Cortical bone is the dense outer layer that forms the protective shell for most bones, accounting for 80% of the total bone mass in an adult skeleton. Also known as compact bone, its structure has minimal gaps and spaces, giving it a smooth and solid appearance. This density allows it to provide structure and support for the body, facilitate movement, and protect internal organs.
The Structure and Composition of Cortical Bone
The strength of cortical bone originates from its microscopic arrangement. The main structural unit is the osteon, or Haversian system, which consists of tightly packed cylindrical structures. These osteons are aligned parallel to the long axis of the bone, providing support and resistance to bending and twisting forces.
Each osteon has several parts. At its center is the Haversian canal, a channel housing blood vessels, nerves, and lymphatic vessels that nourish the bone. Surrounding this canal are concentric layers of bone matrix called lamellae. The collagen fibers within adjacent lamellae are arranged at perpendicular angles, a design that helps the osteon resist torsional forces.
Cortical bone is a composite of organic and inorganic substances. The inorganic component makes up about 60-70% of the bone’s dry mass and is mainly hydroxyapatite, a mineral that gives bone its hardness and rigidity. The organic matrix consists of Type I collagen fibers, which provide flexibility and tensile strength, preventing brittleness. This combination of hard mineral and flexible protein creates a material that is both strong and resilient.
Primary Functions of Cortical Bone
A primary function of cortical bone is providing structural support for the body. It forms the main shaft of long bones, like the femur and humerus, where it bears the body’s weight. This structure withstands the forces generated during activities like walking, running, and lifting.
Another function of cortical bone is protection. This tissue forms a hard, durable shield for the body’s organs. For example, the cranium is composed of cortical bone to protect the brain from impact. The rib cage also uses cortical bone to protect the heart and lungs from physical trauma.
Cortical bone also serves as a lever system to enable movement. Muscles attach to the bone’s surface via tendons. When these muscles contract, they pull on the bones, generating movement at the joints. The smooth, solid surface of cortical bone provides a stable attachment point for these muscles, allowing for efficient motion.
Distinguishing Cortical and Trabecular Bone
The human skeleton has two major types of bone tissue: cortical and trabecular. Trabecular bone, also known as cancellous or spongy bone, is found inside the ends of long bones, within vertebrae, and in the pelvis. While cortical bone is dense with a porosity of less than 10%, trabecular bone has a porous, honeycomb-like structure and can be up to 90% porous. This structural difference determines their distinct roles in the body.
The density of cortical bone provides the rigidity for its supportive and protective roles. In contrast, the lattice-like network of trabecular bone is lighter and helps with shock absorption, distributing forces that occur during movement. This spongy interior also houses bone marrow, where blood cells are produced.
The two bone types also differ in their metabolic activity. Due to its porous nature, trabecular bone has a much larger surface area than cortical bone. This high surface area makes it more metabolically active, meaning it is broken down and rebuilt more quickly. This higher turnover rate is important for mineral exchange and the skeleton’s response to metabolic influences.
Cortical Bone Health and Remodeling
Bone is a dynamic tissue that constantly undergoes a renewal process called bone remodeling. This process involves two main cell types: osteoclasts, which resorb old bone tissue, and osteoblasts, which form new bone. This continuous cycle ensures the skeleton adapts to mechanical stresses and repairs microscopic damage.
Remodeling occurs in cortical bone, though at a slower pace than in trabecular bone. Within cortical bone, osteoclasts drill tunnels, forming cutting cones. These are then refilled by osteoblasts to create new osteons. A balance between the activity of osteoclasts and osteoblasts maintains bone mass and strength.
An imbalance in the remodeling cycle, where bone resorption outpaces bone formation, leads to a net loss of bone mass. In cortical bone, this causes a decrease in thickness and an increase in porosity, which weakens the bone’s structure. This deterioration is a feature of conditions like osteoporosis and increases the risk of fractures, particularly in older adults.