Alterations of cortical and trabecular architecture are associated with fractures in postmenopausal women, partially independent of decreased BMD measured by DXA: the OFELY study

High-resolution peripheral quantitative computed tomography can assess microstructural and mechanical properties of human distal tibial bone

High-resolution peripheral quantitative computed tomography (HR-pQCT) is a newly developed in vivo clinical imaging modality. It can assess the 3D microstructure of cortical and trabecular bone at the distal radius and tibia and is suitable as an input for microstructural finite element (microFE) analysis to evaluate bone's mechanical competence. In order for microstructural and image-based microFE analyses to become standard clinical tools, validation with a current gold standard, namely, high-resolution micro-computed tomography (microCT), is required. Microstructural measurements of 19 human cadaveric distal tibiae were performed for the registered HR-pQCT and microCT images, respectively. Next, whole bone stiffness, trabecular bone stiffness, and elastic moduli of cubic subvolumes of trabecular bone in both HR-pQCT and microCT images were determined by microFE analysis. The standard HR-pQCT patient protocol measurements, derived bone volume fraction (BV/TV(d)), trabecular number (Tb.N*), trabecular thickness (Tb.Th), trabecular spacing (Tb.Sp), and cortical thickness (Ct.Th), as well as the voxel-based direct measurements, BV/TV, Tb.N*, Tb.Th*, Tb.Sp*, Ct.Th, bone surface-to-volume ratio (BS/BV), structure model index (SMI), and connectivity density (Conn.D), correlated well with their respective gold standards, and both contributed to microFE-predicted mechanical properties in either single or multiple linear regressions. The mechanical measurements, although overestimated by HR-pQCT, correlated highly with their gold standards. Moreover, elastic moduli of cubic subvolumes of trabecular bone predicted whole bone or trabecular bone stiffness in distal tibia. We conclude that microstructural measurements and mechanical parameters of distal tibia can be efficiently derived from HR-pQCT images and provide additional information regarding bone fragility.

Finite element analysis performed on radius and tibia HR-pQCT images and fragility fractures at all sites in postmenopausal women

Assessment of bone strength at the radius by micro-finite element analysis (muFEA) has already been associated with wrist fractures. In this study, the analysis has been extended to the distal tibia, and to a larger group of subjects to examine the association with several types of fragility fractures. We have compared muFEA based on in vivo HR-pQCT measurements of BMD and microarchitecture at the radius and tibia, in a case-control study involving 101 women with prevalent fragility fracture and 101 age-matched controls, from the OFELY cohort. Areal BMD was measured by DXA at the radius and the hip. All parameters were analyzed in a principal component (PC) analysis (PCA), and associations between PCs and fractures were computed as odds ratios (OR [95% CI]) per SD change. Radius (tibia) PCA revealed three independent components explaining 76% (77%) of the total variability of bone characteristics. The first PC describing bone strength and quantity, explained 50% (46%) of variance with an OR=1.84 [1.27-2.67] (2.92 [1.73-4.93]). The second PC including trabecular microarchitecture, explained 16% (10%) of variance, with OR=1.29 [0.90-1.87] (1.11 [0.82-1.52]). The third PC related to load distribution explained 10% (20%) of variance, with OR=1.54 [1.06-2.24] (1.32 [0.89-1.96]). Moreover, at the radius, vertebral fractures were associated with trabecular microarchitecture PC with OR=1.86 [1.14-3.03], whereas nonvertebral fractures were associated with bone strength and quantity PC with OR=2.03 [1.36-3.02]. At the tibia, both vertebral (OR=2.92 [1.61-5.28]) and nonvertebral fracture (2.64 [1.63-4.27]) were associated to bone strength and quantity PC. In conclusion, muFEA parameters at the radius and tibia were associated with all types of fragility fractures. We have also shown that muFEA parameters obtained with distal tibia data were associated with prevalent fractures with a similar magnitude that with parameters obtained at the radius.

Assessment of bone quality within the tuberosities of the osteoporotic humeral head: relevance for anchor positioning in rotator cuff repair

BACKGROUND

Tears of the rotator cuff are highly prevalent in patients older than 60 years, thereby presenting a population also suffering from osteopenia or osteoporosis. Suture fixation in the bone depends on the holding strength of the anchoring technique, whether a bone tunnel or suture anchor is selected. Because of osteopenic or osteoporotic bone changes, suture anchors in the older patient might pull out, resulting in failure of repair.

HYPOTHESIS

The aim of our study was to analyze the bone quality within the tuberosities of the osteoporotic humeral head using high-resolution quantitative computed tomography (HR-pQCT).

STUDY DESIGN

Descriptive laboratory study.

METHODS

Thirty-six human cadaveric shoulders were analyzed using HR-pQCT. The mean bone volume to total volume (BV/TV) as well as trabecular bone mineral densities (trabBMDs) of the greater tuberosity (GT) and the lesser tuberosity (LT) were determined. Within the GT, 6 volumes of interest (VOIs) within the LT, and 2 VOIs and 1 control volume within the subchondral area beyond the articular surface were set.

RESULTS

Comparing BV/TV of the medial and the lateral row, significantly higher values were found medially (P < .001). The highest BV/TV, 0.030% + or - 0.027%, was found in the posteromedial portion of the GT (P < .05). Regarding the analysis of the LT, no difference was found comparing the superior (BV/TV: 0.024% + or - 0.022%) and the inferior (BV/TV: 0.019% + or - 0.016%) portion. Analyzing trabBMD, equal proportions were found. An inverse correlation with a correlation coefficient of -0.68 was found regarding BV/TV of the posterior portion of the GT and age (P < .05).

CONCLUSION

Significant regional differences of trabecular microarchitecture were found in our HR-pQCT study. The volume of highest bone quality resulted for the posteromedial aspect of the GT. Moreover, a significant correlation of bone quality within the GT and age was found, while the bone quality within the LT seems to be independent from it.

CLINICAL RELEVANCE

The shape of the rotator cuff tear largely determines the bony site of tendon reattachment, although the surgeon has distinct options to modify anchor positioning. According to our results, placement of suture anchors in a medialized way at the border to the articular surface might guarantee a better structural bone stock.

Quantifying the material and structural determinants of bone strength

The ability of a bone to resist fracture depends on the amount of bone present, the spatial distribution of the bone mass as cortical and trabecular bone and the intrinsic properties of the bone material. Whereas low areal bone mineral density (aBMD) predicts fractures, its sensitivity and specificity is low, as over 50% of fractures occur in persons without osteoporosis by BMD testing and most women with osteoporosis do not sustain a fracture. New non-invasive imaging techniques, including three-dimensional (3D) assessments of bone density and geometry, microarchitecture and integrated measurements of bone strength such as finite element analysis (FEA), provide estimates of bone strength that can be used to increase the sensitivity and specificity of fracture risk assessment. Initial observations have shown that these techniques provide information that will improve our understanding of the pathophysiology of skeletal fragility and suggest that these techniques are likely to have a role in the clinical management of individuals at risk for fracture.

Hormone predictors of abnormal bone microarchitecture in women with anorexia nervosa

Osteopenia is a complication of anorexia nervosa (AN) associated with a two- to three-fold increase in fractures. Nutritional deficits and hormonal abnormalities are thought to mediate AN-induced bone loss. Alterations in bone microarchitecture may explain fracture risk independent of bone mineral density (BMD). Advances in CT imaging now allow for noninvasive evaluation of trabecular microstructure at peripheral sites in vivo. Few data are available regarding bone microarchitecture in AN. We therefore performed a cross-sectional study of 23 women (12 with AN and 11 healthy controls) to determine hormonal predictors of trabecular bone microarchitecture. Outcome measures included bone microarchitectural parameters at the ultradistal radius by flat-panel volume CT (fpVCT); BMD at the PA and lateral spine, total hip, femoral neck, and ultradistal radius by dual energy X-ray absorptiometry (DXA); and IGF-I, leptin, estradiol, testosterone, and free testosterone levels. Bone microarchitectural measures, including apparent (app.) bone volume fraction, app. trabecular thickness, and app. trabecular number, were reduced (p<0.03) and app. trabecular spacing was increased (p=0.02) in AN versus controls. Decreased structural integrity at the ultradistal radius was associated with decreased BMD at all sites (p<or=0.05) except for total hip. IGF-I, leptin, testosterone, and free testosterone levels predicted bone microarchitecture. All associations between both IGF-I and leptin levels and bone microarchitectural parameters and most associations between androgen levels and microarchitecture remained significant after controlling for body mass index. We concluded that bone microarchitecture is abnormal in women with AN. Endogenous IGF-I, leptin, and androgen levels predict bone microarchitecture independent of BMI.

