Osteoporosis : How To Detect

Dr Nachiket Kulkarni

Osteoporosis is a disease characterized by low bone mass and thinning and decreased strength of bone tissue, leading to enhanced bone fragility and consequent increase in fracture risk. It is a silent disease, reflected only in a low bone density, till a fracture occurs. Much in the manner that asymptomatic conditions such as hypertension and dyslipidaemia predispose to stroke and myocardial infarction, respectively, a low bone density (reflecting poor bone health) predisposes to osteoporotic fractures.
Measuring bone mineral density (BMD) is a tricky business. Prior to invention of modern gadgets osteoporosis was diagnosed based on occurrence of a fragility fracture. This did not give power to detect & prevent early low BMD and its complications. In order to meet this need many modalities were studied. The available ones are as described in the table 1.

Table 1: Methods of Measuring Bone Mineral Density

Modality	SubType	Test
Ionizing	Gamma radiation	Single-energy photon absorptiometry Dual-energy photon absorptiometry Neutron activation analysis Compton scattering
	Radiographs	Single-energy x-ray absorptiometry Dual-energy x-ray absorptiometry(DEXA) Quantitative computed tomography(QCT) Radiogrammetry
Nonionizing		Magnetic resonance imaging Spectroscopy Quantitative magnetic resonance imaging Ultrasonography(USG)

Of the above mentioned list only DEXA, QCT & USG have commercial availability. All others serve as tools in research.
The variations in interpersonal readings and to make a clinical relevant meaning of the findings of these tests WHO developed a definition for Osteoporosis based on BMD. For this purpose two scores were developed:

1. T Score: The T score compares one patient's BMD with the average peak BMD in healthy young persons of the same gender. It determines the fracture risk.
2. Z Score: The Z score compares one patient's BMD with the mean BMD for persons of the same age and gender. It is particularly useful in elderly patients who may be osteoporotic by the T score but are average for their age by the Z score.

WHO defines osteoporosis as T-score less than minus 2.5 Standard Deviation. These scores are now assessed by the above mentioned modalities to give a meaningful result for clinical decision making. We would now discuss the individual BMD measuring modality.

Dual-Energy X-ray Absorptiometry (DEXA)
DEXA is the most widely used technique for measuring BMD. The WHO criteria were established largely with DEXA in mind. Measurements can be obtained from any site in the body, but the standard being the lumbar spine, the proximal femur and the distal forearm. The high level of precision of this technique allows not only for diagnosis, but also for monitoring response to therapy. While doing a DEXA Scan no pre procedure preparation is required.
Candidate is expected to lie down on an examination table and special x ray sequences are taken. The radiation exposure though is very minimal as compared to usual X ray examination.

Quantitative Computed Tomography (QCT)
QCT measures the lumbar spine as well as peripheral sites. The results are less likely to be affected by degenerative spinal changes than DEXA scanning. Also, unlike DEXA, QCT allows for selective assessment of both cortical and trabecular bone. Trabecular bone, because of its higher rate of turnover shows metabolic changes earlier. Its ability to enable prediction of spinal fracture, however, is equal to that of DEXA scanning; the cost and level of radiation exposure are higher.

Ultrasonography
Bone USG can measure BMD and changes in trabecular or cortical architecture. Fracture risk discrimination by ultrasound examination is equivalent to that of DXA, especially for hip fracture. But variability is one of its major negative points. The following tables would show the comparison of these modalities:

Table 2: Comparison of Different Modalities for Assessing Bone Fracture

Factor	DEXA	QCT	USG
Cost	Intermediate	High	Low
Radiation	Low	High	None
Portability	Limited	No	Yes
Parts measured	Spine, hip, wrist	Spine, hip	Calcaneus
Precision	Excellent	Good	Low
Monitoring of	Excellent	Good	Low
treatment
response

DEXA = dual-energy x-ray absorptiometry;
QCT = quantitative computed tomography;
USG = ultrasonography.
Early diagnosis of osteoporosis is of great importance because institution of therapy can reduce the morbidity and mortality of osteoporotic fractures. I hope this article helps you in understanding the basics of different modalities for measuring BMD and fracture risk. More importantly to quell any fear of the tests which are simple and non invasive. Best of luck for your BMD check-up.