- Email: [email protected]
An Overlying Fat Panniculus Affects Femur Bone Mass Measurement
Journal of Clinical Densitometry, vol. 6, no. 3, 199–204, 2003 © Copyright 2003 by Humana Press Inc. All rights of any nature whatsoever reserved. 1094-6950/03/6:199–204/$25.00
Original Article
An Overlying Fat Panniculus Affects Femur Bone Mass Measurement Neil Binkley, MD,*,1 Diane Krueger, BS,1 and Nellie Vallarta-Ast, RT(R), CDT2 1University
of Wisconsin Institute on Aging, 2Department of Radiology, Wm. S. Middleton VAMC, Madison WI
Abstract Dual-energy X-ray absorptiometry (DXA) is currently the gold standard technique for osteoporosis diagnosis. However, DXA has limitations, including artifacts, such as degenerative disease or metallic foreign bodies, that may confound bone mineral density (BMD) results. Because fat folds overlying the proximal femur may alter soft-tissue density in a nonuniform manner, this may be a currently unappreciated confounder of proximal femur BMD measurement. This possibility was evaluated in 127 patients (52 women/75 men) referred for routine BMD measurement who were identified as having a fat panniculus overlying their proximal femur scan area. Presence of a fat panniculus within the scan field was confirmed by visual assessment of images obtained utilizing a GE Lunar Expert-XL. Subsequently, these individuals were rescanned while retracting their fat panniculus away from the femur scan area without other repositioning between scans. In 49% of the men, and 56% of the women, either the femoral neck, trochanter, or total femur BMD differed by more than the least significant change at our facility. No pattern was observed to predict whether BMD would increase or decrease upon fat retraction. Subsequently, 30 patients were scanned using the standard and retracted technique twice, with repositioning between scans to establish precision. Retracted and standard precision was similar. In conclusion, an overlying fat panniculus may alter proximal femur BMD measurement, which would be expected to impair the ability to accurately diagnose low bone mass and monitor osteoporosis therapy. When a fat panniculus overlays the proximal femur scan area, its retraction should be part of routine densitometric practice. Key Words: Bone density; obesity; DXA; femur.
Introduction
bone mass measurement at the lumbar spine, such as degenerative arthritis, compression fracture, laminectomy, and contrast material, are widely recognized (2–5). However, factors spuriously altering measured bone mass at the proximal femur, such as degenerative arthritis (6) and degree of internal rotation (7), are less commonly appreciated. An additional confounding factor could be changes in soft-tissue radiation absorption. Consistent with this, dramatic alteration of measured hip BMD has been reported by placement of soft-tissue gluteal silicon implants (8). Similarly, differences in overlying softtissue thickness or fat might be expected to alter
Central bone mineral density (BMD) measurement utilizing dual-energy X-ray absorptiometry (DXA) is currently the gold standard for osteoporosis diagnosis and therapy monitoring (1). Although DXA is an accurate and precise test, confounders to
Received June 10, 2002; Accepted October 30, 2002 *Address correspondence to Neil Binkley, University of Wisconsin, Institute on Aging, 1300 University Avenue 2245 MSC, Madison, WI 53706. E-mail: [email protected]
199
200 measured BMD (9–11). This was initially suggested by a clinical case in which serial femur scans demonstrated a marked BMD change outside of what was expected given the patient’s clinical history. Further evaluation of these scans revealed variation in placement of the patient’s fat panniculus on the follow-up scan; this prompted investigation of a potential effect of overlying fat on proximal femur BMD measurement. Subsequently, the effect of overlying fat on proximal femur BMD measurement was evaluated in a series of 127 patients.
Materials and Methods Subjects The study population consists of 127 patients (52 women/75 men) who were referred to the Wm. S. Middleton VAMC for clinically indicated bone mass measurement in whom a fat panniculus could be visualized on DXA scan to overlay the proximal femur region. Their age (yr; mean ± SEM) was 65.2 ± 1.6 and 67.8 ± 1.1 for the women and men, respectively. Mean weight and body mass index (BMI) was 201 lbs ± 4.3 (SEM); 35 ± 0.7 kg/m2 (SEM) for the women and 227 lbs ± 4.4 (SEM); 34 ± 0.5 kg/m2 (SEM) for the men. The presence of a fat fold within the proximal femur scan field was confirmed on visual assessment of the DXA image by one technologist (NVA). Required approval was obtained from the University of Wisconsin Health Sciences Institutional Review Board prior to the initiation of research. Informed consent was obtained from all research subjects prior to data collection. Routine clinical DXA scanning at the Middleton VAMC consists of measuring BMD at the lumbar spine, proximal femur, and radius. World Health Organization (WHO) diagnostic criteria using the lowest T-score at the L1–L4 spine or proximal femur (femur neck, trochanter, or total) were applied to these patients; of the 52 women, 21 had normal BMD, 21 were osteopenic, and 10 were osteoporotic. In the 75 men, 19 had normal BMD, 30 were osteopenic, and 26 were osteoporotic.