Assessment of trabecular and cortical architecture and mechanical competence of bone by high-resolution peripheral computed tomography: comparison with transiliac bone biopsy

We compared microarchitecture and mechanical competence parameters measured by high-resolution peripheral quantitative computed tomography (HR-pQCT) and finite-element analysis of radius and tibia to those measured by histomorphometry, micro-CT, and finite-element analysis of transiliac bone biopsies. Correlations were weak to moderate between parameters measured on biopsies and scans.

INTRODUCTION

HR-pQCT is a new imaging technique that assesses trabecular and cortical bone microarchitecture of the radius and tibia in vivo. The purpose of this study was to determine the extent to which microarchitectural variables measured by HR-pQCT reflect those measured by the "gold standard," transiliac bone biopsy.

METHODS

HR-pQCT scans (Xtreme CT, Scanco Medical AG) and iliac crest bone biopsies were performed in 54 subjects (aged 39 +/- 10 years). Biopsies were analyzed by 2D quantitative histomorphometry and 3D microcomputed tomography (microCT). Apparent Young's modulus, an estimate of mechanical competence or strength, was determined by micro-finite-element analysis (microFE) of biopsy microCT and HR-pQCT images.

RESULTS

The strongest correlations observed were between trabecular parameters (bone volume fraction, number, separation) measured by microCT of biopsies and HR-pQCT of the radius (R 0.365-0.522; P < 0.01). Cortical width of biopsies correlated with cortical thickness by HR-pQCT, but only at the tibia (R = 0.360, P < 0.01). Apparent Young's modulus calculated by microFE of biopsies correlated with that calculated for both radius (R = 0.442; P < 0.001) and tibia (R = 0.380; P < 0.001) HR-pQCT scans.

CONCLUSIONS

The associations between peripheral (HR-pQCT) and axial (transiliac biopsy) measures of microarchitecture and estimated mechanical competence are significant but modest.

Application of high-resolution skeletal imaging to measurements of volumetric BMD and skeletal microarchitecture in Chinese-American and white women: explanation of a paradox

Asian women have lower rates of hip and forearm fractures despite lower areal BMD (aBMD) by DXA compared with white women and other racial groups. We hypothesized that the lower fracture rates may be explained by more favorable measurements of volumetric BMD (vBMD) and microarchitectural properties, despite lower areal BMD. To address this hypothesis, we used high-resolution pQCT (HRpQCT), a new method that can provide this information noninvasively. We studied 63 premenopausal Chinese-American (n = 31) and white (n = 32) women with DXA and HRpQCT. aBMD by DXA did not differ between groups for the lumbar spine (1.017 +/- 0.108 versus 1.028 +/- 0.152 g/cm(2); p = 0.7), total hip (0.910 +/- 0.093 versus 0.932 +/- 0.134 g/cm(2); p = 0.5), femoral neck (0.788 +/- 0.083 versus 0.809 +/- 0.129 g/cm(2); p = 0.4), or one-third radius (0.691 +/- 0.052 versus 0.708 +/- 0.047 g/cm(2); p = 0.2). HRpQCT at the radius indicated greater trabecular (168 +/- 41 versus 137 +/- 33 mg HA/cm(3); p = <0.01) and cortical (963 +/- 46 versus 915 +/- 42 mg HA/cm(3); p < 0.0001) density; trabecular bone to tissue volume (0.140 +/- 0.034 versus 0.114 +/- 0.028; p = <0.01); trabecular (0.075 +/- 0.013 versus 0.062 +/- 0.009 mm; p < 0.0001) and cortical thickness (0.98 +/- 0.16 versus 0.80 +/- 0.14 mm; p < 0.0001); and lower total bone area (197 +/- 34 versus 232 +/- 33 mm(2); p = <0.001) in the Chinese versus white women and no difference in trabecular number, spacing, or inhomogeneity before adjustment for covariates. Similar results were observed at the weight-bearing tibia. At the radius, adjustment for covariates did not change the direction or significance of differences except for bone, which became similar between the groups. However, at the tibia, adjustment for covariates attenuated differences in cortical BMD and bone area and accentuated differences in trabecular microarchitecture such that Chinese women additionally had higher trabecular number and lower trabecular spacing, as well as inhomogeneity after adjustment. Using the high-resolution technology, the results provide a mechanistic explanation for why Chinese women have fewer hip and forearm fractures than white women.

Automated simulation of areal bone mineral density assessment in the distal radius from high-resolution peripheral quantitative computed tomography

SUMMARY

An automated image processing method is presented for simulating areal bone mineral density measures using high-resolution peripheral quantitative computed tomography (HR-pQCT) in the ultra-distal radius. The accuracy of the method is validated against clinical dual X-ray absorptiometry (DXA). This technique represents a useful reference to gauge the utility of novel 3D quantification methods applied to HR-pQCT in multi-center clinical studies and potentially negates the need for separate forearm DXA measurements.

INTRODUCTION

Osteoporotic status is primarily assessed by measuring areal bone mineral density (aBMD) using 2D dual X-ray absorptiometry (DXA). However, this technique does not sufficiently explain bone strength and fracture risk. High-resolution peripheral quantitative computed tomography (HR-pQCT) has been introduced as a method to quantify 3D bone microstructure and biomechanics. In this study, an automated method is proposed to simulate aBMD measures from HR-pQCT distal radius images.

METHODS

A total of 117 subject scans were retrospectively analyzed from two clinical bone quality studies. The distal radius was imaged by HR-pQCT and DXA on one of two devices (Hologic or Lunar). Areal BMD was calculated by simulation from HR-pQCT images (aBMD(sim)) and by standard DXA analysis (aBMD(dxa)).

RESULTS

The reproducibility of the simulation technique was 1.1% (root mean-squared coefficient of variation). HR-pQCT-based aBMD(sim) correlated strongly to aBMD(dxa) (Hologic: R (2) = 0.82, Lunar: R (2) = 0.87), though aBMD(sim) underestimated aBMD(dxa) for both DXA devices (p < 0.0001). Finally, aBMD(sim) predicted aBMD at the proximal femur and lumbar spine with equal power compared to aBMD(dxa).

CONCLUSION

The results demonstrate that aBMD can be simulated from HR-pQCT images of the distal radius. This approach has the potential to serve as a surrogate forearm aBMD measure for clinical HR-pQCT studies when axial bone mineral density values are not required.

Bone microarchitecture and stiffness in premenopausal women with idiopathic osteoporosis

CONTEXT

Idiopathic osteoporosis (IOP) is an uncommon disorder in which low areal bone mineral density (aBMD) and/or fractures occur in otherwise healthy premenopausal women.

OBJECTIVES

Our objectives were to characterize bone mass, microarchitecture, and trabecular bone stiffness in premenopausal IOP and to determine whether women with low aBMD who have never fractured have abnormal microarchitecture and stiffness.

DESIGN, SETTING, AND PATIENTS

We conducted a prospective cohort study of 27 normal controls and 31 women with IOP defined by low trauma fracture (n = 21) or low BMD (Z score <or=-2.0; n = 10).

MAIN OUTCOME MEASURES

We assessed aBMD by dual-energy x-ray absorptiometry; volumetric BMD and cortical and trabecular microarchitecture of the radius and tibia by high-resolution (82 microm) peripheral quantitative computed tomography; and trabecular bone stiffness (elastic moduli), estimated by micro-finite element analysis.

RESULTS

Fracture subjects did not differ from controls by age or body mass index, which was lower in low-BMD subjects than controls. Fracture subjects also had lower aBMD than controls at all sites (P < 0.05-0.0001). Bone size was similar in controls and fracture subjects but 10.6% smaller in low-BMD subjects (P < 0.05). Every trabecular parameter in both fracture and low-BMD groups was markedly worse than controls (P < 0.01-0.0001). Cortical thickness was significantly lower in both fracture and low-BMD groups at the tibia but not radius. Bone stiffness estimated by micro-finite element analysis was comparably reduced in low-BMD and fracture groups.

CONCLUSION

Premenopausal women with IOP had marked trabecular microarchitectural deterioration at the radius and tibia. Cortical parameters were affected only at the tibia. Although they had not fractured, microarchitectural deterioration was similar in IOP women with low BMD and those with fractures.

