The bone structure of the upper and lower jaw resembles that of the total skeleton. It is therefore assumed that these skeletal components are subject to a similar ageing process of bone loss, thus reduction of the trabecular bone, thinning of the cortical bone and increased porosity. Since the 1960s, it has also been known that the upper and lower jaws are subject to age-related atrophy of the bone. An additional regression of bone mass is observed in the form of tooth loss. Absorptiometric studies have shown that oral bone mass also correlates significantly with the total calcium mass of the body and with the bone density of the forearm and vertebrae.