Cortical bone

Cortical bone, also known as compact bone, forms the dense outer layer of bones and plays a crucial role in providing structural support and protection. It is essential for weight bearing, movement, and overall skeletal stability. Understanding its anatomy, structure, and composition is fundamental in both clinical and research contexts.

Anatomy of Cortical Bone

Location

Cortical bone constitutes the outer shell of all bones, providing strength and rigidity. It is particularly prominent in long bones such as the femur, tibia, and humerus, where it forms a dense layer surrounding the medullary cavity. In flat bones like the skull and pelvis, cortical bone forms the compact layers that sandwich the inner spongy bone, while in irregular bones, it provides a protective outer surface.

Gross Structure

Cortical bone is characterized by its high density and compactness, which distinguishes it from cancellous bone. This dense structure allows it to resist mechanical stress and provide a sturdy framework for muscle attachment. Its thickness varies depending on the bone and location, being thickest in weight-bearing regions and thinner in areas that require flexibility.

Microscopic Structure

At the microscopic level, cortical bone is organized into osteons, also known as Haversian systems. Each osteon consists of concentric lamellae surrounding a central Haversian canal that contains blood vessels and nerves. Volkmann’s canals run perpendicular to Haversian canals, connecting the vascular and nerve supply throughout the bone. Small cavities called lacunae house osteocytes, which communicate through tiny channels called canaliculi, allowing for nutrient and waste exchange.

Composition of Cortical Bone

Organic Components

The organic matrix of cortical bone is primarily composed of type I collagen fibers, which provide tensile strength and flexibility. Non-collagenous proteins such as osteocalcin and osteopontin contribute to bone mineralization, regulate cell function, and support the structural integrity of the matrix.

Inorganic Components

The inorganic portion of cortical bone consists mainly of hydroxyapatite crystals, a mineral form of calcium phosphate. These crystals are embedded within the collagen matrix, providing compressive strength and rigidity. The mineral content is crucial for the mechanical properties of bone, enabling it to resist bending and torsional forces while maintaining structural stability.

Development and Growth

Ossification

Cortical bone develops through two main processes of ossification. Intramembranous ossification occurs primarily in flat bones such as the skull, where mesenchymal cells directly differentiate into osteoblasts to form bone tissue. Endochondral ossification is the process by which most long bones develop, involving a cartilage template that is gradually replaced by bone. This process ensures proper bone length and shape during growth.

Bone Remodeling

Bone remodeling is a continuous process in which cortical bone is broken down and rebuilt to maintain strength and adapt to mechanical demands. Osteoclasts resorb old or damaged bone, while osteoblasts synthesize new bone matrix. Osteocytes, embedded within the bone matrix, act as mechanosensors and regulate remodeling in response to stress and strain. This dynamic process allows cortical bone to repair microdamage and maintain structural integrity throughout life.

Mechanical Properties

Cortical bone exhibits unique mechanical properties that enable it to support the body’s weight and resist external forces. It is highly dense and strong, providing significant resistance to compressive loads. Its arrangement of osteons and mineralized matrix allows it to withstand bending and torsional stresses effectively. Compared to cancellous bone, cortical bone is much stiffer and less deformable, making it the primary load-bearing component of the skeletal system. Its mechanical properties are influenced by age, mineral content, and microstructural organization, which can affect bone strength and fracture risk.

Physiological Functions

Cortical bone plays several essential roles in maintaining overall skeletal and systemic health. Its dense structure provides critical support and stability, enabling upright posture and efficient locomotion. It also protects internal organs, such as the brain, heart, and lungs, by forming rigid protective barriers around them.

In addition to structural support, cortical bone serves as a mineral reservoir, storing calcium, phosphate, and other essential ions. These minerals can be mobilized to maintain blood homeostasis during periods of deficiency. Furthermore, cortical bone contributes indirectly to hematopoiesis by forming the outer shell around the bone marrow, where blood cell production occurs.

Clinical Relevance

Bone Diseases Affecting Cortical Bone

• Osteoporosis: A condition characterized by decreased bone density and increased fragility, affecting cortical bone more prominently in long bones and leading to higher fracture risk.
• Osteomalacia: Softening of bone due to defective mineralization, often caused by vitamin D deficiency, which compromises the mechanical strength of cortical bone.
• Paget’s Disease of Bone: A disorder involving abnormal bone remodeling, resulting in enlarged and weakened cortical bone that may fracture more easily.

Fractures

Cortical bone is commonly involved in fractures, especially in the long bones of the limbs. These fractures often result from high-impact trauma or repetitive stress. Healing of cortical bone involves a complex process of inflammation, callus formation, and remodeling, which restores both structural integrity and function.

Diagnostic Imaging

• X-ray Assessment: Provides a primary tool for evaluating cortical bone thickness, fractures, and deformities.
• CT and MRI Evaluation: Offers detailed visualization of cortical bone microarchitecture, fracture patterns, and surrounding soft tissue involvement, aiding in precise diagnosis and treatment planning.

Research and Advances

Recent research in cortical bone has focused on improving understanding of its structure, function, and potential for regeneration. Bone tissue engineering explores the use of scaffolds, stem cells, and growth factors to repair or replace damaged cortical bone. Advances in biomaterials have led to the development of synthetic bone substitutes that mimic the mechanical properties of natural cortical bone, offering promising options for fracture repair and reconstructive surgery.

Modern imaging techniques, including high-resolution CT, micro-CT, and MRI, allow detailed assessment of cortical bone microarchitecture and mineral density. Biomechanical testing and computational modeling provide insights into stress distribution, fracture risk, and the effects of aging or disease. These advances are improving both clinical management and fundamental understanding of cortical bone biology.

References

1. Rho JY, Kuhn-Spearing L, Zioupos P. Mechanical properties and the hierarchical structure of bone. Med Eng Phys. 1998;20(2):92-102.
2. Marieb EN, Hoehn K. Human Anatomy & Physiology. 11th ed. Pearson; 2019.
3. Clarke B. Normal bone anatomy and physiology. Clin J Am Soc Nephrol. 2008;3(Suppl 3):S131-S139.
4. Robling AG, Castillo AB, Turner CH. Biomechanical and molecular regulation of bone remodeling. Annu Rev Biomed Eng. 2006;8:455-498.
5. Frost HM. Bone “mass” and the “mechanostat”: a proposal. Anat Rec. 1987;219(1):1-9.
6. Bertram JEA, Biewener AA. Bone curvature: sacrificing strength for load predictability? J Theor Biol. 1988;131(1):75-92.
7. Silva MJ, Keaveny TM. Mechanisms and functional significance of bone microarchitecture. Bone. 2005;37(5):503-507.
8. Fink H, Rehberg M, Najeeb S. Cortical bone structure, composition, and clinical implications. Orthop Clin North Am. 2020;51(2):123-136.
9. Standring S, ed. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. Elsevier; 2020.
10. Seeman E, Delmas PD. Bone quality—the material and structural basis of bone strength and fragility. N Engl J Med. 2006;354(21):2250-2261.