This study was conducted to survey the IS prevalence among primary school-aged children in Chaozhou, Southeast of China, and further investigate the factors associated with IS, which was the first time to screen IS in primary school students based on a large-scale epidemiological study. The results indicated that the prevalence of IS among primary school students in Chaozhou was 6.15%, with 4.04% in males and 8.71% in females. In addition, the prevalence of IS increased with age. The female students, with more possibility of trunk asymmetry, had a higher prevalence than the males. Many factors such as BMI, myopia, cognition of scoliosis, self-sensation of scoliosis with symptoms, sleep time, and exercise time were associated significantly with scoliosis.
Gender, age-related prevalence
Our study indicated that females had more possibility to suffer from IS. As reported in literature elsewhere, IS was more common in females. Adolescent girls are easier to suffer from idiopathic scoliosis because the curvature of the spine progress in puberty, and females enter physiological puberty early. Another reason is that boys are more motivated than girls when it comes to doing physical exercise , except in relation to weight and body image and agility or flexibility, which is superior in the female gender. The reduced body weight, especially in girls, strongly predisposes the occurrence of scoliosis. In the annual observation, the girls who had trunk asymmetry, in order to decrease the ATR value, cannot significantly increase their body weight . The studies [19, 20] showed that adolescents with scoliosis were taller than other adolescents in the same age group, and a growth velocity of more than 2 cm per year was associated with curve progression. It is believed that the increased prevalence of scoliosis in girls compared to boys is justified by the fact that girls tend to grow more than boys from ages 11 to 13 . The ratio of females to males was reported to range from 2.3 to 18 [22,23,24]. The previous studies [25, 26] reported that females had a higher prevalence in all age groups than that in males, and both genders had a higher prevalence in the age group 15–16 after puberty, which is consistent with our study.
Weight, BMI, and IS
Weight is one of the important indexes to reflect a children’s health condition. The average weight of the IS cases is 32.74±8.28 kg, which is less than that of the controls (34.35±10.32 kg). At the developmental stage of a child, weight can reflect the nutritional status, muscle development, and bone development to some degree. BMI was a more comprehensive index of reflecting the body shape. The BMI of the IS cases is 16.03±3.22 kg/m2, which is also less than that of the controls (16.91 ±3.95 kg/m2), but both fall within the normal range of the national standard for students’ physical health. The study  indicated a population-based prospective study, and the results suggested that BMI/body weight at age 10 and scoliosis at age 15 had a negative association, with per SD increase in BMI 20% reduced risk of scoliosis. Worthington et al . indicated that malnutrition might play a crucial part in the etiology of IS. But nutrition covers a wide range of areas that still need to be studied in conjunction with other indicators.
Myopia and IS
Students with myopia presented 1.49 times more possibility of IS, when compared with students who did not have myopia. It can be considered as a process of mutual or simultaneous development. Incorrect posture will increase the likelihood of scoliosis and lead to impaired vision. The major cardinal feature is joint hypermobility, or ligamentous laxity, [29, 30]. In the absence of vision rectification, the impaired vision will promote the creation of forced posture, which in turn exacerbated the occurrence of scoliosis . This is in line with Egorova’s  study, which found that the musculoskeletal system of high myopia and impaired vision school-age children was more distorted than the controls, including scoliosis, pelvic dislocation, kyphosis, lordosis, torsion of the column, flatfoot, lower limb, and chest deformation.
Cognition and self-sensation of scoliosis
The IS cases had higher self-sensation of scoliosis symptoms. The most common scoliosis symptoms are physical asymmetry, back pain injury, muscle spasms, and others. According to the studies [33, 34], idiopathic scoliosis may result in a difference in leg length which regards that the difference in leg length can lead to compensated non-progressive lumbar scoliosis. In the published study , smaller leg length differences (≤ 2 cm) may lead to functional or non-fixed scoliosis, which should not be overlooked by medical professionals. Postural imbalances caused by muscle spasms, injury, pain, or any other factor s can also develop into unstructured scoliosis, which is generally considered to be inconsequential . However, if functional scoliosis is not diagnosed and corrected in puberty, it may eventually develop into pathological scoliosis . Even if the screening results were normal, students with physical asymmetry still need to be paid great attention to, especially in puberty.
The cognition and sensation of deformity with IS should be considered cautiously, for the best treatment methods can be applied to reduce the spine curvature or stop the progression of IS [39, 40]. Having good cognition help students redress their bad habits and took corrective measures initiatively. In theory, good posture can reduce the asymmetrical load of column deformity and reverse the vicious cycle of column curvature. Incorrect postures are common in daily life, which will increase the load of column asymmetry .
Sleep time, exercise time, and IS
Scoliosis patients have an unbalanced load that may lead to skeletal muscle fatigue and require more sleep to regain strength . From another perspective, most muscles are relaxed when people are lying. The most obvious change after standing up is the erector spine, which is an important muscle for keeping the body upright and maintaining the stability and balance of the crista e. Long time lying may lead to poor vertical stress training and muscle strength or endurance of skeletal muscl e. The study  showed that decreasing sleep duration results in overexpression of IL-1 and lower bone mineral density and might increase the spine curvature. Furthermore, the relationship of scoliosis, class II malocclusion, and obstructive sleep apnea is worth discussing. The study  showed that the forward head posture associated with scoliosis may lead to stretching of the muscles, skin, and fascia covering the head and neck, impeding the sagittal growth of the mandible and facial skeleton, which leads to class II malocclusion. Besides, class II malocclusion has been reported as a dental feature associated with obstructive sleep apnea which could explain sleeping disturbances in IS patients . The stud y suggested that sleep screening should be taken into consideration for the evaluation and treatment of patients with early-onset scoliosis, because poor-quality and inadequate sleep has a bad influence on children’ behavior, cognitive function, and growth.
With the increase of exercise time, the likelihood of scoliosis decreased gradually, and those who exercised less than 1 h were 7.29 times more likelihood than those with 3 h a day. Since it is found that biomechanics are relative to unbalanced loads and time thresholds, we can reverse the asymmetrical load by restoring normal postures and exercise. According to the study , exercise can reduce the prevalence of scoliosis and patients Cobb angle, especially in early adolescence, because exercise can improve mobility, strength, breathing, and equilibrium. The study  confirmed the efficacy of exercises can reduce spine curvature (mainly in puberty) and improve the Cobb angle. The study [51, 52] showed the effectiveness of spinal stabilization exercises on pain reductions for adolescents with idiopathic scoliosis.
Limitation and strength
This study has several limitations. Firstly, X-ray examination was not performed immediately because of low compatibility and lack of attention. Secondly, the cross-sectional study could not establish the cause-and-effect relationship, while a longitudinal approach might help research the development of the relationship. Third, the number of participants in the case-control study was reduced because it was conducted seven months after screening. Despite these limitations, FBT and ATR of 3D images used in the survey could help us increase the accuracy and objectivity of the screening. More adolescents and age phases are needed to be investigated in the future based on large-scale epidemiological studies to reach more scientific conclusions. To our knowledge, this is the first study conducted in Chaozhou city to screen and investigate influencing factors of IS in primary school children. In the future, a wider group of predictors and broader age groups should be brought into study, to collect more epidemiological data and more clues for the scoliosis intervention.