COVID-19 Morbidity and Mortality and its Association with HIV and Health System Factors in India

Introduction

The COVID-19 pandemic has already infected over 494 million people worldwide, leading to over 6 million deaths [1]. With about one-third of confirmed cases in Asia, India witnessed around 43 million infected cases with over 500,000 deaths as of April 2022 [2]. Ever since the first outbreak in December 2019, researchers have been reporting significant associations between mortality and morbidity due to severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), causing the coronavirus disease 2019 (COVID-19) and various clinical and non-clinical factors, including Human Immunodeficiency Virus (HIV) infection, non-communicable diseases (NCDs), socioeconomic, demographic and health system factors [3-6]. Generation of context and geographic-specific evidence related to the factors associated with the SARS-CoV-2 infection and mortality is critical for policymakers to develop evidence-based strategies and targeted interventions such as age and vulnerability-specific prevention and control programmes. This study presents an analysis of the association between COVID-19 incidence, mortality and clinical and non-clinical factors, intending to provide an insight to support and design public health strategies for its response to the pandemic.

Methods

We compiled the data for two time periods, March to August 2020 and March to June 2021 from multiple secondary data sources for 26 states of India. Data related to COVID-19 incidence and mortality were obtained from the Ministry of Health and Family Welfare (MoHFW) website https://www.mohfw.gov. in/ and https://api.covid19india.org/csv/latest/state_wise.csv (last accessed on 1st July 2021). HIV related data were obtained from various technical reports published by the National AIDS Control Organization (NACO), Ministry of Health and Family Welfare (MoHFW), Government of India [7]. Data related to sociodemographic, economic characteristics, vaccination coverage as a measure for health system performance and average genderspecific body mass index (BMI) were obtained from National Family Health Survey (NFHS-4) factsheets [4,8,9]. As complete information on all variables could be obtained only for 26 states and UTs in the country, they were included in the analysis. We considered outcome variables such as cumulative confirmed COVID-19 incidence (burden) and mortality for the two different periods. HIV incidence, HIV prevalence, HIV mortality, PMTCT needs, general vaccination coverage, Sex-ratio, child sex ratio, the proportion of people in poverty, BMI and air-travel density (risk score) were considered as potential risk factors.

Statistical Analysis

Binary associations between the independent and outcome variables were first explored using scatterplots. The associations were then quantified using spearman’s rank correlation to assess the relationship between the independent variables and outcomes variables such as cumulative COVID-19 incidence and mortality. All correlations were reported and factors that were significant at p-value of 0.1 were included in the ecological analysis. Linear regression models were fitted to assess the quantum of association of the covariates on the COVID-19 incidence and mortality. Separate models were fitted for the two time periods, March- August 2020 and March-June 2021 as there were differences in case identification strategies, reporting and other programmatic efforts. The covariates included in the regression model were: general vaccine coverage (as a measure of health systems performance), sex ratio, percentage below poverty line (as a measure of social determinant of health), BMI (as a measure of nutritional status, obesity), HIV prevalence, HIV incidence, HIV related mortality, PMTCT needs (as measures of population highly vulnerable to infection) and air travel risk. All variables in the regression models were standardized to convert them to the same scale.

Results

Association between COVID-19 Incidence and HIV, Health System Performance and Socio-Demographic, Economic Factors

The correlation analyses (Figures 1 & 2) indicated strong positive association between COVID-19 incidence and HIV prevalence (r=0.9, p<0.001), HIV incidence (r=0.8, p<0.001), HIV related mortality (r=0.8, p<0.001) and Prevention of mother-tochild transmission (PMTCT) needs (r=0.8, p<0.001), during the time period from March to August 2020. Similarly, during the time period from March to June 2021, COVID-19 incidence was positively associated with HIV prevalence (r=0.8, p<0.001), incidence (r=0.7, p<0.001), AIDS mortality (r=0.8, p<0.001) and PMTCT needs (r=0.8, p<0.001). States with higher HIV incidence and AIDS related mortality showed a corresponding higher COVID-19 burden. The general vaccination coverage in urban areas, which is considered as a measure of health system performance indicated an inverse, albeit weak, association with COVID-19 burden (r=-0.2, p=0.4) indicating states with lower vaccination rates may have a higher number of cases in 2020. However, vaccination coverage in rural areas and overall vaccination coverage did not indicate any significant association with COVID-19 case burden in both periods. In terms of the socio-demographic, and economic situation, poverty rate did not indicate any significant association with COVID-19 incidence in both the time periods. However, sex ratio (r =-0.2, r=0) and child sex ratio (r=-0.3, r=-0.2) in both the periods of the analysis showed a weak negative association with COVID-19 incidence. Overall BMI in both the period of analysis (r=0.2, r=0.1) indicated a weak positive association with COVID-19 incidence. However, air travel risk ratio (r=0.7; r=0.7) indicated a strong positive association with COVID-19 incidence.

