Bone Biomechanics

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Types of Bone

Bone consists of two forms: woven and lamella

Woven vs Lamellar Bone

Property	Woven Bone	Lamellar Bone
Definition	"Primitive", immature	"Mature" bone, remodelled from woven bone
Found in	Embryo and new-born, in fracture callus, metaphyseal region of growing bone, tumours, osteogenesis imperfecta, and pagetic bone.	Throughout the mature skeleton in both trabecular and cortical bone.
Composition	Dense coarse-fibred collagen, varied mineral content, greater turnover, more cells per unit volume with random arrangement, no lamellae.	Formed by intramembranous or endochondral ossification, contains collagen fibres.
Organisation	Randomly arranged collagen fibres, disorganised	Highly ordered, stress-oriented collagen fibres
Mechanical Properties	Isotropic	Anisotropic
Strength	Weaker, more flexible, more easily deformed	More stiffness and strength

Woven and lamellar bone are organised structurally into trabecular (spongy or cancellous) bone and cortical (dense or compact) bone.

Trabecular Bone vs Cortical Bone

	Trabecular Bone	Cortical Bone
Mass	Cortical bone has four times the mass of trabecular bone
Metabolic turnover	Trabecular bone has 8 times greater metabolic turnover due to its high surface area as bone turnover happens at the surface.
Found in	Metaphysis and epiphysis of long bones and in cuboid bones such as the vertebrae	Diaphysis of long bones, and envelope of cuboid bones.
Structure	Internal beams/spicules form a 3D branching lattice aligned along areas of mechanical stress.	Compact bone is formed by layers of lamellar bone. Plexiform bone in larger animals is formed by layers of lamellar and woven bone. Haversian bone is composed of vascular channels circumferentially surrounded by lamellar bone, with a unit being an osteon.
Subjected forces	Compression predominates, but is subjected to complex set of stresses and strains	Bending, torsional, and compressive forces.
Density	Proximal tibial trabecular bone: 0.30 g/cm³ Greater percentage deviation in density compared to cortical bone.	Femoral cortical bone: 1.85g/cm³
Porosity	Typically 50% to 90%	Typically 10%. Densities of trabecular and cortical bone can overlap, cortical bone is usually defined as bone with less than approximately 30% porosity.
Architecture	Network of small, interconnected plates and rods of individual trabeculae with relatively large spaces between the trabeculae. Individual trabeculae contains only some of the voids found in cortical bone (canaliculi, lacunae, and rarely Haversian canals)	Solid containing a series of dense voids: Haversian and Volkmann's canals and to a lesser extent lacunae and canaliculi.

Osteons in Haversian Bone

• Osteon: the major structural unit of cortical bone, is an irregular, branching, and anastomosing cylinder composed of a more or less centrally placed neurovascular canal surrounded by cell-permeated layers of bone matrix.
• Osteons are generally oriented in the long axis of the bone
• Haversian canal: The central canal of an osteon. This contains cells, vessels, and occasionally nerves
• Volkmann's canal: the canals connecting osteons
• Vessels: capillaries but smaller vessels resemble lymphatic vessels. Derived from th4e principal nutrient arteries of the bone or epiphyseal and metaphyseal arteries. A vascular network is formed.

Material Properties

Elastic Behaviour

• Isotropic materials: elastic properties do not depend on the orientation of the material with respect to the loading direction. Characterised by Young's modulus. Poisson's ratio measures how much a material bulges with compression or contracts with stretching.
• Anisotropic materials: elastic properties depend on their orientation with respect to the loading direction. Includes bone in the longitudinal direction. Cortical bone is transversely isotropic, i.e. it is isotropic with loading in the transverse plane. Transverse isotropy is a subset of anisotropy.
• Young's modulus of bone: modulus of cortical bone in longitudinal direction is 1.5 times the modulus in the transverse direction, and over 5 times the modulus in the shear direction.
• Poisson's ratio of bone: relatively high indicating that cortical bone bulges more than metals when subjected to uniaxial compression.

Strength