DXA Acquisition All scans were conducted and analyzed by one technologist using a GE Lunar Expert-XL densitometer with software version 1.92. AP femur scans were performed in the supine position in standard Journal of Clinical Densitometry
Binkley et al. manner following manufacturer recommendations. The subsequent scan was immediately obtained with patient manually retracting their fat panniculus from the scan field as shown in Fig. 1A,B. No other repositioning occurred between scans. Standard analyses were performed in routine clinical manner with Tscores derived using the GE Lunar male or female normative database.
Precision Study Thirty patients (22 men/8 women) undergoing routine clinical bone mass measurement were studied to determine precision of the retraction technique. The mean (± SEM) age (yr), weight (pounds), and BMI (kg/m2) of these individuals was 63.8 ± 1.8, 220.6 ± 7.1, and 33.6 ± 0.7, respectively. Each subject was imaged in standard clinical fashion followed by a second scan while retracting their panniculus. The patient then stood from the table; following reposition, two similar images were repeated. Precision and least significant change (LSC) were calculated using published recommendations (12).
Results Panniculus Documentation/Retraction Fat lines owing to an overlying panniculus are easily visible on the computer screen and printed image with a GE Lunar Expert-XL. Using this instrument, it is readily apparent when manual retraction successfully removes the panniculus from the scan field (Fig. 2A,B). Whether the panniculus was present or retracted does not affect point typing of bone, tissue, or neutral (images not shown).
Effect of Panniculus Retraction on BMD In this group of 127 patients, the mean percent change in BMD following panniculus retraction was + 2% at the femur neck, – 2% at the total femur, and – 3% at the trochanter. However, at specific regions within an individual, dramatic BMD changes (up to 74%) were observed (Table 1 and Fig. 3A–C). Similar changes in BMD following panniculus retraction were observed in men and women (Table 1). Upon panniculus retraction, DXA-measured fat decreased from 36.5% ± 0.5 to 33.0% ± 0.6 (mean ± SEM). Concomitantly, no change in percent bone area, but a decrease in bone mineral content, was observed (data not shown). Volume 6, 2003
Overlying Fat Affects Hip BMD
201
Fig. 1. Manual retraction of an overlying fat panniculus. Patients were instructed to use two hands to retract their fat panniculus medially and upward away from the proximal femur scan area. (A) Pre-retraction. (B) Post-retraction.
Fig. 2. Documentation of fat panniculus retraction from scan field. Visual examination of the scan images provide documentation that the panniculus has been successfully removed from the scan image. (A) Pre-retraction. (B) Post-retraction. Arrows indicate the panniculus edge in (A); the fat line is not present in (B).
Journal of Clinical Densitometry
Volume 6, 2003
202
Binkley et al. Table 1 Percent Change in BMD Following Panniculus Retraction
Site Entire group n = 127 Women n = 52 Men n = 75
Femur neck 1.9 (0.8) range –12.0 to +73.8 1.2 (0.9) range –10.8 to 22.6 2.4 (1.2) range –12.0 to +73.8
Total femur –2.2 (0.3) range –15.4 to +7.6 –2.5 (0.4) range –8.1 to +7.6 –2.5 (0.4) range –15.4 to +6.8
Trochanter –3.0 (0.5) range –24.6 to +9.6 –1.6 (0.6) range –15.7 to +8.6 –3.4 (0.6) range –24.6 to +9.6
Data as mean ± (SEM).
