Costal facet variations on the eighth, ninth and tenth thoracic vertebrae: association with sex and shifts in the cranio-caudal pattern of the human axial skeleton

Different variants of the ninth and tenth thoracic vertebrae are described in the literature with little information on their prevalence in human populations. To review this question, 5 osteological samples from different geographical areas were studied for the presence of costal facets on the eighth (T8), ninth (T9) and tenth (T10) thoracic vertebrae. We found that inferior costal facets on vertebral centrum were absent bilaterally in 1.5% of T8 and in 46% of T9; costal facets on both T10 transverse processes were absent in 39% of cases. Absence of inferior costal facets on the T8 and T9 centrum and costal facets on the T10 transverse processes was positively associated with cranial shifts at regional borders of the spine. However, additional analysis revealed that the position of the costocentral articulation at the level of T8 and T9 as well as the position of the most inferior "typical" thoracic vertebra significantly depended on sex (p < 0.001). Sex differences were most pronounced at the level of T9 where females showed very frequent absence of inferior costal facets (76% of cases compared to 47% in males). This suggests a difference in an average 10th rib position in relation to the spine in females. Significant sex differences in the position of the most inferior "typical" thoracic vertebra may be partially explained by the fact that females in general are more likely to develop some forms of cranial shifts. However, according to the literature, the female axial skeleton possesses a complex of morphological features that is seen as an adaptation to pregnancy. In this view, the different position of the female 10th rib may be one component of the complex.