Figure 1: Correlation plot of COVID-19 incidence and mortality with independent variables for the year 2020.

Figure 2: Correlation plot of COVID-19 incidence and mortality with independent variables for the year 2021.

Association between COVID Mortality and HIV, Health System Performance and Socio-Demographic, Economic Factors

The magnitude of association between COVID-19 mortality and HIV related indicators that are HIV prevalence, HIV incidence, HIV related mortality and PMTCT needs were almost the same as COVID-19 incidence. COVID-19 mortality did not indicate any significant correlation with general vaccination coverage in both urban and rural areas. Poverty rate (r=-0.1; r=-0.1) indicated a very weak negative association with COVID-19 mortality in both periods. Similarly, sex ratio (r =-0.3, r= -0.2) and child sex ratio (r=-0.5, r=-0.3) showed a weak negative association in both the time periods with COVID 19 mortality. The analysis indicated a very weak association between BMI (r=0.2; r=0.1) and COVID-19 mortality in both the time periods However, air travel risk ratio (r=0.7, r=0.6) indicated a strong positive association with COVID 19 mortality. Some of the correlations, although not statistically significant, hint at important directional relationships between state-level COVID-19 disease burden, mortality and the covariates.

Factors Associated with COVID-19 Mortality

The regression analysis for the year 2020, indicated that for every additional case of PLHIV in the state, there is an increase of 0.77 units of COVID-19 mortality which is statistically significant. Similarly, new HIV infections, PMTCT needs, AIDS deaths and air travel risk scores of the states were found to be the significant predictors of COVID-19 mortality. Health systems performance, socio-demographic and economic factors did not show any significant association with COVID-19 mortality. Similar trends were observed for the year 2021, where the number of PLHIV, new HIV infections, PMTCT needs, AIDS deaths and air travel risk score of the states were found to be significant predictors of COVID-19 mortality (Table 1). A correlation plot of COVID-19 incidence and mortality with independent variables has been depicted in the choropleth map (Figures 1 & 2).

Table 1: Regression analysis to quantify change in COVID 19 mortality associated with the covariates.

Note: *Percentage of population below poverty line as defined for that year.

Discussion

The study highlights the significant relationship between HIV prevalence, HIV incidence, HIV related mortality, PMTCT needs and COVID-19 incidence and mortality. The findings corroborate with a population-based cohort analysis that indicated 2.9 times (95 CI: 1·96–4·30) higher risk for PLHIV dying from COVID-19 after adjusting for age and sex, compared to the general population [4]. A meta-analysis indicated a higher mortality rate due to COVID-19 among PLHIV (3.44%) compared to COVID-19 patients without HIV infection (0.42%)[10]. In addition, cohorts of hospitalized PLHIV with COVID-19 in London and New York have revealed higher rates of severe disease requiring hospitalization relative to those without an HIV diagnosis and higher mortality even with a suppressed viral load on ART [9,11-14]. The increased risk assumption for adverse COVID-19 outcomes among PLHIV is found to be based on their immunosuppressed clinical status since, HIV infection is long associated with increased susceptibility to opportunistic infections because of the abnormal humoral and T-cell mediated immune responses [15,16].