Classification of trabeculae into three-dimensional rodlike and platelike structures via local inertial anisotropy

Trabecular bone microarchitecture is a significant determinant of the bone's mechanical properties and is thus of major clinical relevance in predicting fracture risk. The three-dimensional nature of trabecular bone is characterized by parameters describing scale, topology, and orientation of structural elements. However, none of the current methods calculates all three types of parameters simultaneously and in three dimensions. Here the authors present a method that produces a continuous classification of voxels as belonging to platelike or rodlike structures that determines their orientation and estimates their thickness. The method, dubbed local inertial anisotropy (LIA), treats the image as a distribution of mass density and the orientation of trabeculae is determined from a locally calculated tensor of inertia at each voxel. The orientation entropies of rods and plates are introduced, which can provide new information about microarchitecture not captured by existing parameters. The robustness of the method to noise corruption, resolution reduction, and image rotation is demonstrated. Further, the method is compared with established three-dimensional parameters including the structure-model index and topological surface-to-curve ratio. Finally, the method is applied to data acquired in a previous translational pilot study showing that the trabecular bone of untreated hypogonadal men is less platelike than that of their eugonadal peers.

Role of trabecular microarchitecture in whole-vertebral body biomechanical behavior

The role of trabecular microarchitecture in whole-vertebral biomechanical behavior remains unclear, and its influence may be obscured by such factors as overall bone mass, bone geometry, and the presence of the cortical shell. To address this issue, 22 human T(9) vertebral bodies (11 female; 11 male; age range: 53-97 yr, 81.5 +/- 9.6 yr) were scanned with microCT and analyzed for measures of trabecular microarchitecture, BMC, cross-sectional area, and cortical thickness. Sixteen of the vertebrae were biomechanically tested to measure compressive strength. To estimate vertebral compressive stiffness with and without the cortical shell for all 22 vertebrae, two high-resolution finite element models per specimen-one intact model and one with the shell removed-were created from the microCT scans and virtually compressed. Results indicated that BMC and the structural model index (SMI) were the individual parameters most highly associated with strength (R(2) = 0.57 each). Adding microarchitecture variables to BMC in a stepwise multiple regression model improved this association (R(2) = 0.85). However, the microarchitecture variables in that regression model (degree of anisotropy, bone volume fraction) differed from those when BMC was not included in the model (SMI, mean trabecular thickness), and the association was slightly weaker for the latter (R(2) = 0.76). The finite element results indicated that the physical presence of the cortical shell did not alter the relationships between microarchitecture and vertebral stiffness. We conclude that trabecular microarchitecture is associated with whole-vertebral biomechanical behavior and that the role of microarchitecture is mediated by BMC but not by the cortical shell.

Accuracy of volumetric bone mineral density measurement in high-resolution peripheral quantitative computed tomography

Accurate bone mineral density (BMD) quantification is critical in clinical assessment of fracture risk and in the research of age-, disease-, and treatment-related musculoskeletal changes. The development of high-resolution peripheral quantitative computed tomography (HR-pQCT) imaging has made possible in vivo assessment of compartmental volumetric BMD (vBMD) and bone micro-architecture in the distal radius and tibia. HR-pQCT imaging relies on a polychromatic X-ray source and therefore is subject to beam hardening as well as scatter artifacts. In light of these limitations, we hypothesize that the accuracy of HR-pQCT vBMD measurement in the trabecular compartment (vBMD(trab)) is not independent of bone density and geometry, but rather influenced by variations in trabecular bone volume fraction and cortical thickness. The goal of this study, therefore, was to evaluate the accuracy of HR-pQCT vBMD(trab) measurement in the radius and tibia, and to determine the dependence of this measurement on geometric and densitometric parameters. Our approach was to use a series of idealized hydroxyapatite (HA) phantoms with varying densities and geometries to quantify the accuracy of HR-pQCT analysis. Two sets of custom-made HA phantoms designed to mimic the distal tibia and distal radius were manufactured. Geometric and densitometric specifications were based on a dataset of healthy volunteers and osteopenic patients. Multiple beam hardening correction (BHC) algorithms were implemented and evaluated in their ability to reduce measurement error. Substantial errors in measured vBMD(trab) were found. Overestimation of vBMD(trab) increased proportional to cortical shell thickness and decreased proportional to insert density. The most pronounced vBMD(trab) overestimation therefore occurred in the phantoms with the lowest insert densities and highest shell thickness, where error was as high as 20 mg HA/cm3 (33%) in the radius phantom and 25 mg HA/cm(3) (41%) in the tibia phantom. Error in vBMD(trab) propagates to the calculation of micro-architectural measures; 41% error in vBMD(trab) will produce 41% error in volume fraction (BV/TV) and trabecular thickness (Tb.Th), and 5% error in trabecular separation (Tb.Sp). BHC algorithms supplied by the manufacturer failed to eliminate these errors. Our results confirm that geometric and densitometric variations influence the accuracy of HR-pQCT vBMD(trab) measurements, and must be considered when interpreting data across populations or time-points.

Assessment of trabecular bone structure using MDCT: comparison of 64- and 320-slice CT using HR-pQCT as the reference standard

Objectives

The aim of our study was to perform trabecular bone structure analysis with images from 64- and 320-slice multidetector computed tomography (MDCT) and to compare these with high-resolution peripheral computed tomography (HR-pQCT).

Materials and methods

Twenty human cadaver distal forearm specimens were imaged on a 64- and 320-slice MDCT system at 120 kVp, 200 mA and 135 kVp, 400 mA (in-plane pixel size 234 µm; slice thickness 500 µm). HR-pQCT imaging was performed at an isotropic voxel size of 41 µm. Bone volume fraction (BV/TV), trabecular number (Tb.N), thickness (Tb.Th) and separation (Tb.Sp) were computed.

Results

MDCT-derived BV/TV and Tb.Sp were highly correlated (r = 0.92–0.96, p < 0.0001) with the corresponding HR-pQCT parameters. Tb.Th was the only structure measure that did not yield any significant correlation.

Conclusion

The 64- and 320-slice MDCT systems both perform equally well in depicting trabecular bone architecture. However, because of constrained resolutions accurate derivation of trabecular bone measures is limited to only a subset of microarchitectural parameters.

Advances in osteoporosis imaging

In the assessment of osteoporosis, the measurement of bone mineral density (BMD(a)) obtained from dual energy X-ray absorptiometry (DXA; g/cm(2)) is the most widely used parameter. However, bone strength and fracture risk are also influenced by parameters of bone quality such as micro-architecture and tissue properties. This article reviews the radiological techniques currently available for imaging and quantifying bone structure, as well as advanced techniques to image bone quality. With the recent developments in magnetic resonance (MR) techniques, including the availability of clinical 3T scanners, and advances in computed tomography (CT) technology (e.g. clinical Micro-CT), in-vivo imaging of the trabecular bone architecture is becoming more feasible. Several in-vitro studies have demonstrated that bone architecture, measured by MR or CT, was a BMD-independent determinant of bone strength. In-vivo studies showed that patients with, and without, osteoporotic fractures could better be separated with parameters of bone architecture than with BMD. Parameters of trabecular architecture were more sensitive to treatment effects than BMD. Besides the 3D tomographic techniques, projection radiography has been used in the peripheral skeleton as an additional tool to better predict fracture risk than BMD alone. The quantification of the trabecular architecture included parameters of scale, shape, anisotropy and connectivity. Finite element analyses required highest resolution, but best predicted the biomechanical properties of the bone. MR diffusion and perfusion imaging and MR spectroscopy may provide measures of bone quality beyond trabecular micro-architecture.

Assessment of material, structural, and functional properties of the human skeleton by pQCT systems

Peripheral quantitative computed tomography (pQCT) systems measure bone parameters noninvasively using low radiation doses. This limits image resolution but is practical for the diagnosis and quantitative monitoring of the properties of the peripheral human skeleton. pQCT determines volumetric bone mineral density separately in trabecular and cortical bone. It may combine densitometry determinations with geometric estimates and use strain-stress indexes, and it may be used to analyze muscle variables in some areas, allowing the study of regional fragility. Experimental and clinical ex vivo studies show that pQCT variables correlate with biomechanical predictors of fragility and/or fractures. Since pQCT was approved by the US Food and Drug Administration in 1997, new skeletal regions (human femur and mandible) have been considered in the development of the system. Basically, pQCT explores intraindividual and interindividual variations in greater detail and compares the impact of skeletal diseases, risk factors, and anabolic and catabolic treatments within a given bone cross section.