Effect of Panniculus Retraction on Interpretation of Significant Change Based on a precision study conducted previously in usual clinical patients using published methodology (12), the least significant change in BMD at the Middleton VAMC is 0.033 g/cm2 at the total proximal femur, 0.040 grams/cm2 at the femoral neck, and 0.046 g/cm2 at the greater trochanter. This least significant change was exceeded at one of the three proximal femur regions in 56% (29/53) of the women and 49% (37/75) of the men (Fig. 3A–C). Furthermore, the measured BMD changed by ≥10% in 17% (22/127) of patients. Panniculus retraction did not consistently increase or decrease measured BMD. Specifically, at the femur neck, measured BMD increased in 69 and decreased in 58; at the total femur it increased in 21 and decreased in 102; and at the trochanter it increased in 30 and decreased in 98. It is apparent that measured BMD may increase at one site and decrease at another in the same patient upon panniculus retraction.
Proximal Femur BMD Precision With and Without Panniculus Retraction With no panniculus retraction, proximal femur LSC values of 0.074, 0.086, and 0.046 g/cm2 were obtained at the femur neck, trochanter, and total proximal femur, respectively. Utilizing the retraction technique in these same individuals, proximal femur BMD LSC was 0.066, 0.106, and 0.052 g/cm2 at the femur neck, trochanter, and total proximal femur, respectively. Therefore, proximal femur BMD precision was comparable when utilizing the retraction technique.
Discussion These data document marked variability in measured femur BMD based on presence or absence of a Journal of Clinical Densitometry
fat panniculus overlying the scan field in both men and women. Specifically, in 49% of men and 56% of women, a BMD change greater than the least significant change in g/cm2 was observed at one of the three proximal femur sites upon retraction of their panniculus from the scan field. Furthermore, greater than a 10% change, which would almost certainly be considered significant by all densitometrists, is observed in 17% of such individuals. Finally, measured BMD following panniculus retraction may change in different directions at various proximal femur regions of interest within the same individual. This effect has obvious ramifications on low bone mass diagnosis and monitoring of osteoporosis treatment efficacy. DXA technology assumes that there are two tissues that absorb radiation: bone and uniform soft tissue (13). However, nonuniform fat distribution is known to occur and affect bone mass measurement accuracy and precision (9,14). It is likely that the observed changes in measured BMD reflect different radiographic absorption from nonuniformity of softtissue thickness produced by a fat panniculus within the scan field. Similarly, alteration of soft-tissue radiation absorption by gluteal silicon implants affects measured femur BMD (8). As such, presence of an overlying fat panniculus must be added to the list of known proximal femur BMD measurement confounders, which currently includes improper rotation, metallic foreign bodies, and osteoarthritis (12). It should be noted that the excellent image quality obtained with the Expert-XL densitometer allowed easy visualization of overlying fat folds, thereby permitting identification of a potential explanation for the observed BMD differences. However, it is not established that overlying fat can be visualized as readily when utilizing other densitometers providing lower resolution images. In fact,
Volume 6, 2003
Overlying Fat Affects Hip BMD
203
Fig. 3. Effect of fat panniculus retraction on measured proximal femur BMD. Approximately 50% of men and women demonstrated a BMD difference greater than the least significant change when their fat panniculus was retracted. No pattern to increase or decrease measured BMD at either the (A) femoral neck, (B) total proximal femur, or (C) greater trochanter was reliably evident.
it is our anecdotal experience that fat lines are much more difficult, or impossible, to visualize on other instruments. Additionally, routine point typing of bone and soft tissue did not disclose abnormalities
Journal of Clinical Densitometry
to explain these observed differences in BMD. Thus, technologists and physicians may not be able to rely on the images to detect an overlying fat panniculus. Finally, it should be noted that alteration of
Volume 6, 2003
204 measured BMD following fat retraction is not unique to GE Lunar densitometers; similar observations were recently reported in a small number of patients imaged on a Hologic densitometer (15). Because body weight is a major determinant of bone mass (16), it might be assumed that overweight individuals would be protected from the development of osteoporosis. Clearly, this is an incorrect assumption, as demonstrated by the observation that 69% of the patients in this study were either osteopenic or osteoporotic. Given the high and increasing prevalence of obesity in the United States, currently approx 27% nationwide (17), it is probable that femoral BMD measurement errors owing to different positioning of overlying fat are common but underappreciated. A limitation of this study is the absence of direct bone mass documentation. Thus, it is not clear that the retracted BMD measurement is more accurate. However, to obtain documentation of accuracy would require cadaveric studies of DXA-measured bone mass compared with ash mineral content. It seems unlikely that such studies will ever be conducted in individuals with an overlying fat panniculus. Given that standardization of DXA measurements was performed largely in individuals without overlying soft tissue, it is intuitive that the retracted BMD value be used for diagnosis. Additionally, because overlying fat may induce unpredictable BMD variation, and precision following retraction is similar to the nonretracted state, a recommendation to routinely retract an overlying fat panniculus seems warranted. In conclusion, overlying fat may dramatically alter DXA-measured BMD at the total femur, femoral neck, and/or trochanteric regions. Whether the measured BMD will increase, decrease, or be unaffected by panniculus retraction cannot be predicted reliably. As such, when technologists observe this situation, retraction of the overlying fat panniculus should be standard densitometric practice. Additionally, when physicians detect a nonphysiologic change in proximal femur BMD on serial DXA scans, in the absence of other technical explanations, different positioning of overlying fat should be a consideration.
