DEXA Use in Sports Medicine

Position Statement – DEXA Use in Sports Medicine

DEXA (Dual Energy Xray Absorptiometry) has been available for over 25 years and it is now an established diagnostic tool for measuring bone mineral density (BMD) and body composition. It therefore has a role in both Sports Medicine and Sports Science.

DEXA uses a thin beam of xrays which exposes the recipient to a very low radiation dose. There are two different xray beam methods. The pencil beam system gives a radiation dose of less than 1 µSv for a standard DEXA (Spine and Hip). The fan beam system gives a radiation dose, for a similar test, between 1 – 10 µSv. (By comparison the dose for a chest xray is between 20 – 50 µSv).  The dose for a whole body scan can vary between about 10 – 60 µSv.

It is a painless procedure with the subject lying face-up on a cushioned table while the scanner passes above. It usually takes between 5 and 20 minutes.

DEXA provides a very reproducible measurement with a coefficient of variation of 1 – 3%.

Due to the low radiation dosage, repeat measurements on the same patient are not usually a problem. However, the  smallest detectable difference in bone density means that there should be a gap of at least one year between scans, ideally longer. The time difference may be less for body composition measurements, but this has not yet been established.

By providing an accurate measure of bone mineral density, typically in the lumbar spine and hip, it can be used to  assess bone health and in the diagnosis and management of osteoporosis.  This can be helpful in patients with low body weight, the female athlete triad, bone stress injuries and stress fractures. Changes are easier to identify in trabecular rather than cortical bone.

The BMD can be recorded using the T- and Z-scores. The T-score is the best measure for a fully skeletally mature adult, i.e. from about 30 years onwards, whereas the Z-score should be used in a younger population:

i. The T-score measures the BMD relative to the adult mean value and then documents it in terms of Standard Deviations (SD) above or below that mean value, i.e. (measured BMD – young adult mean BMD)/ young adult population SD.

ii. The Z-score measures the BMD relative to an age matched mean value and then documents it in terms of Standard Deviations above or below that mean value, i.e. (measured BMD – aged matched mean BMD)/ age-matched population SD.

iii. For an adult a T-score of -1.0 to -2.5 is regarded as osteopenia and a score of less than -2.5 as osteoporosis.

iv. For a younger person a Z-score of less than -2.0 is regarded as abnormal. Some argue that a Z-score of less than -1.0 should be regarded as low for an athlete.

v. DEXA scanning is less reliable as a measure of BMD in children and gradually becomes more reliable during the teenage years.

DEXA can also provide a good estimate of body composition and is better than most other techniques as a measure of body fat. It can therefore be used as a tool to assess patients with eating disorders and other nutritional problems.

It can also be used in the assessment of training and performance. There is an inverse correlation between distance running performance and body fat. There is a correlation between muscle mass and BMD but not muscle mass and muscle strength.

Lateral spine DEXA (compared to the AP view that is routinely used) can be used to identify structural abnormalities of the vertebrae such as crush fractures.

Whole body DEXA can act as a screening test for abnormalities of the skeleton as it can sometimes identify regional abnormalities of bone.

DEXA is now a well validated tool able to provide information regarding BMD and body composition. This information can be used to help provide an assessment of the athlete’s bone health and their response to training.

©Faculty of Sport and Exercise Medicine UK – DEXA Use in Sports Medicine May 2015, to be reviewed May 2018.

Authors: Dr Roger Wolman MD (Res) FRCP FFSEM Consultant in SEM and Rheumatology

References:

Blake GM, Fogelman I. The role of DXA bone density scans in the diagnosis and treatment of osteoporosis.Postgrad Med J 2007; 83:509–517.

Mattila VMTallroth K et al.  Physical fitness and performance. Body composition by DEXA and its association with physical fitness in 140 conscripts. Medicine and Science in Sports and Exercise. 2007; 39(12):2242-2247.

El Maghraoui A, Roux C. Review: DXA scanning in clinical practice. Q J Med. 2008; 101:605–617.

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