Measuring Bone Health

The ultimate goal of maintaining bone health is to prevent fractures.  We use a test called a bone density to help assess fracture risk.  It is a rather simple test to have done.  It requires no needles and is not painful.  It is similar to taking an x-ray but instead of taking a picture it takes a measurement.  The measurements are averaged and compared to the bones of young healthy adults.  The comparison is quantified mathematically in the form of a logarithm, called a T score.  Now before you roll your eyes and stop reading this, all you need to know is that the further you get away from zero, the worse you get.  Zero is the normal for that young healthy adult.  Osteoporosis (brittle bones) is labeled at -2.5 and osteopenia begins at -1.0.  Since it is logarithmic, -2.0 doesn't mean it is twice as bad, it means it is twenty times as bad and -3.0 is three hundred times as bad.

Ideally a bone density scan should be done on the same machine by the same technician.  There is significant variation between different types of machines.  The least accurate measurements are from the ones that look at the heel or the wrist.  Those should only be done as a quick screen and if abnormal should prompt a full evaluation with the standard scan done on the hip and spine.  The heal and wrist scans should not be used as a determinant to guide therapy.  Our bones are not uniformly thick, therefore variations in where the beam is directed can results in different values.  The technicians are trained to perform the test in the same area but if it is possible, having the same technician will lead to the most precise results.


One condition that will affect the accuracy of the test is arthritis of the spine.  In this condition, there is a build up of calcium deposits on the sides of the bone which will make the bone appear to be thicker and stronger than it really is.  In those patients who have a significant amount of arthritis, the scan should only be done on the hip.  Arthritis of the hip does not have the same problem because the measurement is not taken through the joint where the build up occurs.

It is important to understand that the number on the test is not the only determinant of your fracture risk.  A person with a nontraumatic hip fracture may have a T score of -2.2.  That would put them in the thin but not brittle group.  However, this person has already had a fracture so the future risk of additional fractures is going to be much higher.  Regardless of the T score, this person has osteoporosis.  Someone else with the same score and no history of fracture would be at a much lower risk.  Other risk factors should be considered such as whether a person is a woman who has gone through menopause, one's level of activity, whether or not a person is a smoker, one's family history is important and body size and composition.  A tool to add risk factors to one's T score to more accurately measure fracture risk is called the FRAX calculator.  This can be easily computed on line.  A score of 3% or higher is considered to be high risk and would warrant treatment.

Who should be screened for osteoporosis and how frequently does a bone density need to be repeated?  Men without risk factors are typically not screened.  Women should be screened for a baseline at menopause, repeating the test every two to five years depending on the score and their overall risk.  Those at higher risk due to medical conditions, certain medications, unprovoked fractures, etc should be screened earlier.  If in doubt, discuss this with your physician.  You should know your risk.  If you are at an above average risk for fracture, taking care of your bones now can prevent horribly painful conditions later when it will be too late to build back your bones.