Acknowledgment This work was presented in abstract form at ISCD 2002. Journal of Clinical Densitometry
Binkley et al.
References 1. Kanis JA, Melton LJ, Christiansen C, Johnston CC, Khaltaev N. 1994 Perspective The diagnosis of osteoporosis. J Bone Miner Res 9:1137–1141. 2. Antonacci MD, Hanson DS, Leblanc A, Heggeness K. 1997 Regional variation in vertebral bone density and trabecular architecture are influenced by osteoarthritic change and osteoporosis. Spine 22:2393–2402. 3. Orwoll ES, Oviatt SK, Mann T. 1990 The impact of osteophytic and vascular calcifications on vertebral mineral density measurements in men. J Clin Endocrinol Metab 70:1202–1207. 4. Yu W, Gluer CC, Fuerst T. 1995 Influence of degenerative joint disease on spinal bone mineral measurements in postmenopausal women. Calcif Tissue Int 57:169–174. 5. Adler RA, Nordlie R, Burke TS. 1998 Increased bone mineral density in a man with known compression fractures. J Clin Densitom 1:275–278. 6. Preidler KW, White LS, Tashkin J, et al. 1997 Dual-energy xray absorptiometric densitometry in osteoarthritis of the hip. Acta Radiol 38:539–542. 7. Girard MS, Sartoris DJ, Moscona AV, Ramos E. 1994 Measured femoral density by dual-energy x-ray absorptiometry as a function of rotation. Orthop Rev 1:38–40. 8. Hauache OM, Vieira JGH, Alonso G, Martins LRF, Brandao C. 2000 Increased hip bone mineral density in a woman with gluteal silicon implant. J Clin Densitom 3:391–393. 9. Tothill P, Pye DW. 1992 Errors due to non-uniform distribution of fat in dual x-ray absorptiometry of the lumbar spine. Br J Radiol 65:807–813. 10. Tothill P, Avenell A. 1994 Errors in dual-energy x-ray absorptiometry of the lumbar spine owing to fat distribution and soft tissue thickness during weight change. Br J Radiol 67:71–75. 11. Svendsen OL, Hassager C, Skodt V, Christiansen C. 1995 Impact of soft tissue on in vivo accuracy of bone mineral measurements in the spine, hip and forearm: A human cadaver study. J Bone Miner Res 10:868–873. 12. Bonnick SL, Lewis LA. 2002 Precision in Bone Densitometry. In Bone Densitometry for Technologists. Humana Press, Totowa, NJ. 169–181. 13. Hansen MA, Hassager C, Overgaard K, Marslew U, Riis BJ, Christiansen C. 1990 Dual energy x-ray absorptiometry: a precise method of measuring bone mineral in the lumbar spine. J Nucl Med 31:1156–1162. 14. Valkema R, Verheij LF, Blokland JAK, et al. 1990 Limited precision of lumbar spine dual-photon absorptiometry by variations in the soft-tissue background. J Nucl Med 31:1774–1781. 15. Jankowski L, O’Brien-Schmack E. 2002 Panniculus adiposus: an unpredictable and often unrecognized source of precision and accuracy errors on DXA scans of the proximal femur. J Clin Densitom 5:SupplS57. 16. Holbrook TL, Barrett-Connor E. 1993 The association of lifetime weight and weight control patterns with bone mineral density in an adult community. Bone Miner 20:141–149. 17. Centers for Disease Control. 2002 Obesity and Overweight: A Public Health Epidemic. Centers for Disease Control, Washington, DC. Volume 6, 2003
Original Article
An Overlying Fat Panniculus Affects Femur Bone Mass Measurement Neil Binkley, MD,*,1 Diane Krueger, BS,1 and Nellie Vallarta-Ast, RT(R), CDT2 1University
of Wisconsin Institute on Aging, 2Department of Radiology, Wm. S. Middleton VAMC, Madison WI
Abstract Dual-energy X-ray absorptiometry (DXA) is currently the gold standard technique for osteoporosis diagnosis. However, DXA has limitations, including artifacts, such as degenerative disease or metallic foreign bodies, that may confound bone mineral density (BMD) results. Because fat folds overlying the proximal femur may alter soft-tissue density in a nonuniform manner, this may be a currently unappreciated confounder of proximal femur BMD measurement. This possibility was evaluated in 127 patients (52 women/75 men) referred for routine BMD measurement who were identified as having a fat panniculus overlying their proximal femur scan area. Presence of a fat panniculus within the scan field was confirmed by visual assessment of images obtained