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Bone Structure
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Bone Forms
Bone consists of two forms: woven and lamella
| 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 | 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 the principal nutrient arteries of the bone or epiphyseal and metaphyseal arteries. A vascular network is formed.
Blood Supply
The three major blood supplies to long bones are:
- Nutrient artery: These penetrate the cortex of the shaft and divide into ascending and descending medullary arteries. The medullary arteries supply blood to the inner two thirds of the cortex.
- Metaphyseal arteries: These supply blood to the metaphysis and anastomose with the medullary microvasculature.
- Periosteal arteries: These enter the cortex along heavy muscle and fascial attachments, and supply blood tot he outer one third of the cortex.
