An Evaluation of the Limitations of the Technique of Dual Energy Absorptiometry in the Measurement of Bone Disease.

Abstract

Many osteoporotic bone fractures would be largely preventable if a screening programme were successful in identifying those most at risk or in the early stage of the disease. Assessment of bone mass by bone mineral density measurement has developed rapidly in the last 15 years from Single Photon Absorptiometry to Dual Photon Absorptiometry (DPA) and currently Dual X-ray Absorptiometry (DEXA). This study examines the results from patients scanned on both Novo Lab 22a (DPA) and Hologic QDR 1000 (DEXA) machines used at this hospital and gives a high correlation coefficient (r = 0.96, p=<0.001).A comparison was made of previous studies on precision values. To convey the significance of patient's Bone Mineral Density (BMD) scans to the Clinician requires an accurate set of normal reference data. Several studies have indicated thatinternational variations may be significant so a small local study was conducted to see if any trend were identifiable. In both male and female locals of the 70+ age group significantly higher values of BMD were found in the lumbar spine and all hip regions, producing 10-27% higher Z-scores. The percentage of the local population with undiagnosed, abnormal lumbar spine findings likely to effect the results of the DEXA scans was studied. Of 500 sequential investigations, 100 subjects acted as controls. Of these 6% exhibited ambiguous results.Of the 400 clinical cases remaining, 19.25% produced complications in scan analysis.Measurement of the hip may be open to greater errors in patient positioning than the spine and a study comparing standard scanning angle of the femur (20° internal rotation) to increases of 20° (40° internal rotation) and decreases of 20°, 40° and 60° (0° rotation, 20° external rotation and 40° external rotation respectively) showed significant % increases. For femoral neck region the % change in BMD ranged from +2.77 to +9.23. For Ward's Triangle region increases were +3.79% (40° external rotation), +6.16% (40° int. rot.) and +8.05% (0°rot).Where abnormalities exist and may be undetected, particularly in the older population, a trained operator may be needed to instigate radiographic backup. Patients with hip disease need particular care in positioning of the femur and further work is needed in designing a device to limit movement of the femur.This study indicates that the limitations of the technique of Dual Energy Absorptiometry lie mainly in the comparison of clinical scans with normal data, correct positioning of the patient and well trained operating personnel to minimise the effects of artefacts and misleading results.

Date of Award	Mar 1996
Original language	English