utilizing a GE Lunar Expert-XL. Subsequently, these individuals were rescanned while retracting their fat panniculus away from the femur scan area without other repositioning between scans. In 49% of the men, and 56% of the women, either the femoral neck, trochanter, or total femur BMD differed by more than the least significant change at our facility. No pattern was observed to predict whether BMD would increase or decrease upon fat retraction. Subsequently, 30 patients were scanned using the standard and retracted technique twice, with repositioning between scans to establish precision. Retracted and standard precision was similar. In conclusion, an overlying fat panniculus may alter proximal femur BMD measurement, which would be expected to impair the ability to accurately diagnose low bone mass and monitor osteoporosis therapy. When a fat panniculus overlays the proximal femur scan area, its retraction should be part of routine densitometric practice. Key Words: Bone density; obesity; DXA; femur.
Introduction
bone mass measurement at the lumbar spine, such as degenerative arthritis, compression fracture, laminectomy, and contrast material, are widely recognized (2–5). However, factors spuriously altering measured bone mass at the proximal femur, such as degenerative arthritis (6) and degree of internal rotation (7), are less commonly appreciated. An additional confounding factor could be changes in soft-tissue radiation absorption. Consistent with this, dramatic alteration of measured hip BMD has been reported by placement of soft-tissue gluteal silicon implants (8). Similarly, differences in overlying softtissue thickness or fat might be expected to alter
Central bone mineral density (BMD) measurement utilizing dual-energy X-ray absorptiometry (DXA) is currently the gold standard for osteoporosis diagnosis and therapy monitoring (1). Although DXA is an accurate and precise test, confounders to
Received June 10, 2002; Accepted October 30, 2002 *Address correspondence to Neil Binkley, University of Wisconsin, Institute on Aging, 1300 University Avenue 2245 MSC, Madison, WI 53706. E-mail: [email protected]
199
200 measured BMD (9–11). This was initially suggested by a clinical case in which serial femur scans demonstrated a marked BMD change outside of what was expected given the patient’s clinical history. Further evaluation of these scans revealed variation in placement of the patient’s fat panniculus on the follow-up scan; this prompted investigation of a potential effect of overlying fat on proximal femur BMD measurement. Subsequently, the effect of overlying fat on proximal femur BMD measurement was evaluated in a series of 127 patients.
Materials and Methods Subjects The study population consists of 127 patients (52 women/75 men) who were referred to the Wm. S. Middleton VAMC for clinically indicated bone mass measurement in whom a fat panniculus could be visualized on DXA scan to overlay the proximal femur region. Their age (yr; mean ± SEM) was 65.2 ± 1.6 and 67.8 ± 1.1 for the women and men, respectively. Mean weight and body mass index (BMI) was 201 lbs ± 4.3 (SEM); 35 ± 0.7 kg/m2 (SEM) for the women and 227 lbs ± 4.4 (SEM); 34 ± 0.5 kg/m2 (SEM) for the men. The presence of a fat fold within the proximal femur scan field was confirmed on visual assessment of the DXA image by one technologist (NVA). Required approval was obtained from the University of Wisconsin Health Sciences Institutional Review Board prior to the initiation of research. Informed consent was obtained from all research subjects prior to data collection. Routine clinical DXA scanning at the Middleton VAMC consists of measuring BMD at the lumbar spine, proximal femur, and radius. World Health Organization (WHO) diagnostic criteria using the lowest T-score at the L1–L4 spine or proximal femur (femur neck, trochanter, or total) were applied to these patients; of the 52 women, 21 had normal BMD, 21 were osteopenic, and 10 were osteoporotic. In the 75 men, 19 had normal BMD, 30 were osteopenic, and 26 were osteoporotic.
