Failure load of thoracic vertebrae correlates with lumbar bone mineral density measured by DXA

Fractures of the thoracic spine account for a large portion of vertebral fractures in the elderly, yet noninvasive measurements of bone mineral properties are limited to the L2–L4 vertebral bodies. The purpose of this investigation was to determine whether bone mineral properties of the umbar spine correlate with the failure properties of thoracic ertebrae. Cadaveric lumbar segments were scanned using dual-energy x-ray absorptiometry (DXA) from both the latcrol and anteroposterior projections. Three-body segments L1–L3 and T10–T12 were then compressed to create crush tractures in the L2 and T11 vertebral bodies, and linear corelation analyses were performed to compare each DXA measure with the failure properties of L2 and T11. Lumbar BMD from the lateral view correlated significantly with T11 altimate load (r=0.94, P<0.001), as did lumbar BMD from the anteroposterior projection (r=0.83, P=0.001). Significant correlations were also found between both lumbar BMD and BMC and the stiffness and energy to failure of I'll. Furthermore, BMD and BMC measured at L2 correlated significantly with L2 ultimate load, stiffness, and energy to failure. We conclude that bone mineral properties measured at the lumbar spine provide a valid assessment of the compressive strength of both thoracic and lumbar vertebrae. Lumbar BMD may therefore be used to derive an index for the prediction of thoracolumbar fractures to aid in the early intervention of vertebral fractures.Portions of this work were presented at the 40th Annual Meeting of the Orthopaedic Research Society and appeared in abstract form in the conference proceedings.     

Bone mineral density in feline mucopolysaccharidosis VI measured using dual-energy X-ray absorptiometry

Dual-energy X-ray absorptiometry (DXA) was used to determine the in vivo bone mineral density (BMD) and bone mineral content (BMC) of lumbar vertebrae in six cats affected with the inherited lysosomal storage disease mucopolysaccharidosis VI (MPS VI). DXA was also performed on MPS cats that had a bone marrow transplant (BMT) and total body irradiation (TBI) (MPS+BMT;n=7), normal cats that had a bone marrow transplant, and TBI (control+BMT; n=8) and normal cats (control; n=14). Following euthanasia, one of the lumbar vertebrae that had been scanned (L5) was harvested and bone volume (BV/TV%) was determined by histomorphometry. The in vivo BMD and BMD measurements were compared with the BV/TV%. There was a greater BMD and BMC in the MPS+BMT cats compared with the MPS cats but the difference was not statistically significant. However, there was a greater BV/TV% in the MPS+BMT cats compared with the MPS cats and the difference was significant (P=0.0152). Correlation between the noninvasive in vivo DXA measurements of BMD and BMC and the BV/TV% was significant (r ²=0.767, P<0.0001; r ²=0.504, P<0.0001). Noninvasive in vivo DXA was a rapid and precise method for measuring the lumbar BMD and BMC in cats and it correlated well with histomorphometric determination of bone mass. Further, the response of inherited storage diseases such as MPS VI to therapy, such as BMT, could be monitored in a longitudinal fashion using DXA.     

糖皮质激素对成年大鼠骨量的影响

目的利用双能X线吸收法(DXA)探讨成年大鼠接受糖皮质激素后骨量变化的规律。方法 21只44周龄SD雌性大鼠分别假性去卵巢+未注射糖皮质激素(SHAM组)、摘除双侧卵巢(OVX组)或注射甲基强的松龙[2.5 mg/(kg·d)](PRED组),应用扇形束DXA(QDR-4500A)每4周测定一次全身骨密度(BMD)、骨矿含量(BMC)、骨骼面积(AREA);术后12周处死,测定离体腰椎、股骨、胫骨及其兴趣区的BMD、BMC、AREA。压缩试验测定第二腰椎最大载荷和弹性模量。结果 (1)术后8周开始OVX组体重显著重于同龄SHAM组(8周时,P0.05,12周时P0.01),术后4周开始PRED组体重显著轻于同龄SHAM组(P0.05);(2)术后12周OVX组整体BMC显著高于SHAM组(P0.05),术后8、12周OVX组整体BMC显著高于PRED组(P0.05);(3)术后12周OVX组离体第5、6腰椎BMD及第6腰椎BMC显著低于SHAM组和PRED组(P0.05),PRED组离体各腰椎BMD、BMC、AREA与SHAM组无明显差异;(4)术后12周与SHAM组比较,OVX组离体股骨(-7.42%)、股骨远端(-10.85%)和近端(-6.92%)、胫骨近端(-11.40%)BMD显著降低(P0.05),其中股骨、股骨远端、胫骨近端BMC也显著降低(P0.05);(5)术后12周与SHAM组比较,PRED组离体股骨及各区BMD、BMC、AREA无显著性差异,整体胫骨及各区BMD无显著性差异;(6)术后12周与SHAM组比较,OVX组及PRED组胫骨中远端骨量增加;(7)与SHAM组比较,OVX组最大载荷和弹性模量显著降低,PRED组最大载荷显著降低。结论成熟期大鼠接受甲基强的松龙后,皮质骨和松质骨骨量没有显著变化,DXA检查难以发现其骨丢失情况;力学性能改变提示糖皮质激素更多的是引起骨质量的改变而导致了力学性能的下降及骨折的发生。     

