In order to understand what role bone graft products carry in the process of bone fracture repair, it is important to understand the architecture of bone and the different categories of bone involved in the healing process. Like most complex subjects, there are many ways to characterize bone. This blog describes the three main methods of categorizing bone.
- Macroscopic Appearance: Cancellous Bone vs. Cortical Bone
- Degree of Maturity: Woven vs. Lamellar Bone
- Embryological Development: Membranous vs. Endochondral Bone
Macroscopic Appearance: Cancellous Bone vs. Cortical Bone
On the macroscopic level, bone can be classified as either cancellous bone or cortical bone. Synonyms for cancellous bone are trabecular or spongy bone. Cortical bone may be referred to as dense or compact bone. When looking at bones with an unaided eye, it is easy to see distinct differences in the porosity or density. Cancellous bone tissue is typically found on the interior of the bone whereas cortical bone is found on the exterior (Figure 1). With a severe bone fracture, often both cortical and cancellous bone are broken.
Cancellous bone porosity typically ranges from 75-95% with an average pore size of 200-600μm in diameter. This gives it a honeycombed, spongy appearance and light weight. It is found in the inner chamber of most bones, typically at the ends, near joints. This type of bone is made out of trabeculae, which are curved beams or arches arranged specifically to evenly distribute biomechanical loads onto the articular surfaces of joints.
Cancellous bone’s low density causes it to be more fragile than cortical bone, but it is also more flexible. In engineering terms, it has a lower modulus of elasticity. This cushioning effect prevents or delays arthritis of the more fragile and non-regenerative tissues, specifically cartilage or spinal discs. Cancellous bone’s high level of porosity also serves as a reservoir for bone marrow, which is vital for the regeneration of a variety of tissues. Lastly, cancellous bone serves as a source for storing calcium and phosphorus for use throughout the body.
In contrast to cancellous bone, cortical bone is very dense being only 5-10% porous. Therefore, it is heavier in weight. The pores are very small, typically ranging from 10-100μm in diameter. In fact, the pores sizes of the channels that feed the osteocytes, called canaliculi, are less than 500 nm (0.5μm). For the most part, the pores of cortical bone are not visible without magnification. These pores are just large enough in diameter to allow blood and lymphatic vessels, as well as nerves, to snake throughout cortical bone to support all the osteocytes and other cells found in bone. Due to its high density, cortical bone serves as a hard protective layer around the internal bone marrow cavity and carries most of the biomechanical loads applied to our bones.