On the other hand, HIV/SARS‐CoV‐2 co-infected patients may have mortality benefits from the immunosuppressive state [17,18]. Though concerns were raised by the World Health Organization (WHO) and Center for Disease Control and Prevention (CDC) for population at high risk including PLHIV for severe health outcomes due to COVID-19, factors like immunological and virological status of PLHIV with consumption of antiretroviral treatment (ART) might play a role in the outcome of COVID-19 infection [19,20]. The high prevalence of critical underlying co-infections among PLHIVs, in comparison to HIV-negative individuals, is found responsible for higher mortality rates due to COVID-19 and not only the HIV positive status of individuals [21]. However, adequate caution and care are required while managing COVID-19 patients with immunosuppressive conditions [15,22,23]. This study indicates protective effect of general vaccination in the reduction of COVID-19 deaths. According to literature, universal Bacillus Calmette-Guerin (BCG) vaccination policy in countries, and the rate of BCG vaccination are correlated with reduced mortality rates due to COVID-19 [24-26]. Further, it has shown to produce broad protection against viral infections and sepsis [27]. Studies have also highlighted reduced COVID-19 infection, severity and death rates among patients vaccinated with measles mumps rubella (MMR) compared to the population in the same age range without vaccination [28-31]. Supporting these findings, another study indicated higher death rates due to COVID-19 with delayed MMR vaccination programs [32].

In terms of socio-demographic and economic variables, sex ratio and child sex ratio were found to be negatively associated with COVID-19 mortality. These findings are in corroboration with a few studies indicating that infectious disease threats and deaths including COVID-19 disproportionately affect the population from less developed geographies [33-35]. According to the study, a very weak positive association was found between BMI which is a proxy for co-morbidity of obesity, and mortality due to COVID-19. However, several studies indicated higher mortality among older, people with obesity and diabetes with complications [8,36]. According to a study, frequency of obesity (BMI > 30kg/m2, 47.6%) and severe obesity (BMI > 35 kg/m2, 28.2%) was found to be higher among patients with COVID-19 infection compared to non-SARSCov- 2 respiratory disease patients (25.2% and 10.8%, respectively) [37]. Similarly, another study demonstrated higher mean values of BMI of COVID-19 infected patients who needed ICU care (25.5kg/ m2), compared with the general group (22.0kg/m2) [38]. Another systematic review suggested obesity as a prospective predictor of poor outcomes in patients with COVID-19, in all continents [39]. Data from China’s Centers for Disease Control shows that 7.3% of those with diabetes who were later diagnosed with COVID-19 died and for those with no other co-morbidity, the mortality rate was lower at 0.9% [40]. Similar studies in Italy documented that 99% of deaths were among those with at least one or other health condition, showing the highest rates among patients with three or more illnesses [41].

Conclusion

The study gained insights on the associated factors that could increase the risk of COVID-19 incidence and mortality. The study provided evidence that PLHIVs are at higher risk for COVID-19 incidence and mortality, which suggests the need for focused interventions among PLHIV, especially during a pandemic of this kind. It is also critical that there is uninterrupted access to treatment services such as ART and treatments for co-infections and comorbidities to mitigate the impact of COVID -19. As there is a higher risk for COVID-19 incidence and mortality among people with underlying chronic illness and comorbidities, public health strategies should focus on early detection, diagnosis and timely initiation of treatment.

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Efficacy of Treatment Regimens for Sodium-Glucose Counter Transporter 2 Inhibitor (Emaglyf) with Metformin and DPP-4 Inhibitor (Januvia) in Patients with Type 2 Diabetes Mellitus with Stage 1-3 Chronic Kidney Disease

Background

As is known, the general goals of the treatment of type 2 diabetes mellitus (DM 2) are to avoid acute metabolic decompensation, prevent or delay complications, reduce premature mortality and maintain quality of life [1]. Pharmacological treatment options for T2DM are divided into:

a) Non-insulin therapies, including

1) Insulin sensitizers (metformin, thiazolidinediones [TZDs]).

2) Secretion stimulants (sulfonylureas [SUs]).

