Bone Structure and Strength Reviewed
In order for researchers to develop therapies to prevent or treat bone fractures, it's important for them to understand what makes up a bone and why. In this article, the author discusses the make-up of different types of bones and why they act as they do.
The numerous bones in the human body have different roles. While some are for support, others are for weight-bearing. Yet others, like the bones in our ears, are used to transmit sounds. Each role requires a different type of bone. The bones also have to accommodate certain requirements. They must be strong enough not to break unless under extreme force, yet they must be able to absorb impacts and to resist becoming deformed.
To be strong enough for its role, the bone material stiffness and flexibility is determined by its mineral content. For example, the bones in the ear are much more mineralized than the bones in your hips. Some bones are tubular, while others are flat and formed. According to what researchers know about bones, the larger bones have a lower bone mineral density (BMD) than do the smaller ones, and the longer bones are more resistant to bending because of their composition. These longer bones are, therefore, stiffer than others.
The bone growth in females generally stops earlier than in males, but the bone composition still seems to be the same. However, researchers have found that there is a difference in one of the layers of the bone called the cortex.
As the bones grow, they adapt to their assigned job and their shapes are defined by the loads that they have to bear. For example, the bone shape at the thigh. This area bears a lot of weight and it is involved in intensive bending, so the shape of the bone is oval and the cortex layer is thick inside the bone, rather than outside. As you move down the femur (the thigh bone), the shape and make up of the bone changes to adapt to its role of just weight-bearing. Another example is in the spine. These bones, the vertebrae don't play the same weight-bearing and bending role as does the femur at the thigh level, but the act more as shock absorbers. The vertebrae of men are generally wider than in women, and in some races than in others. Of course, as much as the bones adapt to their job in the body, genetics also play a role.
Since researchers have a good idea of how the bones are formed and why, the focus now comes on how to keep the bones strong or how to make them stronger as we get older. The author says, "The emergence of bone fragility during aging may be regarded as the net result of accumulating abnormalities caused by disease, hormonal deficiency, and excess exposure to risk factors." He also says that "bone fragility can be viewed as a disorder of adaptation."
Researchers have found that the first sign of a change in bone structure is the decrease in the way the bone is reforming and rebuilding itself. This is seen a lot in women who have gone through menopause, but it has also been seen to begin in some form as early as age 19 years. As the bone formation after growth has complete, the need for constant regeneration is not as strong, so it slows down. As bone loss begins, this too is different between men and women. In men, the loss is more in the actual mass and the bones thin out. In women, the loss is in the connectivity of the strands of bone. Many women end up with fractures in their vertebrae, as opposed to men, and this can be due to the vertebrae seem to feel the loss of this connectivity a lot more than the other bones in the body.
Another difference between male and female skeletons is the larger skeleton itself. The larger male skeleton is able to handle a larger load than the smaller female skeleton. As the women age, their bones become more fragile and this can lead to fragility fractures, something not often seen in men.
The authors concludes by pointing out that while bones need to be stiff enough to do their job, they also must be flexible enough to bend a bit and absorb shocks. When there is less mineral content in the bone due to aging or bone loss, these abilities are decreased. There is still much research needed to clearly understand the process and the differences between different races.
Ego Seeman. Bone's material and structural strength. In Current Opinion in Orthopaedics. September 2007. Vol. 18. No. 5. Pp. 494-498.