Precision and accuracy of in vivo bone mineral measurement in rats using dual-energy X-ray absorptiometry

The aim of this study was to evaluate the precision and accuracy of dual-energy X-ray absorptiometry (DXA) for measuring bone mineral content at different sites of the skeleton in rats. In vitro the reproducibility error was very small (<1%), but in vivo the intra-observer variability ranged from 0.9% to 6.0%. Several factors have been shown to affect in vivo reproducibility: the reproducibility was better when the results were expressed as bone mineral density (BMD) rather than bone mineral content (BMC), intra-observer variability was better than the inter-observer variability, and a higher error was observed for the tibia compared with that for vertebrae and femur. The accuracy of measurement at the femur and tibia was assessed by comparing the values with ash weight and with biochemically determined calcium content. The correlation coefficients (R) between the in vitro BMC and the dry weight or the calcium content were higher than 0.99 for both the femur and the tibia. SEE ranged between 0.0 g (ash weight) and 2.0 mg (Ca content). Using in vitro BMC, ash weight could be estimated with an accuracy error close to 0 and calcium content with an error ranging between 0.82% and 6.80%. TheR values obtained between the in vivo and in vitro BMC were 0.98 and 0.97 respectively for femur and tibia, with SEE of 0.04 and 0.02 g respectively. In conclusion, the in vivo precision of the technique was found to be too low. To be of practical use it is important in the design of experimentation to try to reduce the measurement error. This can be achieved by performing measurements in the same position, by repeating measurements several times and by using the mean values of several BMD calculations performed by the same observer on each BMD measurement. Furthermore, better reproducibility can be obtained on the vertebra or the femur than on the tibia.     

绝经后妇女骨质疏松性椎体骨折与腰椎骨密度的关系

目的探讨绝经后妇女骨质疏松性椎体骨折与腰椎骨密度的关系。方法选择骨质疏松性椎体骨折的绝经后妇女23例为骨折组,无椎体骨折的25例绝经后骨质疏松妇女为对照组。两组的年龄、绝经年限、身高、体重、体重指数差异无显著性,均行胸腰椎正侧位X线摄片。用双能X线吸收仪（DXA）测量的腰椎（L2-4）前后位骨密度（BMD）、骨矿含量（BMC）和T值。结果骨折组BMD、BMC和T值均低于对照组（P〈0.01）。结论腰椎BMD降低与绝经后妇女的骨质疏松性椎体骨折相关。绝经后骨质疏松妇女应重视BMD变化,预防椎体骨折的发生。     

Failure strength of human vertebrae: prediction using bone mineral density measured by DXA and bone volume by micro-CT