3) Incretin-based therapies (receptor agonists glucagon-like peptide-1 [RAs GLP-1], dipeptidyl peptidase-4 inhibitors [DPP4-is]), and

4) Insulin-sparing agents such as α-glucosidase inhibitors (AGis) and sodium glucose cotransporter-2 inhibitors (SGLT-2is); and

b) Insulin therapy. Until recently, stepwise and combination therapy were the two guidelines for pharmacological approaches in T2DM [2-5]. Due to the lack of sufficient data on the use of early combination therapy, stepwise treatment intensification has been the standard approach to achieve glycemic control, as recommended by the ADA/EASD consensus treatment algorithm. Asia, China, Hong Kong, Taiwan, Korea and Japan follow similar rules [6].

The AACE and ADA/EASD guidelines recommend intensifying treatment with an additional drug if monotherapy fails to achieve or maintain the target HbA1c level after 3 months. Preferred third-line therapy includes insulin or a triple combination of oral antidiabetic drugs [5,6]. The AACE treatment algorithm recommends that patients with an HbA1c level of 7.5% or higher (≥59 mmol/mol) be started on combination therapy with metformin plus an additional antidiabetic agent [5]. The 2018 ADA/EASD Position Statement recommends combination treatment only if HbA1c is more than 17 mmol/mol (1.5%) above an individual’s target [7]. In line with the latest data, the 2019 update recommends early recruitment of patients with newly diagnosed T2DM to start combination therapy through shared decision making. [four]. In Taiwan, combination therapy with metformin and another antidiabetic drug is recommended for patients with an HbA1c level of 8.5% or higher (≥69 mmol/mol) at the time of diagnosis [8]. In Hong Kong and Korea, combination therapy with metformin is recommended for patients with HbA1c 7.5% or higher (≥59 mmol/mol) [9,10].

Almost all classes of hypoglycemic drugs, such as metformin, SU, AGi, GLP-1 RA, DPP4-i, and SGLT2-i, can be used in combination. Most early combination therapies use metformin as baseline therapy. The efficacy and safety of various combination therapies have been reviewed and evaluated in detail in meta-analyses [11,12]. Positive Effects of SGLT2 on the kidneys was first shown in the EMPAREG, CANVAS and DECLARE cardiovascular trials (CVOT). These studies initially focused on assessing cardiovascular safety in patients with type 2 diabetes with renal outcomes as a secondary endpoint [Barnett AH, et al. 2014]. The efficacy and renal outcomes of SGLT2 inhibitors in patients with type 2 diabetes mellitus and chronic kidney disease were studied in a 2019 US multicenter study [Michael S, et al. 2019]. However, the effectiveness remains unexplored. SGLT2 in combination with other drugs at the stage before hemodialysis in patients with DM 2 and CKD. The above was the reason for the present study.

Purpose of the Study

To study the effectiveness of combination therapy of sodiumglucose counter transporter 2 inhibitor -SGLT-2- (Emaglyf) with metformin and DPP-4 inhibitor (Januvia) in patients with stage 1-3 chronic kidney disease associated with DM2.

Material and Research Methods

A total of 40 patients with type 2 diabetes and CKD grades 1-4 were selected. To study the effect of various schemes of nephroprotective therapy on the functional state of the kidneys in DM2, patients were divided into 2 therapeutic groups:

• Group 1 consisted of 20 patients with DM 2 and CKD 1-3 tbsp. receivingSGLT-2 (emoglyph) + metformin.

• Group 2 consisted of 20 patients with DM 2 and CKD 1-3 tbsp. receivingSGLT-2 (Emoglyph) + DPP 4 (Januvia)

In the work, general clinical, clinical and biochemical (AL, AST, bilirubin, PTI, urea, creatinine, GFR, C-reactive protein, etc.), hormonal (insulin, C-peptide), immunological (uromodulin) methods of blood tests, as well as instrumental methods of examination - ultrasound of internal organs, Ultrasound and dopplerography of renal vessels, as well as statistical methods. We also evaluated the results of ECG in 12 conventional leads and echocardiography (EchoCG) (dimensions of the chambers of the heart, the thickness of its walls and myocardial contractility). The control group consisted of 20 healthy individuals. For kidney ultrasound, an Aloka ultrasound machine with a 4L convex probe (2–5 MHz) was used. The renal resistive index in segmental arteries was assessed as described by the authors. The average value of RI was calculated from 2-3 measurements in the upper, middle and lower sections of the renal sinus. Renal perfusion was assessed using the DTPM method.

