Bones

Purpose and Function of Bones

Human bones serve a variety of essential functions including:

• Supportive Framework: Bones create a strong, supportive, structural framework for the body. This includes creating an important protective shield for vital organs in the chest and pelvis.
• Facilitating Movement: Bones provide a strong site for muscles and tendon to attach and allow for smooth gliding joints at their ends. Together this allows for muscle contractions to apply force through the tendons to move joints. This allows the body to balance itself, lift objects, walk, and run.
• Create Blood: Bone marrow helps produce elements that are essential for human function, including the creation of red blood cells, white blood cells, and platelets.
• Mineral Storage: Bones serves as an important source of mineral storage. Essential minerals such as calcium and phosphorus are stored in bones. These minerals can be released from bones when their levels in the blood become low.

Types of Bones

Based on their shape and function the bones of the human skeleton can be placed into a variety of categories including:

• Long bones: These are long, narrow bones with joints on either end. They are often divided into large and small long bones. Large bones include the: femur, tibia, fibula, humerus, radius, and ulna. Smaller bones include the metacarpal and phalangeal bones of the hand, as well as the metatarsal and phalangeal bones of the foot.
• Short bones: These are smaller square and oblong shaped bones. They include the carpal bones of the wrist, and the mid and hindfoot bones of the foot.
• Flat bones: These bones are seen in the skull and pelvis, and often helped to protect vital organs.
• Irregular shaped bones: These bones include the irregular shaped vertebral bones that make up the spine. Their unique shape allows the spine to move in many directions and also serves to protect the spinal cord.
• Sesamoid bones: These are unusual bones that occur within the substance of a tendon. The most well-known sesamoid bone is the kneecap (patella).

Make Up of Bones

Bones tend to have a hard-outer surface (cortical or compact bone) with a lattice work (trabecular bone) and bone marrow in the middle (medullary canal). The hard cortical bone is covered by a thin layer of tissue called the periosteum -imagine a plastic wrap covering the surface of the bone. The periosteum has a rich blood supply, nerve endings, and bone producing cells (osteoblasts). Long bones (Figure 1) have a shaft (diaphysis) with flaring (metaphysis) and a joint surface at the end (epiphyseal region). The hard-outer surface of bone provides strong structural support. Bone is a living dynamic structure that is constantly remodeling. Some bone cells (osteocytes) remove bone while other bone cells (osteoblasts) make new bone. When a bone is subject to increased load (ex. increased walking or running) it will get stronger. If it’s subject to less load (ex. bedrest, outer space) it will get weaker.

Bone Problems

The bones in humans can be affected by a variety of problems including:

• Fractures: Even though bones are very strong, they can be broken with a great force (ex. fall from a height, or car accident) or excessive repetitive force (stress fracture). It is interesting to note that when attempting to bend or compress a bone they are quite strong. However, they have less strength when they are twisted. Therefore spiral (twisting) fractures are reasonably common.
• Bone infections (osteomyelitis): Although uncommon bones can become infected. This may occurs following a bone injury that breaks the skin such as an open fracture. However, it can also occur if an infection in the bloodstream takes hold in an area of the bone, usually the flared metaphyseal area which often a poor or sluggish blood supply. Bone can also be infected if a nearby total joint replacement becomes infected. Bone infections can be quite challenging because the bacteria will tend to wall off an area. This prevents blood from reaching the infected bone. Blood carries immune fighting elements and antibiotics, so a lack of local blood supply often means the bone infection will persist.
• Bone tumors: Bone can be affected by abnormal growth. This abnormal bone growth (tumor) can be benign or malignant. Malignant tumors can be primary (arising from the bone itself), or secondary (metastatic) from the spread of a tumor that has started elsewhere. Bone tumors that do not grow much such as many benign tumors often do not create discomfort. Whereas a rapidly growing bone tumor will often create localized pain. Either type of bone tumor can lead to a weakening of the bone if it gets large enough, and this in turn can lead to a fracture through the weakened bone (pathological fracture).
• Bone disease: Bones can be affected by disease. Osteoporosis is a weakening of the bone which tends to occur commonly in older patients, particularly women. Rickets (in children), Padget disease, and osteomalacia are all abnormal bone conditions which fortunately are relatively uncommon today.

Edited by Stephen Pinney MD October 15th, 2024