Investigating the relationship between dental caries, microbial plaque, and unknown infertility in men and women: A case–control study

Infertility is defined as a condition in which a couple’s reproductive system is unable to conceive after 12 months of regular unprotected sex. Besides, a healthy woman should have a successful pregnancy and give birth to a healthy child.[ 1 ] In 2010, 48.5 million couples were estimated to be infertile worldwide, but today, the rate of infertility has greatly increased, affecting about 60–80 million couples.[ 2 ] Fertility issues have significant financial expenses and put families in a dire situation. Population growth, marriage happiness, sexual pleasure, and mental and social well-being may all be impacted by infertility. In addition, infertile couples can experience feelings, such as anger, sadness, anxiety, depression, guilt, shame, loss of control, and incompleteness, as well as changes in self-esteem, social isolation, and increased marital anxiety.[ 3 ] In infertile people, failure to have children causes feelings of social isolation and shame.[ 4 5 ] In addition, infertility in couples disrupts their identity and perspective on being parents and their fertility, regarding the purpose and meaning of life in men or women, or both.[ 6 ] In infertile couples, social pressure and the stigma of childlessness have serious effects on the couple’s mental and physical health.[ 7 ] One of the most important oral health problems in the world is tooth decay, which is caused by the interaction between bacteria and fermentable carbohydrates and can lead to the destruction of the hard parts of the tooth.[ 8 ] Despite being preventable, dental caries remain prevalent, affecting approximately half of the world’s population, or 3.5 billion people, with 40% of cases going untreated.[ 9 ] Unfortunately, tooth decay has become a significant public health problem worldwide, with high rates of decay reported among children and adults, which can cause pain and discomfort.[ 10 ] For the first time, a microbial plaque was described as gelatinous on the tooth surface, which is formed in terms of the accumulation of 100s of diverse bacterial species exclusively found in the oral cavity.[ 11 ] Studies have reported various factors like lifestyle, such as eating habits, increased consumption of sweets, poor oral hygiene, low level of oral health knowledge, lack of access to quality dental care, and social factors in increasing tooth decay.[ 10 12 13 ] Oral and dental diseases are a group of chronic infections primarily caused by dental caries and chronic periodontists, a disease of the tooth’s supporting structures, especially cementum, gingiva, periodontal ligament, and alveolar bone. Globally, 3.9 billion people are affected by oral disorders, with untreated caries being the most prevalent illness. Oral and dental conditions have been linked to several systemic conditions during the last 30 years, including arteriosclerosis, heart attacks, and diabetes mellitus.[ 14 ] Among the biological systems affected by oral disease are pulmonary, urinary, respiratory, and reproductive systems because oral inflammation may contribute to systemic inflammation.[ 15 ] The main causes of female infertility are ovulation problems, fallopian tube problems, endometriosis, and unexplained infertility.[ 16 ] Some studies indicated a positive relationship between periodontal disease and infertility.[ 17 ] Male infertility is a multifactorial disorder, but about 30% of male infertility cases are without a known cause despite careful examinations.[ 18 ] Results of studies conducted on this issue showed a positive relationship between chronic periodontists and infertility in men. In three studies, a positive relationship has been reported between unknown infertility and odontogenic infections associated with pulp necrosis and radicular cysts.[ 14 ] Based on reviewed studies, there is a relationship between male infertility and dental health.[ 19 ] It is believed that periodontal tissue bacteria and inflammatory mediators, which are created in response to them through the bloodstream, cause an inflammatory process in the body, including the reproductive system, ultimately leading to infertility.[ 20 ] Hard dental tissue is destroyed by dental caries, a bacterial infectious illness that may ultimately extend to the tooth pulp. The decayed, missing, and filled teeth (DMFT) index often assesses dental caries. It should be emphasized that microbial plaque is the root cause of both periodontal disorders and tooth decay. In dentistry, it was proven that chronic teeth infection may have destructive effects on parts of the body that are distanced from the oral cavity. In recent years, studies have been conducted on the relationship between periodontal diseases and infertility, but very few studies have been conducted globally on the relationship between dental caries and infertility. Of course, these studies also have errors in the research methodology, and the results of these studies have been contradictory. According to researchers in our country, Iran, no study has been conducted in this field so far. The study by Tatar and colleagues 2020 was conducted only in the female group, but unlike all studies, our study was conducted in both the female and male groups. Therefore, considering the importance of this topic and the lack of studies in this field, the present study was conducted to investigate the relationship between dental caries, microbial plaque, and unknown male and female infertility in Hamadan City.