The renal artery was assessed at seven points: at the exit from the aorta, in the proximal, middle and distal segments, as well as the apical, middle and inferior segmental arteries. Peak systolic (PSV) and end diastolic (EDV) blood flow velocities, resistivity index (RI), acceleration time (AT), acceleration index (PSV/AT) were calculated. Statistical processing was carried out on a personal computer using the Microsoft Excel-2019 software package using the methods of parametric and non-parametric statistics. With mild renal failure (GFR> 50 ml / min, approximately corresponding to the content of serum creatinine <1.7 mg / dl in men, <1.5 mg / dl in women) Januvia dose adjustment is not required. In moderate renal failure (GFR >30 mL/min but <50 mL/min, roughly corresponding to serum creatinine >1.7 mg/dL but <3 mg/dL in men, >1.5 mg/ dL, but <2.5 mg/dl in women) the dose of Januvia is 50 mg 1 time per day. When taking Emaglif, it is recommended to monitor kidney function before starting treatment (at least once a year), as well as before prescribing concomitant therapy that may adversely affect kidney function. Patients with renal insufficiency less than 45 ml / min / 1.73 m2) receive Emaglyf is contraindicated.

Research Results and Discussion

Table 1 shows the distribution of patients by sex and age. As can be seen from Table 1, patients in the age group from 45 to 74 years old both among men and women predominated - 25/15 cases, respectively. Table 2 gives general characteristics of patients included in the study in groups. As can be seen from Table 2, there were no significant differences in the general characteristics of the initial indicators in the studied groups (p>0.05). The mean glomerular filtration rate (GFR) was significantly lower in all groups. Next, we studied the biochemical parameters by groups before treatment (Table 2). As can be seen from Table 2, the initial data on carbohydrate metabolism indicated its decompensation in the studied groups. The next step was to conduct dopplerography of the renal arteries before and after treatment (Table 3). As seen from the data shown in Table 3 showed significant differences between the Doppler values of the renal arteries in the groups compared to the control. After 6 months of treatment according to the above schemes, we studied the effectiveness of therapy in the study groups, for which we studied the dynamics of biochemical and Doppler parameters (Tables 4 & 5). As can be seen from Table 4, after 6 months of therapy, the indicators of carbohydrate metabolism reached normalization in both groups, while the best results were observed in group 2 patients. The next step was to conduct dopplerography of the renal arteries before and after treatment (Table 5).

Table 1: Distribution of patients by sex and age.

Table 2: Mean biochemical blood parameters of patients by groups before treatment

Note: P - significance of differences compared with control data, where * - p <0.05.

As can be seen from the data, given in Table 5, after 6 months of treatment, between the Doppler values of the renal arteries in the groups, a significant improvement in the parameters was revealed.peak systolic (PSV) and end-diastolic (EDV) blood flow velocity, resistivity index (RI), acceleration time (AT), acceleration index (PSV/AT),namely, in group 2, the best results were obtained (in comparison with control data p>0.05). Thus, our study showed nephroprotective effect of both schemes. Our results confirm the literature data. Thus, according to Italian authors, antidiabetic drugs with potential nephroprotective effects, namely DPP-4 inhibitors, incretin analogues and SGLT-2 inhibitors, can have a nephroprotective effect regardless of glycemic control. Sodiumglucose co-transporter (SGLT) 2 inhibitors act at multiple sites that may affect kidney function, according to other sources. The canagliflozin Cardiovascular Assessment Study (CANVAS) showed a 27% reduction in albuminuria progression, a 40% reduction in eGFR, need for renal replacement therapy, or death from renal causes associated with canagliflozin use. All of the above confirms the high relevance of this study and dictates the need for its further continuation.

Table 3: Doppler parameters of the kidneys in patients included in the study (M ± m).

Table 4: Mean biochemical blood parameters of patients by groups after 6 months of treatment.

Note: P - significance of differences compared with control data, where * - p <0.05. , **-p<0.001 after 6 months of treatment.

Table 5: Doppler parameters of the kidneys in patients included in the study (M ± m).

