part, the highest and lowest stresses were observed as 27.074 MPa and 9.85 MPa, respectively, in the unreinforced models. In the C–D part, the highest stress was found to be 27.821 MPa in the woven carbon epoxy model and the lowest stress was 4.19 MPa in the unreinforced model, as shown in Figure 14.

When the stress distributions of the models in the midline under 230 N occlusal load at 70 °C were investigated, in the A–B path, the maximum stress was 27.364 MPa in the unreinforced model. On the C–D path, the maximum stress was 18.456 MPa in the unreinforced model as in Figure 15.

Depending on temperature and force loading on the reinforced models, the tensile stresses resulting from the application of force and the compressive stresses resulting from the application of the temperature compensated each other, leading to lower stress values.

Stress distributions through the lateral line of the maxillary denture under 230 N occlusal force at 0 °C and 70 °C are shown in Figure 16 and Figure 17, respectively.

3.4. Stress Analysis in Critical Notchs of the Dentures Under 230 N and Thermal Loading

After investigating the maximum stress distributions on maxillary dentures, it was observed that maximum stresses occurred in the incisal notch and the labial frenal notch of maxillary dentures for all the models (Figure 18). Therefore, failure may begin at these points. In maxillary dentures without reinforcement material under a 230 N loading only, the highest tensile stress recorded was 31.15 MPa at the tip of the incisal notch, while the labial frenal notch showed the highest compressive stress of −53.32 MPa. It was seen that using reinforcement materials in maxillary dentures led to reductions in stresses at the critical notches, in varying proportions. The greatest reduction was 76.15% in the CrCo-reinforced denture due to CrCo’s high bending stiffness. For all the dentures with reinforcing materials shown in

Table 3. Maximum and minimum stress values in critical notches of the dentures under 230 N and thermal loading.

Reinforcement Material of PMMA Denture and Value of Temperature	The Tensile Stresses at the Tip of the Incisal Notch, Maximum Stress (MPa)	The Compressive Stresses at Labial Frenal Notch, Minimum Stress (MPa)	Percentage Change in Maximum Stress According to the Unreinforced Denture (%)	Percentage Change in Minimum Stress According to the Unreinforced Denture (%)
Unreinforced 0 °C	30.76	−52.85	-	-
Unreinforced 36 °C	31.15	−53.32	-	-
Unreinforced 70 °C	31.52	−53.76	-	-
CrCo 0°	15.16	−43.94	50.72	16.86
CrCo 36 °C	7.43	−39.80	76.15	25.36
CrCo 70 °C	4.77	−35.88	84.87	33.26
UniCarbon/Epoxy 0 °C	19.11	−51.01	37.88	3.48
UniCarbon/Epoxy 36 °C	9.94	−42.20	68.09	20.86
UniCarbon/Epoxy 70 °C	10.69	−34.26	66.09	36.27
UniGlass/Epoxy 0 °C	23.14	−52.31	24.78	1.02
UniGlass/Epoxy 36 °C	16.01	−45.85	48.61	14.01
UniGlass/Epoxy 70 °C	11.07	−40.02	64.88	25.56
UniKevlar/Epoxy 0 °C	21.10	−51.03	31.5	3.44
UniKevlar/Epoxy 36 °C	12.47	−44.17	59.97	17.16
UniKevlar/Epoxy 70 °C	11.60	−37.89	63.2	29.52
WovenCarbon/Epoxy 0 °C	20.70	−49.91	32.71	5.56
WovenCarbon/Epoxy 36 °C	11.93	−44.14	61.7	17.22
WovenCarbon/Epoxy 70 °C	9.22	−38.75	70.75	27.92
WovenGlass/Epoxy 0 °C	24.70	-51.89	19.7	1.82
WovenGlass/Epoxy 36 °C	19.34	−47.74	37.91	10.47
WovenGlass/Epoxy 70 °C	14.27	−43.83	54.73	18.47
WovenKevlar/Epoxy 0 °C	24.15	−51.92	23.38	1.76
WovenKevlar/Epoxy 36 °C	17.78	−47.09	42.93	11.68
WovenKevlar/Epoxy 70 °C	12.04	−42.53	61.8	20.89

, it was observed that tensile stresses at the tip of the incisal notch and compressive stresses at the labial frenal notch decreased considerably with increasing temperatures from 0 °C to 70 °C.

