The Effect of Fluorosis on the Rate of Orthodontic Space Closure - A Prospective Study

Introduction

Fluorine is one of the essential nutrients for normal growth and development of the body. The safe limit of fluoride consumption is 1.5 ppm. According to the National Health Profile 2019, over four lakh students in Karnataka's schools have dental fluorosis, accounting for almost 81% of the nation's total number of affected students. The government's calculations show that approximately 11.7 million people are at risk, while grassroot groups advise that the risk is considerably more pervasive, impacting nearly 60 million people nationwide.¹

Adjusted levels of fluoride in drinking water reduced dental caries and proved to be an effective method in controlling dental caries.² Fluoride is predominantly present in calcified tissues such as bone and teeth. The amount of fluorine in such structures is directly proportional to the fluoride concentration of water. The issue worsens with time and could end in permanent disability as the bones become more brittle.¹

Orthodontic tooth movement depends on bone deposition and resorption.³ Nazam and Vandana KL observed significant differences in the trabecular pattern, reversal and resting lines, and the number of osteoclasts in non fluorosed bones when compared with fluorosed bones. Cellularity of cortical and cancellous bone was found to be statistically significant in non-fluorosed group (10.72±4.10, 8.74±2.34) when compared to fluorosed group (6.61±3.31, 5.69±1.31), respectively.⁴

Variables that can affect the rate of tooth movement are extrinsic and systemic factors such as medication history, age, genetic factors, and metabolic bone diseases can alter key molecules that regulate bone remodelling.^5,6 Fluoride's impact on these processes is unknown.

Fluoride enhances alkaline phosphatase activity of bone cells. Fluoride action on bone formation is mainly mediated by stimulating the proliferation and differentiation of committed osteoblast precursors in bone marrow.⁷

Customary diagnostic records to evaluate the rate of tooth movement include dental casts, intraoral clinical examinations, and radiographic data such as lateral cephalograms. Most clinicians prefer 2D radiographs instead of 3D because of their lower radiation doses.

Fluoride is a common natural chemical with biphasic activities. Excessive systemic exposure (from endemic water fluoride) can lead to detrimental effects such as enamel fluorosis and disruptions in bone homeostasis. Lower fluoride dosages are advantageous for bone health and are frequently used to treat osteoporosis. The effects of dental fluorosis are presented in two reviews of decades of research studies.^8,9 The fluoride effects on various dental tissues are less studied as compared to its effect on dental caries. Enamel and bone are target tissues for the harmful effects of excessive fluoride in endemic geographic water fluoride belts. The possibility that orthodontic treatment in dental fluorosis subjects may differ due to mineralization and density changes on fluorosed bone led us to conduct this clinical work to compare the orthodontic treatment in dental fluorosed and non fluorosed subjects.

While fluoride has been shown to minimize the pace of orthodontic tooth movement in animals in previous studies,¹⁰ the effect of fluorosis in humans is yet to be explored.

Medline search revealed scarcity in the literature on the effect of dental fluorosis on OTM (orthodontic tooth movement) by using keywords such as fluorosis, orthodontic tooth movement, space closure and bone density.

In the present study, we compared the rate of tooth movement in FL (fluorosed) and NFL (non-fluorosed) patients clinically and radiographically using study models and in addition to it, estimated the bone density using Image J Analyser in fluorosed and non fluorosed patients.

Materials and Methods

This prospective clinical longitudinal study was approved by the Ethical Committee Board of the College of Dental Sciences (CODS/ 3210 2019-2020) and no patient was exposed to supplementary radiations for study purposes.

Subjects

The sample of this study consisted of 40 subjects who were referred to the Department of Orthodontics and Dentofacial Orthopedics, College of Dental Sciences, Davanagere with complaints of having forwardly placed upper front teeth. The treatment plan included extraction of all first bicuspids to reduce the proclination of upper and lower teeth.

The patients included in this study were in the age group of 18-24 years with a full complement of permanent dentition with no radiographic bone loss. The subjects suffering from dental fluorosis with stained teeth and permanent residents of endemic high water fluoride belt, with mild crowding were included. The treatment plan involved bilateral maxillary first premolar extraction with maximum anchorage. Patients had a regular attendance of their orthodontic appointments and had records that included complete diagnostic and treatment notes, pre-treatment and post-treatment orthopantomogram (OPG), lateral cephalograms, and study models. Fluoride concentration in drinking water was in the range of 1.5-3 ppm, assessed by Chemical Department, Bapuji Institute of Engineering and Technology.

Patients having complex systemic diseases, having hypodontia, midline shift, those who underwent palatal expansion or orthognathic surgery and extractions done for other reasons before the start of orthodontic treatment were excluded from this study.