Conclusion

1) After 6 months of therapy, the indicators of carbohydrate metabolism reached normalization in both groups, while the best results were observed when using the SGLT-2 + DPP4 regimen.

2) After 6 months of treatment, significant differences were found between the Doppler values of the renal arteries in the groups, namely, when using the SGLT-2 + DPP4 scheme.

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Comparison of Two Types of Dual Resin Cements in Cantilever Dental Bridge Compressive Stress Distribution: Finite Element Analysis

Introduction

Nowadays, it has become common to find young patients with missing teeth [1]. In these cases, dental implants are the first option [2], since they have a high success rate (94%) [3] and allow aesthetically pleasing results to be achieved [4]; however, it is an expensive treatment [5]; therefore, as an alternative, the resinbonded fixed dental prostheses (RBFDP) can be used [6]. The latter is characterized by the fact that, unlike the conventional fixed prosthesis, it requires a more conservative wear of the surface of the abutment teeth [7]; in addition, it is economical compared to the implant [5]. RBFDPs began to be known in the world of dentistry in the 1970s [8] and their design evolved over time [6]. Within this group, we can mention the Maryland bridge, [6] which was created by professors from the university of the same name [9] and its main characteristic is to have two palatal retainers; There is also the cantilever, whose peculiarity is based on the possession of a single palatal retainer [6]. Currently, they are used to replace missing teeth in the anterior sector [10]; however, at some point an attempt was made to extend their indication to the posterior sector [11], but several investigations have shown unfavorable results [12]. There are studies that have compared the success of the Maryland bridge and the cantilever in the anterior sector, where the latter demonstrated more satisfactory results [13]. This is because the teeth in that area they receive forces in different directions during chewing; therefore, in the case of the cantilever, the pontic is free to move in the same direction as the retainer [13].

The union of the adhesive fixed prosthesis is mainly based on adhesion [6]; for this reason, the choice of cement is important. This is evidenced in a variety of systematic reviews, where it is detailed that the most recurrent cause of failure for this type of prosthesis is detachment (33.7%) [14]. Likewise, masticatory force is a factor that should be consider when choosing the cement, because, during chewing, different types of forces are exerted [15], which can cause disunion between the prosthesis and the tooth. There are few studies that have evaluated the distribution of compressive stresses in RBFDP when using different types of cements through finite element analysis, [2,16,17] despite being a useful tool for this purpose. This method allows the evaluation of the mechanical properties of structures such as fixed prostheses or implants [18] through the use of irregular geometry solids software, isolating the variable of interest from various external factors that may affect the result.16 In turn, it is widely used in the area of dentistry since, unlike other experimental studies, it is precise, easy to perform and requires less time [19]. Therefore, the purpose of this study was to compare the distribution of compressive stresses in a cantilever bridge using two different types of dual resin cement, designed through finite element analysis.

Materials and Methods

This research was approved by the Research Ethics Committee of the Universidad Cientifica del Sur, with the approval code 521- 2021-PRE8. The models were built following these steps: first, the construction of the models was carried out, then the forces were applied and at the end, the results were analysed. Two virtual models were designed using SOLIDWORKS® software version 2017 (SolidWorks Corporation, France), which consisted of a central incisor and an upper left canine, fixed on a surface. In both models, a lithium disilicate cantilever bridge was simulated with a pontic (lateral incisor), a palatal retainer (upper left canine) and a 16mm2 connector [20]. The difference between both models was the dual resinous cement, since Relyx U200 cement was used in model A and Relyx ARC cement in model B. For both models, a 1mm preparation was made in the centre of the palatal aspect of the upper left canine, covering only enamel to be conservative, with a supragingival termination line. The cement layer was patterned with a uniform thickness of 0.1mm. The mechanical properties that were considered in the study are the modulus of elasticity and the Poisson’s ratio, which were summarized in Table 1 [17,21- 23]. Finally, a force of 100N was applied to the palatal aspect of the lateral incisor (pontic) at 2mm below the incisal surface of the tooth, at an angulation of 45° and a horizontal force of 100N on the palatal aspect of the canine, simulating the direction of forces that this sector receives. After all this, the results of the stress fields and displacement of the evaluated prosthesis were obtained. These data were obtained through Von Mises stress analysis, whose criterion is based on the conception of internal energy.