This study was conducted as a case–control study on 332 fertile and infertile women and men in two case groups and two control groups. Table 1 demonstrates that there is no statistically significant difference in the demographic factors of age, age at marriage, monthly income, occupation, spouse’s occupation, education, and spouse’s education between the case and control groups. The demographic characteristics of the study groups were matched. Demographic characteristics of study samples The average DMFT index was 11.33 in the female case group and 8.96 in the control group. This figure was 11.36 in the male case group and 9.06 in the male control group, which showed a significant difference between female and male case and control groups ( P < 0.001). The average number of decayed teeth was 6.3 in the female case group and 3.36 in the female control group, while it was 5.71 in the male case group and 3.71 in the male control group, which indicated a significant difference in the number of decayed teeth between male and female case and control groups ( P < 0.001). There was a statistically significant difference between the male and female case and control groups, with the mean plaque index being 28.84 in the female case group, 24.81 in the female control group, 34.97 in the male case group, and 28.8 in the male control group. Table 2 shows a significant difference between two male and female case and control groups in terms of the level of secondary infertility, infertility treatment, type of infertility treatment, the number of infertility treatments, and history of surgery related to infertility in women and men. The results showed no statistically significant difference between female ( P = 0.261) and male ( P = 0.263) case and control groups due to visits to the dentist. No statistically significant difference was observed between female ( P = 0.11) and male ( P = 0.506) case and control groups regarding compliance with oral hygiene. Infertility-related variables in the study samples IUI=Intrauterine insemination; IVF= In vitro fertilization Tables 3 and 4 show a statistically significant difference in the number of decayed teeth, healthy teeth, and the DMFT index between the female and male case and control groups. Regarding the number of repaired and lost teeth, a significant difference was also discovered between the case and control groups for females and males. Concerning plaque index, the findings likewise revealed a statistically significant difference between the female and male case and control groups [ Table 4 ]. However, the results of the Mann–Whitney test showed no statistically significant difference between the infertile men with normal and abnormal spermogram indexes in terms of average D, M, and F components, DMFT index, and plaque index ( P > 0.05). About the main study objective, Table 5 shows that with the increase in DMFT, dental caries, plaque index, and the chance of infertility will also increase. Variables related to DMFT of men in two case and control groups SD=Standard deviation; DMFT=Decayed, missing, and filled teeth Variables related to DMFT of women in two case and control groups Plaque index (PI) of women and men in the case and control groups SD=Standard deviation The multiple logistic regression test results showed that the chance of infertility increases with the increase in DMFT, dental caries, and plaque index [ Table 6 ]. Relationship between DMFT, PI, and infertility PI=Plaque index; DMFT=Decayed, missing, and filled teeth

According to the study findings and considering the importance of dental caries in the prevention of infertility in couples of childbearing age, it is suggested to have educational programs to observe oral and dental hygiene and correct nutritional style from childhood to adulthood. As a result, effective planning should be made for dental caries prevention, oral and dental health education, and prompt detection of dental caries by midwives and dentists in health centers. All authors were involved in concepts, design, definition of intellectual content, literature search, clinical studies, experimental studies, data acquisition, data analysis, statistical analysis, manuscript preparation, manuscript editing, manuscript review, and guarantor. The authors have no conflicts of interest.

Infertility is one of the health problems that have a great effect on marital relationships, mental health, and self-confidence of affected persons.[ 3 ] The prevalence of infertility has increased significantly in recent years, so the global prevalence of infertility is between 10–15 percent.[ 21 ] In 2010, the number of infertile couples in the world was estimated at 48.5 million, but today, infertility has affected about 60–80 million couples.[ 2 ] According to a book entitled; “Oral Health Surveys” published by the World Health Organization, the mean DMFT in 35–44-year-old adults is classified into four categories: very low (13.9).[ 22 ] Thus, as the mean DMFT was 10.2, we can argue that it was moderate. The average DMFT index was 11.33 in the female case group and 8.96 in the control group. This figure was 11.36 in the male case group and 9.06 in the male control group. Based on the classification made by the World Health Organization, women in the case group were in the middle category, and women in the control group were in the lower category. Based on this classification, the men in the case group were in the middle category, and the men in the control group were in the low category. It may be concluded that men and women have similar health behaviors in the field of oral and dental care, and inappropriate health behaviors can affect their reproductive health and fertility. The present study showed a positive relationship between dental caries, microbial plaque, and unknown infertility in men and women. The study conducted by Maybodi et al .[ 3 ] on the relationship between poor oral health and infertility issues revealed a direct correlation between oral and dental health and sexual health; therefore, they argued that reducing oral inflammation could improve fertility. Based on Miller’s focal infection theory, oral bacteria can enter the body system through the bloodstream and negatively affect different body organs. Eastham and Seymour’s[ 23 ] study, which examined the relationship between oral health and sexual health, also showed that the extent and severity of periodontal diseases can be an important risk factor for erectile dysfunction and sperm mobility. This result is consistent with the findings of the present study. Bieniek et al .,[ 24 ] who investigated the relationship between oral and dental health and sperm count in 76 men using dental and periodontal examination and sperm analysis, showed a relationship between oral health and an abnormal number of sperms among all age groups of infertile men. These results are also in line with the results of the present study. In a study conducted by Machado et al .