The new DEXA scan helps early diagnosis of osteoporosis

In the diagnosis and treatment of osteoporosis, a disease that is variously called the silent thief or porous bones, there has emerged a new diagnostic aid that helps detect the condition early on. The DEXA scan — Dual Energy Xray Absorptiometr y — currently the best available method for checking bone density has a major advantage over the x-ray.

Says Dr. Ravi Subramaniam, Orthopaedic Surgeon, Dr. Soundarapandian Bone and Joint Hospital, “Conventionally, it was the radiologist who would report osteoporosis in the x-rays. Now we know that to get a radiological evidence of osteoporosis, it requires a loss of 35 to 40 per cent of the bone mass at which stage it becomes difficult to treat, whereas the Dexascan can measure Bone Mineral Density which is widely accepted as an indicator of bone strength and detect it much before that.”

“Osteoporosis means weak bones which are prone to fracture. The most important reason for diagnosing the condition early is to prevent fractures due to trivial trauma,” says Dr. Shriraam Mahadevan, Consultant Endocrinologist, Associates in Clinical Endocrinology, Education and Research (ACEER). The Dexascan is a quick, (a scan of the spine, hip or total body requires two to four minutes), painless, non-invasive, outpatient procedure which does not require any sedative or medication and the amount of radiation used is extremely small (less than one tenth the dose of a standard chest x-ray.)

How the test is done

“For the test, the patient is made to lie down on an examination table fully clothed and the scanner rapidly directs x-ray energy from two different sources towards the bone being examined in an alternating fashion at a set frequency. The mineral density of the patient’s bone weakens or prolongs the transmission of these two sources of x-ray energy through a filter onto a counter. The greater the bone mineral density, the greater the signal picked up by the counter. The use of two different x-ray energy sources improves the precision and accuracy of the result. Special software compute and display the bone density measurements,” explains Dr. Ravi Subramaniam.

Even small and subtle changes can show up on a Dexascan. People with osteoporosis have substantially lower bone density than others in their age group. The results are in the form of a T score and a Z score in which the patient’s bone mineral density values are compared with that of a young normal patient and with an age matched normal patient of the same ethnicity. This comparison throws up vital clues. By comparing a patient’s bone density with a young normal person, the risk of fracture can be predicted and by comparing it with that of peers, one can determine in the case of a low score whether there are reasons other than age related bone loss. According to WHO criteria, a normal T score would be Bone Mineral Density of -1.0 (minus1.0), a T score of -1.0 to -2.5 would indicate low bone mass and a T score of -2.5 would mean osteoporosis.

The drawback

The only drawback, according to Dr. Shreedhar, is that the standard figures are for a white population or those from a similar ethnic background. Normative values of bone density for Indians are yet to be completed. Factors that put people at risk for osteoporosis include Asian race (Indians have a lower bone density than their western counterparts), malnutrition, early menopause, smoking, prolonged use of steroids, anti epileptic drugs, thyroid medication and anti coagulants and women on prolonged Hormone Replacement Therapy. Usually, the DEXA is repeated once in two years or earlier for people with established osteoporosis and once in five to ten years in normal people. “The DEXA is also useful in monitoring patients on treatment which x-rays cannot do,” says Dr. Shriraam Mahadevan.