Table 1: The mechanical properties of the materials used in this study

Results

Table 2: Results of the compressive stress distribution (MPa) in a prosthesis cemented with Relyx U200.

Table 3: Results of the compressive stress distribution (MPa) in a prosthesis cemented with Relyx ARC.

Note: Maximum value (maximum peaks of compressive stress).

The results can be seen as colorimetric graphs in megapascals (MPa) scale for compressive stresses and in millimetres (mm) for displacement. Likewise, in tables it can be analysed that both the compressive stresses (Tables 2 & 3) and the displacement (Table 4) for both models followed the same pattern after the loads were applied. Regarding the compressive stress calculated in the prosthesis that was cemented with Relyx U200 dual resin cement (Table 2), higher stress values were observed when applying oblique forces (660,891 MPa) compared to horizontal forces (16.6MPa) (Figures 1 & 2). The same happened with respect to the compressive stress calculated in the prosthesis that was cemented with Relyx ARC dual resin cement (Table 3), where higher stress values were also produced when applying oblique forces (660,891MPa) (Figures 3 & 4). Regarding the displacement of the cantilever bridge (Table 4), the same results were obtained in both models, where it was greater when receiving oblique forces (0.014mm) compared to horizontal forces (0.00066mm) (Figures 5 & 6); however, in both cases, the values obtained were minimal (Table 4).

Table 4: Comparison of displacement of the cemented cantilever bridge with Relyx U200 (Model A) and Relyx ARC (Model B).

Figure 1: Results of the compressive stress distribution in a prosthesis cemented with Relyx U200 after the application of oblique forces of 100N.

Figure 2: Results of the compressive stress distribution in a prosthesis cemented with Relyx U200 after the application of horizontal forces of 100N.

Discussion

The analysis of both models when applying oblique forces, showed that the maximum stress in the prosthesis was located at the level of the pontic and on the incisal edge adjacent to the pontic. This result agrees with other studies such as that of Uraba A, et al. [24] where they evaluated 3 models of fixed adhesive zirconium prostheses in the anterior sector by means of a finite element analysis and obtained as a result that, in all the models, the maximum stress concentrated on the incisal side of the connector, applying a force of 200N at 45° from the pontic. Likewise, Wei Y, et al. [25] in their systematic review on the failure and complication rate of different adhesive fixed prosthesis designs in the anterior sector, recommended that a larger dimension connector should be used to improve the properties of the material in that area. Regarding the displacement of the prosthesis, the results obtained support the study by Keulemans F, et al. [26], who carried out a finite element analysis comparing five cantilever bridges made of different materials, including vitrioceramic and zirconium, obtaining as a result a greater displacement of the prosthesis in the pontic part when applying a stress of 90MPa, at an angle of 45° to the incisal edge of the pontic. In addition, the homogeneity of the results of the present investigation agrees with the finite element study by Penteado M, et al. [2], who compared six types of cements with different modulus of elasticity and observed that the displacement pattern was the same for all the models evaluated.

Figure 3: Results of the compressive stress distribution in a prosthesis cemented with Relyx ARC after the application of oblique forces of 100N.

Figure 4: Results of the compressive stress distribution in a prosthesis cemented with Relyx ARC after the application of horizontal forces of 100N.

Figure 5: Results of the displacement of the cantilever bridge cemented with Relyx U200 after the application of forces.

Figure 6: Results of the displacement of the cantilever bridge cemented with Relyx ARC after the application of forces.

