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Dual energy X-ray absorptiometry of the spine—decubitus lateral versus anteroposterior projection in osteoporotic women: Comparison to single energy X-ray absorptiometry of the forearm
Bone Vol. 16, No. 2 February 1995:255-260 ELSEVIER
Dual Energy X-Ray Absorptiometry of the Spine Decubitus Lateral versus Anteroposterior Projection in Osteoporotic Women: Comparison to Single Energy X-Ray Absorptiometry of the Forearm K. BJARNASON, L. NILAS, C. H A S S A G E R , and C. CHRISTIANSEN Center for Clinical and Basic Research, Ballerup Byvej 222, 2750 Ballerup, Denmark
more valuable than measurements at other locations. Measurements of spinal BMD have thus in some studies been superior to appendicular measurements in discriminating between normals and patients with nontraumatic spinal compression fracture (Riggs et al. 1993; Eastell et al. 1989; Heuck et al. 1989). Other studies, however, have found equal diagnostic abilities (Melton et al. 1993; Gotfredsen et al. 1989; Nordin et al. 1988; Ott et al. 1987). These ambiguous findings might result from innate imperfections of the methods applied for the determination of spinal BMD (Gotfredsen et al. 1988; Genant et al. 1987). Dual energy X-ray absorptiometry (DXA) of the lumbar spine offers a small short- and long-term precision error, low radiation, and rapid execution. Conventional DXA of the spine in the anteroposterior projection (AP DXA), however, includes the vertebral arch and the spinous process in the BMD measurement. These posterior elements probably do not contribute much to the ability of the vertebrae to withstand compression. Furthermore, it has been suggested that aortic calcifications and degenerative changes in the facets might increase the AP results (Reid et al. 1991; Drinka et al. 1992). DXA of the lumbar spine in the lateral view (LAT DXA) makes it possible to evade some of these problems, as only the vertebral body is included in the BMD measurement. Furthermore, the vertebral body contains a higher proportion of trabecular bone than the posterior elements (Nottestad et al. 1987) and may therefore be subject to greater bone loss (Genant et al. 1988; Duboeuf et al. 1994; Harper et al. 1993; Uebelhart et al. 1990; Slosman et al. 1990; Rupich et al. 1990). LAT DXA may thus have diagnostic advantages (Duboeuf et al. 1994; Harper et al. 1993; Uebelhart et al. 1990; Slosman et al. 1990; Rupich et al. 1990). In the present study, we compared the discriminatory ability of decubitus LAT DXA vs. AP DXA for vertebral fractures. Furthermore, some technical problems of LAT DXA in the decubitus position were addressed. For comparison, data on forearm BMD were included.
We investigated the discriminatory ability of dual energy X-ray absorptiometry (DXA) of the lumbar spine in the anteroposterior vs. decubitus lateral projection in 53 elderly women with at least one vertebral fracture and 63 agematched women without fracture. Twenty-three premenopausal healthy women served as a reference group. Spine measurements were compared to forearm measurements by single energy X-ray absorptiometry (SXA). Bone mineral density (BMD) of the women with fractures was 16% lower in the AP projection and 17% lower in the decubitus lateral projection compared to age-matched women without fractures, t-Scores (deviation from normal premenopausai values in SDs) were - 2 . 1 to - 2 . 3 in the women without fractures, and - 3 . 9 (AP projection), - 3 . 1 (lateral projection) and - 3 . 8 (forearm) in the women with vertebral fractures. t-Scores as well as z-scores for bone mineral content of the vertebral body and the vertebral posterior elements (both measured with the lateral projection) were similar. ROC analysis showed no significant difference between the AP and the lateral projection of the spine. When subjects with vertebral endplate sclerosis (about 25% in each group, with considerably elevated spinal BMD) were excluded, it did not significantly change the diagnostic abilities for fractures. We found no diagnostic advantage of the lateral projection as compared to the AP projection or forearm measurement. Future studies will reveal whether absorptiometry in the lateral projection can be improved by supine lateral scanning.
(Bone 16:255-260, 1995) Key Words: Single energy X-ray absorptiometry; Dual energy X-ray absorptiometry; Bone mineral content; Bone mineral density; Endplate sclerosis; Osteoporosis. Introduction Bone strength is closely related to bone mineral density (BMD) (Chalmers & Weaver 1966; Arnold 1973) and accordingly the risk of fracture is commonly predicted from measurements of BMD (Black et al. 1992; Melton et al. 1993; Cummings et al. 1993). Measurement at the site of fracture to be predicted may be
Materials and Methods Participants
All participants were recruited within 14 months from municipalities in the vicinity of Glostrup Hospital, and the study was carried out at Center for Clinical and Basic Research, Ballerup. All were in good general health, as judged from medical history,
Address for correspondence and reprints: Ketil Bjarnason, Center for Clinical and Basic Research, Ballerup Byvej 222, 2750 Ballerup, Denmark.
© 1995by ElsevierScienceInc.
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K. Bjarnason et al. Lateral vs. anteroposterior projection in spinal DXA
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Table 1. Demographic data and bone densities in the three groups of women (data for women without endplate sclerosis are given in parentheses) (mean ± 1 SD) Premenopansal women (group A) N Age YSM Height Weight AP BMD LAT BMD Forearm BMD
23 30.6 ± 4.6 -166.2 ± 5.4 60.7 ± 6.9 1.131 ± 0.091 0.730 -+ 0.093 0.467 ± 0.042
Elderly women without fracture (group B)
Elderly women with fracture (group C)
63 (45) 69.4 20.8 161.1 64.0 0.925 0.533 0.372
± ± ± ± ± ±
5.5 5.9 5.7 ~ 9.9 (NS) 0.186 a 0.143 ~ 0.064 ~
53 (39)
(68.7 (20.5 (161.6 (63.1 (0.861 (0.496 (0.362
-+ 5.6) -+ 6.1) ± 5.6) ~ ± 9.9) (NS) ± 0.138) ~ ± 0.120)" ± 0.051) ~
70.2 21.0 158.5 61.2 0.777 0.440 0.307
± 4.6 (NS) ± 5.9 (NS) ± 5.6 b --_ 9.8 (NS) - 0.163 d ± 0.135 d _+ 0.057 d
(69.3 (20.7 (157.5 (60.5 (0.738 (0.423 (0.311
± 4.6) (NS) ± 5.9) (NS) - 5.3) c _+ 9.8) (NS) ± 0.142) d ± 0.117) ~ -+ 0.058) ~
YSM = years since menopause. NS = not significant. ap < 0.001, compared to premenopausal women. bp < 0.05, ~p < 0.01, '~p < 0.001, compared to elderly women without fractures using Student's t-test for unpaired data. physical examination (including a gynecological examination in the older groups) and standard blood chemistry (electrolytes, glucose, liver parameters and lipids) and hematology. None were hospitalized or immobilized, or were suffering from diseases or taking medicine known to influence bone metabolism. The study was approved by the local ethics committee. The study comprised three groups of women.
