Causes

What causes a bone to break?

We can think of this in three ways:

• situations in which fractures are most likely to happen
• mechanical forces that actually cause the bones to break
• risk factors that make some people more likely to have a fracture

Situations

When the bones are normal, breaks occur when they are subjected to unusually high forces. We often divide these situations into High and Low Energy causes. High Energy trauma is caused by motor vehicle accidents, falls from heights and violence (e.g., gunshot wound - GSW) and the injuries are more likely to be multiple.

Low Energy trauma results from falls, childhood play and most sports injuries. The breaks caused by these type of events tend to be localized to one bone or area.

Mechanical Forces

Most times a combination of forces is applied to bones to cause the fracture. Here we simplify the situation to explain what would happen if only one type of force occurred.

Avulsion

Muscles, ligaments, and tendons attach to bone. If they pull too hard the bony attachment may break off. This happens most commonly when joints such as the knee or elbow dislocate, imposing severe strains on the ligaments that normally hold the joint together. It can also happen from the direct pull of a muscle in the spine, shoulder, elbow, hand, hip, knee, heel, or foot. This is usually a low energy injury but the result may depend on healing the pulled-off piece of bone back to the correct place.

Compression

Crushing forces can cause fractures in the vertebrae, knee and heel bone (sometimes together) when you land on your feet after a fall. These bones tend to squash down. This is called "impacted". When compression is applied along the length of a long bone you tend to get a transverse fracture pattern. This is quite common in the forearm (low energy) and the thighbone (high energy) but can occur in any long bone. The fracture fragments are often displaced and overlapped.

Twisting

A twist severe enough to break a long bone results in a spiral fracture pattern. This is particularly common in the lower leg bone (tibia) and the upper arm bone (humerus). It takes less force to break a bone by twisting it so most of these injuries are low energy. However the amount of displacement at the time of injury can be spectacular and there is often a significant soft tissue component to the injury. Twisting forces near joints can cause fractures; the ankle is a common example.

Bending

Bending forces can be applied either by a direct impact on the middle of a bone or by opposing forces being applied to each end of the bone. In a pedestrian vs motor vehicle accident, for example, the fender of the vehicle may strike the pedestrian’s shin with force enough to bend and break the bone. This is a high energy injury and the bone often breaks out through the skin in this situation (see open fracture below). In a fall on the outstretched hand (FOOSH) the downward momentum of the body weight is applied to the elbow end of the bone and the upward ground reaction force is applied to the hand end. The result is bending of the forearm bones and a low energy bending fracture of the wrist, forearm or elbow. The fracture is often angulated. At the wrist and elbow there is usually a compression element as well.

Violence

Blast and gunshot injuries cause severe fractures with major open wounds, splintering of the bone and terrible damage to the muscles. Blood vessels and nerves are often torn by the spreading shockwave from projectiles. Child abuse causes injuries in a depressingly large number of cases and recognition is very important. The fractures themselves are less of a problem than the danger the child faces if the situation is not discovered. Twisting fractures in infants need to be carefully evaluated because they don’t often occur in the normal course of events.

Risk Factors

There are a variety of reasons why some people are more likely than others to have a fracture. Either their bones are weaker or they undertake riskier activities.

Behavior

Four groups of people have markedly higher risks of breaking bones. The elderly are more likely to fall, particularly if they are also confused. Their bones are more fragile too. Young adult and post-teen males are more likely to take risks on the road, at work and play. They too have a higher than normal risk of fracture. The third group is children at play. The normal rough and tumble of childhood does increase the risk of fracture. With fortunately few exceptions fractures in children heal well and without long-term problems. People with substance abuse problems also have a higher risk of suffering a fracture because of lifestyle issues and higher risk of falling.

Bone abnormality

Bone is a living tissue made up of a gristly matrix hardened by calcium. Both the matrix and the calcium salts are slowly reabsorbed and re-deposited by living bone cells. This is called bone remodelling. Bone remodelling is a normal process occurs constantly throughout our lives. As with all living tissue, the normal situation can be altered by disease processes. This usually leaves the bone weaker than normal.

Osteoporosis and other Metabolic Bone Diseases

In the elderly, bone gets steadily thinner and weaker. The metabolic balance of bone changes. Less new bone is laid down and more old bone is reabsorbed. The bone gets weaker and weaker with time. Certain metabolic diseases can make this process occur faster. The process of osteoporosis is the most common of several metabolic bone diseases. Others are Paget's disease and osteomalacia in older people; rickets, osteogenesis imperfecta and osteopetrosis usually are first noted in childhood.

Deposits in Bone

Bones are weakened when normal tissue is replaced by benign or malignant tumors. Benign deposits are the most common in children. In adults, especially the elderly, secondary deposits from cancers that start elsewhere are more common. Rarely, a fracture through a secondary deposit is the first sign that there is a cancer in the body. In all age groups primary bone cancer, meaning that the cancer started first in the bone, is rare.