DXA Acquisition All scans were conducted and analyzed by one technologist using a GE Lunar Expert-XL densitometer with software version 1.92. AP femur scans were performed in the supine position in standard Journal of Clinical Densitometry
Binkley et al. manner following manufacturer recommendations. The subsequent scan was immediately obtained with patient manually retracting their fat panniculus from the scan field as shown in Fig. 1A,B. No other repositioning occurred between scans. Standard analyses were performed in routine clinical manner with Tscores derived using the GE Lunar male or female normative database.
Precision Study Thirty patients (22 men/8 women) undergoing routine clinical bone mass measurement were studied to determine precision of the retraction technique. The mean (± SEM) age (yr), weight (pounds), and BMI (kg/m2) of these individuals was 63.8 ± 1.8, 220.6 ± 7.1, and 33.6 ± 0.7, respectively. Each subject was imaged in standard clinical fashion followed by a second scan while retracting their panniculus. The patient then stood from the table; following reposition, two similar images were repeated. Precision and least significant change (LSC) were calculated using published recommendations (12).
Results Panniculus Documentation/Retraction Fat lines owing to an overlying panniculus are easily visible on the computer screen and printed image with a GE Lunar Expert-XL. Using this instrument, it is readily apparent when manual retraction successfully removes the panniculus from the scan field (Fig. 2A,B). Whether the panniculus was present or retracted does not affect point typing of bone, tissue, or neutral (images not shown).
Effect of Panniculus Retraction on BMD In this group of 127 patients, the mean percent change in BMD following panniculus retraction was + 2% at the femur neck, – 2% at the total femur, and – 3% at the trochanter. However, at specific regions within an individual, dramatic BMD changes (up to 74%) were observed (Table 1 and Fig. 3A–C). Similar changes in BMD following panniculus retraction were observed in men and women (Table 1). Upon panniculus retraction, DXA-measured fat decreased from 36.5% ± 0.5 to 33.0% ± 0.6 (mean ± SEM). Concomitantly, no change in percent bone area, but a decrease in bone mineral content, was observed (data not shown). Volume 6, 2003
Overlying Fat Affects Hip BMD
201
Fig. 1. Manual retraction of an overlying fat panniculus. Patients were instructed to use two hands to retract their fat panniculus medially and upward away from the proximal femur scan area. (A) Pre-retraction. (B) Post-retraction.
Fig. 2. Documentation of fat panniculus retraction from scan field. Visual examination of the scan images provide documentation that the panniculus has been successfully removed from the scan image. (A) Pre-retraction. (B) Post-retraction. Arrows indicate the panniculus edge in (A); the fat line is not present in (B).
Journal of Clinical Densitometry
Volume 6, 2003
202
Binkley et al. Table 1 Percent Change in BMD Following Panniculus Retraction
Site Entire group n = 127 Women n = 52 Men n = 75
Femur neck 1.9 (0.8) range –12.0 to +73.8 1.2 (0.9) range –10.8 to 22.6 2.4 (1.2) range –12.0 to +73.8
Total femur –2.2 (0.3) range –15.4 to +7.6 –2.5 (0.4) range –8.1 to +7.6 –2.5 (0.4) range –15.4 to +6.8
Trochanter –3.0 (0.5) range –24.6 to +9.6 –1.6 (0.6) range –15.7 to +8.6 –3.4 (0.6) range –24.6 to +9.6
Data as mean ± (SEM).
