This study compared the structural complexity of the bone trabeculae in children residing in areas with different sunlight exposure conditions, using DF (fractal dimension) and IP (irregularity parameter) values derived from digital panoramic radiographs. The calculation of DF and IP in imaging exams has been previously employed in dentistry to assess bone trabeculae, primarily in adults, cadavers, and animals [23, 24, 30, 31]. To the best of our knowledge, only one study [18] has utilized these methods to evaluate bone architecture in children. Given that sunlight exposure has the benefit of promoting the production of vitamin D3 (the "sunshine vitamin"), a crucial micronutrient for bone metabolism, especially in growing children, assessing trabecular bone alterations through DF and IP in children with varying levels of sun exposure becomes relevant and justified.
In the reviewed literature, no study has examined variations in trabecular bone complexity based on sunlight conditions using panoramic radiographs. Existing studies have predominantly employed more complex imaging methods such as computed tomography (CT), densitometry, and dual-energy X-ray absorptiometry (DXA), focusing on bones like the radius and vertebrae [12, 32]. However, digital panoramic radiographs are cost-effective, easily executed by professionals, and well-accepted by children [33, 34]. Importantly, they pose lower risks, as radiation exposure is significantly reduced compared to the methods, making them ideal for use in children [16]. A recent systematic review indicated that childhood exposure to CT scans may be associated with an increased risk of cancer, while no significant association was observed with two-dimensional radiographic exams [14]. This current study stands as the first to evaluate trabecular bone complexity, utilizing FD and PI calculations in digital panoramic radiographs of children exposed to different sunlight conditions.
A significant difference in trabecular bone density has been reported between female and male adults, being greater in men [35]. Therefore, in the present study, the influence of sex on FD and PI results was investigated. However, no differences were observed between boys and girls, as in the study by Arrepia et al., (2023). This result can be explained by the metabolic similarity of the bone growth phase between boys and girls in the age range studied (6 to 9 years), as more significant changes occur after this age [36]. This was also the reason for choosing the age group researched; since possible confounding factors in the detection of bone trabecular variations, inherent to the different metabolism between the sexes with advancing age, especially in the growth spurt phase, could influence the results.
FD is widely used in the analysis of bone trabecular structure and several studies have demonstrated its ability to verify [37, 38]. The boxcounting method is the most used and recommended for calculating FD as it is easily reproducible [10]. This method requires a pre-established tool [23, 29] and is present in the ImageJ® program. As it is free and easily accessible software, it was chosen to be used in this work. In addition to FD, PI can also be calculated in ImageJ®, using the Histogram tool, and adds important information about the density of the analyzed region.
The sum of FD values, considering the selected ROIs, showed a significant difference between children more and less exposed to the sun, rejecting the null hypothesis of the study. In children less exposed to sunlight, the total FD value was higher than in more exposed children. This result seems controversial when analyzing data from other studies that showed that an increase in the FD value may be associated with greater structural complexity [39, 40]. However, other studies have shown that ROIs marked in areas of pathological bone formation, such as osteosarcoma, areas of implant osseointegration, and post-orthognathic recovery, present higher FD values compared to a region of normal bone [41–43]. Thus, higher FD values are not always related to increased bone density, nor can they be correlated with increased PI [18]. It is suggested, with this result, that low exposure to sunlight may be a predisposing factor to changes in the complexity of bone trabeculation, as it causes changes in bone metabolism, mainly due to the lack of vitamin D3.
Sunlight is the greatest source of vitamin D, which is responsible for regulating calcium absorption and bone mineralization [44, 45]. Maintaining controlled and regular exposure to sunlight is of great importance, especially during childhood and adolescence, as it is a critical period of bone development, thus avoiding changes such as rickets and fractures [46, 47]. However, measuring an individual's real exposure to sunlight involves many variables, such as investigating bone mineral density (BMD), a method considered invasive and high risk due to the ionizing radiation of the tests required for this purpose. Therefore, choosing only the place where the children live may be a limitation of the study, but it is a method that does not pose any risk to the participants and provides examinations from an image bank of panoramic X-rays that are constantly requested in dental practice.