Group A: normal premenopausal women. Twenty-three premenopausal women, aged 25-39 years, relatives of employees at the clinic. Group B: elderly women without fracture. Sixty-three women with no vertebral fractures were recruited from 2241 women who had returned a general questionnaire on health status sent to 2580 women. Participants were invited in the order in which the questionnaires were returned, until about 60 eligible women had been examined. Group B was age matched to group C with regard to range of age and age profile. Group C: elderly women with fracture. Fifty-three postmenopausal out-patients, aged 59-75 years, were recruited from 257 female outpatients who, within 2 years, had had an X-ray of the spine at a department of radiology. The X-ray showed at least one vertebral fracture. Sixty women were eligible and consented to participate. Seven of these women were later excluded due to fractures in L-2 to L-5. W o m e n with a history of significant trauma to the spine were not included. Precision was determined in 10 healthy premenopausal subjects aged 21-50 years. Methods Fractures were verified/excluded for groups B and C by lateral X-ray of the spine visualizing all vertebrae from Th-4 to L-5 inclusive. One radiologist diagnosed all X-rays blindly. A n t e r i o r and p o s t e r i o r h e i g h t s of the v e r t e b r a e were measured. A fracture was defined as a reduction of more than 25% in the anterior and posterior heights of the vertebrae compared with adjacent vertebrae, or a reduction of more than 25% in the anterior to posterior height (Melton et al. 1989). Sclerosis of vertebral endplates and adjacent bone was diagnosed for L - 2 L-4 by lateral X-ray. Ambiguous cases were classified as negative. Endplate sclerosis was invariably accompanied by disc degeneration. Bone mineral density of the lumbar spine was measured in the
anteroposterior (AP) and the lateral (LAT) projections by DXA with a QDR-1000 (Hologic Inc., Waltham, MA), utilizing software version 4.47. L-2-L-4 were included in the AP B M D measurements. In the LAT projection only L-2 and L-3 could be consistently included because of frequent superimposition of the iliac crest on L-4. LAT B M D was measured in the highresolution mode. The subjects were placed in the left decubitus position with knees and hips flexed 90 °, back and thighs supported by vertical plates, hands above and in front of the head. Rotation of the spine was prevented by cushions placed between knees and elbows and below the head, which also ensured that the pelvis was in a stable vertical position. In addition to these standard procedures we also determined total vertebral bone mineral content (BMC) in the lateral projection (LAT BMCtotal) by defining the entire vertebrae as the region of interest. Furthermore, we determined BMC of the posterior vertebral elements (LAT BMCpo~t) as the difference between LAT BMCtota1 and BMC of the vertebral body (LAT BMCboay ) determined by standard procedures. Finally, B M C of the posterior elements was also determined directly by defining only the posterior vertebral elements as the region of interest. We used BMC instead of BMD in the comparison of the vertebral ABMD (%) -IX
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AP LAT ARM Figure 1. The percentage bone "loss" in the lumbar spine (in the AP and the lateral projection) and in the forearm relative to premenopausal values, in elderly women without and with fractures, before and after exclusion of subjects with vertebral endplate sclerosis. - fx: (D), Elderly women with no vertebral fractures (n = 63); ( l ) , elderly women with no vertebral fractures, after exclusion of subjects with vertebral endplate sclerosis (n = 45). + fx: (~) Elderly women with at least one vertebral compression fracture (n = 53); (1~), elderly women with at least one vertebral compression fracture, after exclusion of subjects with vertebral endplatc sclerosis (n = 39).
Bone Vol. 16, No. 2 February 1995:255-260
K. Bjarnason et al. Lateral vs. anteroposterior projection in spinal DXA
257
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t LAT ...... t Figure 2. The mean bone mineral density (BMD) of single vertebrae in the AP and the lateral projection relative to premenopausal values, in elderly women without fractures and with fractures, before and after the exclusion of subjects with vertebral endplate sclerosis. - fx: (C]), Elderly women with no vertebral fractures (n = 63); ( l ) , elderly women with no vertebral fractures, after exclusion of subjects with vertebral endplatesclerosis (n = 45). +fx: ([]), Elderly women with at least one vertebral compression fracture (n = 53); (Ill), elderly women with at least one vertebral compression fracture, after exclusion of subjects with vertebral endplate sclerosis (n = 39). Mean (+SEM); *p < 0.05; Student's t-test for unpaired data. body and the posterior vertebral elements (see Table 3), because B M D calculations were inappropriate for the posterior vertebral elements due to their inhomogeneous structure. BMD of the nondominant forearm was measured by single energy X-ray absorptiometry (DTX-100, Osteometer A/S, R~dovre, Denmark) and was determined as the mean of six scans 4 m m apart just proximal to the site where the distance between the ulna and radius is 8 mm. The short-term precision error in vivo of this method is 1% (Kelly et al. 1994). Short-term precision errors of AP B M D and decubitus LAT B M D by QDR-1000 were estimated by duplicate measurements done within 12 days in 10 subjects. Statistics
The short-term precision error of duplicate measurements was calculated as the coefficient of variation (CV%). To assess and compare the diagnostic capabilities of decubitus LAT DXA, AP D X A and forearm SXA, B M D values were expressed in terms of deviation from the mean value of the normal premenopausal women, expressed in the SD of the latter (t-scores). Fracture cases were similarly expressed in terms of deviation from the mean value of age-matched nonfracture cases in SDs (z-scores). Receiver operating characteristic (ROC) curves were plotted for the elderly women with fracture versus the elderly women without fracture. A ROC curve is generated by plotting the true positive fraction (sensitivity) against the false positive fraction (1 minus specificity), while successively changing the cut-off level. The method with the largest area under the curve (AUC) is best able to discriminate between women witli fracture and women without fracture. AUC was calculated by the trapezoidal rule, and standard errors and the significance of differences between the AUCs were calculated according to Hanley and McNeil (1982, 1983). Differences of mean B M D and t-scores between subgroups were tested by Student's t-test for unpaired data. Results
The short-term precision error in vivo by Hologic QDR-1000 was 2.8% in the decubitus lateral projection and 0.8% in the AP