All the patients were divided into two groups of 20 each based on the presence of dental fluorosis. Non-fluorosed group (NFL) included patients showing no signs of fluorosis while flourosed group (FL) consisted of patients having dental fluorosis. All patients were treated with a 0.022-inch slot Pre-Adjusted Straight Wire Appliance (MBT, 3M Unitek, California, USA) using sliding mechanics. Anchorage was reinforced using a transpalatal arch in all the patients. After leveling and aligning, 0.019×0.025 inches stainless steel arch wire was engaged in the slots and ligated with 0.010 inch stainless steel wire. Passive tiebacks were placed for one month. Later, active tiebacks using elastomeric modules applying a force of 150-200 g were used for space closure in both the groups. Patients were called every month for reactivation of active tiebacks until the space was closed completely.

Study models were taken after the placement of active tie-backs (T1) and after the completion of space closure (T2). Baseline space was measured using a digital Vernier caliper from the distal of canine to mesial of second bicuspid (Figure 1). The rate of space closure was measured by dividing the baseline space by the total duration required to completely close the space.

Additionally, bone density was compared in both the groups using Image J analyzer software (Version 1.38) on the radiograph of the patients.

File > open (select the radiographic file) > select the radiographic area to be evaluated > select Analyze > choose histogram. The X-axis represents the possible gray values and the Y-axis shows the number of pixels found for each gray value (Figure 2). The mean values of all radiographs were assessed and compared for density changes.

Statistical analysis

The data collected were subjected to SPSS software statistical analysis with a 95% confidence interval. Results were represented as mean±standard deviation and percentages. The intragroup comparison was conducted using paired t-test.

Results

A total of 40 patients were included in this study to evaluate the effect of fluorosis on the rate of tooth movement. All the measurements were done using a digital Vernier caliper and density changes were analyzed using Image J analyzer.

The statistical analysis showed no significant difference in baseline space between the study groups (NFL and FL). Rate of tooth movement between NFL (mean- 7.9 months) and FL groups (mean-14.8 months) showed highly significant difference between the groups (P-value=0.000<0.01). Bone density changes assessed using the radiographs showed highly significant changes in NFL and FL group (P-value= 0.000<0.01).

Discussion

In the earth’s crust, Fluorine is the 13th most abundant trace element (0.07%).^{11, 13-15} In the human body, over 99% of fluoride is stored in bone, enamel, dentin, and cementum.¹² The fluorine concentration in hard tissues depends on factors like the level of fluoride intake, duration of fluorine exposures, and interdependent factors such as growth rate, tissue developmental stage, and the surface area. The cementum has been found to have the highest fluorine concentration amongst mineralized tissues, which increases with age.^16-25

Dental fluorosis is staining of teeth that is the most plausible pointer reflecting admission of higher than needed degrees of fluorides, essentially through drinking water. Consistent exposure to fluorides might prompt skeletal fluorosis and other serious results, for example, harming occurrences including demise. Fluorides have a great affinity for calcified structures. Fluorine can stimulate bone growth which explains the changes observed in the skeleton as a result of prolonged exposure.¹

There seems to be limited literature comparing the effect of fluoride on OTM which caught our interest.

Due to the endemic fluorosis belt in our district, most of the patients who visit the department for treatment are with dental fluorosis and forwardly placed teeth.

In the present study, forty patients were divided into two groups based on the presence of dental fluorosis. Patients were treated by extracting four first bicuspids using fixed mechanotherapy MBT prescription 0.022 slot. The magnitude of tooth movements surrounding the first premolar extraction site was used in this study to determine the rate of space closure in both the groups. Results of this study showed a mean rate of space closure for fluorosed and non-fluorosed teeth. From this study, we also concluded that there is a significant reduction in the rate of tooth movement in fluorosed teeth. The present study also assessed the bone density values in both the groups using Image J analyzer software. It concluded that there is an obvious increase in bone density in fluorosed teeth when compared with non-fluorosed teeth. According to Li KQ et al. (2016), radiological presentation of dental fluorosis includes increased bone density.²⁶ A CBCT study conducted by KL Vandana (2016) found that the buccal cortical plate thickness in fluorosed and non-fluorosed healthy groups showed similar measurements which varied from 0.93 to 1.1 mm from coronal to apical areas. The lingual cortical plate thickness varied from 3 to 2.0 mm coronoapically, the bone density was found to be higher in fluorosed healthy group 526.34 Hounsfield unit in comparison to non-fluorosed healthy group 474.4 HU.²⁷

Fluorosed patients usually go through frequent debonding of metal brackets. A recent study by Basalamah et al. (2021) found that the shear bond strength of fluorosed teeth (6.5 Mpa) is low when compared with non-fluorosed teeth (8.14 Mpa). This could explain the reason for the frequent debonding of brackets.²⁸

The experimentally induced fluorosis effects in animals are generally found in the literature. It appears that the chronic exposure of fluoride in endemic fluorosis belts would provide a different response to orthodontic teeth movement as there are critical changes in bone and teeth of subjects hailing from endemic fluoride areas like the subjects of current study. So far, such studies are not found in literature.

