~ Pergamon Appl. Radiat. lsot. Vol. 49. No. 5/6, pp. 685-686, 1998

@ 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain

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Variation in Bone Mineral Density Between Different Anatomical Sites in a Normal Local

Population C. F. N JEH and C. M. BOIV IN*

Department of Nuclear Medicine, Queen Elizabeth Hospital, Birmingham B15 2TH, U.K.

Bone mineral density (BMD) measured using dual energy X-ray absorptiometry (DXA) can be expressed in terms of standard deviations, above or below mean young adult T-scores and above or below age-matched Z-scores. The differences between the left neck of the femur (LN), right neck of the femur (RN) and lumbar spine (L2 L4) were statistically significant, irrespective of whether expressed as T-scores or Z-scores. Therefore skeletal status assessment should involve as many sites as practically possible. Considering the low radiation dose and short scanning time, it is suggested that at least the two femurs and lumbar spine BMD be used in routine osteoporosis risk assessment. ~) 1998 Elsevier Science Ltd. All rights reserved

Introduction Osteoporosis is characterised by low bone mass and micro-architectural deterioration of bone tissue, leading to increased risk of fragility fractures. Bone mineral density (BMD) measured using DXA is currently the most widely used method of quantifying the development and degree of osteoporosis. Some of the clinically available machines such as the Lunar DPX-L have the capability to measure BMD at various sites, including spine-PA, lateral spine, proximal femur, hand and total body. Using the WHO definition (WHO, 1994), a patient is os- teoporotic if their BMD is 2.5 SD or more below the young adult mean (i.e. T-score < - 2.5). It has been reported that using a T-score of - 2.5 as a cut-off point, the number of patients classified as os- teoporotic varied between sites (Njeh et al., 1996). Also, it has been reported that degenerative joint disease affects BMD measurements (Yu et al., 1995). To reduce variation due to pathological conditions such as rheumatoid arthritis and osteoarthritis, we restricted our study to a local normal population. The most commonly measured sites are the spine and proximal femur. However, the number of sites measured varies between centres. This study investi- gated retrospectively site differences in measured BMD in a local normal population.

Method BMD at the spine (L2 L4), right and left proximal

femur (neck of femur, Ward's triangle and trochanter) were measured using a Lunar DPX-L

*To whom all correspondence should be addressed.

densitometer. The manufacturer's recommended positioning and analysis protocols were used. Local ethical approval was granted for this study. Healthy volunteers were recruited from the local population by sequential referral by GP and the measurements were carried out with their informed consent. 1740 male and female subjects with a mean age of 51.3 + 13.0 yr were recruited. Each subject was given an explanation of the procedure and X-ray radiation dose involved. A questionnaire was used to eliminate those with pathological conditions that might affect their BMD. Subjects were also excluded if they regularly used certain medications, had a history of fracture, alcohol abuse or prolonged immobilisation. The BMD values were expressed as T- and Z-scores. Simple linear correlations were computed for different regions. The difference between sites was examined using the Student paired t-test.

Results and Discussion There were highly significant correlations between

BMD at all regions in both left and right femurs with the coefficients varying from 0.74 to 0.93 (p < 0.0001) (Table 1). There was a consistently better correlation between identical regions on the two femurs (r = 0.92, SEE = 0.057 g/cm2). A moderate corre- lation between femur regions and the lumbar spine (r = 0.69-0.70, p < 0.0001, SEE = 0.106-0.136 g/ cm 2) was observed. These results are consistent with previous reports (Faulkner et al., 1995a). The mean (+ SD) T-scores of the left femoral neck, right femoral neck and L2-L4 lumbar spine were - 0.480__ 1.198, - 0.430 + 1.197 and -0 .580 _+ 1.574, respectively. The differences be- tween these T-scores were statistically significant

685

686 C.F. Njeh and C. M. Boivin

Table 1. Linear regression correlations between regions in the left femur, right femur and lumbar spine (L2-L4), p < 0.0001

Right Left Spine Ward's triangle Trochanter Neck Ward's triangle Trochanter L2-L4

Right neck 0,93 0.83 0.92 Right Ward's triangle 0,79 0.87 Right trocbanter 0.80 Left neck Left Ward's triangle Left trocbanter

0.88 0.79 0.70 0.92 0.74 0.70 0.75 0.92 0.70 0.93 0.83 0.70

0.79 0.70 0.69

Table 2. A Student paired t-test demonstrating the difference between the T-scores and Z-scores of the left neck of the femur (LN), right neck of the femur (RN) and lumbar spine (L2-L4). At 95%

confidence, absolute significant t-value is > 1.96

T-scores Z-scores

t-Value p-Value t -Value p-Value

LN RN -4.276