projection. Superimposition of ribs on L-2 was observed in 16% of the cases and ribs were visible in the scan area anterior to L-2 in 62% of the cases. Sclerosis of the endplates and adjacent bone was seen in 29% of the elderly women without fracture and in 26% of those with fracture. On average, women with endplate sclerosis had 24% higher B M D in the AP projection (p < 0.001) and 22% higher BMD in the decubitus lateral projection (p < 0.01) than women without endplate sclerosis. There was no significant difference in the lower forearms. T a b l e 1 shows the mean age, height, weight and B M D values in the three groups (data for women without endplate sclerosis are given in parentheses). BMD of the women with fracture was 16% (14%) lower in the AP projection and 17% (15%) lower in the lateral decubitus projection compared to age-matched women without fracture, and 31% (35%) and 40% (42%) lower than in the premenopausal group. In the elderly women with fracture, forearm B M D was 17% (14%) lower than that of the age-matched healthy women and 34% (33%) lower than that of the premenopausal group. The percentage decreases in B M D in the elderly women (with and without fractures and with and without degenerative endplate sclerosis) at the different measurement sites compared to the premenopausal women are summarized in F i g u r e 1. Figure 2 visualizes B M D of the individual vertebrae given in
Table 2. Correlation coefficients and SE% between bone densities of individual vertebrae, in both projections, in 53 women with fracture and 63 women without fracture
L-3 AP BMD L-4 AP BMD L-2 LAT BMD L-3 LAT BMD
L-2 AP BMD
L-3 AP BMD
L-4 AP BMD
L-2 LAT BMD
0.93 (8.4) 0.87 (11.3) 0.81 (18.3) 0.76 (21.0)
--0.91 (9.3) 0.72 (21.7) 0.75 (21.5)
---
---
-0.64 (23.8) 0.69 (23.4)
---0.79 (19.9)
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K. Bjarnason et al. Lateral vs. anteroposterior projection in spinal DXA
percent of premenopausal values, in each group and in both projections, In premenopausal women there is no difference in BMD between the different vertebrae (data not shown). The figure shows that with age more bone is lost from L-2 than from L-3, which again has lost more bone than L-4. T a b l e 2 shows the correlations between BMD values of individual vertebrae (r = 0.64-0.93). AP and lateral decubitus BMD measurements were significantly correlated (all vertebrae; r = 0.79, SEE = 18.5%). The coefficients of correlation were insignificantly different in the elderly women with and without fracture (r = 0.71 vs. 0.80). The SE% were higher for the LAT than for the AP measurements, which again had virtually the same SE% as the forearm when related to AP measurements. Both AP and LAT measurements were modestly related to height (r = 0.17 vs. 0.19, p < 0.05), weight (r = 0.27 vs. 0.27, p < 001) and age (r = - 0 . 2 3 vs. - 0 . 1 9 , p < 0.01). Figure 3 visualizes the t-scores of the spine and forearm measurements, and cases with endplate sclerosis indicated. Overall, the t-scores of the AP, lateral decubitus and forearm measurements were similar. Delta t (i.e., the difference in t-scores between the group with fractures and the group without fractures) was significantly larger in the AP than in the LAT projection (p < 0.01). In the spine, but not in the forearm, the groups with no endplate sclerosis had significantly lower t-scores than the groups with endplate sclerosis. Table 3 compares LAT BMCbody and LAT BMCpost, regarding t-scores for the two elderly groups and z-scores for the frac-
Bone Vol. 16, No. 2 February 1995:255-260 ture group. LAT BMCpost and LAT BMCbody had similar t- and z-scores. Virtually identical results (data not shown) were obtained when LAT BMCpost was calculated directly from the region of interest instead of as LAT BMCtota ] - LAT BMCboay. LAT BMCbody had higher t-scores than LAT BMD, partly due to higher premenopausal SD of the former. F i g u r e 4 shows the ROC diagrams. There was no significant difference in the AUCs between AP and decubitus lateral measurements. Exclusion of subjects with endplate sclerosis smoothed the AP curve without significantly altering the AUCs (data not shown). The forearm measurements had significantly larger AUC than the decubitus lateral measurements.
Discussion Fracture is the clinical consequence of osteoporosis. A primary goal of bone mass measurement is therefore to assess the risk of future fracture. Single measurements at various skeletal sites, the forearm, heel, spine and hip, have been shown to predict fractures at many skeletal sites (Black et al. 1992; Melton et al. 1993; Cummings et al. 1993). Each standard deviation decline in bone mass increases the risk of fracture by 50-100%, depending on both measurement and fracture site (Black et al. 1992; Melton et al. 1993; Cummings et al. 1993). How the prediction of vertebral fracture risk can be optimized has been investigated. One suggestion is to measure bone mineral in the vertebral body only, by using the lateral measurement
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Figure 3. Bone mineral density of the lumbar spine and the forearm, expressed in t-scores relative to premenopausal women. Mean values for the entire population are given in the figure. Mean values for the two subpopulations and p-values (for the difference between them) are given in the table. (0), Subjects with endplate sclerosis; (O), subjects without endplate sclerosis; - fx: 63 elderly women with no vertebral fractures; + fx: 53 elderly women with at least one vertebral compression fracture.
Bone Vol. 16, No. 2 February 1995:255-260
K. Bjarnason et al. Lateral vs. anteroposterior projection in spinal DXA
259
Table 3. t-Scores and z-scores of the lumbar spine mineral content (BMC) measured in the anteroposterior projection (AP BMC) compared with BMC of the vertebral body (LAThody BMC) and BMC of the vertebral posterior elements (LATr,o~t BMC) measured with lateral projection (mean ± SD)
APBMC LATbody BMC LATpo~t BMC
t-Scores + Fx
+ Fx
z-Scores + Fx
- 1 . 1 ± 1 . 4 ( - 1 . 5 ± 1.1) - 1 . 2 ± 1.4 ( - 1 . 6 ± 1.2) - 1.5 ± 1.2 ( - 1.6 ± 1.2)
- 2 . 3 ± 1.1 ( - 2 . 6 ± 1.0) -2.1 ± 1.1 ( - 2 . 3 ± 0.9) - 2 . 4 ± 1.0 ( - 2 . 5 ± 0.9)
- 0 . 8 ± 0.8 ( - 1 . 0 - - - 0.7) - 0 . 7 ± 0.8 ( - 0 . 8 ± 0.7) - 0 . 8 ± 0.8 ( - 0 . 9 ± 0.7)
t-Scores: relative to premenopausal values; z-scores: fracture patients relative to nonfracture age-matched women. -Fx: postmenopausal women without vertebral fractures; + Fx: postmenopausal women with vertebral fractures. Data after exclusion of subjects with endplate sclerosis are given in parentheses.
approach, which could add increased relevance to the measurement (Uebelhart et al. 1990; Slosman et al. 1990; Rupich et al. 1990). In the present study, the decubitus lateral projection and the traditional AP projection were compared with respect to their ability to distinguish between age-matched women with and without vertebral fracture. This kind of study has the inborn problem that a fraction of the control group (age-matched women without fracture) may suffer from osteoporosis, but they have not yet presented their first vertebral fracture. Furthermore, fracture risk cannot be estimated from a cross-sectional study. This should be b o m e in mind when considering the diagnostic validity results of such studies. For comparison of different methods, however, the above-mentioned problem is less important. The women entered in the present study were recruited at the same time from the same geographical area, and all underwent the same thorough medical examination. The groups of elderly
All elderly women 1.0 Arm AP t-
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Figure 4. ROC curves showing the true positive fraction (sensitivity) vs. the false positive fraction (1-specificity) at increasing cut-off levels, in 63 elderly women with no fractures and 53 elderly women with at least one fracture. AP and LAT: the lumbar spine in the AP and the lateral projection; ARM: the lower forearm. The area below the ARM curve is larger than the areas below the AP and LAT curves, indicating greater diagnostic value of the forearm measurement.
