Cortical bone, also known as compact bone, is a dense and strong type of bone tissue that forms the outer layer of bones. It provides structural support and protection due to its high density and minimal porosity. Cortical bone is composed of tightly packed osteons or Haversian systems, which are cylindrical structures that contain a central canal surrounded by concentric rings of calcified matrix. These osteons are responsible for the hardness and strength of cortical bone, making it crucial for weight-bearing and resistance to bending and torsion.
Location
Cortical bone is located on the outer surfaces of all bones, forming a protective layer around the internal cancellous (spongy) bone. It is most prominent in the shafts of long bones, such as the femur and humerus, where it provides significant strength and support. Cortical bone also constitutes the outer layer of flat bones, such as those in the skull, and forms the thickened areas around the joints, like the femoral neck and the vertebral bodies.
Structure and Anatomy
Cortical bone, also known as compact bone, is characterized by its dense and solid structure. Unlike cancellous bone, which has a porous and trabecular architecture, cortical bone is highly organized and contributes significantly to the strength and rigidity of the skeletal system.
Composition
Osteons (Haversian Systems)
Osteons are the fundamental structural units of cortical bone. Each osteon consists of a central canal surrounded by concentric lamellae (layers) of calcified matrix.
Central (Haversian) Canal
The central canal runs longitudinally through the osteon and contains blood vessels, nerves, and lymphatic vessels, providing nourishment and communication to the bone cells.
Lamellae
Lamellae are concentric rings of calcified matrix that surround the central canal. They are composed of collagen fibers and mineral deposits, which provide the bone with its strength and resilience.
Lacunae
Small spaces called lacunae are located between the lamellae. Each lacuna contains an osteocyte, a mature bone cell responsible for maintaining the bone matrix.
Canaliculi
Tiny channels called canaliculi radiate from the lacunae, connecting osteocytes with each other and with the central canal. This network allows for the exchange of nutrients and waste products between bone cells and blood vessels.
Interstitial Lamellae
Interstitial lamellae are remnants of old osteons that have been partially resorbed during bone remodeling. They fill the spaces between the new osteons, adding to the overall density of cortical bone.
Circumferential Lamellae
These lamellae are found just beneath the periosteum and encircle the entire bone. They contribute to the thickness and strength of the cortical bone.
Perforating (Volkmann’s) Canals
Volkmann’s canals run perpendicular to the central canals of osteons. They connect the blood and nerve supply of the periosteum to that of the central canals, facilitating the distribution of nutrients and signals.
Outer Layers
Periosteum
The periosteum is a dense, fibrous membrane that covers the outer surface of cortical bone. It consists of an outer fibrous layer and an inner cellular layer (osteogenic layer) that is involved in bone growth and repair. The periosteum is richly supplied with blood vessels, nerves, and lymphatic vessels.
The endosteum is a thin, vascular membrane lining the inner surface of the cortical bone, including the medullary cavity and the Haversian canals. It contains osteoblasts and osteoclasts, which are involved in bone remodeling.
Microscopic Structure
Bone Matrix
The bone matrix in cortical bone is composed of an organic component (mainly collagen fibers) and an inorganic component (primarily hydroxyapatite crystals). The organic component provides flexibility, while the inorganic component gives the bone its hardness.
Osteocytes
Osteocytes are mature bone cells located within the lacunae. They maintain the bone matrix and communicate with other bone cells through the canaliculi.
Osteoblasts and Osteoclasts
Osteoblasts are bone-forming cells found in the periosteum and endosteum. Osteoclasts are bone-resorbing cells that break down bone tissue during the remodeling process. Both cell types are crucial for the maintenance and repair of cortical bone.
Mechanical Properties
Density and Porosity
Cortical bone is highly dense with low porosity, typically containing less than 10% porosity. This density is essential for its load-bearing capabilities.
Orientation of Fibers
The collagen fibers in the lamellae are oriented in different directions in successive layers, providing tensile strength and resistance to various mechanical stresses.
Regional Variations
In long bones, the diaphysis (shaft) is primarily composed of thick cortical bone, providing the necessary strength for weight-bearing and movement.
Metaphysis and Epiphysis
The metaphysis and epiphysis regions, located near the ends of long bones, have a thinner cortical layer surrounding a core of cancellous bone, which helps in distributing joint loads and facilitating bone growth.
Function
Structural Support
Weight-Bearing
Cortical bone provides the necessary strength to support the body’s weight. Its dense and solid structure ensures that the bones can withstand the mechanical stresses of daily activities such as standing, walking, and running.