Effect of Panniculus Retraction on Interpretation of Significant Change Based on a precision study conducted previously in usual clinical patients using published methodology (12), the least significant change in BMD at the Middleton VAMC is 0.033 g/cm2 at the total proximal femur, 0.040 grams/cm2 at the femoral neck, and 0.046 g/cm2 at the greater trochanter. This least significant change was exceeded at one of the three proximal femur regions in 56% (29/53) of the women and 49% (37/75) of the men (Fig. 3A–C). Furthermore, the measured BMD changed by ≥10% in 17% (22/127) of patients. Panniculus retraction did not consistently increase or decrease measured BMD. Specifically, at the femur neck, measured BMD increased in 69 and decreased in 58; at the total femur it increased in 21 and decreased in 102; and at the trochanter it increased in 30 and decreased in 98. It is apparent that measured BMD may increase at one site and decrease at another in the same patient upon panniculus retraction.
Proximal Femur BMD Precision With and Without Panniculus Retraction With no panniculus retraction, proximal femur LSC values of 0.074, 0.086, and 0.046 g/cm2 were obtained at the femur neck, trochanter, and total proximal femur, respectively. Utilizing the retraction technique in these same individuals, proximal femur BMD LSC was 0.066, 0.106, and 0.052 g/cm2 at the femur neck, trochanter, and total proximal femur, respectively. Therefore, proximal femur BMD precision was comparable when utilizing the retraction technique.
Discussion These data document marked variability in measured femur BMD based on presence or absence of a Journal of Clinical Densitometry
fat panniculus overlying the scan field in both men and women. Specifically, in 49% of men and 56% of women, a BMD change greater than the least significant change in g/cm2 was observed at one of the three proximal femur sites upon retraction of their panniculus from the scan field. Furthermore, greater than a 10% change, which would almost certainly be considered significant by all densitometrists, is observed in 17% of such individuals. Finally, measured BMD following panniculus retraction may change in different directions at various proximal femur regions of interest within the same individual. This effect has obvious ramifications on low bone mass diagnosis and monitoring of osteoporosis treatment efficacy. DXA technology assumes that there are two tissues that absorb radiation: bone and uniform soft tissue (13). However, nonuniform fat distribution is known to occur and affect bone mass measurement accuracy and precision (9,14). It is likely that the observed changes in measured BMD reflect different radiographic absorption from nonuniformity of softtissue thickness produced by a fat panniculus within the scan field. Similarly, alteration of soft-tissue radiation absorption by gluteal silicon implants affects measured femur BMD (8). As such, presence of an overlying fat panniculus must be added to the list of known proximal femur BMD measurement confounders, which currently includes improper rotation, metallic foreign bodies, and osteoarthritis (12). It should be noted that the excellent image quality obtained with the Expert-XL densitometer allowed easy visualization of overlying fat folds, thereby permitting identification of a potential explanation for the observed BMD differences. However, it is not established that overlying fat can be visualized as readily when utilizing other densitometers providing lower resolution images. In fact,
Volume 6, 2003
Overlying Fat Affects Hip BMD
203
Fig. 3. Effect of fat panniculus retraction on measured proximal femur BMD. Approximately 50% of men and women demonstrated a BMD difference greater than the least significant change when their fat panniculus was retracted. No pattern to increase or decrease measured BMD at either the (A) femoral neck, (B) total proximal femur, or (C) greater trochanter was reliably evident.
it is our anecdotal experience that fat lines are much more difficult, or impossible, to visualize on other instruments. Additionally, routine point typing of bone and soft tissue did not disclose abnormalities
Journal of Clinical Densitometry
to explain these observed differences in BMD. Thus, technologists and physicians may not be able to rely on the images to detect an overlying fat panniculus. Finally, it should be noted that alteration of
Volume 6, 2003
204 measured BMD following fat retraction is not unique to GE Lunar densitometers; similar observations were recently reported in a small number of patients imaged on a Hologic densitometer (15). Because body weight is a major determinant of bone mass (16), it might be assumed that overweight individuals would be protected from the development of osteoporosis. Clearly, this is an incorrect assumption, as demonstrated by the observation that 69% of the patients in this study were either osteopenic or osteoporotic. Given the high and increasing prevalence of obesity in the United States, currently approx 27% nationwide (17), it is probable that femoral BMD measurement errors owing to different positioning of overlying fat are common but underappreciated. A limitation of this study is the absence of direct bone mass documentation. Thus, it is not clear that the retracted BMD measurement is more accurate. However, to obtain documentation of accuracy would require cadaveric studies of DXA-measured bone mass compared with ash mineral content. It seems unlikely that such studies will ever be conducted in individuals with an overlying fat panniculus. Given that standardization of DXA measurements was performed largely in individuals without overlying soft tissue, it is intuitive that the retracted BMD value be used for diagnosis. Additionally, because overlying fat may induce unpredictable BMD variation, and precision following retraction is similar to the nonretracted state, a recommendation to routinely retract an overlying fat panniculus seems warranted. In conclusion, overlying fat may dramatically alter DXA-measured BMD at the total femur, femoral neck, and/or trochanteric regions. Whether the measured BMD will increase, decrease, or be unaffected by panniculus retraction cannot be predicted reliably. As such, when technologists observe this situation, retraction of the overlying fat panniculus should be standard densitometric practice. Additionally, when physicians detect a nonphysiologic change in proximal femur BMD on serial DXA scans, in the absence of other technical explanations, different positioning of overlying fat should be a consideration.
