All About Bone Density

Introduction

Bone densitometry is a noninvasive technology that is used to measure bone mass. Bone mass, simply put, is the weight of the skeleton, overall or in specific regions. Bone mineral density, or BMD, reveals a risk factor for fractures. BMD is usually expressed as the amount of mineralized tissue in the area scanned (g/cm2); with some technologies it is expressed as the amount per volume of bone (g/cm3). A bone density more than 2 standard deviations below the reference population indicates osteoporosis.

In the United States, osteoporosis affects over 25 million people, and is associated with more than 1.3 million fractures annually. The prevention, detection and treatment of osteoporosis will become an increasingly important medical concern for the next decade, as the American population ages in record numbers. Bone measurement tests are simple, painless and cost-effective, usually costing less than $200. Increased reimbursement for screening would have a direct impact on the the billions of dollars a year spent on direct medical costs for osteoporosis and related fractures.

Clinical Indications for Bone Densitometry
Currently, bone densitometry tests are the only accurate measurements of bone mass and fracture risk. Bone mass cannot be deduced accurately from any other clinical data, and risk factors for osteoporosis based on medical history or physical examination are not good predictors of bone mass.

Bone mass measurements also influence the choice of therapy. Bone mass measurements can influence clinical decisions in four key indications: to decide about hormone replacement therapy in estrogen deficient women, to diagnose spinal osteoporosis, to adjust therapy of patients on long-term glucocorticoids, and to decide about surgery in patients with primary hyperparathyroidism.

Bone mass measurements should lead to fracture reduction. Bone loss can be reduced by treatment, but it is difficult, if not impossible, to restore the biomechanical competence of the skeleton once bone has been lost because bone architecture has been disrupted. Thus, bone loss must be prevented, which needs to be done sooner rather than later in life.

Bone Mass Measurement Equipment

The most commonly used tool to diagnose low bone density is the DEXA (Dual Energy X- ray Absorptiometry) densitometer, a specialized x-ray device that precisely quantifies bone at the spine, femur and other skeletal sites. DEXA scans are non-invasive and comfortable for the patient, with very low radiation. The patient lies on the scanner bed while a small beam of radiation passes through the region of interest. Patient bone density is measured automatically. The entire examination for the spine and the femur requires about 2-5 minutes.

Dual-energy x-ray absorptiometry (DEXA)

DEXA, the gold standard in densitometry, uses one of two methods to create a dual-energy spectrum from an x-ray source. One method involves alternating pulses of low and high kV that are applied to the x-ray tube. The low- and high-energy spectra are then measured separately. The other method applies a constant potential to the x-ray source while using a K-edge filter to separate the energy spectrum into two narrow energy bands. An energy-discriminating detector with a dual-channel analyzer counts the resultant photons. The use of two energies allows bone mineral to be assessed independently of soft-tissue inhomogeneities. Bone mineral content (BMC) and bone mineral density (BMD) are calculated in g/cm and g/cm2, respectively.

DEXA scanners use either a pencil beam coupled to a single detector (first generation) or a fan beam coupled to a linear array of detectors (second generation). The pencil-beam scanner performs a two-dimensional raster scan, while the fan-beam scanner performs a single sweep across the patient.

Spine or hip?

Bone density measurements can be done at the hip (proximal femur), total body, spine, radius, and calcaneus. Recent research has focused on comparisons of the hip and spine measurements to maximize the practitioner's abililtiy to get the most accurate BMD measurement for each patient.

Spine BMD may be preferable in none rodded children. With most antiresporptive therapies there is a greater change at the spine than at the hip. The spine does have a higher percentage of trabecular bone that is more metabolically active.

With newer techniques the hip is about as reproducible as the spine, so the hip is now considered the best overall site to measure. The total hip has the best ability to predict hip fractures and can predict spine fractures as well as the spine density. Because both bisphosphonates and estrogens will increase bone density at the hip as well as the spine, this site can also be used to follow most patients.

Some investigators have argued that both spine and hip should be measured to achieve greater sensitivity in prediction of fractures. A large study which prospectively measured fracture incidence documented that this is not true. To increase sensitivity (to detect more patients who might get a hip fracture) one could measure just the hip and choose the higher threshold.

There are some clinical situations in which the spine and the hip would biologically be different, and in these infrequent cases measurements of both the spine and the hip are justified. The most common is hyperparathyroidism, both primary and secondary, in which the bone density at the hip is more affected than at the spine. This is because PTH tends to increase bone loss from cortical bone more than trabecular bone. In fact, PTH may increase trabecular bone mass while decreasing cortical bone mass. Fluoride also preferentially increases trabecular bone mass. Corticosteroids, on the other hand, may have a greater effect on the trabecular bone, but spine fractures may falsely increase the spinal bone density.

Dual energy x-ray absorptiometry scan of the proximal femur in a 37-year old woman with femoral-neck osteopenia (T-score = -1.6).

Dual energy x-ray absorptiometry scan of the lumbar spine in a 37-year old woman with lumbar spine osteopenia (T = -1.8)