Framework for the Body
Acting as the primary framework, cortical bone maintains the shape and form of the body. It supports the attachment of muscles and ligaments, enabling movement and stability.
Support for Soft Tissues
The cortical bone also supports and protects the soft tissues and organs by providing a rigid structure that helps maintain their position and function.
Protection
Protection of Vital Organs
The dense structure of cortical bone protects the vital organs located within the thoracic and abdominal cavities. For instance, the ribcage, composed of cortical bone, safeguards the heart and lungs, while the skull protects the brain.
Shock Absorption
Although primarily rigid, cortical bone can absorb and dissipate impact forces, reducing the risk of fractures and injuries during high-impact activities.
Movement and Locomotion
Leverage for Muscles
Cortical bone acts as a lever system for muscles. When muscles contract, they pull on bones, generating movement. The rigid nature of cortical bone ensures efficient transmission of these forces.
Joint Stability
Cortical bone contributes to the stability of joints by forming the articulating surfaces that fit together. This stability is essential for smooth and controlled movements.
Mineral Storage
Reservoir for Minerals
Cortical bone serves as a reservoir for essential minerals, primarily calcium and phosphorus. These minerals are vital for various physiological processes, including nerve transmission, muscle contraction, and blood clotting.
Regulation of Mineral Balance
The bone matrix can release minerals into the bloodstream as needed, helping to maintain the balance of minerals in the body. This dynamic process is crucial for homeostasis.
Hematopoiesis
Bone Marrow Cavity
Although less prominent in cortical bone compared to cancellous bone, the medullary cavity within long bones contains bone marrow. This marrow is responsible for the production of blood cells, including red blood cells, white blood cells, and platelets.
Support for Marrow Function
The rigid structure of cortical bone provides a protective environment for the bone marrow, ensuring it can effectively produce and release blood cells into the circulation.
Endocrine Function
Hormone Regulation
Bone cells, including osteoblasts and osteocytes, are involved in the regulation of hormones that affect bone formation and resorption. This endocrine function helps regulate bone density and strength over time.
Regulation of Bone Remodeling
Cortical bone is constantly being remodeled through the actions of osteoclasts and osteoblasts. This process is influenced by hormonal signals, ensuring that old or damaged bone is replaced with new, healthy bone tissue.
Protection of the Nervous System
Skull and Vertebrae
The skull, made of cortical bone, encases and protects the brain from mechanical injury. Similarly, the vertebrae protect the spinal cord, a crucial part of the central nervous system.
Foramina and Canals
Cortical bone contains various foramina (holes) and canals that allow the passage of nerves. These structures ensure that nerves are protected and can transmit signals efficiently throughout the body.
Clinical significance
Cortical bone, with its dense and compact structure, plays a pivotal role in various clinical contexts. Its primary function in providing structural support and protection makes it a critical focus in the study and treatment of musculoskeletal disorders. One of the most common clinical concerns related to cortical bone is osteoporosis, a condition characterized by decreased bone density and increased fragility. Osteoporosis predominantly affects postmenopausal women and the elderly, leading to a higher risk of fractures, particularly in the hip, spine, and wrist. Understanding the properties of cortical bone is essential for diagnosing osteoporosis, monitoring its progression, and evaluating the effectiveness of treatment strategies.
Fractures involving cortical bone, such as those in the femur, tibia, and humerus, are significant clinical events that require careful management. These fractures can result from trauma, falls, or pathological conditions like bone tumors. The healing process of cortical bone involves a complex interplay of biological factors that mediate bone repair and regeneration. Clinical interventions, including surgical fixation, bone grafting, and the use of orthopedic implants, are often necessary to ensure proper alignment and stabilization of the fractured bone. Advances in biomaterials and surgical techniques continue to improve outcomes for patients with cortical bone fractures.
The cortical bone’s role in load-bearing and mechanical support is also crucial in the context of joint replacement surgeries, such as hip and knee arthroplasty. The success of these procedures depends on the integration and stability of prosthetic components within the cortical bone. Ensuring adequate bone quality and density is vital for achieving long-term success and functionality of joint replacements. Preoperative assessments, including bone density scans and imaging studies, are essential for planning surgical interventions and optimizing patient outcomes.
In the realm of oncology, cortical bone is often involved in primary bone cancers (such as osteosarcoma) and metastatic bone diseases. The aggressive nature of these malignancies can lead to significant bone destruction and pathological fractures. Clinical management includes a combination of surgery, chemotherapy, and radiation therapy. Understanding the structural integrity and response of cortical bone to these treatments is crucial for developing effective therapeutic strategies and improving patient prognosis.
