A. Benvenuti

Reconstruction of Extensive Long-Bone Defects in Sheep Using Porous Hydroxyapatite Sponges

Abstract. The capacity of hydroxyapatite (HA) implants to support large defect repair in weight-bearing long bones of large size animals was investigated. Diaphyseal resections 3.5 cm of the tibia were performed in five adult sheep. They were substituted with HA macroporous ceramic cylinders anatomically shaped, and an external fixator was assembled. The sheep were sacrificed at 20, 40, 60, 120, and 270 days after surgery, respectively. Histology and micro X-ray study of resected implants and adjacent tissues showed proper integration of ceramic with newly formed periosteal bone as early as 20 days after surgery. In one sheep, the external fixator was removed 5 months after surgery. The animal gained the ability to walk with no functional impairment until it was sacrificed 4 months later. At this time, extensive integration of ceramic with bone was detected radiographically and confirmed by a morphological study of the resected sample. Our data indicate that large defects in a weight-bearing long bone can be repaired to the extent necessary for full functional recovery in large animals. These data set the stage for further intervention on material properties as well as for preliminary attempts to use ceramic prostheses for reconstruction of large bone defects in humans.

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.

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.

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.

X-Ray Diffraction on Cyclically Loaded Osteons

Abstract. The results of a study on the fine structural distortion due to the two previously observed types of degradation in cyclically loaded single osteons (i.e., stiffness degradation and pinching effect) are presented. Fully calcified longitudinal and alternate osteons were isolated from 350-μm-thick longitudinal sections of human femoral cortical bone. The samples were prepared from 500-μm-long central cylindrical portions of an osteon, whose two ends were penetrating into rectangular lugs for fixation to an electromechanical device that cyclically loaded the samples. This device was connected to a microwave micrometer and a recorder. The structural distortions induced by cyclic loading were investigated by high- and low-angle X-ray diffraction on conventional and synchrotron radiation sources. Cyclic loading results in a reduction in the degree of orientation of apatite crystallites, especially in longitudinal osteons, in which the most abundant longitudinal lamellae are not protected against buckling by transverse lamellae as they are in alternate osteons. In contrast, the degree of orientation of collagen fibrils does not seem to be affected by cycling loading in the two osteon types, possibly because the disorientation of collagen fibrils is, within limits, a reversible process. Finally, the contrast between the disorientation of inorganic crystallites and the apparently unaltered distribution of collagen fibrils suggests that the degradation of cyclically loaded osteons may be due to a separation of the crystallites from the fibrils.

Ablation of hard dental tissues by ArF and XeCl excimer lasers

The paper investigates the effects of UV laser irradiation on hard dental tissues. 12 sagittal cross sections 1 mm thick of teet extracted have been irradiated with pulsed ArF (A.=193 nm) and XeCl (A.=308 nm) excimer lasers. The beam has been focused to obtain a spot of about 1 mm on the enamel and dentin of the teet. The depth and the internal profile of the holes have been measured with a profilometer and correlated with the number of pulses and beam energy for each wavelength. No cracks were induced by thermal stresses nor preferential concentrations of Ca and F have been evidenced on the enamel and dentin after UV irradiation. These results show that the ablation of hard dental material can be successfully carried out with excimer laser irradiation without heat damage of the underlying tissues.