Our study found that the incidence of DDH is significantly higher in regions with lower average yearly temperature, minimum monthly temperature, or maximum monthly temperature. This finding is the most important result and is also the greatest predictor in our final multiple regression analysis. One possible explanation for these findings is related to the use of swaddling [20–22], which is still common in many countries with cold weather, despite long-term policies promoting newborn health care [13, 23]. Swaddling involves wrapping the baby in tighter clothing or blankets to protect them from the cold and promote uninterrupted sleep [20, 24]. However, this can lead to hip instability, as the tight clothing or swaddling can extend the position of the hips, resulting in Ortolani-positive hip instability [16, 25]. In contrast, the incidence of DDH is lower in high-temperature regions such as Hong Kong, Bangkok, and Malawi, where back-carrying is used instead of swaddling [14, 26]. Back-carrying involves carrying the baby in a flexion, abduction position that is similar to the Pavlik harness, which is used as a prophylaxis for DDH development [27]. Another hypothesis is related to abnormal levels of nutrients such as calcium, vitamin C, and vitamin D, which are associated with cold temperatures and correlated with joint laxity and delayed bone remodeling, potentially leading to the development of DDH [9, 11].
The univariate analysis revealed a significant correlation between the incidence of DDH and latitude, but this was not found to be significant in the final multiple regression analysis. This may be due to the fact that even at approximately the same latitude, there can be different environmental factors, such as temperature, precipitation, humidity, and climate between two cities (for example, Mashhad in Iran and Takmaya in Japan). This heterogeneity of environmental factors can obscure the correlation between latitude and DDH incidence, leading to different patterns of variation.
In a study from Indiana, United States, Loder et al. investigated seasonal variation in DDH in their institute and found results similar to our study, with a single winter peak in most children. However, approximately 20% of infants with DDH who did not experience a winter peak refuted the cold temperature hypothesis. Loder et al. suggested that multiple factors may interact to contribute to DDH, leading to these findings [11]. Another study by Lee et al. (12) examined the association between cold weather in the first three months of life and the incidence of surgically treated DDH using the National Health Insurance Research Database in Taiwan. Their results showed a strong association between cold weather and the incidence of surgically treated DDH [12]. However, as Taiwan is a subtropical country, it is difficult to generalize this relationship to tropical or temperate zones. By collecting DDH incidence data from worldwide literature, we believe our findings are representative and applicable across different geographic areas.
Cold weather can pose health risks to the pediatric population globally, such as asthma exacerbations, allergic rhinitis, and atopic dermatitis [28]. Additionally, cold weather and reduced sunlight exposure can affect cutaneous vitamin D synthesis in children, leading to vitamin D deficiency [29]. This deficiency is a risk factor for nutritional rickets, one of the most common pediatric bone diseases worldwide [30]. To prevent and treat nutritional rickets, some areas with high latitudes and cold weather have established guidelines for vitamin D supplementation. For example, in North America, infant formula is fortified with vitamin D, and additional vitamin D is recommended for children who do not consume enough vitamin D-fortified milk and have limited sunlight exposure [31]. The results of our study suggest that DDH is also related to environmental factors, similar to nutritional rickets. Birth in cold weather should be considered a risk factor for DDH in future screening programs.
Our study has several strengths and limitations. One of its strengths is that it includes many global studies on the incidence of DDH, allowing for investigation and consideration of risk factors without geographical limitations. Additionally, the large sample size in our study can be representative and provide insight into the environmental factors associated with the incidence of DDH.
Some limitations of the current study should be noted. First, the selection bias may exist, that only approximately 20% of the literature reviewed is from tropical or subtropical zones, with the remaining 80% from temperate zones. Second, the study did not include some important information, such as the definition of DDH, the age of the children, and the screening methods used. Additionally, the diagnosis criteria for DDH were not included in the analysis, which may have led to overestimation or underestimation of the incidence of DDH. Future studies should consider using consistent diagnosis criteria and surgical intervention information in a larger geographic area to further explore the relationship between environmental factors and the incidence of DDH.