Quantitative computed tomography (QCT) of the forearm using general purpose spiral whole-body CT scanners: accuracy, precision and comparison with dual-energy X-ray absorptiometry (DXA)

BACKGROUND

Dual-energy X-ray absorptiometry (DXA) allows clinically relevant measurement of bone mineral density (BMD) at central and appendicular skeletal sites, but DXA has a limited ability to assess bone geometry and cannot distinguish between the cortical and trabecular bone compartments. Quantitative computed tomography (QCT) can supplement DXA by enabling geometric and compartmental bone assessments. Whole-body spiral CT scanners are widely available and require only seconds per scan, in contrast to peripheral QCT scanners, which have restricted availability, limited spatial resolution, and require several minutes of scanning time. This study evaluated the accuracy and precision of whole-body spiral CT scanners for quantitatively assessing the distal radius, a common site of non-vertebral osteoporosis-related fractures, and compared the CT-measured densitometric values with those obtained from dual-energy-X-ray absorptiometry.

SUBJECTS AND METHODS

A total of 161 postmenopausal women with baseline lumbar spine BMD T-scores between -1.0 and -2.5 underwent left forearm QCT using whole-body spiral CT scanners twice, 1 month apart. QCT volumes of interest were defined and analyzed at 3 specific radial regions: the ultradistal region by using slices at 8, 9, and 10 mm proximal to the ulnar styloid tip; the distal region by a slice 20 mm proximal; and the middle region by a slice 40 mm proximal. BMD, bone mineral content (BMC), volume, and average cortical thickness and circumference were measured. We evaluated QCT accuracy and precision and also report correlations between QCT and DXA for BMD and BMC.

RESULTS

Overall accuracy and precision errors for BMD, BMC and volume were consistent with known skeletal QCT technology precision and were generally less than 3%. BMD and BMC assessed by QCT and DXA were correlated (r=0.55 to 0.80).

DISCUSSION

Whole-body spiral CT scanners allow densitometric evaluations of the distal radius with good accuracy and very good precision. This original and convenient method provides a tool to further investigate cortical and trabecular bone variables in the peripheral skeleton in osteoporotic patients. These assessments, coupled with evaluation of the effects on cortical and trabecular bone measured in response to therapies for osteoporosis, may advance our understanding of the contributors to non-vertebral fracture occurrence.

Correlation of tibial low-frequency ultrasound velocity with femoral radiographic measurements and BMD in elderly women

The ultrasonic axial transmission technique has been proposed as a method for cortical bone characterization. Using a low enough center frequency, Lamb modes can be excited in long bones. Lamb waves propagate throughout the cortical bone layer, which makes them appealing for characterizing bone material and geometrical properties. In the present study, a prototype low-frequency quantitative ultrasonic axial transmission device was used on elderly women (n = 132) to investigate the relationships between upper femur geometry and bone mineral density (BMD) and tibial speed of sound. Ultrasonic velocities (V) were recorded using a two-directional measurement set-up on the midtibia and compared with dual-energy X-ray absorptiometry measurements and plain radiographs of the hip. Statistically significant, but weak, correlations were found between V and femoral shaft cortex thickness measured from radiographs (r = 0.20-0.26). V also correlated significantly with various BMD and bone mineral content parameters (r = 0.20-0.35). Femoral BMD and geometry were found to be significant independent predictors of V (R(2) = 0.07-0.16, p < 0.01). This study showed that femoral geometry and BMD affect significantly the axial ultrasound velocity measured at the tibia. In addition, the results confirmed, for the first time, a relationship between tibial ultrasound velocity and cortical bone thickness at the proximal femur.

Maintenance of exercise-induced benefits in physical functioning and bone among elderly women

SUMMARY

This study showed that about a half of the exercise-induced gain in dynamic balance and bone strength was maintained one year after cessation of the supervised high-intensity training of home-dwelling elderly women. However, to maintain exercise-induced gains in lower limb muscle force and physical functioning, continued training seems necessary.

INTRODUCTION

Maintenance of exercise-induced benefits in physical functioning and bone structure was assessed one year after cessation of 12-month randomized controlled exercise intervention.

METHODS

Originally 149 healthy women 70-78 years of age participated in the 12-month exercise RCT and 120 (81%) of them completed the follow-up study. Self-rated physical functioning, dynamic balance, leg extensor force, and bone structure were assessed.

RESULTS

During the intervention, exercise increased dynamic balance by 7% in the combination resistance and balance-jumping training group (COMB). At the follow-up, a 4% (95% CI: 1-8%) gain compared with the controls was still seen, while the exercise-induced isometric leg extension force and self-rated physical functioning benefits had disappeared. During the intervention, at least twice a week trained COMB subjects obtained a significant 2% benefit in tibial shaft bone strength index compared to the controls. A half of this benefit seemed to be maintained at the follow-up.

CONCLUSIONS

Exercise-induced benefits in dynamic balance and rigidity in the tibial shaft may partly be maintained one year after cessation of a supervised 12-month multi-component training in initially healthy elderly women. However, to maintain the achieved gains in muscle force and physical functioning, continued training seems necessary.

Vertebral fractures are associated with increased cortical porosity in iliac crest bone biopsy of men with idiopathic osteoporosis

In men, vertebral fractures are poorly associated with bone density, and both cortical and trabecular micro-architectural changes could contribute to bone fragility. Bone histomorphometry makes it possible to investigate both the thickness and porosity of cortical bone, which has been reported to have a major impact on the biomechanical properties of bone. We therefore conducted a cross sectional study using iliac crest biopsies to investigate the trabecular and cortical bone structure in men with or without vertebral fractures. We selected 93 bone biopsies from men with idiopathic osteoporosis (defined as a T-score <-2.5), between 40 and 70 years of age. Patients were divided into two groups on the basis of the presence (n=46) or absence (n=47) of prevalent vertebral fracture (VFX). We measured micro-architectural indices in trabecular and cortical bone by histomorphometry at the iliac crest. Patients with VFX had lower trabecular bone volume (BV/TV: 12.4+/-3.8 versus 14.7+/-3.1 % (m+/-SD)), p<0.01), higher trabecular separation (Tb.Sp: 871+/-279 versus 719+/-151 microm, p<0.01), and higher marrow star volume (V*(m.space): 1.617+/-1.257 versus 0.945+/-0.466 mm(3), p<0.01). Cortical thickness (Ct.Th) was the same in patients with or without VFX, whereas cortical porosity (Ct.Po) was higher in patients with VFX (6.5+/-2.6 versus 5.0+/-2.0 %, p<0.01), because their Haversian canals had higher mean areas (8291+/-4135 versus 5438+/-2809 microm(2), p<0.001). There was no correlation between any trabecular and cortical micro-architectural parameters. Using a logistic regression model, we evaluated the VFX as a function of the V*(m.space) and Ct.Po, adjusted for age. The odds-ratio of having a VFX was 3.89 (95% CI 1.19-12.7, p=0.02) for the third tertile of V*(m.space) (adjusted on age and Ct.Po), and 4.07 (95% CI 1.25-13.3, p=0.02) for the third tertile of Ct.Po (adjusted on age and V*(m.space)). Our data show that both trabecular and cortical bone microarchitecture contribute independently to vertebral fractures in men with idiopathic osteoporosis. In contrast to data reported in women, in men it is cortical porosity, and not cortical width, that is associated with vertebral fractures. This suggests that the cortical deficit is different in men and in women with fragility fractures.

A structural approach to skeletal fragility in chronic kidney disease

Renal osteodystrophy is a multifactorial disorder of bone metabolism in chronic kidney disease (CKD). As CKD progresses, ensuing abnormalities in mineral metabolism result in distortions in trabecular microarchitecture, thinning of the cortical shell, and increased cortical porosity. Recent studies have shown significantly increased hip fracture rates in CKD stages 3 and 4, in dialysis patients, and in transplant recipients. The majority of studies of bone loss in CKD relied on dual-energy x-ray absorptiometry (DXA) measures of bone mineral density. However, DXA summarizes the total bone mass within the projected bone area, concealing distinct structural alterations in trabecular and cortical bone. Recent data have confirmed that peripheral quantitative computed tomography (pQCT) measures of cortical density and thickness provide substantially better fracture discrimination in dialysis patients, compared with hip or spine DXA. This review summarizes the growing evidence for bone fragility in CKD stages 3 through 5, considers the effects of CKD on trabecular and cortical bone structure as it relates to fracture risk, and details the potential advantages and disadvantages of DXA and alternative measures of bone density, geometry, and microarchitecture, including pQCT, high-resolution pQCT, and micro-magnetic resonance imaging for fracture risk assessment in CKD.

