Single- and dual-photon absorptiometry in osteoporosis and osteomalacia

A Review of the Relationship Between Parenteral Nutrition and Metabolic Bone Disease

Metabolic bone disease (MBD) refers to the conditions that produce a diffuse decrease in bone density and strength because of an imbalance between bone resorption and bone formation. MBD can be a potential complication in patients receiving chronic parenteral nutrition (PN) therapy and the management of this condition presents a challenge for many clinicians. The etiology of PN-associated MBD is poorly understood, but traditional risk factors can include malnutrition, vitamin and mineral deficiencies, toxic contaminants in the PN solution, concomitant medications, and presence of certain disease states. Although additional studies are warranted to further elucidate the development and management of this condition, the following review discusses some of the important factors that may play a role in the genesis of PN-associated MBD and evaluates some potential strategies for the diagnosis and treatment of this complication.

Role of cortical bone in bone fragility

PURPOSE OF REVIEW

Trabecular bone loss and vertebral fractures are historical hallmarks of osteoporosis. During the past 70 years, this view has dominated research aiming to understand the structural basis of bone fragility. We suggest this notion needs to be revised to recognize and include the role of cortical bone deterioration as an important determinant of bone strength throughout life.

RECENT FINDINGS

About 80% of the fragility fractures involve the appendicular skeleton, at regions comprising large amounts of cortical bone. Up to 70% of the age-related bone loss at these locations is the result of intracortical remodeling that cavitates cortical bone producing porosity. It is now possible to accurately quantify cortical porosity in vivo and use this information to understand the pathogenesis of bone fragility throughout life, assist in identifying patients at risk for fracture, and use this as a potential marker to monitor the effects of treatment on bone structure and strength.

SUMMARY

Cortical bone has an important role in determining bone strength. The loss of strength is the result of intracortical and endocortical remodeling imbalance that produces cortical porosity and thinning. Studies are needed to determine whether porosity is an independent predictor of fracture risk and whether a reduction in porosity serves as a surrogate of antifracture efficacy.

Growth and Age-Related Abnormalities in Cortical Structure and Fracture Risk

Vertebral fractures and trabecular bone loss have dominated thinking and research into the pathogenesis and the structural basis of bone fragility during the last 70 years. However, 80% of all fractures are non-vertebral and occur at regions assembled using large amounts of cortical bone; only 20% of fractures are vertebral. Moreover, ~80% of the skeleton is cortical and ~70% of all bone loss is cortical even though trabecular bone is lost more rapidly than cortical bone. Bone is lost because remodelling becomes unbalanced after midlife. Most cortical bone loss occurs by intracortical, not endocortical remodelling. Each remodelling event removes more bone than deposited enlarging existing canals which eventually coalesce eroding and thinning the cortex from 'within.' Thus, there is a need to study the decay of cortical as well as trabecular bone, and to develop drugs that restore the strength of both types of bone. It is now possible to accurately quantify cortical porosity and trabecular decay in vivo. The challenges still to be met are to determine whether measurement of porosity identifies persons at risk for fracture, whether this approach is compliments information obtained using bone densitometry, and whether changes in cortical porosity and other microstructural traits have the sensitivity to serve as surrogates of treatment success or failure.

Increased metabolism of bone collagen in post-menopausal female osteoporotic femoral heads

Our previous studies demonstrated that the residual collagen in osteoporotic bone was not normal but possessed higher levels of lysine hydroxylation and modified cross-linking. However, the mechanism for these changes was not clear. In the current investigation, an assessment of bone collagen metabolism in osteoporosis (OP) revealed an increase in the overall metabolism of collagen relative to age-matched controls. The increased metabolism accounts for the observed post-translational modifications of collagen which lead to a more fragile bone matrix. The rate of bone metabolism is therefore an important aspect of the pathogenesis of osteoporosis, the greater the turnover the greater the propensity of a more fragile tissue. Clearly, the quality of bone tissue does not depend solely on adequate bone density but also on the state of the collagenous matrix.

