Bone is the hard, calcified connective tissue that makes up the human skeleton. Bones act as columns, beams, plates, and 3D structures. Along with cartilage, bones form the skeletal system that carries loads during activity and protects the vital organs. There are 206 separate bones in the adult human skeleton, some of which are described in detail in this Bio Lexicon.
Unlike engineering structural materials, bones are alive. Healthy bones constantly remodel or turnover, and the rate of turnover is related to the mechanical stress on the bone. Wolf’s law describes how change in the function of bone is followed by changes in its internal architecture and external conformation. Areas of high stress are therefore stronger in response to chronic force.
Stress on bones occurs from internal (e.g., muscle contraction) and external (e.g., gravity) forces and results in increased bone mineral (calcium) density. Bones with higher mineral density are typically stronger and able to resist greater forces without breaking. A lack of bone stress can have the opposite effect, resulting in bone weakening.
“Stress shielding” can occur with orthopedic implants (e.g., prostheses for joint replacements) because the hard metal implant materials (typically titanium) significantly reduce the weight-bearing load of the surrounding bone. Stress shielding results in localized resorption and weakening of this surrounding bone, ultimately causing the implant to loosen. Stress shielding and postoperative infection are two of the main reasons for surgical failure requiring revision surgery.
Several types of cells are important to bone turnover. Osteocytes maintain the health of the bone, and osteoblasts are involved in forming new bone and increasing mineralization. Osteoclasts break down and remove old bone tissue, releasing calcium into the bloodstream, in the process referred to as bone resorption.
Calcium Reservoir
Calcium is critical for bone turnover. It is used for several physiological functions, such as nerve signaling and muscle contraction (these will be discussed later). In addition to acting as a structural element, bone also serves as a reservoir to store calcium and other minerals. Calcium levels are tightly regulated within the body by a negative feedback system. When calcium is consumed in our diet, a portion is absorbed into the bloodstream, increasing the calcium ion concentration. Excess calcium is stored within bones; when there is not enough calcium in the bloodstream, it is resorbed from the bone under the action of the osteoclasts. Osteoporosis is a condition where the bone calcium is excessively resorbed over a long period, leading to porous and weak bones.
As in many engineering systems, there is a range of total body calcium within which the body will function properly. When the value contained in the bloodstream and reserves becomes too low or too high, physiological symptoms will result—think of the effect of automobile oil on a car’s functioning.
The outer layer of bone is a hard tissue called compact or cortical bone. Within the interior of bone is a porous matrix called trabecular bone, which has a high percentage of open space similar to a block of Swiss cheese. An engineer can think of it like a 3D truss system. Trabecular bone is also referred to as cancellous or spongy bone.
The exterior of bone is covered by a tough fibrous layer called the periosteum that is able to integrate with fibers of muscle tendons and ligaments. In addition, the periosteum functions to aid bone repair, nourish the bone, and signal pain sensation.
Long bones have a shaft whose structure is similar to a pipe, with the compact or cortical bone on the exterior able to resist bending movements and twisting forces. Bones that are flat, cubical, or irregularly shaped typically do not have a pipe-like shaft, but are composed of the same compact/cortical bone on the exterior and trabecular/spongy bone on the interior.
Bone is living tissue that is constantly being remodeled. Remodeling of bone enables growth during childhood and adolescence, and repair of tissue throughout life. This process repairs micro fractures and allows complete fractures to be repaired if the bone fragments are kept immobilized. One goal of engineering is to develop self-repairing materials like bones in the human body.