Deleterious effect of late menarche on distal tibia microstructure in healthy 20-year-old and premenopausal middle-aged women

Late menarche is a risk factor for fragility fractures. We hypothesized that pubertal timing-dependent alterations in bone structural components would persist from peak bone mass to menopause, independent of premenopausal bone loss. We studied the influence of menarcheal age (MENA) on femoral neck BMD (FN aBMD) by DXA and microstructure of distal tibia by HR-pQCT in healthy young adult (YAD; 20.4 +/- 0.6 [SD] yr, n = 124) and premenopausal middle-aged (PREMENO; 45.8 +/- 3.4 yr, n = 120) women. Median of MENA was 13.0 +/- 1.2 and 13.1 +/- 1.7 yr in YAD and PREMENO, respectively. In YAD and PREMENO (n = 244), FN aBMD (R = -0.29, p = 0.013), as well as total volumetric BMD (Dtot; R = -0.23, p = 0.006) and cortical thickness (Ct.Th; R = -0.18, p = 0.011) of distal tibia were inversely correlated to MENA. After segregation by the median of MENA in EARLY and LATE subgroups, the significant influences of both MENA (p = 0.004) and chronological age (p < 0.0001) were observed for FN aBMD and trabecular bone volume fraction of the distal tibia with similar differences in T-scores between LATE and EARLY subgroups in YAD (-0.36 and -0.31 T-scores) and PREMENO (-0.35 and -0.42 T-scores) women. Ct.Th was negatively influenced by MENA, whereas trabecular thickness (Tb.Th) was negatively influenced by chronological age. There was a striking inverse relationship between cross-sectional area and Ct.Th (R = -0.57, p < 0.001). In conclusion, the negative influence of late menarcheal age at weight-bearing sites as observed by the end of skeletal growth remains unattenuated a few years before menopause and is independent of premenopausal bone loss. Alterations in both bone mineral mass and microstructural components may explain the increased risk of fragility fractures associated with later menarcheal age.

Once-monthly oral ibandronate improves biomechanical determinants of bone strength in women with postmenopausal osteoporosis

CONTEXT

Bone strength and fracture resistance are determined by bone mineral density (BMD) and structural, mechanical, and geometric properties of bone. DESIGN, SETTING, AND OBJECTIVES: This randomized, double-blind, placebo-controlled outpatient study evaluated effects of once-monthly oral ibandronate on hip and lumbar spine BMD and calculated strength using quantitative computed tomography (QCT) with finite element analysis (FEA) and dual-energy x-ray absorptiometry (DXA) with hip structural analysis (HSA).

PARTICIPANTS

Participants were women aged 55-80 yr with BMD T-scores -2.0 or less to -5.0 or greater (n = 93).

INTERVENTION

Oral ibandronate 150 mg/month (n = 47) or placebo (n = 46) was administered for 12 months.

OUTCOME MEASURES

The primary end point was total hip QCT BMD change from baseline; secondary end points included other QCT BMD sites, FEA, DXA, areal BMD, and HSA. All analyses were exploratory, with post hoc P values.

RESULTS

Ibandronate increased integral total hip QCT BMD and DXA areal BMD more than placebo at 12 months (treatment differences: 2.2%, P = 0.005; 2.0%, P = 0.003). FEA-derived hip strength to density ratio and femoral, peripheral, and trabecular strength increased with ibandronate vs. placebo (treatment differences: 4.1%, P < 0.001; 5.9%, P < 0.001; 2.5%, P = 0.011; 3.5%, P = 0.003, respectively). Ibandronate improved vertebral, peripheral, and trabecular strength and anteroposterior bending stiffness vs. placebo [7.1% (P < 0.001), 7.8% (P < 0.001), 5.6% (P = 0.023), and 6.3% (P < 0.001), respectively]. HSA-estimated femoral narrow neck cross-sectional area and moment of inertia and outer diameter increased with ibandronate vs. placebo (respectively 3.6%, P = 0.003; 4.0%, P = 0.052; 2.2%, P = 0.049).

CONCLUSIONS

Once-monthly oral Ibandronate for 12 months improved hip and spine BMD measured by QCT and DXA and strength estimated by FEA of QCT scans.

Ultrasonic assessment of the radius in vitro

The overall objective of this research is to develop an ultrasonic system for noninvasive assessment of the distal radius. The specific objective of this study was to examine the relationship between geometrical features of cortical bone and ultrasound measurements in vitro. Nineteen radii were measured in through transmission in a water bath. A 3.5 MHz rectangular (1 cm x 4.8 cm) single element transducer served as the source and a 3.5 MHz rectangular (1 cm x 4.8 cm) linear array transducer served as the receiver. The linear array consisted of 64 elements with a pitch of 0.75 mm. Ultrasound measurements were carried out at a location that was 1/3rdrd of the length from the distal end of each radius and two net time delay parameters, tau(NetDW) and tau(NetCW), associated with a direct wave (DW) and a circumferential wave (CW), respectively, were evaluated. The cortical thickness (CT), medullar thickness (MT) and cross-sectional area (CSA) of each radius was also evaluated based on a digital image of the cross-section at the 1/3rd location. The linear correlations between CT and tau(NetDW) was r = 0.91 (p < 0.001) and between MT and tau(NetCW) - tau(NetDW) was r = 0.63 (p < 0.05). The linear correlation between CSA and a nonlinear combination of the two net time delays, tau(NetDW) and tau(NetCW), was r = 0.95 (p < 0.001). The study shows that ultrasound measurements can be used to noninvasively assess cortical bone geometrical features in vitro as represented by cortical thickness, medullar thickness and cross-sectional area.

Trabecular bone microarchitecture: a review

The bone mass is constituted during the life by the modeling and remodeling mechanisms. Trabecular bone consists in a network of trabeculae (plates and rods) whose distribution is highly anisotropic: trabeculae are disposed parallel to the resultant of stress lines (Wolff's law). Trabecular microarchitecture appears conditioned by mechanical strains, which are exerted on the bones of the skeleton. However, few methods are currently clinically validated to appreciate and follow the evolution of microarchitecture in bone diseases. The most developed studies relate to microarchitectural measurements obtained by bone histomorphometry with the use of new algorithms, which can appreciate 2D various characteristics of the trabeculae, such as thickness and connectivity. Several works have shown that microarchitecture parameters should be obtained by using several independent techniques. X-ray microtomography (microCT), micro-RMI, synchrotron also allow the measurement in 3D of the trabecular microarchitecture in a nondestructive way on bone specimens. This review describes the evolution of our knowledge on bone microarchitecture, its role in bone diseases, such as osteoporosis and the various methods of histological evaluation in 2D and 3D.

High-resolution pQCT analysis at the distal radius and tibia discriminates patients with recent wrist and femoral neck fractures

We depict a fragility bone state in two primitive osteoporosis populations using 3D high-resolution peripheral in vivo QCT (HR-pQCT). Postmenopausal women (C, controls, n = 54; WF, wrist, n = 50; HF, hip, n = 62 recent fractured patients) were analyzed for lumbar and hip DXA areal BMD (aBMD), cancellous and cortical volumetric BMD (vBMD), and microstructural and geometric parameters on tibia and radius by HR-pQCT. Principal component analysis (PCA) allowed extracting factors that best represent bone variables. Comparison between groups was made by analysis of covariance (ANCOVA). Two factors (>80% of the entire variability) are extracted by PCA: at the radius, the first is a combination of trabecular parameters and the second of cortical parameters. At the tibia, we found the reverse. Femoral neck aBMD is decreased in WF (8.6%) and in HF (18%) groups (no lumbar difference). WF showed a approximately 20% reduction in radius trabecular vBMD and number. Radius cortical vBMD and thickness decrease by 6% and 14%, respectively. At the tibia, only the cortical compartment is affected, with approximately 20% reduction in bone area, thickness, and section modulus and 6% reduction in vBMD. HF showed same radius trabecular alterations than WF, but radius cortical parameters are more severely affected than WF with reduced bone area (25%), thickness (28.5%), and vBMD (11%). At the tibia, trabecular vBMD and number decrease by 26% and 17.5%, respectively. Tibia cortical bone area, thickness, and section modulus showed a >30% decrease, whereas vBMD reduction reached 13%. Geometry parameters at the tibia displayed the greatest differences between healthy and fractured patients and between wrist and hip fractures.

Considerations for development of surrogate endpoints for antifracture efficacy of new treatments in osteoporosis: a perspective

Because of the broad availability of efficacious osteoporosis therapies, conduct of placebo-controlled trials in subjects at high risk for fracture is becoming increasing difficult. Alternative trial designs include placebo-controlled trials in patients at low risk for fracture or active comparator studies, both of which would require enormous sample sizes and associated financial resources. Another more attractive alternative is to develop and validate surrogate endpoints for fracture. In this perspective, we review the concept of surrogate endpoints as it has been developed in other fields of medicine and discuss how it could be applied in clinical trials of osteoporosis. We outline a stepwise approach and possible study designs to qualify a biomarker as a surrogate endpoint in osteoporosis and review the existing data for several potential surrogate endpoints to assess their success in meeting the proposed criteria. Finally, we suggest a research agenda needed to advance the development of biomarkers as surrogate endpoints for fracture in osteoporosis trials. To ensure optimal development and best use of biomarkers to accelerate drug development, continuous dialog among the health professionals, industry, and regulators is of paramount importance.