Significant relationships exist between areal bone mineral density (BMD) derived from dual energy X-ray absorptiometry (DXA) and bone strength. However, the predictive validity of BMD for osteoporotic vertebral fractures remains suboptimal. The diagnostic sensitivity of DXA in the lumbar spine may be improved by assessing BMD from lateral-projection scans, as these might better approximate the objective of measuring the trabecular-rich bone in the vertebral body, compared to the commonly-used posterior-anterior (PA) projections. Nowadays, X-ray micro-computed tomography (μCT) allows non-destructive three-dimensional structural characterization of entire bone segments at high resolution. In this study, human lumbar cadaver spines were examined ex situ by DXA in lateral and PA projections, as well as by μCT, with the aims (1) to investigate the ability of bone quantity measurements obtained by DXA in the lateral projection and in the PA projection, to predict variations in bone quantity measurements obtained by μCT, and (2) to assess their respective capabilities to predict whole vertebral body strength, determined experimentally. Human cadaver spines were scanned by DXA in PA projections and lateral projections. Bone mineral content (BMC) and BMD for L2 and L3 vertebrae were determined. The L2 and L3 vertebrae were then dissected and entirely scanned by μCT. Total bone volume (BV(tot)=cortical+trabecular), trabecular bone volume (BV), and trabecular bone volume fraction (BV/TV) were calculated over the entire vertebrae. The vertebral bodies were then mechanically tested to failure in compression, to determine ultimate load. The variables BV(tot), BV, and BV/TV measured by μCT were better predicted by BMC and BMD measured by lateral-projection DXA, with higher R(2) values and smaller standard errors of the estimate (R(2)=0.65-0.90, SEE=11%-18%), compared to PA-projection DXA (R(2)=0.33-0.53, SEE=22%-34%). The best predictors of ultimate load were BV(tot) and BV assessed by μCT (R(2)=0.88 and R(2)=0.81, respectively), and BMC and BMD from lateral-projection DXA (R(2)=0.82 and R(2)=0.70, respectively). Conversely, BMC and BMD from PA-projection DXA were lower predictors of ultimate load (R(2)=0.49 and R(2)=0.37, respectively). This ex vivo study highlights greater capabilities of lateral-projection DXA to predict variations in vertebral body bone quantity as measured by μCT, and to predict vertebral strength as assessed experimentally, compared to PA-projection DXA. This provides basis for further exploring the clinical application of lateral-projection DXA analysis.     

Differential Effects of Bone Mineral Content and Bone Area on Vertebral Strength in a Swine Model

Since the biomechanical competence of a vertebral body may be closely related to the content and distribution of the bone mineral, we have evaluated the effects of projected vertebral bone area (BA) and bone mineral parameters [bone mineral content (BMC) or bone mineral density (BMD)] on their biomechanical competence. We used dual-energy X-ray absorptiometry (DXA) to assess the bone mineral parameters of 36 swine thoracic vertebrae (T1–T12) and 15 lumbar vertebrae (L1–L5) after removal of the posterior elements. The failure load, compressive stress, and the stored strain energy of these vertebral bodies were assessed by a uniaxial compressive test using an MTS 810 testing system. Multiple regression analysis showed a significantly negative effect of BA and significantly positive effect of BMC on the biomechanical competence (compressive stress, r²= 0.67, P < 0.0001; failure load, r²= 0.75, P < 0.0001). However, the stored strain energy was only related to the BMC (r²= 0.35, P < 0.0001). The contributory effects of BMC and BA on the biomechanical competence were not equal. The effects of BMC was larger than BA in determining the failure load and stored strain energy, whereas the reverse was found for the compressive stress. Using the log-transformed parameters as the regressors resulted in similar results. These results suggested the differential effects of BA and BMC in determining the biomechanical competence of vertebral bodies. We recommend the use of both parameters instead of BMD alone for evaluation of the vertebral biomechanical competence. Received: 26 June 1997 / Accepted: 8 January 1998     

New or worsening lumbar spine vertebral fractures increase lumbar spine bone mineral density and falsely suggest improved skeletal status.

Changes in lumbar spine bone mineral density (BMD) are determined by follow-up dual-energy x-ray absorptiometry (DXA) assessments. Inclusion of new or worsening vertebral fractures in follow-up measurements may increase BMD. To test this hypothesis, we examined pooled data from the placebo groups of two clinical trials that involved postmenopausal women with osteoporosis. DXA measurements of lumbar spine BMD, bone mineral content (BMC), and area were obtained at baseline and at two years in the Multiple Outcomes of Raloxifene Evaluation (MORE) Trial and at baseline and study endpoint in the Fracture Prevention Trial. In these trials, fractured vertebrae identified by expert radiologists during posterioranterior (PA) spine DXA assessment were excluded from the BMD assessment. Lateral spine radiographs were graded using a semi-quantitative (SQ) scale. Most new or worsening vertebral fractures (84%) diagnosed from lateral spine radiographs were not identified by PA spine DXA. While the follow-up BMD of vertebrae without new or worsening fractures did not change significantly, each unit increase in SQ grade was associated with an approximate 7.0% increase in the BMD of affected vertebrae (p < 0.001). Increases in BMD were highly correlated with increases in BMC (r = 0.87, p < 0.001). Inclusion of new or worsening vertebral fractures increased PA spine BMD measurements at follow-up, with the impact being related to the magnitude of change in SQ score. It is difficult to reliably identify vertebral fractures from PA spine DXA assessments. Inclusion of new or worsening vertebral fractures in follow-up DXA measurements may falsely suggest an improvement in spine BMD. Our suggestion is to perform lateral spine imaging concurrently with any assessment of PA spine BMD in patients who, in the opinion of the health care provider, may have vertebral fractures.     

