Why do Bones Fracture?

Bone growth source: http://training.seer.cance...

Dual-energy X-ray absorptiometry (DEXA) assess...

Dual-energy X-ray absorptiometry (DEXA) scan assessment of bone mineral density of the femoral neck.

X-ray showing the distal portion of a fracture...

X-ray showing the distal portion of a fractured tibia and intramedular nail.

Bone is basically a simple lever, but it is also a spring. Bone needs to be both flexible and stiff to withstand normal loading.It is when the forces on bone are too strong that we get a fracture. This can be as a result of an unexpected stress on bone that is greater than the force that it is constructed to withstand.

Or it can be as a result of weakening of the bone through disease, infection or aging. Diseases of the bone, such as cancer or infection, can prematurely age or weaken bone.

Bone Function and Composition
Bone is flexible so it can absorb energy through deformation without cracking. It is stiff to allow it to resist bending without cracking.
The ability to do this depends on the unique material composition of bone and its three dimensional conformation.
Different bones have different function and position in the body and the material composition as a consequence varies.
Long bones are levers that are light and rigid with the cortex replaced by a marrow cavity. Vertebral bones are springs with a honeycomb structure.
Abnormalities in bone mineral density make bones fragile. Some patients have a fracture because their bone mineral density is too low. They break a bone because it bends too much. This is what happens when they have osteoporosis, which literally means porous bone.

What is bone mineral density (BMD)?
BMD is a measure of the density of bone or the bony material in bone. There are a number of methods used to measure bone density and the most common is a DEXA scan (dual energy x-ray absorptiometry). BMD is usually given as a T-score, Z-score or in grams per cubic centimetre. The T-score is the most common score used when assessing for osteoporosis and risk of a fracture. A T-score of -1 is defined as normal and of -2.5 or more is defined as at heightened risk of an osteoporotic fracture.

Aging of Bone
Change in bone structure make them more fragile as we get older. This makes us more prone to fractures. As we age the load on the bone decreases because our muscle mass decreases, but the area of bone built to tolerate that load also decreases. Therefore, the stress on the bone increases.
Bone turnover is an essential process. The balance between bone modelling and remodelling is what maintains healthy bone, but our bone balance goes awry as we get older. There is a definite tilt of the scales towards less production of bone and more resorption of bone. This means increased bone loss and more fragility.

Women at Risk
Women are at greater risk of a bone fracture than men as they lose bone density faster than men as they grow older.
It is after the menopause when endocortical resorption increases and there is very little rebuilding of bone that women have greater loss of bone. As a consequence the bone starts to become compromised in terms of strength. This is why postmenopausal women are particularly vulnerable to an osteoporotic fracture.

Collagen and Fractures
Others have a fracture because their bone is too stiff or dense. Collagen which makes bones flexible decreases with age and reduced collagen in bone has been implicated in this process. Some experts suggest this is the reason why BMD measurements are not a good predictor of fracture risk while others maintain that dense bones are a less common risk for a fracture than the typical osteoporotic risk. For them the poor predictivity of BMD measurement is that osteopenia, a BMD measurement of -1 to -2.5 should be considered as a measure of being at increased risk of fracture.



  1. Bone & Bowel « Life is Mysterious - February 15, 2013

    […] Why do Bones Fracture? (conorcaffrey.wordpress.com) […]

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