Bone is a dynamic tissue that constantly remodels throughout life. It provides the framework for the human body, enabling movement and protecting internal structures. Cortical bone, also known as compact bone, forms the dense and robust outer shell of most bones, providing mechanical strength and rigidity. It is a fundamental component of the skeletal system, supporting various physiological processes.
Where Cortical Bone is Found
Cortical bone forms the primary structural component of the human skeleton. It appears as a solid, continuous layer that covers nearly every bone. This dense outer layer is particularly prominent in the shafts of long bones, such as the femur and tibia, where it forms the diaphysis.
The thickness of cortical bone varies depending on the bone and its mechanical demands. Long bones, which bear significant weight and stress, have thick cortical walls. In contrast, flat bones, such as the skull, also have an outer layer of compact bone, though it might be thinner. This distribution ensures the skeleton provides protection and efficient leverage for muscle action.
The Internal Organization of Cortical Bone
The highly organized structure of cortical bone contributes to its strength and rigidity. Its primary structural unit is the osteon, or Haversian system, which consists of concentric layers of bone matrix arranged around a central Haversian canal. These canals house blood vessels, nerves, and lymphatic vessels, providing nutrients and communication pathways.
Surrounding the Haversian canal are concentric rings of calcified matrix called lamellae. These lamellae are composed of collagen fibers oriented in alternating directions, enhancing resistance to twisting forces. This layered arrangement provides mechanical integrity, allowing it to withstand stresses.
Within these lamellae are small, interconnected spaces called lacunae, housing osteocytes. Osteocytes connect to each other and the Haversian canal via microscopic channels called canaliculi. Canaliculi facilitate the exchange of nutrients, waste, and signals among osteocytes and the blood supply, maintaining bone health and facilitating remodeling.
Essential Functions of Cortical Bone
Cortical bone serves several roles. Its primary function is to provide structural support and rigidity, forming the framework that supports body weight and maintains posture. Its dense composition allows it to withstand compressive forces and bending moments, essential for daily activities.
Beyond structural support, cortical bone offers protection for internal organs. For example, the skull shields the brain, while the ribs protect the heart and lungs from impact. This protection results from its compact, robust nature.
Cortical bone also acts as attachment points for muscles and tendons, enabling movement. Its strength allows it to endure pulling forces from muscle contractions, converting them into leverage for motion. This interaction enables a wide range of movements, from walking to lifting objects.
Cortical bone plays a role in mineral storage for calcium and phosphorus. These minerals are vital for physiological processes, including nerve function, muscle contraction, and blood clotting. The bone acts as a reservoir, releasing minerals into the bloodstream when needed and absorbing them when levels are high, regulating mineral balance.
Cortical Bone Versus Spongy Bone
Bone tissue is broadly categorized into two main types: cortical bone and cancellous (spongy or trabecular) bone. These types differ significantly in density, structure, and location. Cortical bone is characterized by its high density and compact nature, forming the solid outer layer of bones and accounting for about 80% of total bone mass in adults.
In contrast, spongy bone is less dense and highly porous, featuring a network of interconnected bony struts called trabeculae. Trabeculae create an open, lattice-like structure, making spongy bone lighter. While cortical bone is found in long bone shafts, spongy bone is typically in long bone ends (epiphyses), flat bones, and vertebrae.
Functionally, cortical bone is designed for strength and protection, bearing mechanical stress. Its compact structure provides rigidity and resistance to bending and compression. Spongy bone, with its trabecular network, is adapted for flexibility and shock absorption, distributing stress. The spaces within spongy bone also house red bone marrow, involved in blood cell production, a function not associated with cortical bone.