The accuracy of volumetric bone density measurements in dual X-ray absorptiometry

New developments in dual x-ray absorptiometry (DXA) allow the performance of high precision anteroposterior (AP) and lateral scans of spinal bone mineral density (BMD, units: g/cm²) without the patient moving from the supine position. Data from both projections may be combined to give an estimate of the true volumetric bone mineral density (VBMD, units: g/cm³) of the lumbar vertebral bodies. This report presents a cadaver study designed to validate DXA measurements of volumetric bone density. Sections of whole lumbar spine were scanned in AP and lateral projections in a water tank to simulate soft tissue. Individual vertebrae were then divided to separate the vertebral body from the neural arch, and vertebral body volume was measured using the displacement of sand. The bone mineral content (BMC) of vertebral bodies and neural arches was measured by ashing at 250°C for 60 hours followed by 500°C for a further 24 hours. The results showed that DXA scanning systematically underestimated ashing data by 14% for AP BMC, 33% for vertebral body BMC, 23% for vertebral body volume, and 12% for VBMD. Despite these significant systematic errors, the DXA measurements and ashing values were highly correlated (r=0.979-0.992). The results suggested that after allowing for the systematic errors, lateral DXA parameters related closely to true BMC, volume, and VBMD.     

Accuracy and precision of bone mineral density and bone mineral content in excised rat humeri using fan beam dual-energy X-ray absorptiometry.

The aim of this study was the evaluation of fan beam dual-energy X-ray absorptiometry (DXA) for measuring bone mineral density (BMD) and bone mineral content (BMC) of isolated rat humeri. Defleshed rat humeri from male Lewis rats were examined with a Hologic QDR 4500 A (Hologic, Inc., Bedford, MA) high-resolution densitometer both in water and 0.9% saline solution. The small animal scan protocol with the regional high-resolution mode was used. BMC measured by DXA was compared with bone dry weight, ash weight, and bone calcium content. Furthermore, DXA BMD and BMC precision were determined. We also evaluated the effect of salinity of the water bath in which the bones were measured. Correlations (r(2)) of BMC, as determined by DXA with dry weight, ash weight, and bone calcium content, were 0.978, 0.988, and 0.890, respectively. DXA overestimated ash weight by 5%-9%. Precision errors for BMC (BMD) were 0.90% (0.76%) without and 1.3 (0.86) with repositioning. Changes in the salinity of the water bath had a significant influence on the DXA results: At the 0.9% physiological level, BMC (-4.4%) and area (-4.1%), but not BMD, values were significantly lower (p < 0.005) compared with measurements in tap water. Fan beam DXA is a highly accurate and precise technique for measuring BMC and BMD in excised small animal bones. A physiological saline concentration in the water bath had a significant impact on BMC and area, but not on BMD, and should therefore be strictly controlled to avoid an underestimation of BMC.     

