What is bone density, and why is it important?
Our bones are strong, and are able to resist various mechanical loads that we place on them during our activities of daily living. Bone tissue is composed two key materials: a meshwork of protein collagen and that is interspersed with the mineral, calcium hydroxyapatite. Collagen gives our bones the ability to resist bending or stretching. The mineral helps our bone resist compression. The strength of our bones depends on the amount of collagen and mineral present. Reduced amounts of either can increase the risk of a fracture. Seemingly minor accidents or falls may result in fractures that can be potentially life-threatening. Your doctor can evaluate your bone health by ordering a bone density test, and use this information to help manage your bone health through proper nutrition, exercise, and medication if necessary. But how is bone density measured? And, what is a T- or Z-score?
How is bone density measured?
Bone density tests are an indispensable tool for evaluating bone health. The most common bone density test is a DEXA (pronounced ‘DECK-sa’) scan, which uses x-rays to determine how much bone mineral is present. The test will be performed by a certified clinical densitometrist. The test is non-invasive and completely painless and takes about 15 minutes. During the test, involves laying on a padded table with your knees slightly bent or propped on support. During the test, DXA machine collects x-ray images of your skeleton. Some scanners can also measure the amount of lean and fat tissue present, and certain structural parameters that may predict bone strength. The densitometrist uses these x-ray images to measure the amount of bone mineral in several regions. The most common locations for measurement are the lumbar spine (lower back) and hip. For some patients, such as those who have had prior spine surgery or hip replacement, these sites may not be ideal for measurement, so the densitometrist can make measurement at the wrist or heel. The densitometrist will then record your bone density as the amount of bone mineral present. This information is used to calculate your ‘T-score’.
So…what does my T- (or Z-) score actually mean?
The World Health Organization (WHO) has assembled a massive data collection from adults (18+ years old) of all ages, ethnicities sex and economic status, and how these measurements predict fracture risk. From this data, the WHO has established a very comprehensive picture of what is normal and abnormal bone density. The T-score is a way to express how your bone density compares to a ‘young, normal’ person of the same sex. The Z-score expresses how you bone density compares to others of your age, sex and ethnicity. Your doctor will decide which score is most appropriate to use to make help you manage your bone health and prevent fragility fractures. For both scoring systems, a score of 0 is the average. Scores greater than 0 indicate higher bone density, and scores 0 indicate lower bone density. A T- or Z- score ranging from -1 to +1 is completely normal, and higher scores are generally indicate lower risk of fracture. In contrast, score less than -1 can indicate ‘osteopenia’ (low bone density). This means that you may be at increased risk for fracture, and will may alert your doctor to closely watchyour bone health and perhaps order some blood tests to gather more information. A score of less than -2.5 indicates osteoporosis and high risk for fracture, and would prompt your doctor to order some blood tests that will help decide which medications might be best to prevent further bone loss and/or increase new bone formation.
I know my score - Now what do I need to do to keep my bones healthy?
In our upcoming next Bone blog posts we will explore how diet, exercise and lifestyle affects our skeletal health, and identify some easy ways you can improve your bone health naturally. Of course, you should always consult with your doctor before making any major lifestyle changes.
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