Since the maximum stresses occurring at the critical points of the denture do not exceed the tensile and compressive strengths, no damage occurs for all models.

When the temperature of the maxillary denture with reinforcing material was decreased from 36 °C to 0 °C under 230 N, it was seen that the stresses of the critical notches increased. However, when the temperature of the maxillary denture with reinforcing material was increased from 36 °C to 70 °C under 230 N, it was seen that the stresses of the critical notches decreased in all models except for unidirectionally reinforced Carbon/epoxy due to its negative thermal expansion coefficient in the fiber direction.

3.5. Failure Analysis of Reinforcement Materials

After the strength values of reinforcement materials were entered into the program, a failure analysis of the reinforcement materials was carried out. Since reinforcement materials have orthotropic material properties, the Tsai-Wu failure criterion was used. According to the criterion, if the value of the Tsai-Wu strength ratio index is less than 1, then the material will not fracture. However, if this value is higher than 1, then damage to the material may occur. Maximum and minimum stresses occurring in reinforcement materials and values of the Tsai-Wu strength ratio index are presented in

Table 4. Maximum and minimum stresses occurring in reinforcement materials and values of the Tsai-Wu strength ratio index.

Reinforcement Material of PMMA Denture and Value of Temperature	Maximum Stress	Minimum Stress	The Tsai-Wu Strength Ratio Index
CrCo 0°	371.98	−267.34	0.4895
CrCo 36 °C	281.06	−232.11	0.2852
CrCo 70 °C	276.64	−198.84	0.3409
UniCarbon/Epoxy 0 °C	267.51	−152.17	0.1599
UniCarbon/Epoxy 36 °C	321.75	−170.44	0.2359
UniCarbon/Epoxy 70 °C	372.97	−187.71	0.4693
UniGlass/Epoxy 0 °C	140.53	−63.77	0.5008
UniGlass/Epoxy 36 °C	183.57	-65.91	0.8991
UniGlass/Epoxy 70 °C	224.21	−67.92	1.4656
UniKevlar/Epoxy 0 °C	119.14	−99.67	0.1183
UniKevlar/Epoxy 36 °C	271.21	−106.86	0.2090
UniKevlar/Epoxy 70 °C	339.26	113.65	0.3249
WovenCarbon/Epoxy 0 °C	202.5	−92.37	0.1071
WovenCarbon/Epoxy 36 °C	238.75	−103.65	0.1852
WovenCarbon/Epoxy 70 °C	272.98	−114.30	0.2902
WovenGlass/Epoxy 0 °C	113.04	−39.27	0.3437
WovenGlass/Epoxy 36 °C	139.1	−40.50	0.6937
WovenGlass/Epoxy 70 °C	163.70	−45.06	1.1504
WovenKevlar/Epoxy 0 °C	125.98	−45.14	0.4692
WovenKevlar/Epoxy 36 °C	153.48	−48.27	0.4260
WovenKevlar/Epoxy 70 °C	179.44	−59.42	0.7665

. It can be seen from the table that thermal stresses with a load of 230 N increase the risk of damage to the reinforcement material. In general, it has been seen that the Tsai-Wu strength index value increased with the increase in temperature in all reinforcement materials. For example, for unidirectional reinforced glass/epoxy composite, the Tsai-Wu index value at 36 °C was 0.8991, while the Tsai-Wu index value at 70 °C was greater than 1, at 1.4656. A similar result was obtained for woven glass/epoxy composite. In other reinforcement materials, the variation in temperature changed the Tsai-Wu index values in the reinforcement materials, but it was observed that the Tsai-Wu index values were less than 1.

4. Discussion

The strengthening of acrylic denture base materials with different fiber reinforcements has been the subject of many investigations [2,3,4,5,7,9,10,11,14,16,17,18,20,32,33]. In our study, it was noticed that the stresses in critical regions of maxillary dentures with different fiber reinforcements under concentrated occlusal load varied considerably with high-temperature changes. This issue is hardly mentioned in the literature

For a long time, PMMA has been the most popular denture base material, despite the development of alternative materials to PMMA [9,14]. PMMA is a material that has ease of manipulation, low cost, lightness, excellent aesthetic properties, low water absorption, and ease of repair. However, low thermal conductivity, drawbacks in mechanical strength, brittleness, a high thermal expansion coefficient, and a low elastic modulus are important features to be developed [14].

