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Cortical Bone – Structure, Function & Conditions
Cortical bone is the hard, dense outer layer of bone that provides strength and stability to the skeleton. It accounts for approximately 80% of total bone mass in the human body.
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Cortical bone is the hard, dense outer layer of bone that provides strength and stability to the skeleton. It accounts for approximately 80% of total bone mass in the human body.
What is cortical bone?
Cortical bone – also referred to as compact bone or substantia compacta – is the dense, rigid outer shell that surrounds every bone in the human body. It forms a solid protective layer around the inner cancellous (trabecular) bone tissue and accounts for approximately 80% of total skeletal bone mass. In contrast to the spongy, porous trabecular bone found in the interior, cortical bone is highly dense with a porosity of only around 5–10%, giving it its characteristic hardness and mechanical strength.
Structure and composition
At the microscopic level, cortical bone is organized into cylindrical structural units called osteons (also known as Haversian systems). Each osteon consists of concentric layers (lamellae) of mineralized bone matrix surrounding a central channel – the Haversian canal – through which blood vessels and nerves pass.
- Osteon (Haversian system): The basic functional unit of cortical bone.
- Haversian canal: A central longitudinal channel containing blood vessels and nerve fibres.
- Volkmann canals: Transverse channels connecting adjacent Haversian canals.
- Lacunae and canaliculi: Small cavities that house bone cells (osteocytes), which communicate via fine channels throughout the bone matrix.
Functions of cortical bone
Cortical bone serves several vital functions in the human body:
- Mechanical protection: It shields internal organs and sensitive tissues from external forces and impacts.
- Structural support: It provides the long bones of the limbs (e.g. femur, tibia) with the rigidity needed to bear body weight and enable movement.
- Mineral reservoir: Cortical bone stores significant amounts of calcium and phosphate, which can be mobilized into the bloodstream as needed to maintain mineral homeostasis.
- Protective housing: It encases the inner cancellous bone and, in long bones, the bone marrow cavity.
Cortical bone vs. trabecular bone
The skeleton is composed of two distinct but complementary bone types:
- Cortical bone (compact bone): Dense, solid outer layer; predominantly found in the shafts of long bones such as the femur and radius. Porosity: approximately 5–10%.
- Trabecular bone (cancellous or spongy bone): An open, lattice-like internal network; mainly located in vertebral bodies, the pelvis, and the ends (epiphyses) of long bones. Porosity: 75–95%.
Bone remodeling
Cortical bone is not a static tissue – it undergoes continuous renewal through a process called bone remodeling. This process is regulated by two main cell types working in coordinated balance:
- Osteoclasts: Cells responsible for breaking down (resorbing) old bone tissue.
- Osteoblasts: Cells that form new bone matrix and mineralise it.
Bone remodeling allows the skeleton to adapt to mechanical demands, repair microdamage, and regulate calcium balance. In healthy adults, cortical bone is renewed at a rate of approximately 2–3% per year.
Clinical relevance and associated conditions
Changes in cortical bone quality and quantity are central to a range of common medical conditions:
Osteoporosis
Osteoporosis is characterized by a progressive reduction in bone mineral density and deterioration of bone microarchitecture, including thinning and increased porosity of the cortical shell. This substantially increases the risk of fragility fractures, particularly at the hip, wrist, and vertebrae.
Fractures
Cortical bone fractures occur when mechanical forces exceed the load-bearing capacity of the bone. Stress fractures can develop from repetitive, cumulative loading – a common issue among athletes and military recruits.
Osteomalacia and rickets
Osteomalacia in adults and rickets in children result from insufficient mineralisation of the bone matrix, rendering cortical bone softer and prone to deformity. The most common underlying cause is deficiency of vitamin D or calcium.
Paget disease of bone
In Paget disease, disordered bone remodeling leads to abnormal enlargement and structural deformity of affected bones, including the cortical compartment, resulting in pain and increased fracture risk.
Diagnostic methods
Several imaging and measurement techniques are used to assess cortical bone:
- DXA (dual-energy X-ray absorptiometry): The standard clinical method for measuring bone mineral density (BMD); widely used in the diagnosis and monitoring of osteoporosis.
- pQCT (peripheral quantitative computed tomography): Enables separate assessment of cortical thickness, density, and geometry.
- HR-pQCT (high-resolution peripheral QCT): Provides detailed three-dimensional imaging of cortical microstructure.
- Conventional X-ray: Used to identify fractures and gross structural abnormalities.
- Bone biopsy: Histological analysis reserved for selected diagnostic questions.
References
- Seeman, E. & Delmas, P.D. (2006): Bone quality – the material and structural basis of bone strength and fragility. New England Journal of Medicine, 354(21), pp. 2250–2261.
- World Health Organization (WHO) (1994): Assessment of fracture risk and its application to screening for postmenopausal osteoporosis. WHO Technical Report Series 843. Geneva: WHO.
- Tortora, G.J. & Derrickson, B. (2017): Principles of Anatomy and Physiology. 15th edition. Wiley, Hoboken.
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