Changes in thoracic cage morphology from birth into adulthood

Thoracic cage morphology correlates with respiration, posture and locomotion. The shape of the thoracic cage and the thoracic kyphosis (TK) changes significantly from infancy into adulthood. Few studies emphasized the rapid changes in thoracic morphol- ogy in the first two years of life, assuming the changes are related to the establishment of an erect posture. Other studies, however, pointed to puberty as a period in which the thoracic cage proportions are changing. There are contradicting evidences in current literature, regarding sexual dimorphism of the thoracic cage morphology. The main goal of the current study is to characterize the morphological changes in the thoracic cage of males and females from birth into adulthood. Material and Methods For this Cross- sectional retrospective CT study, we included chest CT scans of typically developing individuals from birth to 29 years of age, all displaying non-pathological morphology. We combined traditional length and angular measurement with 3D geometric morpho- metric analysis (GMA). For the traditional measurements, we used 176 chest CT scans. Measurements included manubrium level in relation to spinal level, and TK angle. For the GMA we created 50 3D models based on CT scans; 302 3D landmarks and semi- landmarks were placed on each thoracic cage. Using Procrustes method, all models were superimposed to eliminate size-related variation, thus, we observed shape differences. We then used EVAN-Tool box to preform principal component analysis (PCA). Re- sults: Traditional measurements: TK increases significantly with age from 24±5.6° in infants to 36.7±6.5° in adults (p<.000). Most of this change occurred in the first 5 years. We found no difference in TK between genders (p=.323). The level of the manubrium was higher in infancy and lower in childhood and adulthood (p<.000). The manubrium of sub adult and adult females was posi- tioned lower than in males (p=.013). GMA: Our results indicate that the 1st PC accounts for 52% of the total variation, while the 2nd PC accounts for 8.5% of the total variation. The 1st PC nicely separates along an age gradient. The 2nd PC separates males from females, but this becomes evident only from the onset of puberty. The shape of the thoracic cage at infancy is pyramidal (in frontal view), the ribs are horizontal. The thoracic spine is relatively straight and is mildly invaginated into thoracic cage. The posi- tion of the manubrium is high, and the sternum is aligned more horizontally than in adults. Along PC 1, 50% of the morphological changes occurred in the first 3-4 years of life. The shape of the thoracic cage in adults is barrel (in frontal view), and the ribs are antero-inferiorly inclined. The thoracic spine is kyphotic and there is marked invagination of the spine into the thoracic cage. The sternum is aligned more vertically, and the manubrium is lower. Sexual differences in adult thoracic cage are subtle and include mainly more inferiorly inclined ribs together with lower position of the manubrium in females, compared with males. Discussion: Our results clearly indicate shape differences from birth into adulthood, which are more rapid in the early years. The results of this study conform with previous publications, indicating that the establishment of an erect posture is probably a significant factor in thoracic cage morphology, thus, supporting previous publications indicating increased TK and more antero-inferiorly inclined ribs with age. Surprisingly, we also found differences in the orientation and position of the sternum, with age, which, to the best of our knowledge, have not been published before. Regarding sexual dimorphism, our study supports previous publications that have shown shape differences between males and females. These differences are subtle and they only appear with the onset of puberty. The most striking differences are shown in the position of the manubrium and the inclination of the ribs.