The stronger your bones, the greater their ability to withstand fracture. Given the importance of bone strength, what is the best way to measure it? What are the major determinants of bone strength? What is recent research telling us about what you can do to improve the strength of your bones?
For many years, the density of your bones, as measured by a Bone Mineral Density (BMD) test or Dual Energy X-ray Absorptiometry (DXA), has been assigned a significant amount of weight by medical professionals when they diagnose the health of your bones and determine whether you have osteoporosis or not. The diagnosis is frequently used to estimate your risk of fracture, select your best treatment options and determine whether you should take medication.
However, recent research is indicating that the DXA test is not as reliable a measure of bone strength as originally thought.(1)
Shortcomings of the Bone Mineral Density (BMD) Test
In fact, several studies have shown that many fractures occur in people with only moderately decreased Bone Mineral Density. (2,3) Further, half of all postmenopausal fractures occur in women with Bone Mineral Density test results that indicate that their bone density levels are not low enough for them to be categorized as having osteoporosis.(4)
In addition, the results of the BMD test can determine whether you end up on medication or not. The most popular osteoporosis medication, known as bisphosphonates, is frequently prescribed to increase the density of bone. These medications increase bone density but do not necessarily improve the quality of bone.
Recent medical research has shown that the quality of your bone structure is more important than the density of your bones when determining the strength of your bones. The BMD test or DXA does not provide information on the quality of your bone, and it does not provide any insight into the architectural structure of your bones.
Bone Quality: Definitions and Discussion
Before I get too far into the discussion of bone quality, I need to provide some definitions of bone tissue. Bone tissue (or osseous tissue) refers to the bone. There are two types of bone tissue: cortical bone and cancellous bone. An individual bone can be composed of both cortical and cancellous bone, and the relative mix of the two types does vary from bone to bone in your body.
Cortical bone (sometimes referred to as compact bone) forms the cortex or outer shell of most bones. It is much denser, rigid and stronger than the other type of bone tissue, cancellous bone.
Cancellous bone (also referred to as trabecular or spongy bone) appears at the end of long bones, at the points of attachment of joints and within the interior of the vertebrae. It has a greater surface area and is more porous, softer, weaker, less dense and less stiff than cortical bone. As a result, cancellous bone is more affected by osteoporosis than cortical bone.
Vertebral Body
Because the vertebral area is largely composed of cancellous bone, it is at a greater risk of osteoporotic fracture when compared to other parts of the body. The image to the right shows a cross section of the vertebral body. The vertebral body is the portion of the spinal column facing into the body. Note the porous structure of the bone where the black indicates empty space and the light color indicates cancellous bone.
Cancellous bone is composed of many individual and interconnected trabecula. Trabecula is a small, microscopic tissue element. They look like beams (“trabecula” is Latin for “small beam”) and are often in the forms of small rods or plates. The plural of trabecula is trabeculae.
Bone Quality
When we talk about bone quality, we are specifically referring to the quality of the cancellous or trabecular bone. Bone quality is defined as the sum of the structural and material properties of the trabecular bone.
Trabecular Bone
The structural properties of bone refer to the structure of the trabecular bone. I mentioned earlier that the trabecular or cancellous bone is made up of many interconnected trabecula (trabeculae) and that they are either in the shape of a rod or a plate. The illustration to the right shows a microscopic view of a trabecular bone. Note the flatter, plate-like trabecula and the more narrow, rod-like trabecula in the image.
The more of the thicker plate-like trabeculae present in the structure as compared to the thinner, rod-like trabeculae, the better the quality of the bone. The better the quality, the stronger the bone.
36 Year Old Female
74 Year Old Female
Trabecular bone that has seen its plate-like trabecula replaced by rod-like trabecula is weaker, osteoporotic and more at risk of a fracture. This change can occur over time and is dependent upon the individual’s family history, and exercise and nutritional history. The illustrations to the right show the trabecular structure of bone from a 74 year old female and the trabecular bone structure from a 36 year old female. Note the higher frequency of plate-like trabecula in the bone structure of the 36 year old when compared to the bone structure of the 74 year old female. In addition, the 74 year old female has more rod-like structures and more empty space.
The material properties refer to substances or matter such as collagen and minerals that make up the bone.
Measuring Bone Quality
High Resolution peripheral Quantitative Computed Tomography (HR-pQCT) provides high resolution, three-dimensional monitoring of the architectural structure of bone. Because it is able to provide a microscopic view it can be used to determine the quality of the bone.
However, this technology is much more expensive than the BMD test or DXA test. In addition, HR-pQCT examination exposes you to more radiation compared to a DXA and the machines are not as readily available as DXA machines. As a result, the use of HR-pQCT is usually reserved for special cases and research studies.
Given that the measurement of bone quality is largely inaccessible to the majority of people, should we leave bone quality aside and rely upon the BMD as a compass for bone strength? I believe that the answer to that question is “no”. New research is showing that the quality of your bone structure can greatly influence whether you sustain a fracture or not. We also have determined that there are activities we can pursue that will improve the quality of the bone.
Bone Quality Matters
For example, a study (5) presented at the 2010 American Society of Bone Mineral Research (ASBMR) meeting in Toronto explored the differences of the trabecular bone in postmenopausal women who had fragility fractures with those of women in the same age group who did not sustain a fragility fracture.
Researchers compared 101 control subjects who had not fractured to 68 women who had sustained fractures from a fall or slip from a standing height or lower position. The research team discovered that the DXA scores of both groups were similar. However, the quality of the bone in the fracture group was much less than that of the group that did not fracture.
Another study suggests that people who are more active through exercise have better bone quality than people who are inactive: a review of data from over 6000 women of mixed ethnicity between the ages of 50 and 79 years found that the most active women in the group had geometrically stronger femurs, or thigh bones. The bones had more resistance to bending.
Conclusions
High resolution computer imaging technology has helped researchers learn more about bone strength, the importance of bone quality and the limitations of solely relying on bone density to determine bone strength. From these developments, we can conclude the following:
- Try not to get too preoccupied with your bone density and your DXA score. It is one variable in the overall equation in determining your bone strength. But it is not the be all and end all.
- The quality of your bone structure is very important in determining the strength of your bones and their ability to resist fracture.
- It appears that people who are active through exercise have better quality bone structure and that you can influence the quality of your bone through regular and appropriate exercise for your body.
Bottom line: bone quality matters, incorporate physical activity into your bone health regimen and consult with a qualified professional knowledgeable in movement and bone health.
References
- Beck TJ et al. Confounders in the Association between Exercise and Femur Bone in Postmenopausal Women. Med & Sc in Sports & Ex. Post acceptance May 2010
- McDonnell et al. Ann Biomed Eng 2007
- Delmas PD et al. J Bone Miner Res 2005
- Schuit SC et al Bone 2004
- Liu XS et al. Fewer Trabecular Plates and Decreased Connectivity Between Plates and Rods is associated with Reduced Bone Stiffness in Postmenopausal Women with Fragility Fractures. Columbia Univ. NY. Poster presentation at the 2010 ASBMR Toronto.
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