Bone tissue is a specialized connective tissue that continuously remodels throughout a person’s life. This material forms the entire skeletal structure, providing support, protection, and mobility. The skeleton is composed of two primary types of bone tissue: cortical bone and cancellous bone. Understanding the differences between these two types—in their structure, location, function, and activity—is necessary.
Defining the Two Bone Types
Cortical bone is the dense, solid outer layer that forms the exterior shell of virtually all bones. It is often referred to as compact bone due to its low porosity, which typically measures less than 10% of its total volume. This structure contributes to its smooth, white, and robust appearance, making it the primary material for mechanical strength.
In contrast, cancellous bone is the highly porous tissue found on the interior of bones. It is also known as trabecular bone or spongy bone, names that reflect its lattice-like or honeycomb appearance. Its porosity is significantly higher, ranging from approximately 50% to 90% of its volume. This open structure makes it lighter and more flexible than its dense counterpart.
Structural Architecture and Density
The microscopic organization of cortical bone is designed for maximum strength and resistance to mechanical stress. Its primary structural unit is the osteon, also known as the Haversian system, which is a cylindrical arrangement of bone tissue. These osteons consist of multiple concentric layers of bone matrix, called lamellae, surrounding a central Haversian canal that houses blood vessels and nerves. This dense, parallel organization allows cortical bone to provide strong resistance to bending and compression forces.
Cortical bone is highly mineralized, resulting in its high density and strength, and it accounts for about 80% of the total mass of the adult skeleton. The density of this tissue is largely a result of its low porosity, where the matrix is solidly filled with inorganic salts, leaving only tiny spaces for bone cells. This compact arrangement is the basis for the bone’s role as a primary load-bearing material.
Cancellous bone, however, is built on a network of thin, interconnected bony struts and plates called trabeculae. These trabeculae are geometrically aligned along the lines of stress that the bone experiences. This structural arrangement creates a lightweight yet strong internal framework.
The high porosity of cancellous bone means it has a much lower density than cortical bone, but this design helps to reduce the overall weight of the skeleton. Its open structure also enables it to absorb and dampen sudden stresses, which is an important function in areas near joints.
Distinct Locations and Primary Functions
Cortical bone is predominantly found in the shaft, or diaphysis, of long bones, such as the femur and humerus. Its function in these locations is primarily mechanical, acting as the main support column for the limbs. This dense outer shell provides structural rigidity, protects the internal bone marrow, and offers secure attachment sites for muscles and tendons, enabling movement.
The distribution of cancellous bone is concentrated in areas where shock absorption and multi-directional support are required. It is found primarily in the ends, or epiphyses, of long bones, within the interior of vertebrae, and throughout flat bones like the pelvis and ribs. In these locations, the trabecular network acts as a shock absorber, distributing forces transmitted through the joints.
Beyond its mechanical role, cancellous bone serves as a primary metabolic and hematologic center. The large spaces between the trabeculae are often filled with bone marrow, which is where hematopoiesis, the production of blood cells, takes place. Cancellous bone also acts as a major reservoir for the storage and exchange of calcium and phosphate ions, playing a significant part in mineral homeostasis.
Biological Activity and Clinical Relevance
The internal architecture of the two bone types leads to a significant difference in their biological activity. Cancellous bone has a greater surface area-to-volume ratio because of its fine, interconnected trabeculae. This large surface area makes it more metabolically active than cortical bone, as it provides greater access for bone-remodeling cells.
This increased activity translates to a much faster bone turnover rate. The remodeling rate in cancellous bone is estimated to be approximately four to five times faster than in cortical bone. This rapid turnover allows cancellous bone to quickly adapt its structure to changes in mechanical loading.
The difference in remodeling rates has significant clinical implications, particularly concerning bone diseases. Osteoporosis, a condition of low bone mass and microarchitectural deterioration, often manifests first in cancellous bone because of its faster turnover. Common osteoporotic fracture sites, such as the hip and spine, have a relatively high proportion of cancellous bone, making them vulnerable to rapid bone loss.
While cortical bone turns over more slowly, its integrity is important for overall skeletal strength and resistance to torsion fractures. In later stages of osteoporosis, the inner layer of cortical bone can become porous, a process called cortical-to-cancellous bone transformation, which further compromises the bone’s mechanical properties. Both tissue types work in concert: the dense cortical shell provides the main structural column, while the lighter cancellous interior provides metabolic flexibility and internal support.