women with and without fractures were well matched for age and years since menopause. Not surprisingly, the women with spinal fractures were significantly shorter than those without fractures. An effort was made to avoid other confounding factors, such as fractured vertebrae in the area of interest, superimposed ribs and diseases and medication known to influence bone and calcium metabolism. Osteophytosis and osteoarthritis of the facets have been shown to interfere with AP B M D (Drinka et al. 1992; Reid et al. 1991; Uebelhart et al. 1990), and endplate sclerosis and aortic calcification are also presumed to cause erroneously elevated BMD (Uebelhart et al. 1990; Slosman et al. 1990). Such degenerative phenomena are generally present in the age group studied. Endplate sclerosis affects both AP and lateral measurements. To eliminate any distortion of the conclusions due to this, data were analyzed both with and without excluding women with endplate sclerosis. The exclusion of women with endplate sclerosis resulted in an improvement of the mean t-scores of the elderly women (i.e., when compared to premenopausal women), however, without significantly affecting the AUCs in the ROC diagram (i.e., when compared to age-matched controls). The absolute difference in BMD of the groups with and without fracture was significantly greater in the AP than in the LAT projection, but the relative difference was slightly larger in the LAT projection. This relative difference was more pronounced in other studies (Uebelhart et al. 1990; Slosman et al. 1990), suggesting superior diagnostic value of decubitus LAT BMD. In those studies too, however, the percentage coefficient of variation was greater in the lateral decubitus than in the AP projection, thus counteracting the relative advantage of the decubitus lateral projection. Correspondingly, in the present study, t-scores and ROC diagrams indicated no diagnostic advantage of the lateral decubitus projection. Furthermore, as published earlier (Overgaard et al. 1992) forearm measurements had good diagnostic abilities. In this study, the ROC diagrams provide a valuable presentation of sensitivity vs. specificity for pre-existing fractures, which is not based on an arbitrary cut-off level. However, ROC analysis would be even more valuable in a longitudinal study on the occurrence of new spine fractures. The close correlation between the AP and the LAT projections of individual vertebrae indicates that, at age 70, the fraction of bone lost from the vertebral body equals that of the posterior elements. Corresponding to this we found that LAT BMCbody and LAT BMCpost were equally able to discriminate between premenopausal and elderly women as well as elderly women without and elderly women with fractures. This is in line with other studies, which have failed to demonstrate significantly larger bone loss from more trabecular parts of the skeleton as compared to more cortical parts (Nilas et al. 1987). LAT spine
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K. B j a r n a s o n et al. Lateral vs. anteroposterior projection in spinal D X A
measurement may however be more relevant in women within 10 years of menopause where the trabecular bone loss is predominant. The precision data of this study, less than 1% error for AP BMD and less than 3% for LAT BMD, correspond to previous findings (Slosman et al. 1990; Rupich et al. 1990; Hansen et al. 1990). In conclusion, the present cross-sectional study was unable to demonstrate a diagnostic advantage of measuring spinal BMD in the decubitus lateral projection in women aged 70 years to distinguish women with and without fractures. Other studies are required to examine whether the supine lateral projection has a diagnostic advantage, and longitudinal studies are required to verify the potential superiority of lateral spine measurement in women within 10 years of menopause. Forearm measurement with SXA yields diagnostic values comparable to AP and decubitus lateral measurement of the spine. References Arnold, J. S. Amount and quality of trabecular bone in osteoporotic vertebral fractures. J Clin Endocrinol Metab 2:221 238; 1973. Black, D., Cummings, S. R., Genant, H. K., Nevitt, M. S., Palermo, L., and Browner, W. Axial and appendicular bone density predict fractures in older women. J Bone Min Res 7:633-638; 1992. Chalmers, J. and Weaver, J. K. Cancellous bone: Its strength and changes with ageing and an evaluation of some methods for measuring its mineral content. J Bone Joint Surg [Am] 48A:299-308; 1966. Cummings, S. R., Black, D. M., Nevitt, M. C., Browner, W., Cauley, J., Ensrud, K., Genant, H. K., Palermo, L., Scott, J., and Vogt, T. M. Bone density at various sites for prediction of hip fractures. The study of osteoporotic fractures research group. Lancet 341:72-75; 1993. Drinka, P. J., DeSmet, A. A., Bauwens, S. F., and Rogot, A. The effect of overlying calcification on lumbar bone densitometry. Calcif Tissue lnt 50:507510; 1992. Duboeuf, F., Pommet, R., Meunier, P. J., and Delmas, P. D. Dual-energy X-ray absorptiometry of the spine in anteroposterior and lateral projection. Osteoporosis lnt 4:110-116; 1994. Eastell, R., Wahner, H. W., O'Fallon, M., Amadio, P. C., Melton Iti, L. J., and Riggs, B. L. Unequal decrease in bone density of lumbar spine and ultradistal radius in Colles' and vertebral fracture syndromes. J Clin Invest 83:168-174; 1989. Genant, H. K., Steiger, P., Block, J. E., Glueer, C. C., Ettinger, B., and Harris, S. T. Quantitative computed tomography: Update 1987. Calcif Tissue Int 41: 17%186; 1987. Genant, H. K., Ettinger, B., Harris, S. T., Block, J. E., and Steiger, P. Quantitative computed tomography in assessment of osteoporosis. Riggs, B. L.; Melton II1, L. J. eds. Osteoporosis: Etiology, diagnosis, and management. New York: Raven Press; 1988; 221-249. Gotfredsen, A., P~lenphant, J., NCrgaard, H., Nilas, L., Nielsen, V. H., and Christiansen, C. Accuracy of lumbar spine bone mineral content by dualphoton absorptimetry. J Nucl Med 29:248-254; 1988. Gotfredsen, A., Pcdenphant, J., Nilas, L., and Christiansen, C. Discriminative ability of total body bone mineral measure by dual-photon absorptiometry. Scand J Clin Lab Invest 49:125-134; 1989. Hanley, J. A. and McNeil, B. J. The meaning and use of the area under a receiver operating characteristic (ROC) curve. Radiology 143:29-36; 1982.
