The association of height, weight, pubertal stage, calcium intake, and physical activity with bone mineral density (BMD) was evaluated in 500 children and adolescents (205 boys and 295 girls), aged 4-20 yr. The BMD (grams per cm2) of lumbar spine and total body was measured with dual energy x-ray absorptiometry. Lumbar spine volumetric BMD was calculated to correct for bone size. BMD and volumetric BMD increased with age. During puberty, the age-dependent increment was higher. After adjustment for age, the Tanner stage was significantly associated with all three BMD variables in girls and with spinal BMD in boys. In boys, positive correlations were found between BMD and both calcium intake and physical activity after adjustment for age. Stepwise regression analysis with weight, height, Tanner stage, calcium intake, and physical activity as determinants with adjustment for age resulted in a model with Tanner stage in girls and weight in boys for all three BMD variables. The major independent determinant of BMD was the Tanner stage in girls and weight in boys.

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Bone mineral density in children and adolescents: relation to puberty, calcium intake, and physical activity

Modeling the mechanical behavior of vertebral trabecular bone: Effects of age-related changes in microstructure

The ability of the spine to carry load depends on the structural capacity of the vertebrae and on the loading conditions that arise from activities of daily living or trauma. The ratio of the load on the spine to the failure load of the bone is called the factor of risk, a value that indicates whether fracture is likely during a given activity. A high factor of risk can result from weak bone that has a low failure load or from a risk activity that generates a large force. The failure load of the vertebral body depends on the density and architecture of the trabecular bone and on the shape, size, and organization of the vertebral body. Spine bone mineral density determined by dual-energy x-ray absorptiometry correlates strongly with the compressive failure load and therefore should be a convenient and specific indicator of the compressive strength of the vertebrae in vivo. The magnitude of a load applied to the spine depends on the specific activity. Bending and lifting activities generate loads on the spine that exceed the failure load of vertebrae with very low bone mineral density. Biomechanical indications for prevention of age-related fractures point to the need to strengthen bone by maintaining or increasing density and the need to lower the magnitude of forces applied to the spine in the elderly by avoiding such risk activities as lifting.

Biomechanics of osteoporosis and vertebral fracture

STUDY DESIGN A biomechanical study comparing two materials for augmentation of osteoporotic vertebral bodies and vertebral bodies after compression fracture. OBJECTIVES To compare an injected, biodegradable calcium phosphate bone substitute with injected polymethylmethacrylate bone cement for strengthening osteoporotic vertebral bodies and improving the integrity of vertebral compression fractures. SUMMARY OF BACKGROUND DATA Injection of polymethylmethacrylate bone cement into fractured vertebral bodies has been used clinically. However, there is concern about thermal damage to the neural elements during polymerization of the polymethylmethacrylate bone cement as well as its negative effects on bone remodeling. Biodegradable calcium phosphate bone substitutes have been studied for enhancement of fixation in fractured vertebrae. METHODS Forty fresh osteoporotic thoracolumbar vertebrae were used for two separate parts of this study: 1) injection into osteoporotic vertebrae: intact control (n = 8), calcium phosphate (n = 8), and polymethylmethacrylate bone cement (n = 8) groups. Each specimen then was loaded in anterior compression until failure; 2) injection into postfractured vertebrae: calcium phosphate (n = 8) and polymethylmethacrylate bone cement (n = 8) groups. Before and after injection, the specimens were radiographed in the lateral projection to determine changes in vertebral body height and then loaded to failure in anterior bending. RESULTS For intact osteoporotic vertebrae, the average fracture strength was 527 +/- 43 N (stiffness, 84 +/- 11 N/mm), 1063 +/- 127 N (stiffness, 157 +/- 21 N/mm) for the group injected with calcium phosphate, and 1036 +/- 100 N (stiffness, 156 +/- 8 N/mm) for the group injected with polymethylmethacrylate bone cement. The fracture strength and stiffness in the calcium phosphate bone substitute group and those in the polymethylmethacrylate bone cement group were similar and significantly stronger than those in intact control group (P < 0.05). For the compression fracture study, anterior vertebral height was increased 58.5 +/- 4.6% in the group injected with calcium phosphate and 58.0 +/- 6.5% in the group injected with polymethylmethacrylate bone cement as compared with preinjection fracture heights. No significant difference between the two groups was found in anterior vertebral height, fracture strength, or stiffness. CONCLUSION This study demonstrated that the injection of a biodegradable calcium phosphate bone substitute to strengthen osteoporotic vertebral bodies or improve vertebral compression fractures might provide an alternative to the use of polymethylmethacrylate bone cement.

The use of an injectable, biodegradable calcium phosphate bone substitute for the prophylactic augmentation of osteoporotic vertebrae and the management of vertebral compression fractures.

Regional variation in vertebral bone morphology and its contribution to vertebral fracture strength

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

M. Moro

Papers

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