Three-dimensional dynamic bone histomorphometry.

Dynamic bone histomorphometry is the standard method for measuring bone remodeling at the level of individual events. Although dynamic bone histomorphometry is an invaluable tool for understanding osteoporosis and other metabolic bone diseases, the technique's two-dimensional nature requires the use of stereology and prevents measures of individual remodeling event number and size. Here, we used a novel three-dimensional fluorescence imaging technique to achieve measures of individual resorption cavities and formation events. We performed this three-dimensional histomorphometry approach using a common model of postmenopausal osteoporosis, the ovariectomized rat. The three-dimensional images demonstrated the spatial relationship between resorption cavities and formation events consistent with the hemiosteonal model of cancellous bone remodeling. Established ovariectomy was associated with significant increases in the number of resorption cavities per unit bone surface (2.38 ± 0.24 mm⁻² sham surgery versus 3.86 ± 0.35 mm⁻² bilateral ovariectomy [OVX], mean ± SD, p < 0.05) and total volume occupied by cavities per unit bone volume (0.38% ± 0.06% sham versus 1.12% ± 0.18% OVX, p < 0.001), but there was no difference in surface area per resorption cavity, maximum cavity depth, or cavity volume. In addition, we found that established ovariectomy is associated with increased size of bone formation events because of the merging of formation events (23,700 ± 6,890 µm² sham verusus 33,300 ± 7,950 µm² OVX). No differences in mineral apposition rate (determined in 3D) were associated with established ovariectomy. That established estrogen depletion is associated with increased number of remodeling events with only subtle changes in remodeling event size suggests that circulating estrogens may have their primary effect on the origination of new basic multicellular units with relatively little effect on the progression and termination of active remodeling events.

Relative strength of thoracic vertebrae in axial compression versus flexion.

BACKGROUND CONTEXT: Noninvasive strength assessment techniques are the clinical standard in the diagnosis and treatment of osteoporotic vertebral fractures, and the efficacy of these protocols depends on their ability to predict vertebral strength at all at-risk spinal levels under multiple physiological loading conditions. PURPOSE: To assess differences in vertebral strength between loading modes and across spinal levels. STUDY DESIGN/SETTING: This study examined the relative strength of isolated vertebral bodies in compression versus flexion. METHODS: Destructive biomechanical tests were conducted on 30 pairs of donor-matched, isolated thoracic vertebral bodies (T9 and T10; F=19, M=11; 87+5 years old, max=97 years old, min=80 years old) in both uniform axial compression and flexion using previously described protocols. Quantitative computed tomography (QCT) scans were taken before mechanical testing and used to obtain bone mineral density (BMD) and "mechanics of solids" (MOS) measures, such as axial and bending rigidities. RESULTS: Compressive strength was higher than flexion strength for each donor by 940+152N (p<.001, paired t test), and vertebral strengths in the two loading modes were moderately correlated (adjusted R(2)=0.50, p<.001). For both compression and flexion loading modes, adjacent-level BMD and MOS metrics had approximately half the predictive capacity as same-level measurements, and BMD and MOS values were only moderately correlated across spinal levels. CONCLUSIONS: The results of this study are important in designing clinical test protocols for assessing vertebral fracture risk. Because vertebral body flexion and compressive strength are not strongly correlated and flexion strength is significantly less than compressive strength, it is imperative to investigate a patient's spinal structural capacity under bending loading conditions. Furthermore, our work suggests that clinicians using QCT-based measures should perform site-specific strength assessments on each at-risk spinal level. Future work should focus on improving the accuracy of densitometric measures in predicting vertebral strength in flexion and also on examining same- versus adjacent-level strength assessment for radiographic techniques with lower X-ray dosage, such as dual-energy X-ray absorptiometry.

Voxel size and measures of individual resorption cavities in three-dimensional images of cancellous bone.

Cavities formed by osteoclasts on the surface of cancellous bone during bone remodeling (resorption cavities) are believed to act as stress risers and impair cancellous bone strength and stiffness. Although resorption cavities are readily detected as eroded surfaces in histology sections, identification of resorption cavities in three-dimensional images of cancellous bone has been rare. Here we use sub-micrometer resolution images of rat lumbar vertebral cancellous bone obtained through serial milling (n=5) to determine how measures of the number and surface area of resorption cavities are influenced by image resolution. Three-dimensional images of a 1 mm cube of cancellous bone were collected at 0.7x0.7x5.0 microm/voxel using fluorescence based serial milling and uniformly coarsened to four other resolutions ranging from 1.4x1.4x5.0 to 11.2x11.2x10 microm/voxel. Cavities were identified in the three-dimensional image as an indentation on the cancellous bone surface and were confirmed as eroded surfaces by viewing two-dimensional cross-sections (mimicking histology techniques). The number of cavities observed in the 0.7x0.7x5.0 microm/voxel images (22.0+/-1.43, mean+/-SD) was not significantly different from that in the 1.4x1.4x5.0 microm/voxel images (19.2+/-2.59) and an average of 79% of the cavities observed at both of these resolutions were coincident. However, at lower resolutions, cavity detection was confounded by low sensitivity (<20%) and high false positive rates (>40%). Our results demonstrate that when image voxel size exceeds 1.4x1.4x5.0 microm/voxel identification of resorption cavities by bone surface morphology is highly inaccurate. Experimental and computational studies of resorption cavities in three-dimensional images of cancellous bone may therefore require images to be collected at resolutions of 1.4 microm/pixel in-plane or better to ensure consistent identification of resorption cavities.

Quantitative computed tomography-based predictions of vertebral strength in anterior bending.

STUDY DESIGN: This study examined the ability of QCT-based structural assessment techniques to predict vertebral strength in anterior bending. OBJECTIVE: The purpose of this study was to compare the abilities of QCT-based bone mineral density (BMD), mechanics of solids models (MOS), e.g., bending rigidity, and finite element analyses (FE) to predict the strength of isolated vertebral bodies under anterior bending boundary conditions. SUMMARY OF BACKGROUND DATA: Although the relative performance of QCT-based structural measures is well established for uniform compression, the ability of these techniques to predict vertebral strength under nonuniform loading conditions has not yet been established. METHODS: Thirty human thoracic vertebrae from 30 donors (T9-T10, 20 female, 10 male; 87 +/- 5 years of age) were QCT scanned and destructively tested in anterior bending using an industrial robot arm. The QCT scans were processed to generate specimen-specific FE models as well as trabecular bone mineral density (tBMD), integral bone mineral density (iBMD), and MOS measures, such as axial and bending rigidities. RESULTS: Vertebral strength in anterior bending was poorly to moderately predicted by QCT-based BMD and MOS measures (R2 = 0.14-0.22). QCT-based FE models were better strength predictors (R2 = 0.34-0.40); however, their predictive performance was not statistically different from MOS bending rigidity (P > 0.05). CONCLUSIONS: Our results suggest that the poor clinical performance of noninvasive structural measures may be due to their inability to predict vertebral strength under bending loads. While their performance was not statistically better than MOS bending rigidities, QCT-based FE models were moderate predictors of both compressive and bending loads at failure, suggesting that this technique has the potential for strength prediction under nonuniform loads. The current FE modeling strategy is insufficient, however, and significant modifications must be made to better mimic whole bone elastic and inelastic material behavior.