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Hanley, J. A. and McNeil, B. J. A method of comparing the areas under receiver operating characteristic curves derived from the same cases. Radiology 148: 839-843; 1983. Hansen. M. A., Hassager, C., Overgaard, K., Marslev, U., Riis, B. J., and Christiansen, C. Dual-energy X-ray absorptiometry: A precise method of measuring bone mineral density in the lumbar spine. J Nucl Med 31:1156-1162; 1990. Harper, U. D., Lobaugh, B., King, S., and Drezner, M. K. Supine lateral dual energy x-ray absorptiometry (DXA) of the spine: A superior technique to assess bone loss [abstract 954]. J Bone Min Res 8(suppl. 1):$355; 1993. Heuck, A. F., Block, J., Glueer, C. C., Steiger, P., and Genant, H. K. Mild versus definite osteoporosis: Comparison of bone densitometry techniques using different statistical models. J Bone Min Res 4:891-899; 1989. Kelly, T. L., Crane, G., and Baran, D. T. Single X-ray absorptiometry of the forearm: Precision, correlation and reference data. Calcif Tissue Int 54:212218; 1994. Melton III, L. J., Kan, S. H., Frye, M. A., Wahner, H. W., O'Fallon, W. M., and Riggs, B. L. Epidemiology of vertebral fractures in women. Am J Epidemiol 129:10(0)-1011; 1989. Melton, L. J., Atkinson, E. J., O'Fallon, W. M., Wahner, H. W., and Riggs, B. L. Long-term fracture risk prediction by bone mineral assessed at different skeletal sites. J Bone Min Res 8:1227-1233; 1993. Nilas, L., Podenphant, J., Riis, B. J., Gotfredsen, A., and Christiansen, C. Usefulness of regional bone measurements in patients with osteoporotic fractures at the spine and distal forearm. J Nucl Med 28:960-965; 1987. Nordin, B. E. C., Wishart, J. M., Horowitz, M., Need, A. G., Bridges, A., and Bellon, M. The relation between forearm and vertebral mineral density and fractures in postmenopansal women. Bone Min 5:21-31; 1988. Nottestad, S. Y., Baumel, J. J., Kimmel, D. B., Recker, R. R., and Heaney, R. P. The proportion of trabecular bone in human vertebrae. J Bone Min Res 2:221-229; 1987. Ott, S. M., Kilcoyne, R. F., and Chesnut, C. H. Ability of four different techniques of measuring bone mass to diagnose vertebral fractures in postmenopausal women. J Bone Min Res 2:201-210; 1987. Overgaard, K., Hansen, M. A., Riis, B. J., and Christiansen, C. Discriminatory ability of bone mass measurements (SPA and DXA) for fractures in elderly postmenopausal women. Calcif Tissue lnt 50:30-35; 1992. Reid, I. R., Evans, M. C., Ames, R., and Wattie, D. J. The influence of osteophytes and aortic calcification on spinal mineral density in postmenopausal women. J Clin Endocrinol Metab 72:1372-1374; 1991. Riggs, B. L., Wahner, H. W., Dunn, W. L., Mazess, R. B., Offord, K. P., and Melton, L. J. Differential changes in bone mineral density of the appendicular and axial skeleton with aging. J Clin Invest 67:328-335; 1981. Rupich, R., Pacifici, R., Griffin, M., Vered, I., Susman, N., and Avioli, L. V. Lateral dual-energy radiography: a new method for measuring vertebral bone density: A preliminary study. J Clin Eodocrinol Metab 70:1768-1770; 1990. Slosman, D. O., Rizzoli, R., Donath, A., and Bonjour, J. P. Vertebral bone mineral density measured laterally by dual-energy x-ray absorptiometry. Osteoporosis Int 1:23 29; 1990. Uebelhart, D., Duboeuf, F., Meunier, P. J., and Delmas, P. D. Lateral dualphoton absorptiometry: A new technique to measure the bone mineral density at the lumbar spine. J Bone Min Res 5:525-531; 1990.
Date Received: April 28, 1994. Date Revised: July 25, 1994 Date Accepted: September 16, 1994
Dual Energy X-Ray Absorptiometry of the Spine Decubitus Lateral versus Anteroposterior Projection in Osteoporotic Women: Comparison to Single Energy X-Ray Absorptiometry of the Forearm K. BJARNASON, L. NILAS, C. H A S S A G E R , and C. CHRISTIANSEN Center for Clinical and Basic Research, Ballerup Byvej 222, 2750 Ballerup, Denmark
more valuable than measurements at other locations. Measurements of spinal BMD have thus in some studies been superior to appendicular measurements in discriminating between normals and patients with nontraumatic spinal compression fracture (Riggs et al. 1993; Eastell et al. 1989; Heuck et al. 1989). Other studies, however, have found equal diagnostic abilities (Melton et al. 1993; Gotfredsen et al. 1989; Nordin et al. 1988; Ott et al. 1987). These ambiguous findings might result from innate imperfections of the methods applied for the determination of spinal BMD (Gotfredsen et al. 1988; Genant et al. 1987). Dual energy X-ray absorptiometry (DXA) of the lumbar spine offers a small short- and long-term precision error, low radiation, and rapid execution. Conventional DXA of the spine in the anteroposterior projection (AP DXA), however, includes the vertebral arch and the spinous process in the BMD measurement. These posterior elements probably do not contribute much to the ability of the vertebrae to withstand compression. Furthermore, it has been suggested that aortic calcifications and degenerative changes in the facets might increase the AP results (Reid et al. 1991; Drinka et al. 1992). DXA of the lumbar spine in the lateral view (LAT DXA) makes it possible to evade some of these problems, as only the vertebral body is included in the BMD measurement. Furthermore, the vertebral body contains a higher proportion of trabecular bone than the posterior elements (Nottestad et al. 1987) and may therefore be subject to greater bone loss (Genant et al. 1988; Duboeuf et al. 1994; Harper et al. 1993; Uebelhart et al. 1990; Slosman et al. 1990; Rupich et al. 1990). LAT DXA may thus have diagnostic advantages (Duboeuf et al. 1994; Harper et al. 1993; Uebelhart et al. 1990; Slosman et al. 1990; Rupich et al. 1990). In the present study, we compared the discriminatory ability of decubitus LAT DXA vs. AP DXA for vertebral fractures. Furthermore, some technical problems of LAT DXA in the decubitus position were addressed. For comparison, data on forearm BMD were included.
We investigated the discriminatory ability of dual energy X-ray absorptiometry (DXA) of the lumbar spine in the anteroposterior vs. decubitus lateral projection in 53 elderly women with at least one vertebral fracture and 63 agematched women without fracture. Twenty-three premenopausal healthy women served as a reference group. Spine measurements were compared to forearm measurements by single energy X-ray absorptiometry (SXA). Bone mineral density (BMD) of the women with fractures was 16% lower in the AP projection and 17% lower in the decubitus lateral projection compared to age-matched women without fractures, t-Scores (deviation from normal premenopausai values in SDs) were - 2 . 1 to - 2 . 3 in the women without fractures, and - 3 . 9 (AP projection), - 3 . 1 (lateral projection) and - 3 . 8 (forearm) in the women with vertebral fractures. t-Scores as well as z-scores for bone mineral content of the vertebral body and the vertebral posterior elements (both measured with the lateral projection) were similar. ROC analysis showed no significant difference between the AP and the lateral projection of the spine. When subjects with vertebral endplate sclerosis (about 25% in each group, with considerably elevated spinal BMD) were excluded, it did not significantly change the diagnostic abilities for fractures. We found no diagnostic advantage of the lateral projection as compared to the AP projection or forearm measurement. Future studies will reveal whether absorptiometry in the lateral projection can be improved by supine lateral scanning.
(Bone 16:255-260, 1995) Key Words: Single energy X-ray absorptiometry; Dual energy X-ray absorptiometry; Bone mineral content; Bone mineral density; Endplate sclerosis; Osteoporosis. Introduction Bone strength is closely related to bone mineral density (BMD) (Chalmers & Weaver 1966; Arnold 1973) and accordingly the risk of fracture is commonly predicted from measurements of BMD (Black et al. 1992; Melton et al. 1993; Cummings et al. 1993). Measurement at the site of fracture to be predicted may be
Materials and Methods Participants
All participants were recruited within 14 months from municipalities in the vicinity of Glostrup Hospital, and the study was carried out at Center for Clinical and Basic Research, Ballerup. All were in good general health, as judged from medical history,
Address for correspondence and reprints: Ketil Bjarnason, Center for Clinical and Basic Research, Ballerup Byvej 222, 2750 Ballerup, Denmark.
© 1995by ElsevierScienceInc.
