Bone density

Techniques Bone density losses with aging T and Z scores Details about T and Z scores The WHO definitions Bone density and risk of fracture Discordance between spine and hip How to read a DEXA printout, step-by-step BMAD Size, shape and rotation Standardizing results Reproducibility Find the mistakes Clinical indications for BMD Ultrasound Fracture risk calculator

Techniques

Bone Density Loss with Aging

This graph shows how the bone density of the total hip decreases with age. The units are standardized bone density in (mg/cm²). The lines show the average values, and for each age, race and gender a range of values occurs in the ordinary population.

The bone density is important because it can help to predict the risk of getting a fracture. The fracture risk calculator can be used to predict fracture risk from the DEXA results.

Bone density results in teenagers can be found in this web site from Laura Bachrach at Stanford: teen normals

Here is a new REFERENCE RANGE for CHILDREN by Kalkwarf.

Bone density results have been reported in a variety of ways that have been confusing to physicians and patients. The next sections will attempt to clarify these issues, such as T-score, Z-score and the redefinition of "normal".

T and Z scores

T and Z scores are based on the statistical unit of the standard deviation. Shown here is the classical bell-shaped curve with the percent of a population lower than that value shown next to the curve.

The T-score is the number of standard deviations below the average for a young adult at peak bone density. There are different T-scores depending on which group of young adults were used as the reference (for example, Caucasian women, Hispanic men). The Z-score is the number of standard deviations below an average person of the same age. There are also different Z-scores depending on the group used as a reference (for example, the group could include everybody of the same age, or it could be limited to people with the same age, race, gender and weight). Furthermore, a person can have one T-score at the femoral neck, another at the total hip, and another at the spine.

Click to read more details about the relationships between T-score, Z-score and g/cm², including conversion formulas, data tables and some of the misconceptions about the interpretation of the scores.

The WHO definitions

The World Health Organization has defined the following categories based on bone density in white women: Normal bone: T-score better than -1 Osteopenia: T-score between -1 and -2.5 Osteoporosis: T-score less than -2.5 Established (severe) osteoporosis includes the presence of a non-traumatic fracture. The WHO committee did not have enough data to create definitions for men or other ethnic groups. One standard deviation is at the 16th percentile, so by definition 16% of young women have osteopenia! As shown below, by the time women reach age 80, very few are considered normal. It is important to realize that the T-score alone does not predict fractures, and osteopenia is not a disease.

Cutoff values for osteopenia using total Hip BMD T-score Hologic Lunar Norland Standard -1 .820 .886 .800 833 -2.5 .637 .706 .615 648

Here's another way of looking at it. If I took 100 ordinary women at each of three ages (35, 55 and 75 years) and lined them up according to their bone density, they would stand like this. The ones in purple would have a fragility fracture within the next ten years. (Kanis JA)