中等强度跑台运动对去卵巢大鼠后肢骨骨矿物含量和骨密度的双重调节作用

目的 观察中等强度跑台运动对去卵巢大鼠后肢骨骨矿物含量(BMC)和骨密度(BMD)的影响.方法 将60只3月龄未经产雌性SD大鼠按体重随机分为假手术、去卵巢静止、去卵巢运动Ⅰ、去卵巢运动Ⅱ、去卵巢运动Ⅲ和去卵巢运动Ⅳ 6个组.各运动组经1周的跑台适应训练后,按实验设计分别进行为期14周的正式跑台训练.实验结束时,腹主动脉取血处死大鼠,双能χ-射线骨密度仪检测右侧游离股骨和胫骨的BMC和BMD.结果 ①与假手术组相比,去卵巢静止组股骨近端和远端以及胫骨近端BMC和BMD显著下降,但股骨中段以及胫骨中段和远端BMC和BMD无显著变化.②与去卵巢静止组相比,去卵巢运动Ⅰ组股骨近端和远端BMC显著增加,股骨中段以及胫骨3个部位BMC均无显著变化;去卵巢运动Ⅱ组和Ⅲ组股骨和胫骨3个部位BMC 均无显著变化;去卵巢运动Ⅳ组股骨3个部位BMC均无显著变化,而胫骨3个部位BMC均显著下降.③与去卵巢静止组相比,去卵巢运动Ⅰ组股骨近端和远端以及胫骨近端BMD 显著增加, 而股骨中段和胫骨中段和远端BMD无显著变化;去卵巢运动Ⅱ组和Ⅲ组股骨和胫骨任何部位BMD均没有显著变化;去卵巢运动Ⅳ组股骨3个部位BMD无显著变化,而胫骨3个部位BMD却显著下降.结论 较低中等强度跑台运动能减缓去卵巢大鼠股骨近端和远端骨矿物含量和骨密度的下降;而较高中等强度跑台运动却能加速去卵巢大鼠胫骨近端骨矿物含量和骨密度的下降.     

Regional and total body bone mineral content,bone mineral density,and total body tissue composition in children 8–16 years of age

Summary Normative values for total body bone mineral content (TBBM) and total body bone mineral density (TBMD) were derived from measurements on 234 children 8–16 years of age. In addition, bone mineral content (BMC) and bone mineral density (BMD) values for selected regions of interest and soft tissue (bone free lean and fat) for the total body are presented. Bone mineral and soft tissue values were determined by dual energy X-ray absorptiometry (DXA) using a Hologic QDR-2000 in the array mode. Results of a stepwise multiple regression analysis revealed a significant correlation between bone-free lean tissue (BFLT) and BMD (r² = 0.80) in girls. Adding age to the equation accounted for an additional 2% of the variance (P < 0.05) and height accounted for another 1% of the variance (P < 0.05). Body weight and fat tissue (FT) did not account for any additional variance. In boys BFLT correlated significantly with BMD (r² = 0.75;P < 0.05); none of the other predictor variables accounted for additional variance. No significant differences were found in TBBM or TBMD between boys and girls at any age. There was a significant overall gender effect for only three regions of interest. Boys had greater BMC in the head region and had greater BMD in the upper limbs, but post hoc analysis revealed no significant differences for any specific age groups. Girls had greater overall BMD in the pelvis, but this difference was only significant at the 15–6-year age group. The changes in BFLT and FT over the age ranges were consistent with the growth literature.The normative values can be applied to the assessment of children and adolescents with health problems that may impact on the skeleton as well as to research studies investigating bone mineral development in children.     

Validation of peripheral dual-energy X-ray absorptiometry for the measurement of bone mineral in intact and excised long bones of rats.

We evaluated the precision and accuracy of peripheral dual-energy X-ray absorptiometry (DXA) for the measurement of bone mineral density (BMD) and bone mineral content (BMC) in intact and excised femurs and tibias from rats. Thirty-one Sprague-Dawley rats (18F/13M; 114-360 g) were used in the study. Precision and accuracy were determined in 23 rats and prediction equations were evaluated in an independent sample of 8 animals. Precision was determined by measuring the right hindquarter three times with repositioning between scans. The femur and tibia were then excised, cleaned, and scanned in triplicate, with repositioning. CVs ranged from 0.66 to 2.24%. Accuracy of BMC was determined by comparison to bone ash values. BMC values for the intact and excised femur significantly overestimated bone ash (p < 0.001) by 33% and 5.5%, respectively. BMC for the intact tibia overestimated ash by 37% (p < 0.001), whereas BMC for the excised tibia underestimated ash by 1% (p < 0.05). However, BMC and bone ash were highly related for both bones, whether BMC was measured in the intact animal or after excision (r2 > 0.99). Cross-validation of prediction equations in an independent sample showed that there were no significant differences between predicted ash (based on BMC from DXA) and measured bone ash. These results suggest the peripheral DXA is a useful tool for measuring intact and excised rat leg bones.     