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Bone Vol. 16, No. 2 February 1995:255-260~
Table 1. Demographic data and bone densities in the three groups of women (data for women without endplate sclerosis are given in parentheses) (mean ± 1 SD) Premenopansal women (group A) N Age YSM Height Weight AP BMD LAT BMD Forearm BMD
23 30.6 ± 4.6 -166.2 ± 5.4 60.7 ± 6.9 1.131 ± 0.091 0.730 -+ 0.093 0.467 ± 0.042
Elderly women without fracture (group B)
Elderly women with fracture (group C)
63 (45) 69.4 20.8 161.1 64.0 0.925 0.533 0.372
± ± ± ± ± ±
5.5 5.9 5.7 ~ 9.9 (NS) 0.186 a 0.143 ~ 0.064 ~
53 (39)
(68.7 (20.5 (161.6 (63.1 (0.861 (0.496 (0.362
-+ 5.6) -+ 6.1) ± 5.6) ~ ± 9.9) (NS) ± 0.138) ~ ± 0.120)" ± 0.051) ~
70.2 21.0 158.5 61.2 0.777 0.440 0.307
± 4.6 (NS) ± 5.9 (NS) ± 5.6 b --_ 9.8 (NS) - 0.163 d ± 0.135 d _+ 0.057 d
(69.3 (20.7 (157.5 (60.5 (0.738 (0.423 (0.311
± 4.6) (NS) ± 5.9) (NS) - 5.3) c _+ 9.8) (NS) ± 0.142) d ± 0.117) ~ -+ 0.058) ~
YSM = years since menopause. NS = not significant. ap < 0.001, compared to premenopausal women. bp < 0.05, ~p < 0.01, '~p < 0.001, compared to elderly women without fractures using Student's t-test for unpaired data. physical examination (including a gynecological examination in the older groups) and standard blood chemistry (electrolytes, glucose, liver parameters and lipids) and hematology. None were hospitalized or immobilized, or were suffering from diseases or taking medicine known to influence bone metabolism. The study was approved by the local ethics committee. The study comprised three groups of women.
Group A: normal premenopausal women. Twenty-three premenopausal women, aged 25-39 years, relatives of employees at the clinic. Group B: elderly women without fracture. Sixty-three women with no vertebral fractures were recruited from 2241 women who had returned a general questionnaire on health status sent to 2580 women. Participants were invited in the order in which the questionnaires were returned, until about 60 eligible women had been examined. Group B was age matched to group C with regard to range of age and age profile. Group C: elderly women with fracture. Fifty-three postmenopausal out-patients, aged 59-75 years, were recruited from 257 female outpatients who, within 2 years, had had an X-ray of the spine at a department of radiology. The X-ray showed at least one vertebral fracture. Sixty women were eligible and consented to participate. Seven of these women were later excluded due to fractures in L-2 to L-5. W o m e n with a history of significant trauma to the spine were not included. Precision was determined in 10 healthy premenopausal subjects aged 21-50 years. Methods Fractures were verified/excluded for groups B and C by lateral X-ray of the spine visualizing all vertebrae from Th-4 to L-5 inclusive. One radiologist diagnosed all X-rays blindly. A n t e r i o r and p o s t e r i o r h e i g h t s of the v e r t e b r a e were measured. A fracture was defined as a reduction of more than 25% in the anterior and posterior heights of the vertebrae compared with adjacent vertebrae, or a reduction of more than 25% in the anterior to posterior height (Melton et al. 1989). Sclerosis of vertebral endplates and adjacent bone was diagnosed for L - 2 L-4 by lateral X-ray. Ambiguous cases were classified as negative. Endplate sclerosis was invariably accompanied by disc degeneration. Bone mineral density of the lumbar spine was measured in the
anteroposterior (AP) and the lateral (LAT) projections by DXA with a QDR-1000 (Hologic Inc., Waltham, MA), utilizing software version 4.47. L-2-L-4 were included in the AP B M D measurements. In the LAT projection only L-2 and L-3 could be consistently included because of frequent superimposition of the iliac crest on L-4. LAT B M D was measured in the highresolution mode. The subjects were placed in the left decubitus position with knees and hips flexed 90 °, back and thighs supported by vertical plates, hands above and in front of the head. Rotation of the spine was prevented by cushions placed between knees and elbows and below the head, which also ensured that the pelvis was in a stable vertical position. In addition to these standard procedures we also determined total vertebral bone mineral content (BMC) in the lateral projection (LAT BMCtotal) by defining the entire vertebrae as the region of interest. Furthermore, we determined BMC of the posterior vertebral elements (LAT BMCpo~t) as the difference between LAT BMCtota1 and BMC of the vertebral body (LAT BMCboay ) determined by standard procedures. Finally, B M C of the posterior elements was also determined directly by defining only the posterior vertebral elements as the region of interest. We used BMC instead of BMD in the comparison of the vertebral ABMD (%) -IX
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AP LAT ARM Figure 1. The percentage bone "loss" in the lumbar spine (in the AP and the lateral projection) and in the forearm relative to premenopausal values, in elderly women without and with fractures, before and after exclusion of subjects with vertebral endplate sclerosis. - fx: (D), Elderly women with no vertebral fractures (n = 63); ( l ) , elderly women with no vertebral fractures, after exclusion of subjects with vertebral endplate sclerosis (n = 45). + fx: (~) Elderly women with at least one vertebral compression fracture (n = 53); (1~), elderly women with at least one vertebral compression fracture, after exclusion of subjects with vertebral endplatc sclerosis (n = 39).