Ultrasound simulation in the distal radius using clinical high-resolution peripheral-CT images

The overall objective of this research is to develop an ultrasonic method for noninvasive assessment of the distal radius. The specific objective of this study was to examine the propagation of ultrasound through the distal radius and determine the relationships between bone mass and architecture and ultrasound parameters. Twenty-six high-resolution peripheral-CT clinical images were obtained from a set of subjects that were part of a larger study on secondary osteoporosis. A single midsection binary slice from each image was selected and used in the two-dimensional (2D) simulation of an ultrasound wave propagating from the anterior to the posterior surfaces of each radius. Mass and architectural parameters associated with each radius, including total (trabecular and cortical) bone mass, trabecular volume fraction, trabecular number and trabecular thickness were computed. Ultrasound parameters, including net time delay (NTD), broadband ultrasound attenuation (BUA) and ultrasound velocity (UV) were also evaluated. Significant correlations were found between NTD and total bone mass (R2 = 0.92, p < 0.001), BUA and trabecular number (R2 = 0.78, p < 0.01) and UV and trabecular bone volume fraction (R2 = 0.82, p < 0.01). There was only weak, statistically insignificant correlation (R2 < 0.14, p = 0.21) found between trabecular thickness and any of the ultrasound parameters. The study shows that ultrasound measurements are correlated with bone mass and architecture at the distal radius and, thus, ultrasound may prove useful as a method for noninvasive assessment of osteoporosis and fracture risk.

Clinical interest of bone texture analysis in osteoporosis: a case control multicenter study

We demonstrate the clinical interest of bone texture analysis with a new high resolution X-ray device. We have found that the combination of BMD and texture parameter values provided a better assessment of the fracture risk than that obtainable solely by BMD measurement.

INTRODUCTION

Osteoporosis is characterized by BMD and trabecular bone microarchitecture. We have developed a new high-resolution X-ray device with direct digitization. The aim of this study was to demonstrate in a multicenter case control study the clinical interest of bone texture analysis with this new device.

METHODS

In this cross-sectional multicenter case-control population study in post-menopausal women, 159 osteoporotic fractures were compared with 219 control cases. Images were obtained on calcaneus with a direct digital X-ray device (BMA, D3A Medical Systems). Co-occurrence, run-length matrices and the fractal parameter Hmean were evaluated. BMD was measured at the lumbar spine (LS), femoral neck (FN) and total hip (TH) by DXA.

RESULTS

The three texture parameters were significantly lower in osteoporotic fracture cases than in control cases. These differences persisted after adjustment for TH BMD. Receiver operating characteristic curves were used to compare the discriminant capacity of texture parameters and BMD measurements for fracture. The highest areas under curve (AUC) were 0.721 for TH BMD and 0.706 for Hmean (AUC THBMD vs. AUC Hmean, p = NS). We determined the threshold between high and low Hmean parameter values and then the odds ratios (OR) of fracture for low Hmean, for BMD < or =2.5 SD in the T-score and for combinations of both parameters. The OR of fracture for low H was 2.72 (95% CI, 1.36-5.4). For a FN BMD < or = -2.5 SD, the OR of 4.78 (2.19-10.43) shifted to 14.06 (4.41-44.85) adding H.

CONCLUSIONS

These data confirmed the clinical interest of the combination of BMD and texture parameters to improve the assessment of the risk of fracture other that obtainable by the sole BMD measurement.

Influence of age at menarche on forearm bone microstructure in healthy young women

BACKGROUND

Shorter estrogen exposure from puberty onset to peak bone mass attainment may explain how late menarche is a risk factor for osteoporosis. The influence of menarcheal age (MENA) on peak bone mass, cortical, and trabecular microstructure was studied in 124 healthy women aged 20.4 +/- 0.6 (sd) yr.

METHODS

At distal radius, areal bone mineral density (aBMD) was measured by dual-energy x-ray absorptiometry, and volumetric bone mineral density (BMD) and microstructure were measured by high-resolution peripheral computerized tomography, including: total, cortical, and trabecular volumetric BMD and fraction; trabecular number, thickness, and spacing; cortical thickness (CTh); and cross-sectional area (CSA).

RESULTS

Median MENA was 12.9 yr. Mean aBMD T score of the whole cohort was slightly positive. aBMD was inversely correlated to MENA for total radius (R = -0.21; P = 0.018), diaphysis (R = -0.18; P = 0.043), and metaphysis (R = -0.19; P = 0.031). Subjects with MENA more than the median [LATER: 14.0 +/- 0.7 (+/-sd) yr] had lower aBMD than those with MENA less than the median (EARLIER: 12.1 +/- 0.7 yr) in total radius (P = 0.026), diaphysis (P = 0.042), and metaphysis (P = 0.046). LATER vs. EARLIER displayed lower total volumetric BMD (315 +/- 54 vs. 341 +/- 56 mg HA/cm(3); P = 0.010), cortical volumetric BMD (874 +/- 49 vs. 901 +/- 44 mg HA/cm(3); P = 0.003), and CTh (774 +/- 170 vs. 849 +/- 191 microm; P = 0.023). CTh was inversely related to CSA (R = -0.46; P < 0.001). In LATER reduced CTh was associated with 5% increased CSA.

CONCLUSIONS

In healthy young adult women, a 1.9-yr difference in mean MENA was associated with lower radial aBMD T score, lower CTh without reduced CSA, a finding compatible with less endocortical accrual. It may explain how late menarche is a risk factor for forearm osteoporosis.

RANKL inhibition with osteoprotegerin increases bone strength by improving cortical and trabecular bone architecture in ovariectomized rats

INTRODUCTION

Ovariectomy (OVX) results in bone loss caused by increased bone resorption. RANKL is an essential mediator of bone resorption. We examined whether the RANKL inhibitor osteoprotegerin (OPG) would preserve bone volume, density, and strength in OVX rats.

MATERIALS AND METHODS

Rats were OVX or sham-operated at 3 mo of age. Sham controls were treated for 6 wk with vehicle (Veh, PBS). OVX rats were treated with Veh or human OPG-Fc (10 mg/kg, 2/wk). Serum RANKL and TRACP5b was measured by ELISA. BMD of lumbar vertebrae (L(1)-L(5)) and distal femur was measured by DXA. Right distal femurs were processed for bone histomorphometry. Left femurs and the fifth lumbar vertebra (L(5)) were analyzed by muCT and biomechanical testing, and L(6) was analyzed for ash weight.

RESULTS

OVX was associated with significantly greater serum RANKL and osteoclast surface and with reduced areal and volumetric BMD. OPG markedly reduced osteoclast surface and serum TRACP5b while completely preventing OVX-associated bone loss in the lumbar vertebrae, distal femur, and femur neck. Vertebrae from OPG-treated rats had increased dry and ash weight, with no significant differences in tissue mineralization versus OVX controls. muCT showed that trabecular compartments in OVX-OPG rats had significantly greater bone volume fraction, vBMD, bone area, trabecular thickness, and number, whereas their cortical compartments had significantly greater bone area (p < 0.05 versus OVX-Veh). OPG improved cortical area in L(5) and the femur neck to levels that were significantly greater than OVX or sham controls (p < 0.05). Biomechanical testing of L(5) and femur necks showed significantly greater maximum load values in the OVX-OPG group (p < 0.05 versus OVX-Veh). Bone strength at both sites was linearly correlated with total bone area (r(2) = 0.54-0.74, p < 0.0001), which was also significantly increased by OPG (p < 0.05 versus OVX).

CONCLUSIONS

OPG treatment prevented bone loss, preserved trabecular architecture, and increased cortical area and bone strength in OVX rats.

