Bone looks hard, dense, and solid when viewed from the outside. In reality, bone is more complex. There are two types of bone:
Cortical bone — This is the bone that makes up 80% of our skeleton. Also known as compact bone, its hard outer layer protects the interior of the bone where bone marrow is formed. Cortical bone is the predominant type of bone in the long bones found in our arms and legs. As we age, cortical bone loses mass, but the whole bone adapts to the loss by increasing its total bone width. This change in geometry acts to increase its strength. However, this remarkable adaptation is limited by continued bone thinning, making the bone susceptible to fracture with less and less trauma, such as minor falls.
Trabecular bone — Also known as spongy or cancellous bone, makes up the remaining 20% of the bone in our bodies. Trabecular bone is constructed in a way that is less dense than the harder cortical bone. This more porous structure is the factory for producing blood cells and is found at the end of the long bones, near joints, and in the interior of the vertebrae of the spine and hip. Trabecular bone is primarily the type of bone that we lose as we grow older and the bone that is most adversely affected by poor bone health habits. Weakened trabecular structure in the spine and the vulnerable part of the hip make these bones particularly susceptible to fractures.