Role of trabecular morphology in the etiology of age-related vertebral fractures

Bone quantity and quality in past populations

BACKGROUND

The study of osteoporosis in past populations offers valuable insight into the patterns and prevalence of the disease in both the past and in the present.

METHODS AND RESULTS

A review is made of different paleopathological studies better to understand bone loss in past populations and to examine the contribution such studies can make to our knowledge of osteoporosis in modern populations. The review includes studies of bone mass in past populations from different geographic regions. Nutritionally based hypotheses, traditionally used to explain bone loss in past populations, are reviewed and assessed against the current clinical and epidemiological findings. In general, the various studies have revealed different degrees of low bone mass in past populations; however, the pattern of bone loss and fragility seen in age-related and postmenopausal osteoporosis today is not evident in the past. Bone loss in earlier populations is often found in both sexes, whereas significant bone loss in females occurs often among the young-age categories. In addition, a prevalence of osteoporotic fracture is absent.

CONCLUSIONS

We suggest that, despite influences that may have reduced bone mass in past populations, a protection of bone quality may have occurred, reducing the likelihood of bone fragility and fracture typically seen in modern osteoporotics. It seems evident that, although reduced bone mass may be prevalent in past history, clinically recognized osteoporosis is not.

Quantitative CT in the measurement of bone quantity and bone quality for assessing osteoporosis

The biomechanical strength of the skeleton, and hence the risk of future fractures, depends on both the bone quantity (assessed in terms of bone mineral density) and bone quality (assessed in terms of the integrity of its internal architecture). Precise methods of measuring the calcium-equivalent density of both the trabecular and cortical components of vertebral bone using quantitative computed tomography are presented. For a group of post-menopausal Kuwaiti females, we have shown that the trabecular and cortical components are highly correlated with each other at both the L3 and L4 lumbar vertebral levels, with correlation coefficients of 0.78 and 0.74 (P < 0.0001) respectively. We have explored the anisotropic distribution of trabecular bone and considered the use of the standard deviation and the coefficient of variation of trabecular CT numbers as texture indicators and surrogate measures of bone quality. The coefficient of variation was the better texture indicator: used with bone mineral density values it successfully discriminated between two classes of patients (those with fractures and those without) with high sensitivity (> or = 89%) and specificity (> or = 82%).

Magnetic resonance imaging of the calcaneus: preliminary assessment of trabecular bone-dependent regional variations in marrow relaxation time compared with dual X-ray absorptiometry

RATIONALE AND OBJECTIVES

Marrow transverse relaxation time (T2*) in magnetic resonance (MR) imaging may be related to the density and structure of the surrounding trabecular network. We investigated regional variations of T2* in the human calcaneus and compared the findings with bone mineral density (BMD), as measured by dual X-ray absorpiometry (DXA). Short- and long-term precisions were evaluated first to determine whether MR imaging would be useful for the clinical assessment of disease status and progression in osteoporosis.

METHODS

Gradient-recalled echo MR images of the calcaneus were acquired at 1.5 T from six volunteers. Measurements of T2* were compared with BMD and (for one volunteer) conventional radiography.

RESULTS

T2* values showed significant regional variation; they typically were shortest in the superior region of the calcaneus. There was a linear correlation between MR and DXA measurements (r = .66 for 1/T2* versus BMD). Differences in T2* attributable to variations in analysis region-of-interest placement were not significant for five of the six volunteers. Sagittal MR images had short- and long-term precision errors of 4.2% and 3.3%, respectively. For DXA, the precision was 1.3% (coefficient of variation).

CONCLUSION

MR imaging may be useful for trabecular bone assessment in the calcaneus. However, given the large regional variations in bone density and structure, the choice of an ROI is likely to play a major role in the accuracy, precision, and overall clinical efficacy of T2* measurements.

Trabecular bone morphology from micro-magnetic resonance imaging

Micro-magnetic resonance imaging (micro-MRI) is potentially a widely available tool to image and quantify the three-dimensional structure of trabecular bone. However, it has not been demonstrated that the same quantitative measurements can be obtained using micro-MRI as would be obtained from conventional light microscope images. Bovine trabecular bone from several anatomic sites was imaged with both optical and micro-MRI methods. The six faces of approximately cubic trabecular bone specimens were examined with the light microscope, and the volume of bone internal to these faces was then imaged using an 8.6 T 25 mm bore magnet. Three-dimensional measures of bone morphology were calculated from both the optical and micro-MR images using the method of directed secants. Quantitative measures from the two imaging methods were compared by paired t-tests. Volume fractions (BV/TV) measured by micro-MRI were linearly related to (r2 = 0.81) and did not differ statistically from (p = 0.96) similar measurements from optical images. The trabecular plate number (Tb.N) measured by micro-MRI also was linearly related to (r2 = 0.53) and did not differ statistically from (p = 0.17) similar measurements from optical images. The orientation of trabeculae predicted from micro-MRI was within 6 degrees of that calculated from optical images in 10 out of 16 specimens. The micro-MRI morphology measurements are relatively easy to perform, and since several hundred small-bore high-field strength MRI systems are available, this technique could be used widely to quantify the morphology of trabecular bone.