Lifelong administration of high doses of ibandronate increases bone mass and maintains bone quality of lumbar vertebrae in rats

As part of a long-term safety study the bisphosphonate ibandronate was investigated for its effects on bone quality in lumbar vertebrae in rats. Bone area, bone density and mechanical properties were assessed by peripheral quantitative computed tomography (pQCT), dual-energy X-ray absorptiometry (DXA) and compression tests. Female and male groups of Wistar rats received either vehicle or 3, 7 or 15 mg/kg per day of ibandronate over 104 weeks orally by gavage. Compared with the control group, bone mineral density, compressive strength and stiffness were significantly higher in ibandronate-treated animals, whereas no changes occurred in strain or modulus of elasticity. The increase in vertebral body stress was significant in some of the ibandronate-treated groups. The changes in mechanical properties appear to be due mainly to an increase in bone mass. A highly significant correlation was found between bone mineral density measured either by DXA (r=0.86) or pQCT (r=0.85) and maximal strength in vertebral bodies (p<0.0001 each). In conclusion, we demonstrated that lifelong administration of doses of ibandronate far in excess of any therapeutically intended dose not only increases bone mass and apparent density, but also maintains or even slightly improves bone quality. Bone mineral density measured either by pQCT or DXA can be used as a predictor for ultimate strength in rat lumbar vertebral bodies after treatment with ibandronate.     

Bone measures in HIV-1 infected children and adolescents: disparity between quantitative computed tomography and dual-energy X-ray absorptiometry measurements

Investigators have found that dual-energy X-ray absorptiometry (DXA) of areal bone mineral density (BMD) values in HIV-1 infected children and adolescents are reduced. Volumetric bone density (BD) measured by quantitative computed tomography (CT) in this population has not been studied. This study was designed to evaluate bone measurements in HIV-1 infected children and adolescents using DXA and CT. Fifty-eight children and adolescents (32 females and 26 males with a mean age ± SD of 12.0±3.9 years, age range 5.0–19.4 years) with perinatally acquired HIV-1 infection underwent simultaneous bone area and density evaluation by DXA and CT. Height and weight measurements as well as pubertal assessment were performed on the same day. All but four subjects were receiving highly active antiretroviral therapy (HAART). Subjects were matched with healthy children and adolescents for age, gender, and ethnicity. HIV-1 infected children were significantly shorter ( P <0.001), lighter ( P <0.005), and had delayed puberty ( P <0.001) compared to controls. Using DXA, HIV-1 infected subjects had significantly less bone area ( P <0.001), bone mineral content (BMC) ( P <0.005), and BMD ( P <0.05) at the vertebral level compared to controls. In addition, bone area ( P <0.001), BMC ( P <0.001), and BMD ( P <0.005) of the whole body were also reduced relative to controls. In contrast, using CT, HIV-1 infected subjects had similar vertebral BD compared to controls, but smaller vertebral height and cross-sectional area (CSA) ( P =0.01 and P <0.005, respectively). DXA Z-scores provided values significantly lower than CT Z-scores in the HIV-1 infected population ( P <0.01). After accounting for weight and vertebral height, stepwise multiple regression demonstrated that the prediction of CT BD values of L1 to L3 from DXA values of these vertebrae was significantly improved. HIV-1 infected children and adolescents have lower vertebral and whole body BMC and BMD DXA measures. In contrast, vertebral BD measurements by CT are normal. The lower bone measurements were primarily due to the decreased bone and body size of the HIV-1 subjects.This work was presented in part at the Pediatric Academic Society Annual Meeting, San Francisco, California, 1–4 May 2004.     

Relative contribution of vertebral body and posterior arch in female and male lumbar spine peak bone mass