Metabolic Bone Disease in Inflammatory Bowel Disease

An oral calcium supplement (1000 mg/day) is recommended. Regular exercise should be performed. Ethanol intake should be moderate. Protein intake should be moderate. The patient's vitamin D status should be determined and corrected with an oral supplement when deficiency is present. Baseline and yearly bone density measurement should be taken. Alendronate, 10 mg/d orally, or risedronate, 5 mg/d orally, should be given to patients with osteopenia. Use of corticosteroids, cyclosporin, tacrolimus, and methotrexate should be limited to the short term when possible. Estrogen replacement therapy is recommended in postmenopausal women unless contraindications exist.

Trabecular bone volume fraction mapping by low-resolution MRI

Trabecular bone volume fraction (TBVF) is highly associated with the mechanical competence of trabecular bone. TBVF is ordinarily measured by histomorphometry from bone biopsies or, noninvasively, by means of high-resolution microcomputed tomography and, more recently, by micro-MRI. The latter methods require spatial resolution sufficient to resolve trabeculae, along with segmentation techniques that allow unambiguous assignment of the signal to bone or bone marrow. In this article it is shown that TBVF can be measured under low-resolution conditions by exploiting the attenuation of the MR signal resulting from fractional occupancy of the imaging voxel by bone and bone marrow, provided that a reference signal is available from a marrow volume devoid of trabeculation. The method requires accurate measurement of apparent proton density, which entails correction for various sources of error. Key among these are the spatial nonuniformity in the RF field amplitude and effects of the slice profile, which are determined by B(1) field mapping and numerical integration of the Bloch equations, respectively. By contrast, errors from variations in bone marrow composition (hematopoietic vs. fatty) between trabecular and reference site are predicted to be small and usually negligible. The method was evaluated in phantoms and in vivo in the distal radius and found to be accurate to 1% in marrow volume fraction. Finally, in a group of 12 patients of varying skeletal status, TBVF in the calcaneus was found to strongly correlate with integral bone mineral density of the lumbar vertebrae (r(2) = 0.83, p < 0.0001). The method may fail in large imaging objects such as the human trunk at high magnetic field where standing wave and RF penetration effects cause intensity variations that cannot be corrected. Magn Reson Med 46:103-113, 2001.

Bones and Crohn's: problems and solutions

Inflammatory bowel disease may manifest in various extra intestinal manifestations. Osteopenia and various arthropathies may be debilitating. These may be related to the disease itself, patient genetics, lifestyle, or disease treatment. Calcium and vitamin D malabsorption, vitamin K deficiency, malnutrition, corticosteroid and other immunosuppressive medications, smoking, lack of exercise and postmenopausal state may all play important roles. Treatment may be undertaken to correct nutrient deficiencies, inhibit bone resorption and increase bone formation.

Bone density measurement in major depression

1. Disturbances in cortisol secretory patterns and excessive secretion of cortisol after a variety of neuroendocrine stimulation tests indicate excessive activity of the hypothalamic-pituitary-adrenocortical axis in depression. 2. Peripheral indicators of hypercortisolemia have also been observed (e.g. enlarged adrenal glands, glucocorticoid insensitivity and insulin intolerance). 3. Excessive cortisol production may also result in altered bone metabolism and bone architecture, and a recent study by Michelson et al. (1996) found slightly lower bone density in depressed women with hypercortisoluria versus healthy controls. 4. In this study, the authors examined bone mineral density (BMD) using dual energy radiographic absorptometry (DEXA) technique in 6 depressed patients (3 with and 3 without hypercortisoluria) with a mean (+/- SD) age of 41 +/- 13 years, and in 5 healthy, controls with mean age 38 +/- 4 years). 5. DEXA images of the lumbar vertebrae (L1 to L4) for BMD were acquired over a 5-minute interval. 6. Overall, the authors observed no difference in mean BMD values between patients and controls, nor were differences observed between patients with and without hypercortisoluria.

Bone disease in cholestatic liver disease

Osteopenia in the form of osteoporosis is a common clinical problem associated with chronic cholestatic liver disease, and clinical morbidity from atraumatic fractures is increasing as more patients with PBC and PSC undergo successful liver transplantation. In the absence of symptomatic fractures, the clinical diagnosis may not be evident and must be sought by specific means to assess bone mineral density. The clinical problem has now been defined, but much remains unknown, from etiologic mechanisms to effective therapies. At present, it seems reasonable to provide aggressive supportive therapy in an attempt to maximize skeletal well-being until more effective therapies for osteopenia become available.