The main advantage of the use of adhesive fixed prostheses is the possibility of making conservative wear on the enamel surface [7]. Affuer M, et al. [27], indicated in their study that the average thickness of enamel at the canine level is 2.15mm; therefore, the preparations in this study covered only 1mm of enamel, on the palatal surface of the canine, sufficient space for the lithium disilicate retainer and cement. The present investigation does not agree with the finite element study of Dal Piva A, et al. [17], who made more conservative preparations of 0.5mm for lithium disilicate adhesive fixed prostheses. However, in this study the minimum thickness of the material was also considered, which is 0.8mm [28]. In this study, both self-adhesive and conventional dual resinous cement acted in the same way, and this agrees with that mentioned by Wei Y, et al. [25] who, in their systematic review on the failure rate and complications of different designs of adhesive fixed prosthesis in the anterior sector, mentioned that it is possible that the use of different resinous cements does not influence the risk of prosthesis failure in clinical practice. Likewise, in the study by Upadhyaya V, et al. [29] on the shear resistance of conventional, self-etching and self-adhesive resinous cements, they concluded that total-etching resinous cement is the most reliable and clinically recommended to achieve a long-lasting bond. duration between a lithium disilicate restoration and the tooth. On the other hand, the results of this study do not agree with the research of Penteado M, et al. [2], who evaluated cements with different modulus of elasticity using RBFDP of lithium disilicate and observed a variation in the compressive stress, unlike the present investigation where the results were the same. These data may be discrepant due to retainer design; since in the present study the thickness was greater; therefore, this factor should also be evaluated.

The forces applied in this study were 100N in a horizontal direction at the level of the palatal aspect of the canine and 100N in an oblique direction, at an angle of 45° to 2mm from the incisal edge of the pontic, as in other finite element studies that evaluated fixed adhesive prostheses in the anterior sector, such as those of Penteado M, et al. [2] and Dal Piva A, et al. [17]. In this investigation, a normal bite force was applied as in the study by Toman M, et al. [30] and a parafunctional force was not considered because Tezulas E, et al. [13] reported that this type of prosthesis is contraindicated in such cases. There is great controversy regarding the design of the adhesive fixed prosthesis, since it may have one or two palatal retainers; however, as reported in the literature, the survival rate is higher in cases of structures with a retainer [31,32]. In the same way, in the finite element study by Dal Piva, et al. [17] where they evaluated three different adhesive fixed prosthesis designs, two models with a palatal retainer (one at the level of the canine and the other at the level of the central incisor) and a model with two palatal retainers in the upper left sector, obtaining as a result that the highest stress values were found in structures with two retainers. The explanation of this event is based on the micro-movements of each tooth in the mouth; that is, it is due to the degree of physiological mobility that the abutment teeth present during their function when they come into contact with the opposing pieces [25]; therefore, in the case of a cantilever bridge, the pontic moves in the same direction with the tooth in which it has been retained, as it only has one pillar [12]. Likewise, Tezulas E, et al. [13] and Chen J, et al. [33] mentioned that the canine is the best abutment for a cantilever bridge in this sector, because this piece has a long root and would adequately support lateral forces during chewing. For this reason, in the present study all the data mentioned above were taken into consideration and a cantilever bridge cemented in the canine tooth was selected for evaluation.

Despite the benefits granted by virtual design studies, these present limitations, the main one being the impossibility of simulating a real situation in the mouth, since in this case factors such as humidity, temperature or pH are not valuated, which if they could influence a clinical scenario. Likewise, the polymerization contraction, the management of the adhesive material and the presence of temporomandibular disorders are elements that also influence and may even modify the prognosis of these rehabilitation treatments in the future.

Finite element analysis is a useful tool for the evaluation of the mechanical behaviour of the fixed adhesive prosthesis. These are considered minimally invasive, aesthetic and economic treatments.34 Based on the findings of this study, it is indicated that the mechanical behaviour of different dual resinous cements, with a different clinical protocol, behave in the same way when receiving masticatory forces. However, it is suggested to follow this line of research considering that cements, being found in an oral environment, can be affected by various factors such as saliva, which were not considered in the present investigation as they are virtual simulation.

Conclusion

It is concluded that the use of dual resinous cement does not influence the resistance of the cantilever bridge with a palatal retainer, finding equality between the distribution of compressive stresses and displacement of the prosthesis when using Relyx U200 cement such as Relyx ARC through finite element analysis. In the same way, new thesis students are recommended to evaluate the dynamics of the cantilever bridge cemented in the palatal side of the canine, considering the lateral forces that these pieces receive, since the mechanical behaviour could vary; likewise, it is suggested to carry out an in vitro study using the results of the present study, where the behaviour of the different types of dual resinous cements could be evaluated; and also, carry out finite element studies based on the design used in this research, where the retainer has different thicknesses.

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