Peak bone mass (PBM) is an important reference value in the diagnosis of osteoporosis. It is usually established by determining the areal bone mineral density (BMD in g/cm²) for a given site of the skeleton in young healthy adults. This measurement takes into account both the thickness and the integrated mineral density of the bone scanned. It should therefore be a major determinant of the resistance to mechanical stress. However, in lumbar spine the mean BMD as determined by dual-energy either isotopic or X-ray (DXA) absorptiometry in antero-posterior (ap) view was repeatedly found not to be different between male and female young healthy adults despite the greater volume of lumbar vertebral bodies in males. A greater contribution of the posterior vertebral arch to areal BMD-ap in females than in males could account for such an apparent discrepancy. In order to clarify this issue we have determined in 65 (32 male and 33 female) young healthy adults aged 20–35 years the relative contribution of the vertebral body (VB) and posterior vertebral arch (VA) to BMD and bone mineral content (BMC) of L2–3 measured by both antero-posterior and lateral (lat) scanning using DXA. In young healthy adults mean BMC in antero-posterior view was found not to be significantly different from the total BMC determined by lateral scanning including both VB and VA. This allowed us then to calculate the VA BMC by substracting VB BMC-lat from BMC-ap. The results indicated that the mean value for males was significantly greater than that for females for BMC-ap (male/female ratio (mean±SEM): 1.16±0.05,p<0.01), BMC-lat (1.38±0.07,p<0.001) and VB BMD-lat (1.16±0.04,p<0.001). In sharp contrast, no sex difference was found in BMD-ap (male/female ratio: 0.99±0.03) and VA BMC (male/female ratio: 0.97±0.06). VA BMC represented 44% and 53% (p<0.001) of BMC-ap in males and females, respectively. Furthermore, in neither sex was any correlation between VA BMC and VB BMC found. In summary, this study indicates that the relative contribution of the posterior vertebral arch to the bone mineral content of L2–3 is significantly smaller in males than in females. This difference could partly explain the absence of a sex difference in areal BMD as measured in antero-posterior view. In agreement with lumbar anthropomorphometric data this study further shows that the sex difference in vertebral body size, an important component in mechanical resistance, is expressed when areal BMD is measured in lateral but not in antero-posterior scanning. Finally, the data analysis underlines the quantitative importance of the vertebral arch in the value of areal BMD as measured by DXA in the classical antero-posterior view, and demonstrates the absence of a significant quantitative relationship between the bone mineral content of the vertebral body and that of the posterior vertebral arch.     

Accuracy of DXA scanning of the thoracic spine: cadaveric studies comparing BMC, areal BMD and geometric estimates of volumetric BMD against ash weight and CT measures of bone volume

Biomechanical studies of the thoracic spine often scan cadaveric segments by dual energy X-ray absorptiometry (DXA) to obtain measures of bone mass. Only one study has reported the accuracy of lateral scans of thoracic vertebral bodies. The accuracy of DXA scans of thoracic spine segments and of anterior-posterior (AP) thoracic scans has not been investigated. We have examined the accuracy of AP and lateral thoracic DXA scans by comparison with ash weight, the gold-standard for measuring bone mineral content (BMC). We have also compared three methods of estimating volumetric bone mineral density (vBMD) with a novel standard–ash weight (g)/bone volume (cm³) as measured by computed tomography (CT). Twelve T5–T8 spine segments were scanned with DXA (AP and lateral) and CT. The T6 vertebrae were excised, the posterior elements removed and then the vertebral bodies were ashed in a muffle furnace. We proposed a new method of estimating vBMD and compared it with two previously published methods. BMC values from lateral DXA scans displayed the strongest correlation with ash weight (r=0.99) and were on average 12.8% higher (p<0.001). As expected, BMC (AP or lateral) was more strongly correlated with ash weight than areal bone mineral density (aBMD; AP: r=0.54, or lateral: r=0.71) or estimated vBMD. Estimates of vBMD with either of the three methods were strongly and similarly correlated with volumetric BMD calculated by dividing ash weight by CT-derived volume. These data suggest that readily available DXA scanning is an appropriate surrogate measure for thoracic spine bone mineral and that the lateral scan might be the scan method of choice.     

Validation and application of dual-energy X-ray absorptiometry to measure bone mineral density in rabbit vertebrae.

The rabbit could be a superior animal model to use in bone physiology studies, for the rabbit does attain true skeletal maturity. However, there are neither normative bone mineral density (BMD) data on the rabbit nor are there any validation studies on the use of dual-energy X-ray absorptiometry (DXA) to measure spinal BMD in the rabbit. Therefore, our aim was twofold: first, to investigate whether DXA could be used precisely and accurately to determine the bone mineral content (BMC). bone area (BA). and BMD of the rabbit lumbar spine: Second. to evaluate the new generation fan-beam DXA (Hologic QDR-4500) with small animal software by comparing two DXA methodologies QDR-1000 and QDR-4500 with each other, as well as against volumetric bone density (VBMD) derived from Archimedes principle. As expected. there was a magnification error in the QDR-4500 (BMC, BA. and BMD increased by 52%. 38%. and 10%, respectively, when the vertebrae were positioned flat against the scanning table). With the magnification error kept constant (vertebrae positioned 10 cm above the scanning table to match the height in vivo). there were no differences among the mean BMC. BA. and BMD of the rabbit vertebrae (Ll-L7) in vivo and in vitro using the QDR-4500 (p > 0.05). BMC, BA, and BMD differed between QDR-1000 and QDR-4500 in vitro because of a magnification error when the vertebrae were flat on the table (p <0.0001). and, consequently. the machines did not correlate with one another (p > 0.05). However, the BMC, BA, and BMD of the two DXAs did significantly correlate with each other in vivo and in vitro when the magnification error was compensated for (r = 0.44 and 0.52. i2 = 0.45 and 0.63. and 12 = 0.41 and 0.60. respectively. p < 0.05-0.008). The BMC and BMD (in vivo and in vitro) of the rabbit vertebrae measured by QDR-4500 was significantly correlated with VMBD, ash weight, and mineral content (,2 = 0.67-0.90,j <0.01-0.0001). Therefore, the QDR-4500 can be used to yield precise and accurate measurements of the rabbit spine.     

