Cortical bone is a dense, highly organized tissue that forms the protective outer layer of all bones in the human body. Also known as compact bone, it accounts for approximately 80% of the total bone mass in an adult skeleton. This tissue gives bones their characteristic smooth, white, and solid appearance. Its primary role is to provide structural support and integrity to the body, forming a rigid framework that enables movement and shields internal organs.
What is Cortical Bone and Where is it Found?
This tissue is strategically located throughout the body to withstand mechanical stress. It forms the main shaft, or diaphysis, of long bones such as the femur and the humerus. In addition to long bones, cortical bone forms the protective outer casing for all other bone types, including the flat bones of the skull and ribs, and irregular bones like the vertebrae. It creates a sturdy layer around the more porous, honeycomb-like tissue found inside bones, known as trabecular or spongy bone.
The Intricate Structure of Cortical Bone
The microscopic architecture of cortical bone is a highly organized system. Its fundamental structural unit is the osteon, or Haversian system, which consists of cylindrical columns aligned parallel to the bone’s long axis to resist fracturing. Each osteon is made of concentric layers of bone matrix, called lamellae, which surround a central Haversian canal containing blood vessels and nerve fibers.
The lamellae contain small spaces called lacunae, where living bone cells, or osteocytes, reside. These osteocytes are interconnected by a network of tiny channels known as canaliculi. This network allows for communication between cells and the transport of nutrients and waste products.
The bone matrix has both organic and inorganic components. The organic part consists mainly of Type I collagen fibers, which provide flexibility and resistance to tension. The inorganic component is primarily made of hydroxyapatite, a mineral salt of calcium and phosphate, which gives the bone its hardness and compressive strength.
Essential Functions of Cortical Bone
Cortical bone’s primary role is providing mechanical support and protection. Its rigidity forms the skeleton’s framework, supporting the body’s weight against gravity. For example, the cortical bone of the skull protects the brain, while the rib cage shields the heart and lungs from physical trauma.
Cortical bone also serves as a system of levers for movement. Muscles attach to the outer surface of bones via tendons. When these muscles contract, they pull on the bones to generate movement at the joints, and the bone’s strength withstands the forces from these contractions.
Beyond its mechanical roles, cortical bone is a storage site for minerals, particularly calcium and phosphorus. These minerals contribute to the bone’s hardness and can be released into the bloodstream to maintain physiological balance, a process called mineral homeostasis. When the body’s calcium levels are low, bone matrix is broken down to release calcium for functions like nerve signaling and muscle contraction.
How Cortical Bone Grows, Renews, and Heals
Cortical bone is a dynamic tissue that undergoes constant growth, renewal, and repair. During childhood and adolescence, bones widen through appositional growth, where new bone tissue is added to the outer surface. This process allows bones to thicken and strengthen as the body grows.
Throughout life, cortical bone is maintained through a process called remodeling. This involves the coordinated action of osteoclasts, which break down old or damaged bone, and osteoblasts, which synthesize new bone matrix to replace it. This constant turnover allows the skeleton to adapt to changing mechanical loads and repair micro-damage.
When a bone fractures, cortical tissue initiates a complex healing process. The first stage involves forming a hematoma, or blood clot, at the fracture site. This is followed by a soft, fibrocartilaginous callus that bridges the gap between the broken bone ends. Over several weeks, this callus is replaced by a hard, bony callus, which is then remodeled back into organized cortical bone to restore its original strength.
Maintaining Healthy Cortical Bone
Nutrition is foundational for cortical bone health. Adequate intake of calcium and vitamin D is necessary for mineralizing the bone matrix. Protein is also important, as it provides the building blocks for the bone’s collagen framework.
Physical activity, particularly weight-bearing and resistance exercises, stimulates bone to become denser and stronger by signaling cells to increase bone formation. Hormones also play a regulatory role, with estrogen, testosterone, and growth hormone all influencing bone mass.
With advancing age, cortical bone naturally undergoes changes, including a gradual loss of mass and an increase in porosity. This can lead to a condition known as osteoporosis, where bones become weaker and more susceptible to fractures. In osteoporosis, the cortical layer thins, reducing its ability to withstand stress and increasing the risk of breaks from minor falls or impacts.