Mechanical similarities between alternate osteons and cross-ply laminates

Abstract

Fully calcified osteon samples with alternating lamellae were prepared from longitudinal sections of human femoral shafts according to the technique described by Ascenzi and Bonucci. Osteons of this type reveal an alternation of dark and bright lamellae under the polarizing microscope. The samples were loaded by tension in a direction parallel with their longitudinal axis using a specially designed microwave extensimeter based on cavity and pulse techniques. All the samples were tested wet at a temperature of ca. 20°C. The stress-strain curves recorded from each osteon show a change in slope or knee at low stresses. A useful model for understanding this phenomenon is furnished by the mechanical properties of that type of fiber-reinforced plastic material called cross-ply laminate. The comparison of the data obtained from this material with those furnished by an electron microscope investigation carried out on the loaded osteons suggests that the change in slope at low stresses is due to failure of the interfibrillar cementing substance in those lamellae having fiber bundles oriented perpendicularly to the loading direction, and to yielding of the canaliculi previously filled with osteocyte processes. The physiological implications of these results are discussed.