Technology insight: noninvasive assessment of bone strength in osteoporosis

Fractures that result from osteoporosis are an enormous and growing concern for public health systems; as the population ages, the number of fractures worldwide will double or triple in the next 50 years. The ability of a bone to resist fracture depends not only on the amount of bone present, but also on the spatial distribution of the bone mass, the cortical and trabecular microarchitecture, and the intrinsic properties of the materials that comprise the bone. Although low bone mineral density is one of the strongest risk factors for fracture, a number of clinical studies have demonstrated the limitations of using measurements of areal bone mineral density by dual-energy X-ray absorptiometry to assess fracture risk and to monitor responses to therapy. As a result, new, noninvasive imaging techniques that are capable of assessing various components of bone strength are being developed. These techniques include three-dimensional assessments of bone density, geometry and microarchitecture, as well as integrated measurements of bone strength by engineering analyses. Although they show strong potential, further development and validation of these techniques is needed to define their role in the clinical management of individuals with osteoporosis.

Percolation theory relates corticocancellous architecture to mechanical function in vertebrae of inbred mouse strains

Complex corticocancellous skeletal sites such as the vertebra or proximal femur are connected networks of bone capable of transferring mechanical loads. Characterizing these structures as networks may allow us to quantify the load transferring behavior of the emergent system as a function of the connected cortical and trabecular components. By defining the relationship between certain physical bone traits and mechanical load transfer pathways, a clearer picture of the genetic determinants of skeletal fragility can be developed. We tested the hypothesis that the measures provided by network percolation theory will reveal that different combinations of cortical, trabecular, and compositional traits lead to significantly different load transfer pathways within the vertebral bodies among inbred mouse strains. Gross morphologic, micro-architectural, and compositional traits of L5 vertebrae from 15 week old A/J (A), C57BL6/J (B6), and C3H/HeJ (C3H) inbred mice (n=10/strain) were determined using micro-computed tomography. Measures included total cross-sectional area, bone volume fraction, trabecular number, thickness, spacing, cortical area, and tissue mineral density. Two-dimensional coronal sections were converted to network graphs with the cortical shell considered as one highly connected node. Percolation parameters including correlation length (average number of connected nodes between superior and inferior surfaces), chemical length (minimum number of connected nodes between surfaces), and backbone mass (strut number) were measured. Analysis of the topology of the connected bone networks showed that A and B6 mice transfer load through trabecular pathways in the middle of the vertebral body in addition to the cortical shell. C3H mice transfer load primarily through the highly mineralized cortical shell. Thus, the measures provided by percolation theory provide a quantitative approach to study how different combinations of cortical and trabecular traits lead to mechanically functional structures. The data further emphasize the interdependent nature of these physical bone traits suggesting similar genetic variants may affect both trabecular and cortical bone. Therefore, developing a network approach to study corticocancellous architecture during growth should further our understanding of the biological basis of skeletal fragility and, thus, provide novel engineering approaches to studying the genetic basis of fracture risk.

In vivo determination of bone structure in postmenopausal women: a comparison of HR-pQCT and high-field MR imaging

Bone structural measures obtained by two noninvasive imaging tools-3T MRI and HR-pQCT-were compared. Significant but moderate correlations and 2- to 4-fold discrepancies in parameter values were detected, suggesting that differences in acquisition and analysis must be considered when interpreting data from these imaging modalities.

INTRODUCTION

High-field MRI and high resolution (HR)-pQCT are currently being used in longitudinal bone structure studies. Substantial differences in acquisition and analysis between these modalities may influence the quantitative data produced and could potentially influence clinical decisions based on their results. Our goal was to compare trabecular and cortical bone structural measures obtained in vivo by 3T MRI and HR-pQCT.

MATERIALS AND METHODS

Postmenopausal osteopenic women (n = 52) were recruited for this study. HR-pQCT imaging of the radius and tibia was performed using the XtremeCT scanner, with a voxel size of 82 x 82 x 82 microm(3). MR imaging was performed on a 3T Signa scanner using SSFP imaging sequences, with a pixel size of 156 x 156 microm(2) and slice thickness of 500 microm. Structure parameters were calculated using standard HR-pQCT and MRI analysis techniques. Relationships between measures derived from HR-pQCT, MRI, and DXA were studied.

RESULTS

Significant correlations between HR-pQCT and MRI parameters were found (p < 0.0001) and were strongest for Tb.N (r(2) = 0.52), Ct.Th (r(2) = 0.59), and site-specific Tb.Sp (r(2) = 0.54-0.60). MRI and HR-pQCT provided statistically different values of structure parameters (p < 0.0001), with BV/TV and Tb.Th exhibiting the largest discrepancies (MR/HR-pQCT = 3-4). Although differences in the Tb.N values were statistically significant, the mean differences were on the order of our reproducibility measurements. Systematic differences between MRI and HR-pQCT analysis procedures leading to discrepancies in cortical thickness values were observed, with MRI values consistently higher. Minimal correlations were found between MRI or HR-pQCT parameters and DXA BMD or T-score, except between HR-pQCT measures at the radius and the ultradistal radius T-scores, where moderate correlations were found (r(2) = 0.19-0.58).

CONCLUSIONS

This study provides unique insight into two emerging noninvasive tools for bone structure evaluation. Our findings highlight the significant influence of analysis technique on results of in vivo assessment and underscore the importance of accounting for these differences when interpreting results from these modalities.

Bone mass in young adults with Down syndrome

BACKGROUND

Down syndrome (DS) is a frequent cause of intellectual disability. With the increasing life expectancy of these patients, concerns have been raised about the risk of osteoporosis. In fact, several investigators have reported a reduced bone mass in DS. However, the results may be confounded by comorbid diseases, and differences in lifestyle habits and body size. Therefore, we planned to determine anthropometric and lifestyle factors influencing bone mineral density (BMD) in young adults with DS.

METHODS

Thirty-nine patients with DS (mean age 26 years) and 78 controls were studied. Areal BMD was measured by dual x-ray densitometry (DXA); volumetric BMD at the lumbar spine and femoral neck was estimated with published formulae.

RESULTS

DS patients had lower areal BMD than controls at all regions (spine, hip and total body). Height and projected bone area were also lower. There were no differences between both groups regarding estimated volumetric BMD at the femoral neck. However, spine volumetric BMD was also lower in DS than controls. In multivariate analysis, DS, male sex, little physical activity and low sunlight exposure were associated with lower spine volumetric BMD; on the other hand, fat mass and sunlight exposure were associated with femoral neck volumetric BMD.

CONCLUSION

This study shows that patients with DS had a reduced areal BMD, but it is in part a consequence of the reduced body size, particularly at the femoral neck. Physical activity and sunlight exposure are associated to volumetric BMD and should be stimulated in order to maintain an adequate bone mass in these patients.

Finite element analysis based on in vivo HR-pQCT images of the distal radius is associated with wrist fracture in postmenopausal women

BMD, bone microarchitecture, and bone mechanical properties assessed in vivo by finite element analysis were associated with wrist fracture in postmenopausal women.

INTRODUCTION

Many fractures occur in individuals with normal BMD. Assessment of bone mechanical properties by finite element analysis (FEA) may improve identification of those at high risk for fracture.

MATERIALS AND METHODS

We used HR-pQCT to assess volumetric bone density, microarchitecture, and microFE-derived bone mechanical properties at the radius in 33 postmenopausal women with a prior history of fragility wrist fracture and 33 age-matched controls from the OFELY cohort. Radius areal BMD (aBMD) was also measured by DXA. Associations between density, microarchitecture, mechanical parameters and fracture status were evaluated by univariate logistic regression analysis and expressed as ORs (with 95% CIs) per SD change. We also conducted a principal components (PCs) analysis (PCA) to reduce the number of parameters and study their association (OR) with wrist fracture.

RESULTS

Areal and volumetric densities, cortical thickness, trabecular number, and mechanical parameters such as estimated failure load, stiffness, and the proportion of load carried by the trabecular bone at the distal and proximal sites were associated with wrist fracture (p < 0.05). The PCA revealed five independent components that jointly explained 86.2% of the total variability of bone characteristics. The first PC included FE-estimated failure load, areal and volumetric BMD, and cortical thickness, explaining 51% of the variance with an OR for wrist fracture = 2.49 (95% CI, 1.32-4.72). Remaining PCs did not include any density parameters. The second PC included trabecular architecture, explaining 12% of the variance, with an OR = 1.82 (95% CI, 0.94-3.52). The third PC included the proportion of the load carried by cortical versus trabecular bone, assessed by FEA, explaining 9% of the variance, and had an OR = 1.61 (95% CI, 0.94-2.77). Thus, the proportion of load carried by cortical versus trabecular bone seems to be associated with wrist fracture independently of BMD and microarchitecture (included in the first and second PC, respectively).

CONCLUSIONS

These results suggest that bone mechanical properties assessed by microFE may provide information about skeletal fragility and fracture risk not assessed by BMD or architecture measurements alone and are therefore likely to enhance the prediction of wrist fracture risk.

