Bone, the primary tissue of the skeletal system, is not uniform. It comprises two distinct types: cortical bone and cancellous bone. Understanding these types is important for comprehending bone health, movement, and disease progression.
Defining Cortical Bone
Cortical bone, also known as compact bone, forms the dense, solid outer layer of most bones, accounting for approximately 80% of total skeletal mass in adults. Its structure features microscopic cylindrical units called osteons. Each osteon has concentric layers, lamellae, around a central Haversian canal.
These central canals contain small blood vessels and nerve fibers, providing nutrients to osteocytes embedded within the lamellae. Volkmann’s canals, perpendicular to Haversian canals, connect adjacent osteons to the bone’s surface blood supply. Cortical bone provides mechanical strength, resisting bending and torsional forces, and protecting inner bone structures. It is the main component of long bone shafts, like the femur, and forms the outer shell of most other bones.
Defining Cancellous Bone
Cancellous bone, also known as spongy or trabecular bone, is the inner, less dense, and more porous bone tissue, making up about 20% of total skeletal mass. Its structure resembles a honeycomb, composed of a network of thin, bony struts called trabeculae.
The spaces within this network are filled with bone marrow, responsible for hematopoiesis (blood cell production). Cancellous bone’s functions include providing metabolic activity due to its large surface area and absorbing shock. This porous bone is predominantly found at the ends of long bones, within the vertebral bodies of the spine, and in the pelvis.
Key Structural and Functional Distinctions
Cortical and cancellous bone differ in their structure and mechanical roles. Cortical bone is dense with low porosity (5% to 15% of its volume), making it rigid and able to bear weight. In contrast, cancellous bone is less dense, with high porosity (40% to 95% of its volume), giving it a spongy appearance.
Mechanically, cortical bone resists bending and torsional forces, acting as the main load-bearing component of the skeleton. Cancellous bone, with its lattice-like arrangement of trabeculae, distributes loads and absorbs impact at joints, providing flexibility and reducing stress on the denser cortical shell. The trabeculae align themselves to resist the specific mechanical loads a bone experiences.
Cancellous bone also demonstrates a higher rate of metabolic turnover and remodeling compared to cortical bone. This increased metabolic activity is attributed to its larger surface area, exposed to blood flow and bone marrow. Resorption in cancellous bone occurs along its surfaces, while in cortical bone, resorption involves the tunneling of bone itself. Their typical arrangement, such as the dense cortical shaft surrounding a core of cancellous bone at the ends of a long bone, creates a structure that is both strong and relatively lightweight.
Clinical Significance of Bone Type
The properties of cortical and cancellous bone influence their susceptibility to diseases and injuries. Osteoporosis, characterized by reduced bone density, often impacts cancellous bone earlier and more severely. This is due to its higher metabolic turnover rate, which allows it to lose bone mass more rapidly when the balance between formation and resorption is disrupted. Common fracture sites in osteoporosis, such as the vertebrae, wrist, and hip, have a relatively high proportion of cancellous bone.
Fracture patterns and healing also differ based on the bone type involved. Fractures in cancellous bone tend to heal more readily due to its rich blood supply and higher metabolic activity, supporting faster revascularization and new bone formation. Cortical bone fractures, being less vascularized and metabolically active, may take longer to heal and require more stable fixation. In bone grafting, cancellous bone is often preferred for its porous structure, encouraging rapid blood vessel growth and integration with host bone. Cortical bone grafts, while slower to integrate, offer superior mechanical strength and structural support, suitable for defects requiring rigid scaffolding.