Do estrogens improve bone mass in osteoporotic women over ten years of menopause

A retrospective analysis of 24 patients with established osteoporosis and with ten or more years of menopause treated with conjugated estrogen, progesterone and calcium followed for one year has been performed. Treated women received 0.625 mg/day of conjugated estrogen from day 1 to 25, 5 mg/day of medroxiprogesterone from day 13 to 25, of each cycle, plus calcium (500-1000 mg/day), during one year (12 cycles). As control group was used 18 age-matched that received only calcium (500 a 1000 mg/day). All patients had at least two dual-photon spine and proximal femur (neck, Ward's triangle and trocanter) densities measurements performed 12 months apart. Estrogen treatment was associated with increased bone mineral density at spine and trocanter. Control group did not present any statistically change after one year in any site studied. We concluded that women with ten or more years of menopause and established osteoporosis treated with replacement hormonal therapy and calcium results in improvement of bone mineral density. These data support that women with ten or more years of menopause respond to estrogen replacement therapy with absolute increments in bone density similar to those seen in younger women, in the early menopause.

Advantages of peripheral radiogrametry over dual-photon absorptiometry of the spine in the assessment of prevalence of osteoporotic vertebral fractures in women

Since osteoporosis develops in most postmenopausal women and is probably the most important single factor in the pathogenesis of osteoporotic fractures of the spine, hip, and wrist (and at other sites), methods suitable for mass screening should be developed. In this study of 97 women aged 24-79, measurements of the lumbar spine mineral content by dual-photon absorptiometry (DPA) were compared with the summed combined cortical thickness measurements from radiographs of the radius and metacarpal II (MR). There was good correlation between the two methods (r = 0.90). The correlation of age with MR was higher than with DPA. The correlation of years postmenopause was significant with MR but not with DPA. Taking the -2 SD level of the premenopausal means to be previously established vertebral fracture thresholds, 24% of the DPA measurements, but no MR measurements in patients with vertebral compressions, were above the fracture threshold. Since MR measurement requires taking only two small plain radiographs using ordinary x-ray equipment, it is concluded that this less expensive method is better suited to screening for osteoporotic vertebral fracture risk in postmenopausal women than DPA.

A population-based comparison of quantitative dual-energy X-ray absorptiometry with dual-photon absorptiometry of the spine and hip

Dual photon absorptiometry (DPA) is currently the most widely used method for noninvasive bone mineral density (BMD) measurement of the axial skeleton. Dual energy X-ray absorptiometry (DEXA) is a recently developed technique that uses an X-ray tube as a photon source; it has demonstrated several significant advantages over DPA in preliminary studies. We report here a quantitative comparison of the DEXA and DPA technologies using a Hologic DEXA (Hologic QDR model 1000, Waltham, MA) scanner and a Lunar DPA (Lunar Radiation DP3, gandolineum-153 source) scanner at both the proximal femur and lumbar spine sites using bone density measurements from a population-based sample of older white men and women who had complete DEXA and DPA measurements of the hip (n = 217) or the spine (n = 176). To examine the relationship of BMD measured by the DPA scanner to BMD measured on the DEXA scanner, normal least squares linear regression was used to regress the DPA BMD on the DEXA BMD for each site. DEXA values were consistently lower than DPA values, with an average difference of 16%. The squared multiple correlation (R2) values were at or above 0.95 for almost all sites, with Ward's triangle having the lowest value (0.89). The slope for all sites was similar, ranging from 0.94 to 1.1. Research and clinical centers that wish to change to DEXA technology because of its shorter examination time and greater precision can therefore compare DEXA with DPA values using representative conversion factors.

Vertebral fractures in steroid dependent asthma and involutional osteoporosis: a comparative study

BACKGROUND

Reduced bone mass predisposes patients to the development of vertebral fractures. Measurement of bone mass by non-invasive methods is used to detect patients with involutional osteoporosis at risk from fractures. These methods have not been assessed in patients with steroid dependent osteoporosis. The objective of this study was to assess the value of a predictive fracture threshold value of bone density in patients with steroid dependent asthma.

