Clinical Review

The Potential Value of Dual-Energy X-Ray Absorptiometry in Orthopedics

Publish date: May 16, 2018

TAKE-HOME POINTS

DXA is underutilized technology in orthopedics.
More data ("ancillary data") are often available from a DXA scan then typically included in a standard report from a referral center.
Most orthopedists are likely unaware of the detailed body composition data available with a total body scan.
Preoperative DXA scans and knowledge of BMD may be informative when planning the type of fixation and implant metal to used.
Serial follow-up body composition scans can be useful in monitoring the course of bone healing (mineralization) and soft tissue changes (fat and lean mass).

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ABSTRACT

Dual-energy X-ray absorptiometry (DXA) is a well-established technology with an important and well-known role in measuring bone mineral density (BMD) for the purpose of determining fracture risk, diagnosing osteoporosis, and monitoring treatment efficacy. However, aside from the assessment of bone status, DXA is likely underutilized in the field of orthopedics, and most orthopedists may not be aware of the full capabilities of DXA, particularly with regard to total body scans and body composition assessment. For example, DXA would be a valuable tool for monitoring body composition after surgery where compensatory changes in the affected limb may lead to right-left asymmetry (eg, tracking lean mass change after knee surgery), rehabilitation regimens for athletes, congenital and metabolic disorders that affect the musculoskeletal system, or monitoring sarcopenia and frailty in the elderly. Furthermore, preoperative and postoperative regional scans can track BMD changes during healing or alert surgeons to impending problems such as loss of periprosthetic bone, which could lead to implant failure. This article discusses the capabilities of DXA and how this technology could be better used to the advantage of the attending orthopedist.

Dual-energy X-ray absorptiometry, abbreviated as “DXA,” (although usually abbreviated in older literature as “DEXA”) was first introduced in 1987 (Hologic QDR-1000 system, Hologic, Inc) and immediately made all previous forms of radiation-based bone mineral density (BMD) measurement systems obsolete.¹ Since then, there have been many generations of the technology, with the main US manufacturers in 2017 being Hologic, Inc. and GE Lunar. There are 2 forms of DXA, peripheral systems (which usually measure BMD only in the radius, finger bones, or calcaneus) and central systems (which measure the radius, proximal femur [“hip”], lumbar spine, total body, and custom sites). The general principle of how DXA works is based on the differential attenuation of photons by bone, fat, and lean mass.² The DXA technique uses a low- and high-energy X-ray beam produced by an X-ray tube. With the low-energy beam, attenuation by bone is greater than attenuation by soft tissue. With the high-energy beam, attenuation by bone and soft tissues are similar. The dual X-ray beams are passed through the body regions being scanned (usually posterioanteriorly), and the differential attenuation by bone and soft tissue is analyzed to produce BMD estimates. In addition, a high-quality image is produced to enable the operator of the DXA system to verify that the appropriate body region was scanned. It is important to realize that DXA is 2-dimensional (which is sometimes cited as a weakness of DXA), and the units of BMD are grams of mineral per centimeter squared (g/cm²).

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