Bone Vol. 16, No. 2 February 1995:255-260
K. Bjarnason et al. Lateral vs. anteroposterior projection in spinal DXA
257
9° _,,BMD (%) L2
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t LAT ...... t Figure 2. The mean bone mineral density (BMD) of single vertebrae in the AP and the lateral projection relative to premenopausal values, in elderly women without fractures and with fractures, before and after the exclusion of subjects with vertebral endplate sclerosis. - fx: (C]), Elderly women with no vertebral fractures (n = 63); ( l ) , elderly women with no vertebral fractures, after exclusion of subjects with vertebral endplatesclerosis (n = 45). +fx: ([]), Elderly women with at least one vertebral compression fracture (n = 53); (Ill), elderly women with at least one vertebral compression fracture, after exclusion of subjects with vertebral endplate sclerosis (n = 39). Mean (+SEM); *p < 0.05; Student's t-test for unpaired data. body and the posterior vertebral elements (see Table 3), because B M D calculations were inappropriate for the posterior vertebral elements due to their inhomogeneous structure. BMD of the nondominant forearm was measured by single energy X-ray absorptiometry (DTX-100, Osteometer A/S, R~dovre, Denmark) and was determined as the mean of six scans 4 m m apart just proximal to the site where the distance between the ulna and radius is 8 mm. The short-term precision error in vivo of this method is 1% (Kelly et al. 1994). Short-term precision errors of AP B M D and decubitus LAT B M D by QDR-1000 were estimated by duplicate measurements done within 12 days in 10 subjects. Statistics
The short-term precision error of duplicate measurements was calculated as the coefficient of variation (CV%). To assess and compare the diagnostic capabilities of decubitus LAT DXA, AP D X A and forearm SXA, B M D values were expressed in terms of deviation from the mean value of the normal premenopausal women, expressed in the SD of the latter (t-scores). Fracture cases were similarly expressed in terms of deviation from the mean value of age-matched nonfracture cases in SDs (z-scores). Receiver operating characteristic (ROC) curves were plotted for the elderly women with fracture versus the elderly women without fracture. A ROC curve is generated by plotting the true positive fraction (sensitivity) against the false positive fraction (1 minus specificity), while successively changing the cut-off level. The method with the largest area under the curve (AUC) is best able to discriminate between women witli fracture and women without fracture. AUC was calculated by the trapezoidal rule, and standard errors and the significance of differences between the AUCs were calculated according to Hanley and McNeil (1982, 1983). Differences of mean B M D and t-scores between subgroups were tested by Student's t-test for unpaired data. Results
The short-term precision error in vivo by Hologic QDR-1000 was 2.8% in the decubitus lateral projection and 0.8% in the AP
projection. Superimposition of ribs on L-2 was observed in 16% of the cases and ribs were visible in the scan area anterior to L-2 in 62% of the cases. Sclerosis of the endplates and adjacent bone was seen in 29% of the elderly women without fracture and in 26% of those with fracture. On average, women with endplate sclerosis had 24% higher B M D in the AP projection (p < 0.001) and 22% higher BMD in the decubitus lateral projection (p < 0.01) than women without endplate sclerosis. There was no significant difference in the lower forearms. T a b l e 1 shows the mean age, height, weight and B M D values in the three groups (data for women without endplate sclerosis are given in parentheses). BMD of the women with fracture was 16% (14%) lower in the AP projection and 17% (15%) lower in the lateral decubitus projection compared to age-matched women without fracture, and 31% (35%) and 40% (42%) lower than in the premenopausal group. In the elderly women with fracture, forearm B M D was 17% (14%) lower than that of the age-matched healthy women and 34% (33%) lower than that of the premenopausal group. The percentage decreases in B M D in the elderly women (with and without fractures and with and without degenerative endplate sclerosis) at the different measurement sites compared to the premenopausal women are summarized in F i g u r e 1. Figure 2 visualizes B M D of the individual vertebrae given in
Table 2. Correlation coefficients and SE% between bone densities of individual vertebrae, in both projections, in 53 women with fracture and 63 women without fracture
L-3 AP BMD L-4 AP BMD L-2 LAT BMD L-3 LAT BMD
L-2 AP BMD
L-3 AP BMD
L-4 AP BMD
L-2 LAT BMD
0.93 (8.4) 0.87 (11.3) 0.81 (18.3) 0.76 (21.0)
--0.91 (9.3) 0.72 (21.7) 0.75 (21.5)
---
---
-0.64 (23.8) 0.69 (23.4)
---0.79 (19.9)
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K. Bjarnason et al. Lateral vs. anteroposterior projection in spinal DXA
percent of premenopausal values, in each group and in both projections, In premenopausal women there is no difference in BMD between the different vertebrae (data not shown). The figure shows that with age more bone is lost from L-2 than from L-3, which again has lost more bone than L-4. T a b l e 2 shows the correlations between BMD values of individual vertebrae (r = 0.64-0.93). AP and lateral decubitus BMD measurements were significantly correlated (all vertebrae; r = 0.79, SEE = 18.5%). The coefficients of correlation were insignificantly different in the elderly women with and without fracture (r = 0.71 vs. 0.80). The SE% were higher for the LAT than for the AP measurements, which again had virtually the same SE% as the forearm when related to AP measurements. Both AP and LAT measurements were modestly related to height (r = 0.17 vs. 0.19, p < 0.05), weight (r = 0.27 vs. 0.27, p < 001) and age (r = - 0 . 2 3 vs. - 0 . 1 9 , p < 0.01). Figure 3 visualizes the t-scores of the spine and forearm measurements, and cases with endplate sclerosis indicated. Overall, the t-scores of the AP, lateral decubitus and forearm measurements were similar. Delta t (i.e., the difference in t-scores between the group with fractures and the group without fractures) was significantly larger in the AP than in the LAT projection (p < 0.01). In the spine, but not in the forearm, the groups with no endplate sclerosis had significantly lower t-scores than the groups with endplate sclerosis. Table 3 compares LAT BMCbody and LAT BMCpost, regarding t-scores for the two elderly groups and z-scores for the frac-
Bone Vol. 16, No. 2 February 1995:255-260 ture group. LAT BMCpost and LAT BMCbody had similar t- and z-scores. Virtually identical results (data not shown) were obtained when LAT BMCpost was calculated directly from the region of interest instead of as LAT BMCtota ] - LAT BMCboay. LAT BMCbody had higher t-scores than LAT BMD, partly due to higher premenopausal SD of the former. F i g u r e 4 shows the ROC diagrams. There was no significant difference in the AUCs between AP and decubitus lateral measurements. Exclusion of subjects with endplate sclerosis smoothed the AP curve without significantly altering the AUCs (data not shown). The forearm measurements had significantly larger AUC than the decubitus lateral measurements.
Discussion Fracture is the clinical consequence of osteoporosis. A primary goal of bone mass measurement is therefore to assess the risk of future fracture. Single measurements at various skeletal sites, the forearm, heel, spine and hip, have been shown to predict fractures at many skeletal sites (Black et al. 1992; Melton et al. 1993; Cummings et al. 1993). Each standard deviation decline in bone mass increases the risk of fracture by 50-100%, depending on both measurement and fracture site (Black et al. 1992; Melton et al. 1993; Cummings et al. 1993). How the prediction of vertebral fracture risk can be optimized has been investigated. One suggestion is to measure bone mineral in the vertebral body only, by using the lateral measurement
T-score
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Figure 3. Bone mineral density of the lumbar spine and the forearm, expressed in t-scores relative to premenopausal women. Mean values for the entire population are given in the figure. Mean values for the two subpopulations and p-values (for the difference between them) are given in the table. (0), Subjects with endplate sclerosis; (O), subjects without endplate sclerosis; - fx: 63 elderly women with no vertebral fractures; + fx: 53 elderly women with at least one vertebral compression fracture.
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K. Bjarnason et al. Lateral vs. anteroposterior projection in spinal DXA
259
Table 3. t-Scores and z-scores of the lumbar spine mineral content (BMC) measured in the anteroposterior projection (AP BMC) compared with BMC of the vertebral body (LAThody BMC) and BMC of the vertebral posterior elements (LATr,o~t BMC) measured with lateral projection (mean ± SD)
APBMC LATbody BMC LATpo~t BMC
t-Scores + Fx
+ Fx
z-Scores + Fx
- 1 . 1 ± 1 . 4 ( - 1 . 5 ± 1.1) - 1 . 2 ± 1.4 ( - 1 . 6 ± 1.2) - 1.5 ± 1.2 ( - 1.6 ± 1.2)
- 2 . 3 ± 1.1 ( - 2 . 6 ± 1.0) -2.1 ± 1.1 ( - 2 . 3 ± 0.9) - 2 . 4 ± 1.0 ( - 2 . 5 ± 0.9)
- 0 . 8 ± 0.8 ( - 1 . 0 - - - 0.7) - 0 . 7 ± 0.8 ( - 0 . 8 ± 0.7) - 0 . 8 ± 0.8 ( - 0 . 9 ± 0.7)
t-Scores: relative to premenopausal values; z-scores: fracture patients relative to nonfracture age-matched women. -Fx: postmenopausal women without vertebral fractures; + Fx: postmenopausal women with vertebral fractures. Data after exclusion of subjects with endplate sclerosis are given in parentheses.