[ 17 ] on the relationship between periodontitis and infertility among infertile women receiving infertility treatment, the results showed a statistically significant difference between the case and control groups due to the mean percentage of plaque index and periodontal index, which is in line with our study. Shoorgashti et al .[ 25 ] reported in their study that periodontitis can impair reproductive function through systemic bacteremia. According to the results of the studies, it is necessary to assess the oral and dental health status of primary care providers before starting any fertility treatment. Sergio Varela Kellesarian[ 14 ] conducted a study on the relationship between infertility in men and the state of oral and dental health and showed a positive relationship between oral diseases, dental caries, and pulp necrosis. Zhu et al .[ 26 ] studied the relationship between male infertility and chronic periodontitis among 108 infertile and 108 fertile men. They showed that the prevalence of chronic periodontitis in infertile men was 48.3%, and in fertile men was 31.7%, and this difference was statistically significant. They showed a positive relationship between chronic periodontitis, poor dental health, and infertility in men. In another study by Nándor Práger et al .,[ 27 ] which examined the relationship between male idiopathic infertility with periodontal status and tooth decay, 199 men with unknown infertility were examined, of whom 106 had normal sperm parameters, and 93 had some form of sperm pathology. The above study showed that DMFT was not significantly increased in any of the groups of infertile men, but periodontal indexes, such as dental plaque, calculus, and bleeding on probing, were significantly increased. These results are in line with the findings of the present study. Due to the small sample size, lack of comparison between case and control groups, and problems with the study methodology (not considering systemic diseases in the study group), the results of this study cannot be considered completely reliable, and additional research is required in this field. This study refers to the relationship between oral diseases and infertility based on the theory of bacteremia, in the way that local infections cause periodontal or periapical inflammatory reactions, which increase inflammatory cytokines that can contribute to infertility. For this reason, the role of dental caries in inflammatory processes and its relationship with infertility is significant. A study by Klinger et al .,[ 28 ] which examined the relationship between periodontal status and infertility in 75 infertile men, showed no significant increase in the number of missing teeth in infertile men, which is in line with our study. This study showed no significant increase in plaque index. The difference in the results of this study and our study can be due to the small number of samples and the absence of a control group. In the Yildiz Telatar et al .[ 15 ] study, which was conducted as a case–control study on periodontal status and dental caries in unknown infertility, 50 fertile women and 50 women with unknown infertility were examined. The average DMFT in the case and control groups was 7.62 and 6.06, respectively ( P = 0.079). Furthermore, the number of missing teeth in the case and control groups was 1.68 and 1.58, respectively, while the number of fixed teeth in the case and control groups was 2.64 and 2.02, respectively. This study showed no significant difference between the case and control groups regarding the variables mentioned above. The frequency of advanced dental caries in the case and control groups was 37% and 26%, respectively ( P = 0.011), which showed a statistically significant difference between the case and control groups. Moreover, no significant difference was seen in the two groups regarding dental visits, oral hygiene, and using toothbrushes or dental floss. Moreover, a positive relationship was found between periodontitis and infertility risk factors, including increasing age, obesity, and chronic kidney disease. In a study conducted by Gornowicz et al .,[ 29 ] a relationship was found between dental caries and pro-inflammatory cytokines in saliva, so a statistically significant difference was observed in the amount of DMFT between the case and control groups. In another study conducted by Balakesavan et al .,[ 30 ] the increase in dental bacterial infections caused an increase in systemic pro-inflammatory cytokines, such as IL6, TNF-α, and IFN-γ, and these factors were seen in the seminal plasma of the participants. Results of these studies show the relationship between dental bacterial infections and male infertility and show that these bacteria may lead to adverse pregnancy outcomes in pregnant women. Considering the positive relationship between dental caries and infertility in men and women, which was found in this study, we suggest measures, such as teaching how to maintain oral and dental hygiene from early childhood and puberty and implementing pre-pregnancy preventive programs to reduce the percentage of microbial plaque, to be routinely included in the care plans of health centers. Regarding the availability of materials and tools for the relevant experiments and since the researcher had some experience conducting similar projects, we did not face any particular restrictions. The strength of this study is that, given the importance of infertility and the negative messages it can convey, this study is the first to examine the relationship between dental caries and infertility in an Iranian population in both the female and male groups. However, studies conducted in this field have examined oral diseases, not dental caries and plaque, which could also be the strength of our study. Conducting a similar study in other Iranian communities with a larger sample size is suggested. Conducting more studies with a more diverse statistical population in different cities of the country. Evaluation and measurement of inflammatory factors as well as bacterial examination in infertile people compared to fertile people. Conducting a similar study in other Iranian communities with a larger sample size is suggested. Conducting more studies with a more diverse statistical population in different cities of the country. Evaluation and measurement of inflammatory factors as well as bacterial examination in infertile people compared to fertile people.