Acceleration in developments in composite technology has enabled materials to be reinforced with fiber materials [16]. Fiber reinforcements can increase the strength of the denture such as metal fillers, carbon fiber, nylon fiber, aramid fiber, and glass fiber. Fiber-reinforced denture bases frequently exhibit desired properties [14,33].

As the strength of PMMA is poorer in tension than in compression, cracking of dentures is more likely to start from regions with a concentration of tensile stresses [7,9,34]. For the same reason, the strength of a denture in service is also affected by the presence of the incisal notch, which most likely provides the highest stress concentration [2]. Matthews and Wain found that strain was increased at the incisal notch and that it was the prime factor in contributing to the midline fracture of maxillary complete dentures [9]. Smith pointed out that microcracks developed first in areas of stress concentrations where dentures are repeatedly flexed at a small load [2]. The strength of a denture diminishes as a crack propagates. When the maximum load of a cycle exceeds the remaining strength, catastrophic failure of the denture occurs.

Many researchers have studied tensile stress distribution on the denture base. Beyli and von Fraunhofer noted that the study of maxillary denture bases was frequently performed [35]. Tensile and compressive stresses on the anterior and posterior regions of the maxillary denture cause deformations. In addition, they pointed to bending stresses caused by maxillary denture fractures and indicated the incisal notch area as the critical point of this tension. In local critical regions, high tensile stress with microcracks has caused fractures. These fractures can be prevented by strengthening the frontal part of the maxillary denture. The results obtained in their study are align with the distribution and severity of stress in our study.

Hirajima et al. remarked that fractures occur along the midline of the maxillary denture [10]. They emphasized that a decrease in tension along the midline could reduce deformations. CrCo and fiberglass were used as reinforcements to increase the strength of the denture. It assessed the strain under different occlusal forces, with a maximum of 200 N along the midline of the denture identified as critical regions. Reinforcement materials increased the strength of the denture along the midline. CrCo-reinforced specimens gave the best result for decreasing stress values. In our study, CrCO-reinforced material decreased stress values on the incisal notch area at 0 °C, 36 °C, and 70 °C temperatures

Akan et al. reported that occlusal loads of 75 N were applied to the maxillary frontal region and 150 N to the maxillary molar region occlusal load with 0°, 15°, 30°, 45°, 60°, 75°, and 90° angles to the horizontal axis. They were analyzed using the finite element methodology [11]. Thereafter, it was determined where high values of tensile stress occurred. The posterior region along the denture and frontal region were determined. Unidirectional glass fiber, woven glass fiber, unidirectional carbon fiber, and CrCo were used as reinforcement materials. It was found that the CrCo-reinforced specimens along the midline of the denture give the best results in terms of tensile stress and strength. The results of that study align with those obtained in our study, supporting the result of stress reduction with the fiber reinforcements in critical regions of the maxillary denture.

5. Conclusions

In this study, stress analysis of maxillary dentures with and without reinforcements was carried out using the finite element method to determine the effect of fiber reinforcement materials on the magnitude of stresses in a critical part of the maxillary denture base under thermal and occlusal loading. A concentrated occlusal load of 230 N was applied bilaterally on the molar teeth. A 36 °C reference and 0 °C, 36 °C, and 70 °C variable ambient temperatures were applied to the models. CrCo, unidirectional and woven carbon/epoxy, unidirectional and woven glass/epoxy, and unidirectional and woven kevlar/epoxy were used as reinforcing materials in the maxillary denture base made from PMMA.

It has been seen that fiber reinforcements reduced stress values on the maxillary denture under force and thermal loading. With only thermal loading, the reinforcing materials increased the stress values on the maxillary denture because PMMA’s and fibers’ thermal expansion coefficients are different from each other. It has been observed that the thermal properties of reinforcement materials should be considered as an important criterion in material selection. The obtained stress distributions and values could also be the subject of a future study on maxillary denture base materials, such as analyses of the effects of bending fatigue, distortion, and impact strength, which are indicated as the main factors in denture fractures.