Acknowledgment This work was presented in abstract form at ISCD 2002. Journal of Clinical Densitometry
Binkley et al.
References 1. Kanis JA, Melton LJ, Christiansen C, Johnston CC, Khaltaev N. 1994 Perspective The diagnosis of osteoporosis. J Bone Miner Res 9:1137–1141. 2. Antonacci MD, Hanson DS, Leblanc A, Heggeness K. 1997 Regional variation in vertebral bone density and trabecular architecture are influenced by osteoarthritic change and osteoporosis. Spine 22:2393–2402. 3. Orwoll ES, Oviatt SK, Mann T. 1990 The impact of osteophytic and vascular calcifications on vertebral mineral density measurements in men. J Clin Endocrinol Metab 70:1202–1207. 4. Yu W, Gluer CC, Fuerst T. 1995 Influence of degenerative joint disease on spinal bone mineral measurements in postmenopausal women. Calcif Tissue Int 57:169–174. 5. Adler RA, Nordlie R, Burke TS. 1998 Increased bone mineral density in a man with known compression fractures. J Clin Densitom 1:275–278. 6. Preidler KW, White LS, Tashkin J, et al. 1997 Dual-energy xray absorptiometric densitometry in osteoarthritis of the hip. Acta Radiol 38:539–542. 7. Girard MS, Sartoris DJ, Moscona AV, Ramos E. 1994 Measured femoral density by dual-energy x-ray absorptiometry as a function of rotation. Orthop Rev 1:38–40. 8. Hauache OM, Vieira JGH, Alonso G, Martins LRF, Brandao C. 2000 Increased hip bone mineral density in a woman with gluteal silicon implant. J Clin Densitom 3:391–393. 9. Tothill P, Pye DW. 1992 Errors due to non-uniform distribution of fat in dual x-ray absorptiometry of the lumbar spine. Br J Radiol 65:807–813. 10. Tothill P, Avenell A. 1994 Errors in dual-energy x-ray absorptiometry of the lumbar spine owing to fat distribution and soft tissue thickness during weight change. Br J Radiol 67:71–75. 11. Svendsen OL, Hassager C, Skodt V, Christiansen C. 1995 Impact of soft tissue on in vivo accuracy of bone mineral measurements in the spine, hip and forearm: A human cadaver study. J Bone Miner Res 10:868–873. 12. Bonnick SL, Lewis LA. 2002 Precision in Bone Densitometry. In Bone Densitometry for Technologists. Humana Press, Totowa, NJ. 169–181. 13. Hansen MA, Hassager C, Overgaard K, Marslew U, Riis BJ, Christiansen C. 1990 Dual energy x-ray absorptiometry: a precise method of measuring bone mineral in the lumbar spine. J Nucl Med 31:1156–1162. 14. Valkema R, Verheij LF, Blokland JAK, et al. 1990 Limited precision of lumbar spine dual-photon absorptiometry by variations in the soft-tissue background. J Nucl Med 31:1774–1781. 15. Jankowski L, O’Brien-Schmack E. 2002 Panniculus adiposus: an unpredictable and often unrecognized source of precision and accuracy errors on DXA scans of the proximal femur. J Clin Densitom 5:SupplS57. 16. Holbrook TL, Barrett-Connor E. 1993 The association of lifetime weight and weight control patterns with bone mineral density in an adult community. Bone Miner 20:141–149. 17. Centers for Disease Control. 2002 Obesity and Overweight: A Public Health Epidemic. Centers for Disease Control, Washington, DC. Volume 6, 2003