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Assessing the Health of Our Bones - Part I: Bone Density/Quantity

Updated: May 8

By Rachel Ames, MPT and Owner of Missoula OsteoStrong

In celebration of Osteoporosis Awareness month, below is "Part I: Bone Density/Quantity" our four-part educational series on "Assessing the Health of our Bones". Enjoy the read and watch for "Part II: Bone Quality and Bone Health Measurement Tools" next week!

DXA Scan Explained

How DXA Works

DXA is currently considered the “gold standard” for diagnosing osteoporosis. A DXA (Dual X-ray Absorptiometry) scan is an x-ray used to measure the bone density usually in two main areas of weight-bearing in the body - the lumbar spine & the hips. As the arm of the DXA machine passes over the body,  it uses two different x-ray beams which account for three types of tissue: bone mineral, lean tissue, adipose tissue and more.  These two beams have different energies and the difference between the two is what is being measured to calculate BMD (bone mineral density). The scanner translates the bone density measurement data into pictures and graphs. Bone is most easily seen in white, while the fat and muscle tissue look like shadows in the background on the technologist’s computer monitor. Clogged arteries, osteoarthritis, artifacts in the images, bone spurs, scoliosis issues and other factors previously mentioned need to be taken out of the equation.  If this is not performed, mathematical errors occur leading to discrepancies between yearly measurements.  This is why intrarater and interrater reliability can be poor with DXA.

The DXA scan is used to assess your bone density and compare it in relation to normal (or that of a normal healthy 30 year old). This comparison is converted into what is called a T-score.

T-Score 101

While many people are familiar with a T-score, few actually understand what this "score" means. A T-score is used to diagnose either normal bone density, low bone density called Osteopenia, or Osteoporosis. That diagnosis is used to determine the necessity of intervention or treatment. Because the average person will lose 1-3%+ of their bone density per year with no intervention, DXA scans are generally repeated every 18 months to 2 years.*

Refer to the graph to help explain how to interpret your T-score. This number line representing the span of all possible T-scores is just like a Stop Light – Green, Yellow, Red!*

  • Green (-1 and above) = Good To Go! Normal…Time for Prevention!

  • Yellow (-1 to -2.5) = Warning! Low Bone Density…Time for Action! Osteopenia

  • Red (-2.5 and below) = Stop! Osteoporosis & Time for Serious Intervention!

What do the T-Score Numbers Mean?

The easiest way to think about a T-score is to envision that number very roughly as correlating to a percentage of bone lost when comparing to an average 30 year old (remember Peak Bone Mass).

So, for example...

Your T-score is 0, meaning you have no bone loss and normal bone density

Your T-score is +1 meaning 10% better bone density than an average 30 year old

Green and Time for Prevention!

Your T-score is -1 meaning 10% loss in bone density

Your T-score is -2.3 meaning 23% loss in bone density

Yellow - Low Bone Density - Time for Action!

Your T-score is -2.5 meaning 25% loss in bone density

Your T-score is -3.3 meaning 33% loss in bone density

Red - Osteoporosis - Time for Serious Intervention!*

T-Score...Why Does It Matter?

T-scores are used to diagnose osteoporosis, osteopenia and ultimately your fracture risk in the weight-bearing bones – the spine & hips. The less dense your bones are, the more fragile your bones are, and in theory the more prone to fracture. Fractures in the spine or hip lead to a major decrease in quality of life and a 25% mortality rate within the 1st year of those who have had it surgically repaired and 70% for those without surgical intervention.  Knowing your fracture risk is essential in determining intervention and treatment approaches.* 

What is a FRAX Score?

The DXA provides us with a FRAX (Fracture Risk Assessment Tool) to predict the risk of fractures in a person with osteoporosis within the next 10 years. The tool consists of questions relating to 12 external factors, your BMD and BMI. The output is a percentage, and higher values indicate a greater risk of fracture.

However, keep in mind that bone density is not the entirety of the bone health equation.  

BONE STRENGTH = BONE DENSITY/QUANTITY + BONE QUALITY . Fracture prevention is about total bone strength and is more than just bone density. Bone quality has to be considered as well.*

The good news is that you are addressing BOTH bone density AND quality at OsteoStrong!  Here at OsteoStrong we build NEW bone because osteogenic loading naturally influences osteoblasts to build new bone while bisphosphonates simply keep old bone from breaking down/limiting osteoclasts resulting in lower bone quality.  

We hope that you have learned a little more about bone quantity/density and the DXA scan in this article.  While DXA is currently considered the “gold standard” for diagnosing osteoporosis, new and improved testing is now available which we will cover next week in conjunction with understanding bone quality.

Your Questions Answered:

Here are the answers to a few questions that were submitted by our members last week!

Question 1:  What are the femoral neck scores on the DXA?  Where is that on my body?

Answer 1:  The DXA scan normally only analyzes the lumbar spine from L1-L4 (not the cervical spine which is easily confused since the report states neck) and the total hip (includes femoral neck, trochanteric and intertrochanteric regions as well as parts of the pelvis).  The femoral neck is also reported as a number independent of the total hip since it is most fragile for fractures if the bone density is low.  It is not always the same hip so be sure to clarify that at your follow up exams.  See the image for clarification of anatomy.

Question 2:  I’ve had a DXA year after year from the same place but when I moved here, my report stated I changed from severe osteoporosis to osteopenia.  Is that correct?

Answer 2:  This is why DXA, though the “gold standard”, is not so golden.  One most important factor is that the same machine needs to be used for optimal results.  The same technician is also important; however each technician can have variations from person to person even on the same machine.  If they do not analyze what or exactly where is being evaluated the same way as compared to the previous test, the scores can be skewed.  Also, if they do not remove parts that should not be included (ie. clogged arteries, osteoarthritis, artifacts in the images, bone spurs, scoliosis issues, etc), improperly positioning you on the machine or recent intake of calcium can also affect the results. Look for Part II of this four-part series for more information on Bone Health Testing Options.

*content reference: Dr. Rebecca (Becky) Bear, DC - OsteoStrong Sioux Falls



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