Bone is a dynamic organ, highly vascularised and very active metabolically. As the bones are not completely developed at birth, they continue to be formed slowly out of cartilage or connective tissue, which are converted into the hard, lamellar components of the bone. Growth of the bones (modelling) comes to an end at puberty with ossification of the “growth plates”. Modelling is of particular interest as the bone is much more capable of reacting to external loads during growth than at any other time. About 90% of adult bone is formed by the end of adolescence, and subsequent gains during adulthood are very small.