BMD - Bone Mineral Density - A measure of bone health that compares your bone density, or mass, to that of a healthy person who is the same age and sex as you are. First, it is the actual measure of the unit mass of your bone per unit of area and it is written as g/cm2. BMD is what is being determined during a bone densitometry examination. Therefore, BMD is considered an “areal” measurement (derived from “area” since it comes from a flat surface area measurement and not a volumetric measurement). Second, BMD is used to then determine the measurements known as the T-score and the Z-score.
Bone densitometry – the process of measuring the bone density (BMD) utilizing any of the World Health Organization (WHO) standardized methods which include DXA and REMS.
Bone Quality – is a term encompassing microscopic structural properties of bone that include bone microarchitecture, the presence of micro fracturing and collagen crosslinking. At this time bone quality is believed to be one of the factors that determines bone strength (stiffness) which is the ability of a bone to withstand an applied force and not fracture. The strength (stiffness) of a bone is what actually determines if a bone fractures and therefore any measure that provides additional information about the strength of a bone will also help to better predict the fracture risk.
Bone Strength (Stiffness) – is the measured ability of a bone to withstand applied forces and not fail (fracture). Bone strength is determined by two properties of bone – BMD and Bone Quality.
DXA - Dual Energy X-ray Absorptiometry - commonly used method of determining bone density using two x-ray beams of relatively low and differing energy levels to assess the capacity of the bone to attenuate the beams.
FS - Fragility Score - a measure of bone quality and it is a number that is determined along with the BMD during a REMS study. Currently, the FS has the highest level of sensitivity and specificity for fracture risk prediction when compared to DXA-derived or REMS-derived BMD values. It appears to have the best predictive value for fracture risk.
LSC - Least Significant Change - is the error margin of a particular densitometry unit. An appropriately calibrated DXA machine has an LSC of 3% to 5% - it has an error rate up to 5% for any study done. Any changes in a sequential series of DXA-derived BMD values that are less than 5% cannot be considered statistically significant and therefore cannot be used for treatment assessment and/or treatment recommendations. In other words, you would have to have a greater than 5% bone loss between two serial DXA studies in order for that change to be meaningful. The LSC is main reason that DXA scans are indicated every two years - it is statistically more likely that you will lose 5% of your bone mass over two years vs one year. REMS has an LSC of 0.5% to 1.05% and therefore REMS studies, if medically indicated to monitor the effectiveness of an osteoporosis medication, can be done yearly or even every 6 months.
Normal - is the state of bone with good density that was determined by bone densitometry and defined by a T-score of 0 (zero) SD to (–) 1.0 SD.
Osteopenia - is the state low bone density that was determined by bone densitometry and defined as a T-score of (–) 1.1 SD to (–) 2.4 SD.
Osteoporosis - is the state of very low bone density that was determined by bone densitometry and defined as a T-score of ≤ (–) 2.5 SD (standard deviation).
QUS - Quantitative Ultrasound
REMS - Radiofrequency Echographic Multi Spectrometry - an ultrasound-based method of assessing bone density and quality.
ROI - Region of Interest - are the areas of the body that has been determined by the WHO that are scanned during bone densitometry to determine a clinically significant BMD measurement. Both REMS and DXA have the same axial ROIs - vertebral bodies L1 through L4 in the lumbar spine and both hips that include the femoral neck and total hip. DXA has an additional ROI - the distal 1/3 of the non-dominant forearm which can be useful in certain clinical cases.
T-score - is a statistically derived number that also serves two purposes. First, it gives a numerical representation of the comparison of an individual's BMD measurement to that of a reference BMD (the mean BMD value of a large database of 30-year-old Caucasian females is the benchmark for all comparisons). To determine how we compare to that database, we use the concept of standard deviations which represents a certain percent value away from the established average value as determined on a bell curve (at a certain age if we have lost a certain amount of bone as compared to a 30-year-old then we can say that we are a “negative one standard deviation away” or we have a T-score of -1.0; if our BMD matches the mean BMD then the T-score would be 0). The T-score is also used to determine a diagnosis. The WHO has set parameters that a T-score value of 0 to -1.0 would be considered normal bone density, that a T-score value that ranged from -1.1 through -2.4 would constitute “low bone density (or osteopenia) and that a T-score of -2.5 or lower (larger negative number) establishes the diagnosis of osteoporosis.
TBS - Trabecular Bone Score - a method of determining a measure of bone quality of using otherwise unprocessed data obtained during a spinal DXA scan to evaluate bone quality. It is a significant addition to a DXA scan and it obviously has clinical relevance in helping Bone Healthcare providers make better treatment recommendations. Not all DXA units are TBS capable – it is a software add-on. Also, the quality of the TBS is dependent on the quality of the index spine DXA scan.
VFA - Vertebral Fracture Analysis - a spine assessment to evaluate for or to monitor spinal compression fractures. A VFA is performed on a DXA unit (if the DXA has the appropriate software) and it will show the entire spine on a single image. VFA (or lateral x-rays of the thoracic and lumbar spines) are indicated for screening once a woman reaches 70 years old and a man 80 years old. Also, it is indicated for sudden onset of backpain following a traumatic event, severe back pain and/or as a yearly screening tool for someone with a history of compression fractures or any kind of fragility fracture.
Z-score - is conceptually similar to a T-score except it is the method of comparing BMD value to the mean BMD of a 5-year bracketed age and gender specific cohort database (5-year interval). A Z-score would be zero (0) if it falls exactly on the mean (average) BMD value of individuals in the age and gender specific cohort and will be measured in SDs along a bell-curve. The Z-score has no clinical relevance between the ages of 40 and 90 years old. Outside of the 40–90-year-old age bracket, the Z-score establishes a diagnosis – the benchmark for osteoporosis is a Z-score of -2.0 SDs or lower.