Trabecular structure quantified with the MRI-based virtual bone biopsy in postmenopausal women contributes to vertebral deformity burden independent of areal vertebral BMD

In postmenopausal women with a wide range of vertebral deformities, MRI-based structural measures of topology and scale at the distal radius are shown to account for as much as 30% of vertebral deformity, independent of integral vertebral BMD.

INTRODUCTION

Trabecular bone architecture has been postulated to contribute to overall bone strength independent of vertebral BMD measured by DXA. However, there has thus far been only sparse in vivo evidence to support this hypothesis.

MATERIALS AND METHODS

Postmenopausal women, 60-80 yr of age, were screened by DXA, and those with T-scores at either the hip or spine falling within the range of -2.5 +/- 1.0 were studied with the MRI-based virtual bone biopsy, along with heel broadband ultrasound absorption and pQCT of the tibia. The data from 98 subjects meeting the enrollment criteria were subjected to microMRI at the distal tibia and radius, and measures of topology and scale of the trabecular bone network were computed. A spinal deformity index (SDI) was obtained from morphometric measurements in midline sagittal MR images of the thoracic and lumbar spine to evaluate associations between structure and deformity burden.

RESULTS

A number of structural indices obtained at the distal radius were correlated with the SDI. Among these were the topological surface density (a measure of trabecular plates) and trabecular bone volume fraction, which were inversely correlated with SDI (p < 0.0001). Combinations of two structural parameters accounted for up to 30% of the variation in SDI (p < 0.0001) independent of spinal BMD, which was not significantly correlated. pQCT trabecular BMD was also weakly associated, whereas broadband ultrasound absorption was not. No significant association between SDI and structural indices were found at the tibia.

CONCLUSIONS

Structural measures at the distal radius obtained in vivo by microMRI explained a significant portion of the variation in total spinal deformity burden in postmenopausal women independent of areal BMD.

Noninvasive assessments of bone strength

PURPOSE OF REVIEW

Description of new noninvasive technologies or modifications of existing technologies with which individual components of bone strength and bone strength as a whole can be quantified.

RECENT FINDINGS

Although bone mineral density has served as an able surrogate for bone strength, it is clear that aspects of bone strength are either not captured or are not discernible within the measurement of bone density. New, noninvasive technologies have been developed to quantify aspects of bone strength such as biomechanical parameters based on geometry and scale and topological parameters of microarchitecture. Finite element modeling utilizes sophisticated mathematical approaches to predict the strength of the whole bone. At present, most of these technologies remain beyond the reach of clinicians, with the exception of hip structural or strength analysis.

SUMMARY

Hip strength or structural analysis is widely available because of its incorporation with dual energy X-ray absorptiometry and has been extensively used in clinical research. None of these new approaches has been shown to be superior to the measurement of bone density in the prediction of fracture risk. This fact does not diminish their potential to enhance the understanding of the pathophysiology of fracture and the mechanisms of therapeutic efficacy.

Structural determinants of vertebral fracture risk

Vertebral fractures are more strongly associated with specific bone density, structure, and strength parameters than with areal BMD, but all of these variables are correlated.

INTRODUCTION

It is unclear whether the association of areal BMD (aBMD) with vertebral fracture risk depends on bone density per se, bone macro- or microstructure, overall bone strength, or spine load/bone strength ratios.

MATERIALS AND METHODS

From an age-stratified sample of Rochester, MN, women, we identified 40 with a clinically diagnosed vertebral fracture (confirmed semiquantitatively) caused by moderate trauma (cases; mean age, 78.6 +/- 9.0 yr) and compared them with 40 controls with no osteoporotic fracture (mean age, 70.9 +/- 6.8 yr). Lumbar spine volumetric BMD (vBMD) and geometry were assessed by central QCT, whereas microstructure was evaluated by high-resolution pQCT at the ultradistal radius. Vertebral failure load ( approximately strength) was estimated from voxel-based finite element models, and the factor-of-risk (phi) was determined as the ratio of applied spine loads to failure load.

RESULTS

Spine loading (axial compressive force on L3) was similar in vertebral fracture cases and controls (e.g., for 90 degrees forward flexion, 2639 versus 2706 N; age-adjusted p = 0.173). However, fracture cases had inferior values for most bone density and structure variables. Bone strength measures were also reduced, and the factor-of-risk (phi) was 35-37% greater (worse) among women with a vertebral fracture. By age-adjusted logistic regression, relative risks for the strongest fracture predictor in each of the five main variable categories were bone density (total lumbar spine vBMD: OR per SD change, 2.2; 95% CI, 1.1-4.3), bone geometry (vertebral apparent cortical thickness: OR, 2.1; 95% CI, 1.1-4.1), bone microstructure (none significant); bone strength ("cortical" [outer 2 mm] compressive strength: OR, 2.5; 95% CI, 1.3-4.8), and factor-of-risk (phi for 90 degrees forward flexion/overall vertebral compressive strength: OR, 3.2; 95% CI, 1.4-7.5). These variables were correlated with spine aBMD (partial r, -0.32 to 0.75), but each was a stronger predictor of fracture in the logistic regression analyses.

CONCLUSIONS

The association of aBMD with vertebral fracture risk is explained by its correlation with more specific bone density, structure, and strength parameters. These may allow deeper insights into fracture pathogenesis.

Limitations of peripheral quantitative computed tomography metaphyseal bone density measurements

CONTEXT

Peripheral quantitative computed tomography (pQCT) measurements are frequently obtained to assess cancellous bone density in the appendicular skeleton. Large variations in bone morphology associated with skeletal development may limit the interpretation of pediatric pQCT studies based on a single slice.

OBJECTIVE

The objective of the study was to characterize the variability in trabecular bone density values along the length of the metaphysis.

DESIGN

The design was an analysis of pQCT bone density data.

SETTING

The study was conducted at a hospital radiology department.

PATIENTS

The study included 35 children with cerebral palsy aged 6-12 yr.

MAIN OUTCOME MEASURE

Variations in cancellous bone density along the length of the proximal tibial metaphysis were measured.

RESULTS

The patterns of decay in metaphyseal trabecular bone density were different in all subjects, and the density changed from the physis to the shaft at a rate of 16.8 +/- 8.2% per 1 mm (range 8.6-37.9% per 1 mm). The slopes of the density curve drastically changed in some children over a short period of 6 months. Even with a high correlation (r(2) = 0.88) between the density of a slice located a fixed distance from the growth plate and the overall mean metaphysis density, the respective changes in density over 6 months were only moderately correlated (r(2) = 0.58).

CONCLUSIONS

These results underscore the difficulty in interpreting metaphyseal pQCT bone density measurements from a single slice and highlight the need for developing pQCT acquisition techniques that provide more representative bone density determinations in the appendicular skeleton of children.

Contribution of in vivo structural measurements and load/strength ratios to the determination of forearm fracture risk in postmenopausal women

Bone structure, strength and load-strength ratios contribute to forearm fracture risk independently of areal BMD.

INTRODUCTION

Technological and conceptual advances provide new opportunities for evaluating the contribution of bone density, structure, and strength to the pathogenesis of distal forearm fractures.

MATERIALS AND METHODS

From an age-sratified random sample of Rochester, MN, women, we compared 18 with a distal forearm fracture (cases) to 18 age-matched women with no osteoporotic fracture (controls). High-resolution pQCT was used to assess volumetric BMD (vBMD), geometry, and microstructure at the ultradistal radius, the site of Colles' fractures. Failure loads in the radius were estimated from microfinite element (microFE) models derived from pQCT. Differences between case and control women were assessed, and the risk of fracture associated with each variable was estimated by logistic regression analysis.

RESULTS

Given similar heights, estimated loading in a fall on the outstretched arm was the same in cases and control. However, women with forearm fractures had inferior vBMD, geometry, microstructure, and estimated bone strength. Relative risks for the strongest determinant of fracture in each of the five main variable categories were as follows: BMD (total vBMD: OR per SD change, 4.2; 95% CI, 1.4-12), geometry (cortical thickness: OR, 4.0; 95% CI, 1.4-11), microstructure (trabecular number: OR, 2.3; 95% CI, 1.02-5.1), and strength (axial rigidity: OR, 3.8; 95% CI, 1.4-10); the factor-of-risk (fall load/microFE failure load) was 24 % greater (worse) in cases (OR, 3.0; 95% CI, 1.2-7.5). Areas under ROC curves ranged from 0.72 to 0.82 for these parameters.

CONCLUSIONS

Bone geometry, microstructure, and strength contribute to forearm fractures, as does BMD, and these additional determinants of risk promise greater insights into fracture pathogenesis.