Biomechanics of Osteoporosis and Vertebral Fracture

Abstract

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.

Effects of Teriparatide and Alendronate on Vertebral Strength as Assessed by Finite Element Modeling of QCT Scans in Women With Osteoporosis

“…Biomechanically, a treatment-induced increase in strength protects from fracture by moving the bone over or away from its fracture threshold (i.e., the point at which the in vivo forces applied to the bone during a strenuous activity or fall exceed the strength of the bone). (24)(25)(26) In contrast, agerelated decreases in strength increase the risk of fracture by moving the bone closer to or below its fracture threshold. Mosekilde et al (27,28) estimated that ∼10% of vertebral strength is lost per decade, and this loss is exacerbated further in individuals with osteoporosis.…”

“…Biomechanically, any fracture that is prevented for a bone having a pretreated strength below its biomechanical fracture threshold would only be prevented if the increase in vertebral strength was sufficiently large to protect against the applied in vivo loads. (24)(25)(26)30) Patients having numerous and/or severe prevalent vertebral fractures are at increased risk of subsequent fracture, (31,32) presumably because they have reduced strength of multiple vertebrae from the biomechanical deterioration associated with systemic bone loss. Hence, it would be expected that a large percentage of patients with prevalent fractures have other vertebrae that are near or below their biomechanical fracture threshold.…”

The use of sintered bone in spinal surgery

“…Reductions in the apparent density and the bone mineral density of trabecular bone typify ageing and osteoporosis. In many experimental studies, the maximum force during compression of vertebral specimens has been assessed, and the failure force has been related to the apparent density and geometric properties of the vertebrae [9]. Measurements have shown that much lower failure force accompanies ageing and severe osteoporosis.…”

Multiscale modelling and nonlinear finite element analysis as clinical tools for the assessment of fracture risk

“…These are considerations that affect the probability of a fracture occurring and have to be addressed in simulations of femoral neck fractures (Lotz et al 1995;Keyak et al 1998). Estimating physiological loads in the spine is more difficult (Myers & Wilson 1997), as a vast number of muscles and ligaments are interacting in order to provide stability, and there is currently no method available for the non-invasive quantification of muscle forces in vivo. Most often a simplified compression of an individual vertebra is simulated.…”