Three-dimensional nuclear magnetic resonance microimaging of trabecular bone

The conventional approach to measuring structural parameters in trabecular bone rests on stereology from optical images, derived from sections of embedded bone. In order to provide data that are statistically representative of a sufficiently large volume, multiple sections need to be analyzed in each of the three orthogonal planes. In this work, an alternative technique is presented which is based on three-dimensional (3D) volumetric proton nuclear magnetic resonance (NMR) microimaging. The method presented provides from 9 x 9 x 4 mm3 volumes of defatted bone specimens in 15-20 minutes scan time at isotropic resolution corresponding to (78 microm)3 voxel size. Surface-rendered images of bovine and human trabecular bone are shown and an algorithm was developed and implemented for determining the orientation and magnitude of the principle axes of the mean intercept length tensor.

Human vertebral cancellous bone surface distribution

The three-dimensional distribution of bone surface and the bone volume fraction (BV/TV) of 110 human vertebral cancellous bone specimens from seven individuals were measured using a three-dimensional radiographic method (microcomputed tomography). The ratios of the three principal projections of bone surface per total volume were found to be relatively constant for specimens examined in this study. The constancy of the projected surface ratios means that the fraction of the total bone surface oriented in any direction does not change markedly with BV/TV. Bone volume fraction was a good predictor of bone surface per total volume (BS/TV) for a one-parameter nonlinear model (r2 = 0.92). The results of this pilot study suggest that the changes in surface distribution which occur during age-related bone loss are largely predetermined rather than adaptive. The results are also consistent with the idea that cancellous bone tends to maintain a constant ratio of trabecular number for the principal directions. If these inferences from the data are correct, the morphogenetic processes which create the initial adult trabecular pattern become of primary interest. A model was developed which explained the strong relationship between BS/TV and BV/TV. The model was used to demonstrate the importance of morphogenetic processes.

Quantitative analysis of trabecular microstructure by 400 MHz nuclear magnetic resonance imaging

A new approach for the quantitative analysis of trabecular microstructure, based on high-field proton nuclear magnetic resonance (NMR) imaging, is presented. NMR is ideal because it provides high contrast between the marrow proton signal and the bone, which appears with background intensity. Images from 1 cm3 defatted specimens of trabecular bone, suspended in water doped with 1 mM Gd(DTPA) to shorten T1 to about 300 ms, can be obtained at a resolution on the order of 30-50 microns and slice thickness of 150 microns, in 10 minutes at 400 MHz proton frequency. Digital image processing algorithms were designed and evaluated for the measurement of bone area fraction, perimeter length, mean trabecular thickness, and separation. Bone area fraction derived from the NMR images was found to be in excellent agreement with bone volume fraction measured independently (slope = 0.96, r2 = 0.924, p < 0.0001). Errors in the mean trabecular thickness and separation were < 6%. The effects of finite imaging slice thickness and signal-to-noise ratio (SNR) were also evaluated. The data suggest a resolution of 50 x 50 x 200 microns 3 and an SNR on the order of 10 to provide safe margins for precise and accurate structural analysis by means of the algorithms presented in this paper. The method allows simultaneous measurement at multiple locations within the specimen volume without the need for physical sectioning.

Vertebral size in elderly women with osteoporosis. Mechanical implications and relationship to fractures

Reductions in bone density are a major determinant of vertebral fractures in the elderly population. However, women have a greater incidence of fractures than men, although their spinal bone densities are comparable. Recent observations indicate that women have 20-25% smaller vertebrae than men after accounting for differences in body size. To assess whether elderly women with vertebral fractures have smaller vertebrae than women who do not experience fractures, we reviewed 1,061 computed tomography bone density studies and gathered 32-matched pairs of elderly women, with reduced bone density, whose main difference was absence or presence of vertebral fractures. Detailed measurements of the dimensions of unfractured vertebrae and the moment arm of spinal musculature from T12 to L4 were calculated from computed tomography images in the 32 pairs of women matched for race, age, height, weight, and bone density. The cross-sectional area of unfractured vertebrae was 4.9-11.5% (10.5 +/- 1.4 vs 9.7 +/- 1.5 cm2; P < 0.0001) smaller and the moment arm of spinal musculature was 3.2-7.4% (56.4 +/- 5.1 vs 53.1 +/- 4.4 mm; P < 0.0001) shorter in women with fractures, implying that mechanical stress within intact vertebral bodies for equivalent loads is 5-17% greater in women with fractures compared to women without fractures. Such significant variations are very likely to contribute to vertebral fractures in osteoporotic women.