STRUCTURAL-CHANGES WITH AGING IN CORTICAL BONE OF THE HUMAN TIBIA

Structural modifications are considered to play a significant role in the age-related alterations of bone quality and strength, Senescent co mpact bone is characterized by an increasing heterogeneity of aspects, including high numbers of lowly mineralized osteons as well as the pr esence of osteons with hypermineralized lamellae or with a notched hav ersian canal wall, and of double-zone osteons, These latter three type s of osteons are different from the structures involved in the haversi an remodeling. In the present study, blocks of midshaft tibia from 7 y oung men (18-39 years), 14 aged men (50-92 years) and 15 aged women (5 7-96 years) were embedded in methyl methacrylate in order to perform m icroradiographic and histomorphometric analysis of undecalcified secti ons. The intracortical porosity was higher in the aged men than in the young ones, as were the numbers of haversian structures and, to a les ser extent, the diameters of the haversian canals. The aged women show ed the same tendency, with cortical porosity still higher than in the men. The osteons with hypermineralized lamellae, those with a notched canal and the double-zone osteons appear to constitute large subgroups of the total haversian population, even in the early adult life. Amon g them, only the osteons with a notched canal wall increased in freque ncy with age, The 3 types are much more numerous than the structures i nvolved in the typical haversian remodeling, The correlations between their frequencies as well as their significant topographic association corroborates the hypothesis that the hypermineralized lamellae may cr umble down because of their excessive brittleness, giving rise to the haversian canals with notched walls. These enlarged canals could be re filled by bone apposition and result in the double-zone osteons. The 3 types of osteons could constitute different steps of one mechanism of bone desintegration and repair occurring very progressively, which mi ght contribute to modify the bone quality and to increase the intracor tical porosity.