DXA estimates of vertebral volumetric bone mineral density in children: potential advantages of paired posteroanterior and lateral scans.

Dual-energy X-ray absorptiometry (DXA) estimates of areal bone mineral density (BMD) are confounded by bone size in children. Two strategies have been proposed to estimate vertebral volumetric BMD: (1) bone mineral apparent density (BMAD) is based on the posteroanterior (PA) spine scan; (2) width-adjusted bone mineral density (WABMD) is based on paired PA lateral scans. The objective of this study was to compare DXA estimates of vertebral bone mineral content (BMC), volume and volumetric BMD obtained from Hologic PA scans (Hologic, Inc., Bedford, MA) alone, and paired PA lateral scans in 124 healthy children, ages 4 to 20 yr. The PA scans were used to estimate bone volume (PA Volume) as (PA Area)1.5 and BMAD as [(PA BMC)/(PA Volume)]. Paired PA lateral scans were used to estimate width-adjusted bone volume (WA Volume) as [(pi/4)(PA width)(lateral depth)(vertebral height)] and WABMD as [(lateral BMC)/(WA Volume)]. Generalized estimating equations were used to compare the relationship between scan type (PA vs. paired PA lateral) and bone outcomes, and the effects of height and maturation on this relationship. The estimates of BMC and volume derived from PA scans and paired PA lateral scans were highly correlated (r>0.97); WABMD and BMAD were less correlated (r=0.81). The increases in BMC, volume, and volumetric BMD with greater height and maturation were significantly larger (all p<0.001) when estimated from paired PA lateral scans, compared with PA scans alone. The proportion of spine BMC contained within the vertebral body, versus the cortical spinous processes, increased significantly with age (p<0.001) from 28% to 69%. The smaller increases in bone measures on PA scans may have been due to magnification error by the fan beam as posterior tissue thickness increased in taller, more mature subjects, and the distance of the vertebrae from the X-ray source increased. In conclusion, paired Hologic PA lateral scans may increase sensitivity to growth-related increases in trabecular BMC and density in the spine, with less bias due to magnification error.     

Ability of different techniques of measuring bone mass to determine vertebral bone loss in aging female rats

Summary Changes in vertebral trabecular bone were quantified in female Wistar rats. This study utilized single photon absorptiometry for the measurement of bone mineral content (BMC), quantitative computed tomography (QCT) for the measurement of bone mineral density (BMD), and image analysis histomorphometry for the measurement of trabecular bone volume (TBV). The above measurements were accompanied by biochemical assays of protein and calcium concentrations in the tissues. Also, the activity of bone alkaline and acid phosphatases was measured. Lumbar vertebrae (L₄, L₅) in old rats 27 months old, compared with those of young rats 7 months old, showed significant decreases in BMC, BMD, TBV, protein and calcium, and enzyme activity. A high degree of correlation was recorded between the above changes. The various changes were accompanied by a marked reduction in the overall wet weight of the vertebrae. Hence, new noninvasive methods to quantitate bone mass can be appliedin vivo to small laboratory animals such as the rat. These methods are much more accurate than standard radiographs in quantitating bone loss and are, therefore, recommended for experimental longitudinal studies related to aging of the skeleton. This study was presented in part at the 2nd International Congress of Biomedical Gerontology, Hamburg, Federal Republic of Germany, July 1987, and at the 5th Annual Meeting of the Japanese Society for Bone and Mineral Research, Tokyo, Japan, August 1987.