There is a considerable variation in the mineralization of bone; normal, non-pathological compact bone has ash masses ranging from 45 to 85% by mass. This range of mineralization results in an even greater range of mechanical properties. The Young modulus of elasticity can range from 4 to 32 GPa, bending strength from 50 to 300 MPa, and the work of fracture from 200 to 7000 Jm-2. It is not possible for any one type of bone to have high values for all three properties. Very high values of mineralization produce high values of Young modulus but low values of work of fracture (which is a measure of fracture toughness). Rather low values of mineralization are associated with high values of work of fracture but low values of Young modulus and intermediate values of bending strength. The reason for the high value for the Young modulus associated with high mineralization is intuitively obvious, but has not yet been rigorously modelled. The low fracture toughness associated with high mineralization may be caused by the failure of various crack-stopping mechanisms that can act when the mineral crystals in bone have not coalesced, but which become ineffective when the volume fraction of mineral becomes too high. The adoption of different degrees of mineralization by different bones, leading to different sets of mechanical properties, is shown to be adaptive in most cases studied, but some puzzles still remain.

Effects of differences in mineralization on the mechanical properties of bone

Strain-induced damage in bovine laminar bone has been examined using laser scanning confocal microscopy (LSCM) The specimens were loaded in a fluorescein solution, which penetrated the newly formed cracks in the specimen The microcracking, and the larger cracking, induced by strain were very clearly visible The microcracking occurred diffusely in regions of high strain (stress), but was particularly obvious in the vicinity of large machined stressconcentrators The microcracking could be shown not to be artefactual, that is, it was produced by strain, and not by specimen preparation The microcracking interacted with the structure of the bone, often having a wavy appearance related to the histology Microcracks seemed to be particularly associated with the most highly mineralized parts of the bone LSCM is a technique holding great promise for the investigation of the initiation and development of damage in mineralized hard tissues, and other translucent materials

The extent of microcracking and the morphology of microcracks in damaged bone

Recombinant human bone morphogenetic protein- (rhBMP-) can be used to enhance the repair of congenital or acquired bone pathologies when formulated in the appropriate carrier. Poly [D, L-(lactide-co-glycolide)] (PLGA) has been shown to be an effective carrier of rhBMP-. We investigated several particle sizes PLGA and several doses of rhBMP- in a rat orthotopic model. We also investigated the effects of a fibrinolytic inhibitory agent, epsilon aminocaproic acid (EACA), on the healing response. Our data indicate that higher doses of rhBMP- resulted in increased failure torque (408 ± 70 N-mm or 60% of the intact value) and higher incidence of union (100%). The induced bone in femurs treated with the smaller particle size PLGA achieved the greatest torsional stiffness and strength. The presence of rhBMP- was necessary for new bone to form, but the presence of EACA did not change these results; the use of the PLGA carrier appeared to increase bone strength and stiffness. In fact, with higher doses of rhBMP- in PLGA, the stiffness of the new bone was equal to that of intact controls (64 ± 20 N-mm/deg [intact femurs] versus 45 ± 10 N-mm/degree [medium dose in small PLGA], 61 ± 17 N-mm/degree [high dose in small PLGA], and 36 ± 11 N-mm/degree [medium dose in large PLGA]; P > .05). In conclusion, PLGA implanted with rhBMP- effectively aided in healing large segmental defects in rat femurs. © 1994 John Wiley & Sons, Inc.

Healing of large segmental defects in rat femurs is aided by RhBMP-2 in PLGA matrix.

An examination of the micromechanics of failure of bone and antler by acoustic emission tests and Laser Scanning Confocal Microscopy

Tibiae from 60 male Wistar rats, aged 13 ±1 weeks, were divided into six groups for mechanical and histological testing. Bones were loaded repetitively in torsion at 90 deg.s−1. Group 1 was subjected to 5,000 loading cycles at a twist angle of 3.6°, groups 2–5 to 10,000 cycles at 3.6, 5.4, 7.2, and 9.0°, respectively, and group 6 was tested to failure. Six transverse sections from the middiaphysis were then cut, bulk-stained in basic fuchsin, and hand ground to 30–50 μm to examine the presence of microcracks. Cracks were classified as running parallel to lamellae, crossing lamellae, crossing the full thickness of the cortex, or invading vascular canals. Results for fatigue testing showed that the tibiae exhibited a gradual decrease in torque (P<0.05), average stress (P<0.01), stiffness (P<0.01) and energy absorbed (P<0.01) from the initial loading cycle. Analysis of microdamage showed an increase in the variety of cracks from groups, 1–5. Analysis of deviance demonstrated a strong dependence of crack probability on the level of loading for all crack types (P<0.05) except those crossing lamellae. This study reinforces the evidence that yielding of bone observed during repetitive loading is caused by diffuse structural damage such as microcracking or debonding.

Microdamage in response to repetitive torsional loading in the rat tibia.

A previously presented method for measuring the torsional strength of diaphyseal bones has been used to study the effect of direction of twist upon the torsional strength and stiffness of diaphyseal canine bones. No significant effect of the direction of twist upon the studied properties has been found. The reason for the observed twist direction independence of the torsional strength and stiffness is assumed to be the adaptation of the diaphyseal bone architecture to functional demands.

https://www.tandfonline.com/doi/pdf/10.3109/17453677808993257

Torsional Strength and Geometry of Diaphyseal Bone: An Experimental Study on Dogs

The formation of microcracks in entire canine diaphyseal bones under torsional loads was studied. the acoustic emission technique was used to register the stress waves which were generated in the bones by crack formation and/or growth. Acoustic emission signals, characteristic of crack formation and/or growth, were recorded exclusively during the non-linear deformation. This is a strong indication of the elastic behaviour of diaphyseal canine bone material.

Material Reaction of Diaphyseal Bone Under Torsion: An Experimental Study on Dogs

The deformation of entire long bones from dogs has been studied in torsion. The load-deformation relationship is non-linear prior to ultimate failure and repeated successively increased loading and unloading in the non-linear range is found to reduce the stiffness of the bone. The non-linearity of the curve is assumed to be due to a change in the geometry of the bone originating from formation of small cracks in the cortex.The bone material of entire long bones from dogs seems to be linearly elastic for all torques.

Non-linear properties of diaphyseal bone. An experimental study on dogs.

The fracture process of torsion tested entire canine diaphyseal bones was studied with a high speed camera. the speed of the final fracture propagation proved to be incompatible with plastic deformation of macroscopical parts of the bone.

Ultimate failure of diaphyseal bone. An experimental study on dogs.

The deformation of entire long bones from dogs has been studied in torsion. The load-deformation relationship is non-linear prior to ultimate failure and repeated successively increased loading and unloading in the non-linear range is found to reduce the stiffness of the bone. The nonlinearity of the curve is assumed to be due to a change in the geometry of the bone originating from formation of small cracks in the cortex. The bone material of entire long bones from dogs seems to be linearly elastic for all torques.

P. Netz

Papers

Torsional Strength and Geometry of Diaphyseal Bone: An Experimental Study on Dogs

Material Reaction of Diaphyseal Bone Under Torsion: An Experimental Study on Dogs

Non-linear properties of diaphyseal bone. An experimental study on dogs.

Ultimate failure of diaphyseal bone. An experimental study on dogs.

Non-linear properties of diaphyseal bone