approach, which could add increased relevance to the measurement (Uebelhart et al. 1990; Slosman et al. 1990; Rupich et al. 1990). In the present study, the decubitus lateral projection and the traditional AP projection were compared with respect to their ability to distinguish between age-matched women with and without vertebral fracture. This kind of study has the inborn problem that a fraction of the control group (age-matched women without fracture) may suffer from osteoporosis, but they have not yet presented their first vertebral fracture. Furthermore, fracture risk cannot be estimated from a cross-sectional study. This should be b o m e in mind when considering the diagnostic validity results of such studies. For comparison of different methods, however, the above-mentioned problem is less important. The women entered in the present study were recruited at the same time from the same geographical area, and all underwent the same thorough medical examination. The groups of elderly
All elderly women 1.0 Arm AP t-
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1.0
Figure 4. ROC curves showing the true positive fraction (sensitivity) vs. the false positive fraction (1-specificity) at increasing cut-off levels, in 63 elderly women with no fractures and 53 elderly women with at least one fracture. AP and LAT: the lumbar spine in the AP and the lateral projection; ARM: the lower forearm. The area below the ARM curve is larger than the areas below the AP and LAT curves, indicating greater diagnostic value of the forearm measurement.
women with and without fractures were well matched for age and years since menopause. Not surprisingly, the women with spinal fractures were significantly shorter than those without fractures. An effort was made to avoid other confounding factors, such as fractured vertebrae in the area of interest, superimposed ribs and diseases and medication known to influence bone and calcium metabolism. Osteophytosis and osteoarthritis of the facets have been shown to interfere with AP B M D (Drinka et al. 1992; Reid et al. 1991; Uebelhart et al. 1990), and endplate sclerosis and aortic calcification are also presumed to cause erroneously elevated BMD (Uebelhart et al. 1990; Slosman et al. 1990). Such degenerative phenomena are generally present in the age group studied. Endplate sclerosis affects both AP and lateral measurements. To eliminate any distortion of the conclusions due to this, data were analyzed both with and without excluding women with endplate sclerosis. The exclusion of women with endplate sclerosis resulted in an improvement of the mean t-scores of the elderly women (i.e., when compared to premenopausal women), however, without significantly affecting the AUCs in the ROC diagram (i.e., when compared to age-matched controls). The absolute difference in BMD of the groups with and without fracture was significantly greater in the AP than in the LAT projection, but the relative difference was slightly larger in the LAT projection. This relative difference was more pronounced in other studies (Uebelhart et al. 1990; Slosman et al. 1990), suggesting superior diagnostic value of decubitus LAT BMD. In those studies too, however, the percentage coefficient of variation was greater in the lateral decubitus than in the AP projection, thus counteracting the relative advantage of the decubitus lateral projection. Correspondingly, in the present study, t-scores and ROC diagrams indicated no diagnostic advantage of the lateral decubitus projection. Furthermore, as published earlier (Overgaard et al. 1992) forearm measurements had good diagnostic abilities. In this study, the ROC diagrams provide a valuable presentation of sensitivity vs. specificity for pre-existing fractures, which is not based on an arbitrary cut-off level. However, ROC analysis would be even more valuable in a longitudinal study on the occurrence of new spine fractures. The close correlation between the AP and the LAT projections of individual vertebrae indicates that, at age 70, the fraction of bone lost from the vertebral body equals that of the posterior elements. Corresponding to this we found that LAT BMCbody and LAT BMCpost were equally able to discriminate between premenopausal and elderly women as well as elderly women without and elderly women with fractures. This is in line with other studies, which have failed to demonstrate significantly larger bone loss from more trabecular parts of the skeleton as compared to more cortical parts (Nilas et al. 1987). LAT spine
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measurement may however be more relevant in women within 10 years of menopause where the trabecular bone loss is predominant. The precision data of this study, less than 1% error for AP BMD and less than 3% for LAT BMD, correspond to previous findings (Slosman et al. 1990; Rupich et al. 1990; Hansen et al. 1990). In conclusion, the present cross-sectional study was unable to demonstrate a diagnostic advantage of measuring spinal BMD in the decubitus lateral projection in women aged 70 years to distinguish women with and without fractures. Other studies are required to examine whether the supine lateral projection has a diagnostic advantage, and longitudinal studies are required to verify the potential superiority of lateral spine measurement in women within 10 years of menopause. Forearm measurement with SXA yields diagnostic values comparable to AP and decubitus lateral measurement of the spine. References Arnold, J. S. Amount and quality of trabecular bone in osteoporotic vertebral fractures. J Clin Endocrinol Metab 2:221 238; 1973. Black, D., Cummings, S. R., Genant, H. K., Nevitt, M. S., Palermo, L., and Browner, W. Axial and appendicular bone density predict fractures in older women. J Bone Min Res 7:633-638; 1992. Chalmers, J. and Weaver, J. K. Cancellous bone: Its strength and changes with ageing and an evaluation of some methods for measuring its mineral content. J Bone Joint Surg [Am] 48A:299-308; 1966. Cummings, S. R., Black, D. M., Nevitt, M. C., Browner, W., Cauley, J., Ensrud, K., Genant, H. K., Palermo, L., Scott, J., and Vogt, T. M. Bone density at various sites for prediction of hip fractures. The study of osteoporotic fractures research group. Lancet 341:72-75; 1993. Drinka, P. J., DeSmet, A. A., Bauwens, S. F., and Rogot, A. The effect of overlying calcification on lumbar bone densitometry. Calcif Tissue lnt 50:507510; 1992. Duboeuf, F., Pommet, R., Meunier, P. J., and Delmas, P. D. Dual-energy X-ray absorptiometry of the spine in anteroposterior and lateral projection. Osteoporosis lnt 4:110-116; 1994. Eastell, R., Wahner, H. W., O'Fallon, M., Amadio, P. C., Melton Iti, L. J., and Riggs, B. L. Unequal decrease in bone density of lumbar spine and ultradistal radius in Colles' and vertebral fracture syndromes. J Clin Invest 83:168-174; 1989. Genant, H. K., Steiger, P., Block, J. E., Glueer, C. C., Ettinger, B., and Harris, S. T. Quantitative computed tomography: Update 1987. Calcif Tissue Int 41: 17%186; 1987. Genant, H. K., Ettinger, B., Harris, S. T., Block, J. E., and Steiger, P. Quantitative computed tomography in assessment of osteoporosis. Riggs, B. L.; Melton II1, L. J. eds. Osteoporosis: Etiology, diagnosis, and management. New York: Raven Press; 1988; 221-249. Gotfredsen, A., P~lenphant, J., NCrgaard, H., Nilas, L., Nielsen, V. H., and Christiansen, C. Accuracy of lumbar spine bone mineral content by dualphoton absorptimetry. J Nucl Med 29:248-254; 1988. Gotfredsen, A., Pcdenphant, J., Nilas, L., and Christiansen, C. Discriminative ability of total body bone mineral measure by dual-photon absorptiometry. Scand J Clin Lab Invest 49:125-134; 1989. Hanley, J. A. and McNeil, B. J. The meaning and use of the area under a receiver operating characteristic (ROC) curve. Radiology 143:29-36; 1982.
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Date Received: April 28, 1994. Date Revised: July 25, 1994 Date Accepted: September 16, 1994