An integrated approach to the study of long bone morphology and cortical thickness distribution

Past human populations have been characterized by high levels of mobility in contrast with recent populations. From the literature, the reconstruction of mobility and physical activity levels are performed by measuring the biomechanical performance of long at the mid-shaft [1-2]. Over recent decades it has become increasingly clear how complex is the mechanism of bone modelling and remodelling, and that several factors are involved [1,3]. However, it is still unclear what effect do body proportions, physical activity and sexual dimorphism and age have on long bone morphology. Here, we used geometric morphometrics and morphometric maps of cortical thickness to analyse the morphology and the diaphyseal shape of the tibia and femur from a sample of recently deceased modern humans [4]. The dataset includes information on each individual in relation to sex, age, weight, stature, biomechanical length and occupational level. Despite traditional methods of investigation (analysis limited to a few cross-sections) we extensively analysed variations along the diaphysis by applying morphomap R methods [5] which allow to study the entire diaphysis of the bone with a high degree of details. On each of the long bones we extracted 61 cross-sections along the diaphysis from the 20% to the 80% of the total biomechanical length. Each cross-section is defined by 40 pairs of equiangular semilandmarks on the periosteal and on the endosteal contour. The set of 61 cross-sections was used to build a dataset of shape variables and matrices of cortical thickness. A significant correlation by Procrustes ANOVA was observed in both femurs and tibiae with the sex variable (p=0.026 for femurs and p=0.001 for tibiae). Partial Least Squares analysis shows a statistically significant correlation between age and biomechanical length in the femur. In females we found a strong correlation with age in femurs, but the same result is not observed in males. The different results between the sexes may be related to the occurrence of osteoporosis in women which has a marked impact on the femurs. This suggests that the tibia might be more reliable than the femur in the study of mobility because it is less influenced by age. In this communication, we show the potential of using an integrated approach to the extensive study of the diaphysis combining morphometry and biomechanics. This approach is reliable for assessing diaphysis bone morphology not only in modern humans or archaeological remains but has a great potential interest in paleoanthropological and primate studies in reconstructing past loading history.