A. Ascenzi

Distribution of Osteonic and Interstitial Components in the Human Femoral Shaft with Reference to Structure, Calcification and Mechanical Properties

This paper analyzes the distribution of osteons and interstitial bone in the femoral compacta according to their structure, degree of calcification and mechanical properties. Three cross sections, 100 microns thick, each located 1 cm from the next, were prepared by grinding from the middle third of a human femoral shaft. Starting from the premise that, in lamellar bone, lamellae whose fiber bundles and crystallites have a longitudinal course withstand loading by tension, whereas those whose fiber bundles and crystallites have a transversal course withstand loading by compression, each osteon and fragment of interstitial bone has been given a number recording the percentage of its surface consisting of lamellae with transversally oriented fiber bundles and crystallites (bright under the polarizing microscope). The degree of calcification of the same structures was determined micro-radiographically. The distribution of both osteons and interstitial bone was assessed using a tungsten grid for reference. The total surface of each bone microstructure, and the percentage of that surface consisting of bright lamellae, were all calculated using a Zeiss Video-plan. Our results confirm the view that the distribution of both osteons and interstitial bone is mainly related to their structure--and hence to their mechanical properties. In addition, bone remodeling seems to be most active in areas capable of supporting tensile stress.

The bending properties of single osteons

The bending properties of two fully calcified osteon types (longitudinal and alternate) have been investigated in 62 cylindrical samples by applying the technique of three-point bending loading. The bending of each sample was recorded using a microwave micrometer based on the cavity and pulse technique. It has been shown that alternate osteons are better able to withstand bending stress than longitudinal ones. This result offers a definitive explanation for the high concentration of transverse lamellae in pathologically bowed bone shaft.

Orientation of collagen fibers at the boundary between two successive osteonic lamellae and its mechanical interpretation

By applying an original technique, an investigation has been carried out to determine the orientation of collagen fibrils at the boundary between two successive lamellae in alternate osteons. Evidence is reported that the predominant fiber direction does not change abruptly from one lamella to the next; there is an intermediate system of criss-crossed fibers whose main orientation makes an angle of nearly 45 degrees with the direction of the fibers in the two adjacent lamellae. Taking a composite orthogonally reinforced laminate as a model, a mechanical interpretation of this intermediate system of collagen fibers is given.

Orientation of collagen in human tibial and fibular shaft and possible correlation with mechanical properties

The pattern of collagen orientation in two human tibiae and fibulae was studied. Serial 100 microns plane parallel cross-sections at 1 cm intervals at the same level in tibia and fibula were cut using an annular blade saw. Distribution of transversely oriented collagen was mapped using circularly polarized light and an image analyzing computer. In both the tibia and fibula the pattern of collagen orientation is characteristic, which may be related to the distribution of the bending forces normally operative in these bones.

An approach to the mechanical properties of single osteonic lamellae

Abstract A technique is presented for examining the role played by orientation of fiber bundles in the mechanical behavior of single osteonic lamellae. Cylindrical osteon samples are loaded perpendicular to their axis and, when required, the direction of loading can be changed continually. The main results are: (1) In osteon samples whose fiber bundles change direction in successive lamellae through an angle of about 90° (Type I), circular fractures appear in lamellae whose fiber bundles have a marked longitudinal spiral course, while lamellae whose fibers have an almost transversal spiral course are unaffected. (2) In osteon samples whose fiber bundles have a marked longitudinal spiral course in successive lamellae (Type II), fractures spread radially from the central canal toward the periphery of the osteon, until all the lamellae are affected. (3) These findings are independent of the degree of calcification; they go to strengthen the view that the compactness of osteonic bone is strengthened by the presence of lamellae whose fiber bundles have an almost transversal spiral course. (4) The first fractures to appear are running between collagen fibrils, indicating that the interfibrillar substance is considerably less resistant than the fibrils themselves. (5) In osteon samples of Type I, the fractures which appear in lamellae whose fibers have a marked longitudinal spiral course are circular, and this makes it possible to isolate lamellae whose fiber bundles have an almost transversal spiral course.

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.

Mechanical hysteresis loops from single osteons: Technical devices and preliminary results

The purpose of this paper is to describe an original apparatus for recording hysteresis loops from single osteons and a special microgrinding lathe for preparing osteonic samples for testing. The results obtained by testing isolated osteons, and composite bone samples ground to the same size as an osteon sample justify one to draw the following main conclusions: at an equal degree of calcification, longitudinal osteons show larger hysteresis loops under compression and alternate osteons show larger hysteresis loops under tension; there seems to be little chance of acquiring detailed information on the mechanical effects of osteon calcification recording hysteresis loops; collagen orientation in lamellae is the main factor determining the kind of hysteresis loops displayed by a composite bone sample.

Pinching in longitudinal and alternate osteons during cyclic loading

Pinching is a degrading phenomenon which occurs during cyclic loading of certain materials. A change in the slope of the deflection curve reveals pinching lesions, either flexural cracks or bond degradation, cause pinching. This paper investigates pinching for 20 longitudinal and 18 alternate fully calcified osteonic samples of cylindrical shape and 500 micron length. Each sample was axially loaded beyond the proportional limit using an electromechanical device acting as a transducer of the variations in length of the sample into changes in the resonance frequency of a microwave micrometer. A cubic polynomial served as a mathematical model to investigate the stress-strain diagrams at the first and last cycles through the study of strain limits, stiffness and pinching behaviours, and energy absorption. The hysteretic behaviour of the two types of osteons differs and is far from ideal. The presence of pinching may derive from the existence of longitudinal fibrils, in particular the yielding of the incompletely calcified ones. In longitudinal osteons consisting mainly of longitudinal collagen fibrils, the deformation under compression is not protected by lamellae consisting of transverse fibrils, therefore the lesions inducing pinching are magnified. In contrast, in alternate osteons, where the fibrils having a longitudinal orientation are reduced and protected by lamellae containing transversely oriented fibrils, the lesions-inducing pinching are lessened.

Macroscopic shape of, and lamellar distribution within, the upper limb shafts, allowing inferences about mechanical properties

Collagen orientation maps were determined for the long bones of the upper forelimb (humerus, ulna, radius) using previously described techniques. All three bones had characteristic, non-random patterns of collagen fibre orientation. In the humerus, transverse collagen predominated in the medial and posterior cortices proximally and in the anterior and posterior cortices at the mid diaphysis. In the distal radius, the medial and anterior cortices contained mostly transverse collagen. In the proximal ulna, transverse collagen was found in the anterior and anterolateral cortices. These findings may be correlated to the distribution of bending forces that may be operative in these bones.

Evidence of a state of initial stress in osteonic lamellae

Abstract By applying the technique described by Ascenzi et al. (1973), lamellar samples whose fiber bundles had a transverse spiral course were prepared from osteons with alternating lamellae isolated from human femoral shafts. Especially when their diameter is large with respect to their height, lamellar samples lose the cylindrical shape they normally possess in whole osteons, undergoing spontaneous deformations which suggest a state of initial stress. These deformations are more complex than those contemplated by Volterras theory of dislocations. Even so, the main features of this theory have allowed us to analyse the state of initial stress. This stress was eliminated by cutting each lamellar sample along a line parallel to its axis. The new shape of the sample was then studied. Two conclusions have been drawn: (a) the capacity of lamellae whose fiber bundles have a transverse spiral course to support a tensile load oriented parallel to the osteon axis depends essentially on a state of initial stress, and (b) calcification plays no basic role in producing this state of initial stress.