The main function of bones is to offer strength and support for the body. Compact bone is especially hard, and it is well suited for providing this needed support. However, despite their rigidity and hardness, bones are surprisingly fragile and can break given enough force. Children and adults often break their bones due to injuries received while playing sports. Sports that include high-impact bodily contact can provide enough force to break a bone. Older adults, whose bones are more brittle than those of younger people, are more likely to break their bones as a result of falls. Whatever the cause, bones are able to repair themselves through the action of osteoblasts, osteoclasts, and osteocytes.
A break in a bone is called a fracture. If the ends of the bone remain in the same positions, the fracture is called a nondisplaced fracture. If the ends of the bone are not in line with each other, the fracture is called displaced. A complete fracture goes all the way through the bone, while an incomplete fracture goes only part of the way through. A simple fracture does not penetrate the skin, while a compound fracture does.
Fortunately for humans, bones have the remarkable ability to repair themselves. Deposition of new bone material by osteoblasts fuses the broken ends together, and osteoclasts work to reshape the material until the bone is fixed. Due to this reengineering of the bone tissue, the site of the break is now actually stronger and thicker than it was originally. This helps to prevent additional breakage in the same location. Before the bone starts to repair itself, the pieces need to be set. This means the two exposed ends of the broken bone need to be realigned if the fracture is displaced. This can occur via manual adjustments or surgery, depending on the fracture. Once this occurs, bone repair happens in four main steps:
Step 1: The first thing that occurs is the formation of a hematoma, a large blood clot at the site of a break in bone. The hematoma results from the destruction of periosteum and blood vessels at the break site that supply nutrients and oxygen to the bone. The cells in this area are prevented from getting oxygen and nutrients, so they will eventually die. This causes swelling and large amounts of pain at the site of the break.
Step 2: A few days after the break has been set and the hematoma forms, new blood vessels start to form in the area. Chondroblasts, cells that build new cartilage, enter the damaged area from the periosteum and start to produce a matrix of cartilage that is used to bind the two ends of the bone together. This tissue will later calcify, forming a cartilaginous callus, the structure that holds the two pieces of bone together.
Step 3: Once the cartilaginous callus has been formed, osteocytes and osteoblasts start to change it into spongy bone. These cells form trabeculae, or the open lattice of rod-like connective tissue that forms spongy bone, around the cartilage and bring the two pieces of bone together. Over time, the cartilage is completely replaced by bone, forming a callus.Step 4: The production of the callus is not perfect, and much extra material is added to the bone. Over the next several months, osteoclasts will remove that extra material from the shaft of the bone near the break site and from the central medullary canal. Additional compact bone is produced until the bone is completely healed.