This case–control study was conducted from July 28 to November 29, 2022, on 332 fertile and infertile women and men in Hamadan. The study setting included the endometriosis research center of Hamadan University of Medical Sciences in the Fatemieh Hospital, the Neonatal Center of Fatemieh Hospital, and the Faculty of Dentistry at Hamadan University of Medical Sciences. The inclusion criteria were being 30–40 years old, having primary infertility with an unknown cause, having a history of infertility for at least one year, having at least 25 teeth, being willing to participate in the study, and filling out the ethical consent form (S1). Exclusion criteria were having an underlying disease, such as diabetes, having a history of previous pregnancy (for the female sample), having periodontal disease, suffering from complete edentulousness, and being unwilling to continue participating in the study. The samples of case groups were selected by random sampling method among women and men with unknown infertility referred to the infertility center of Fatemieh Hospital in Hamadan province, and samples of the control groups were selected among fertile women and men who met the inclusion criteria. The control and case groups were matched in terms of some demographic and contextual variables. The data collection method included interview and examination (bacterial plaque disclosing tablets), using the professional dentist’s examination tools, such as a mirror, catheter, and disposable latex gloves. To determine the number of samples, Yildiz Telatar et al .[ 15 ] study and the following formula were used in consultation with a statistical expert: In this formula, d = 1.9 was used, taking into account the first type error of 5% and the test power of 80%. So, the required number of samples was determined to be 83 people in each group. Therefore, 332 samples participated in this study, including 83 infertile women and 83 infertile men as case groups, and 83 fertile men and 83 fertile women as control groups. The data collection tool was a questionnaire consisting of three parts (demographic information, obstetrics and gynecology information, and dental information). The first and second parts of the questionnaire were designated to demographic, gynecology, and obstetrics information. The validity of these two parts was ensured via consultation with 10 experts in this field, and their reliability was determined based on Cronbach’s alpha coefficient, which was more than 0.8. The DMFT index, which was used to calculate the number of decayed, filled, and missing teeth (S2), and the World Health Organization’s standard questionnaire were both utilized in the third section of the survey to gather dental information. By prior coordination with the officials of the Neonatal Center of Fatemieh Hospital, 83 pregnant women who were giving birth for the first time were selected as the control group, and 83 women with unknown infertility were referred to the Endometriosis and Endometriosis Research Center of Hamadan University of Medical Sciences in the same hospital, were selected as the case group. The diagnosis of unknown infertility was made using the patient’s medical record and an obstetrician. Furthermore, 83 fertile men who met the inclusion criteria and 83 men with unknown infertility referred to the same center were selected as the control and case groups, respectively. Demographic and gynecological information about the fertile samples was obtained by asking questions from them, and in infertile samples, in addition to asking questions, their medical records were used in addition to asking questions. In infertile men, the results of the sperm analysis test, which the laboratory of Fatemieh Hospital performed, were extracted from the patient’s files. In all samples participating in the study, the bacterial plaque information was obtained using the O’Leary method (using the microbial plaque percentage formula) and later was recorded in the relevant table. Then, a dental examination was performed using a dental mirror, probe, and lamp, and DMFT was calculated according to the World Health Organization guidelines. The project number of this thesis project is 36255. This study was carried out after being approved by the Ethics Committee of Hamadan University of Medical Sciences with the ethics code IR.UMSHA.REC.1400.327. Patients were fully informed about the purpose of the study. They were assured that their personal information would be kept confidential and personal information would not be given to other parties. Then, a written consent was obtained from them. In the completed consent form, there are the name and details of the participating person, but it is only available to the researcher responsible for the study, whose ethical principles and confidentiality have been fully respected, and it has not been given to any other person. However, the participant’s name and details are not mentioned in the completed questionnaires. We tried to get all the information at the same moment of the examination and complete the questionnaire, so there is no request to meet the research sample again. Descriptive statistics (mean, standard deviation, and frequency percentage), and inferential statistics ( t -test, multiple logistic regression, and Mann–Whitney test) were used to analyze the data. Data analysis was performed through SPSS-20 software at the significance level of P ˂ 0.05.