Issues such as latitude, seasons, and even clothing can influence exposure to sunlight [48]. Children living in less sun-exposed areas may have different clothing habits, for instance, due to colder weather. Consequently, the use of coats and clothing covering a large part of the body may inhibit optimal absorption of sunlight through the skin, providing a possible explanation for the study's significant results. However, despite being an invasive examination and not always recommended in children without a diagnosis of bone diseases, it would be ideal to measure the serum levels of vitamin D in the researched children through blood tests, representing a limitation of the present study. It is suggested that further investigations into variations in children's trabecular bone, with measurement of serum vitamin D levels, be conducted.
In the regions of the mandibular angle (ROI2) and body (ROI3), FD values were significantly higher in children less exposed to the sun, highlighting the impact of exposure on trabecular complexity. The mandibular angle region is explored in various studies due to its susceptibility to changes from childhood onwards [49, 50]. The position of the gonion, a point located in this region, changes life phases, leading to dimensional movements of the jaw with individual growth. Several studies have shown that gonial angle values are higher at ages 5–10 and decrease over time until the age range of 15–20 [50, 51]. The gonial angle is commonly assessed to determine jaw rotation and is a significant indicator for diagnosing patients' growth patterns [52, 53]. Considering that low sun exposure can interfere with children's growth, it is suggested that the structural complexity of trabeculae in regions with higher metabolic activity may have been more influenced.
There are studies with growing animals [54, 55], in which the authors assessed the insertion of the masseter in the gonial region and its influence on the rotational pattern of the mandible. The results demonstrated the influence of muscular action on bone growth, meaning that depending on the location of the muscular insertion, mandibular growth can be altered either negatively or positively. However, it is not clear whether these changes affect the rotational growth pattern of the mandible [55], and it cannot be stated whether there is an association between muscular insertion and alterations in bone architecture in the mandibular angle regions (ROI2); thus, further studies in this field are necessary.
Regarding the difference found in the mandibular body (ROI3) concerning the FD results, it is believed that the stages of dental development in the permanent first molar region may have influenced the findings, as this area is constantly undergoing remodeling in the age group studied. This action occurs in conjunction with muscular maturation and craniofacial growth, meaning that biological maturation has not yet been completed at this stage [56]. Therefore, it is suggested that in children with less sun exposure, higher FD values were found and may be associated with lesser development in this region. However, new studies that clinically examine children in the long term, including eruption chronology and Nolla's dental development stages, are recommended.
Although the total PI value did not differ between the images of children more and less exposed to the sun, the mandibular angle region was different, even though it was always measured in cortical bone. Radiographs of less exposed children showed a lower PI value than those of children more exposed to the sun. The average PI value in ROI2 was the lowest found to the others. It is suggested that the constant modeling and bone remodeling that exists in the gonion region [52, 53] may result in less dense images in these areas. Thus, it can be suggested that children less exposed to the sun, with lower PI values, have lower bone density in this region when compared to more exposed children, despite BMD not being assessed in the present research.
Technical interferences related to the choice of imaging exam, such as the possibility of overlapping structures, and formation of ghost images, in addition to image magnification and distortion when using panoramic radiographs, could be listed as limitations. However, panoramic radiography is the most viable choice for this study, as it is a suitable method for children, due to its lower cost and lower risk, due to the low dose of radiation emitted. Possible inaccuracies in obtaining the FD could also be considered limitations since a small deviation when creating the ROI would significantly influence the morphology of tissue structures. Furthermore, the FD calculation may vary depending on the methods applied, such as boxcounting, pixel dilation, and modified pixel dilation. However, to meet these conditions, in the present study, pre-determined and anatomically demarcated ROIs were chosen, as well as the simple, public domain program, ImageJ®, in which the boxcounting method is already included.
Bone fragility and fractures due to pathological conditions in children, such as rickets, are of great concern in the medical and dental fields. Therefore, the development of methods capable of assisting in the early diagnosis of these conditions becomes very important. Therefore, we can state that considering this pioneering study, a promising line of research using panoramic radiographs to observe possible variations in the complexity of bone trabeculation in children living in different sunlight conditions should be strengthened. It was seen that the use of FD and PI are good indicators for detecting bone variations in this population; Furthermore, they are easily accessible and low-cost methods, investigated in two-dimensional examinations, with low radiation doses and frequently requested by dentists in daily clinical practice.