The structure of the trabeculae of cancellous bone. 1. The calcaneus

87 either cylindroid or laminar trabeculae, isolated from the perpendicular or inferior bundles of the human calcaneus, were embedded in methylmatecrylate and serially cut along longitudinal and transversal planes with a rotatory-blade saw. The microscopical study of the sections showed that in 83% of the samples secondary osteons run along the longitudinal axis of the trabeculae and their lumina either form a continuous channel network throughout each trabecula (37% of cases) or are restricted to discrete segments (46% of cases). The trabeculae entirely devoid of osteons (17%) are the thinnest, never exceeding 400 microns in thickness. This value is not even exceeded by the segments devoid of osteons in the trabeculae in which the Haversian canals occur only intermittently; conversely, the segments containing Haversian canals can reach and exceed 600 microns in thickness. The maximum distance of the osteocytic lacunae from filtering surfaces--i.e. outer surface of the trabeculae or inner surface of the Haversian canals--was found to be almost the same in the segments of the trabeculae that enclose or not osteons, even though the average trabecular thickness is greater in the former than in the latter regions. On the basis of these findings the formation of endotrabecular osteons may be viewed as a device that indirectly favours the metabolic exchange of deep-seated osteocytes while increasing the free surface area available for bone tissue reconstruction. It remains doubtful whether the Haversian systems may also contribute to improve the mechanical properties of the trabeculae. The arrangement of the collagen fibrils, which differs between cylindroid and laminar trabeculae, is apparently well suited to ensure the resistance of the trabeculae to mechanical loading through the use of the least amount of building material, in accordance with Wolff's law.