The basic function of fluoride is stimulation of new bone formation in vitro²⁶ and in vivo. ²⁹ NaF (sodium fluoride) is said to inhibit osteoclastic activity.²⁶

The effect of fluoride on OTM was previously investigated by Hellsing and Hammarstrom³⁰ who reported that 15 mg/1000 g body weight/24 hours sodium fluoride delivered via a subcutaneous osmotic pump which delivered 1µl NaF for seven days in rats, reduced the rate of tooth movement. In addition, it was found that the number of osteoclasts on the pressure side of the PDL decreased significantly. Gonzales et al. (2009) conducted a study on male Wistar rats to assess the effect of fluoride intake on OTM and found that OIRR (Orthodontically Induced Root Resorption) affects 100% of orthodontically treated patients as an inevitable sequelae. There is loss of cementum, dentin, and calcified dental tissues because of localized over-compression of periodontal ligament.³¹

The root resorption process has multiple risk factors. It is described that increased density of dental tissues provide resistance to activity of resorption.³²

According to Helsing and Hammarstorm,30 administering NaF only during orthodontic tooth movement may impede tooth movement. According to Gedalia and Zipkin, the commonly understood concept of the fluoride effect on bone is that the incorporated fluoride gives rise to mixed fluorohydroxyapatite, which is two times more resistant to resorption.³³ As a result, the fluoride groups showed a decreased rate of tooth movement and shallower resorption craters in rat models that were fed with fluoride from birth.³¹ The effect of fluoride on osteoclasts plays an important role. A solitary high portion of fluoride could confine from the bone surface and as a result bone resorption is hindered.³⁴

An in vitro study using fluoride with the isolated osteoclasts showed alteration of shapes of resorption pits, decrease in pit depths, and inhibition of osteoclast movement. Further, fluoride reduced the secretory function of osteoclasts, influencing the low pH and reducing the release of Ca and Mg ions from bone.³⁴ A high dose of fluoride (60 mg of NaF/kg) injected into a four-day-old rat can detach osteoclasts from the bone surface and inhibit bone resorption.³⁵ At low pH, the fluoride ion penetrates the cell membrane,³⁶ which would happen in the case of actively resorbing osteoclasts. This leads to a higher level of intracellular fluoride accumulation leading to toxic effects on various enzyme systems in the osteoclasts and even the intracellular fluoride level may reach an extent to be lethal to the cells.³⁷ This process can occur in odontoclasts too which are identical in structure and function to osteoclasts.

The effect of fluoride on bone is better understood than its effect on cementum. Whether fluoride penetrates cementum to make it more resistant to acid decalcification is still unknown. The role of fluoride on OTM is still controversial in animal models. Its role in humans is yet to be understood. In this aspect, our study attempted for the first time to assess the rate of OTM among subjects hailing from endemic fluoride and non-fluoride areas. Further long-term studies will help throw light on effect of fluoride on OTM.

The adaptive and remodeling changes in the periodontium allow the teeth to move through the alveolar bone during orthodontic movement. Several variables that affect the rate of OTM are magnitude, direction, distribution, and duration of free application. The initial tooth displacement is coupled with strain, stress, and biological changes in the periodontium. The role of fluoride on these variables is not clear.

The rate of OTM is influenced by biological, mechanical, and genetic factors. For effective tooth movement, the optimal force to be identified is controversial and problematic. The genetic factors affecting tooth movement report confounding influences in both human and animal studies. Tooth movement is said to be twice faster in growing than in non-growing subjects.

Conclusion

Fluorosis in the enamel of teeth is a condition that lasts a lifetime. However, if the fluoride content in water is sufficient, it can be avoided. The community living in endemic areas would be benefited by an ongoing water testing programme, periodic medical checking camps and continuing fluoride health awareness programmes.

In orthodontics, the following conclusions have been drawn-

• The average rate of OTM is slower in fluorosed patients when compared to non-fluorosed patients.
• Dental fluorosis is positively correlated with skeletal fluorosis.
• Bone density is high in patients with dental fluorosis, which explains the decreased rate of space closure.

The effect of fluoride on osteoclasts is associated with many controversies which leads to inconsistencies in fluoride effects on OTM. Various interactions between the dosage of fluoride, vitamin D, and calcium utility, species variations, fluoride supply to duration, and insensitivity of individuals lead to multifactorial and undetermined issues.

Source of support in the form of grants, equipment

Nil

Conflict of interest

Nil

Acknowledgements

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