METHODS

Three groups of patients were studied. Group 1 (67 patients) had steroid dependent asthma (mean daily dose of prednisone 11.7 mg) and no vertebral fractures, group 2 (32 patients) had steroid dependent asthma (mean daily dose of prednisone 12 mg) and vertebral fractures, and group 3 (55 patients) were not taking steroids but had involutional osteoporosis and a recent non-traumatic vertebral fracture. Bone mineral density was measured by dual photon absorptiometry and vertebral fractures by radiography of the lumbar spine. A fracture threshold was determined in the two groups with fractures as the 90th percentile of the mean bone mineral density measured in the lumbar spine.

RESULTS

Bone mineral density was significantly higher in the steroid dependent group with fractures (group 2) than in group 3 patients, who had involutional osteoporosis and fractures (0.946 (0.18) g/cm2 v 0.830 (0.16) g/cm2). The fracture threshold value was therefore higher for patients with steroid related vertebral fractures (group 2, 1.173 g/cm2) than for those with involutional osteoporosis (group 3, 0.979 g/cm2). Vertebral fractures were more likely to occur in steroid dependent asthmatic patients with bone density above the fracture threshold value (obtained from subjects with involutional osteoporosis) than in subjects in group 3 (34% v 9%).

CONCLUSION

Vertebral fractures occur in patients treated with steroids in the presence of higher bone mineral density than is the case with patients with involutional osteoporosis. The findings suggests that the assessment of the efficacy of preventive treatment requires measurement of bone mineral density and radiology.

Clinically useful and readily available techniques for measurements of bone mineral and body composition by photon or x-ray absorptiometry

Technical develoments in dual-energy x-ray absorptiometry permit the estimation of bone mass or density and of body fat with one single, quick, nontraumatic measurement. Bone density is the best available means of estimating skeletal fracture risk. Improved fat quantification can be made independent from the generally made assumption in such calculations that bone represents a fixed fraction of fat-free body mass.

A comparison of quantitative dual-energy radiographic absorptiometry and dual photon absorptiometry of the lumbar spine in postmenopausal women

Noninvasive bone densitometry is an important aspect in the detection and management of osteoporosis and other forms of metabolic disease of calcified tissue. A system using quantitative dual-energy digital projection radiography (QDR) of the lumbar spine was systematically tested against dual-photon absorptiometry (DPA) of the lumbar spine in 131 women over 55 years of age and free from major risk factors for osteoporosis. All subjects were scanned by both QDR and DPA under the same conditions. Measurements for a given subject were made within 15 minutes of each other. Bone mineral densities (BMD) were determined for four individual levels in the lumbar spine (L1-L4). Regression equations for BMD vs. age, height, and weight were calculated. The results of this investigation indicate that DPA- and QDR-derived BMD values are comparable. BMD values derived by QDR were consistently lower than those obtained by DPA (DPA = 1.115 QDR + 0.137, r = 0.942). The L2 lumbar region was the most strongly correlated determination.

Some effects of basic multicellular unit-based remodelling on photon absorptiometry of trabecular bone

This article offers algorithms and an algebra for estimating effects of bone turnover, remodelling space, undermineralized bone and trabecular surface-to-volume ratio effects on trabecular bone mass estimation by photon absorptiometry. From published histomorphometric data and other evidence the algorithms suggest the amount of mineral in a given bone sample can suggest to absorptiometry an amount of bone that differs from the truth by over 40% in the extreme, and more commonly by 5-15%. They suggest that by reducing a bone's global mineral content high bone turnover causes underestimation of true bone mass. They suggest that by letting mineral return to the remodelling space and undermineralized bone, reduced bone turnover causes apparent gains in bone mass. The commonly suggested 5-15% magnitude of such errors exceeds those assumed in the past. The algorithms suggest that after a challenge to remodelling those bone mineral changes can take from 6 months to over 3 years to reach steady states. Those features could explain why many osteoporosis treatments judged effective from initial absorptiometric evidence failed when used for long periods in patients. Finally the algorithms suggest that a real increase in ideal bone volume can even appear to absorptiometry as no gain or an initial loss, which has already happened in two human experiments.