Biomechanics of osteoporosis and vertebral fracture.

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.