The microscopic determinants of bone mechanical properties

Composition and structure of the bone matrix, i.e. the bony substance surrounding osteocyte microcavities, was considered in the Light of their mechanical implications. A novel classification of bone tissues was proposed, on the basis of recent comparative studies on collagen fiber texture and osteocyte lacunae distribution, performed under polarized light, scanning and transmission electron microscopies. In particular, contrary to the classical view, lamellar bone can no longer be considered a variety of parallel-fibred bone since bony lamellae are not made up of parallely arranged collagen fibers, but of highly interlaced fibers, the lamellation being due to the alternation of acellular collagen-rich and cellular collagen-poor layers, namely of dense and loose lamellae, respectively. It is postulated that the deposition of woven- and lamellar-bone depends on the manner of recruitment of the osteocyte-differentiating osteoblasts from the osteogenic laminae and not on the spatial orientation by osteoblasts of collagen fibers. The possible role played by osteocyte as mechanical sensor in selecting the type of bone tissue laid down on bone surfaces is discussed.