Cancellous bone is a type of tissue that visually resembles a natural sponge or honeycomb, with a light, porous structure enclosing many large spaces. This design gives it a spongy appearance, which is why it is also frequently referred to as spongy or trabecular bone. This internal, lattice-like framework provides structural support and flexibility without the substantial weight of more solid bone tissue.
Structure and Location of Cancellous Bone
The distinct, porous structure of cancellous bone comes from a network of small, interconnected beams or struts called trabeculae. These trabeculae form a three-dimensional latticework that is not random; instead, it is organized along lines of mechanical stress, providing strength where it is most needed. The spaces within this intricate framework are filled with bone marrow and blood vessels, making it a highly vascularized tissue. This internal arrangement results in a high porosity, which contributes to the bone’s relatively low weight.
This type of bone is strategically located within the skeletal system. It is most commonly found at the ends of long bones, such as the femur and humerus, particularly near the joints where forces are transmitted. It is also the major component of the vertebrae, ribs, and the flat bones of the pelvis and skull. This placement allows it to function effectively in areas that require a degree of flexibility and shock absorption.
Primary Functions
One of the main roles of cancellous bone is metabolic. Its large surface area, created by the extensive trabecular network, makes it well-suited for the rapid exchange of minerals like calcium with the bloodstream. This allows it to play a part in mineral homeostasis, as the body can deposit or draw upon these mineral reserves as needed, facilitated by the bone’s rich blood supply.
The porous interior of cancellous bone also serves as the primary residence for red bone marrow. This marrow is where hematopoiesis—the production of red blood cells, white blood cells, and platelets—takes place. The network of blood vessels transports newly formed blood cells from the marrow into the systemic circulation.
The unique architecture of cancellous bone provides significant structural benefits. It offers strength and support while being considerably lighter than solid bone. At the joints, its honeycombed structure is adept at dampening and distributing sudden stresses, acting as a natural shock absorber that protects the articular surfaces of the bones.
Comparison to Cortical Bone
The most apparent difference between cancellous and cortical bone is density. Cancellous bone is porous and spongy, while cortical bone is dense and solid, forming the protective outer shell of most bones. This structural variance dictates their location, with cancellous bone found on the interior and cortical bone forming the hard external layer. Cortical bone constitutes about 80% of the skeleton, compared to the 20% made up by cancellous bone.
Cortical bone’s primary roles are protection and providing levers for muscle action, owing to its great strength and resistance to bending and torsion. In contrast, cancellous bone’s main functions are metabolic and hematopoietic, due to its high surface area and marrow content. While cortical bone bears the majority of the direct load, cancellous bone assists by distributing forces out to the cortex.
Cancellous bone has a much higher rate of metabolic turnover, meaning minerals are exchanged and the tissue is remodeled more quickly than in cortical bone. This is a direct result of its greater surface area and vascularity. Consequently, processes that affect bone metabolism often impact cancellous bone more rapidly and extensively.
Role in Health and Disease
The properties of cancellous bone make it central to certain health conditions, particularly osteoporosis. Because of its high metabolic turnover rate, cancellous bone is often the first to be affected by bone density loss. Its porous structure becomes even more sparse and fragile, leading to a heightened risk of fractures, especially in the vertebrae and the ends of long bones where it is most prevalent.
Its rich blood supply makes it an active participant in healing, delivering the necessary cells and nutrients to facilitate repair when a fracture occurs. This same quality makes it a preferred material for bone grafting procedures. Surgeons often use cancellous bone to fill bone defects or encourage fusion because its porous structure provides an excellent scaffold for new blood vessels and bone cells to grow into, promoting successful integration and healing.