Effects of osteoporosis treatments on bone quality

to fracture. However, bones fracture for different reasons, so there are several different biomechanical definitions of bone fragility. One function of bones is to carry loads. Fractures occur when loads exceed the bone strength, so weakened bones should be considered fragile. For example osteoporotic vertebral bodies might fracture during normal daily activities like opening a window or rising from a chair. These non-traumatic or fragility fractures result from substantially weakened bone. On the other hand, hip fractures result mainly from trauma associated with falls or impact. During a traumatic loading, like a fall to the ground, fracture will occur if the energy from the fall exceeds the mechanical energy that the bone can absorb. Consequently, with trauma even strong bones should be considered fragile if they are unable to absorb energy due to excessive brittleness. This may sound like a contradictory statement – how can strong bones be fragile? – but from a biomechanical perspective, fragility is not defined only by bone strength. The biomechanical definition of bone fragility includes at least three components: strength, brittleness and work to failure. (A fourth biomechanical measure, stiffness, also is used to assess mechanical integrity of bones, but is not a direct measure of fragility.) Skeletal diseases can cause fragile bones by affecting bone structure in different ways. For instance, osteopetrosis and osteomalacia both cause increased risk of fracture. However, these diseases result in grossly different bone biomechanical characteristics. Osteopetrosis causes stiff, brittle bones. Osteopetrotic bones absorb very little energy before breaking (reduced work to failure) and are therefore more susceptible to fracture resulting from trauma. Osteomalacia results in weak, ductile bones. Like osteopetrosis, osteomalacia reduces work to failure. However, osteomalacic bones can deform considerably before the fracture. Due to their weakness the bones often bend under load and deform, which can result in the bowed long bones seen in rickets patients. No such bowing is possible in osteopetrotic bones, instead fracture occurs after very little deformation. How can bone fragility be reduced? There are at least three ways to make the skeleton stronger. First, increase bone mass – larger bones can carry more load. Second, distribute bone mass effectively, i.e., put bone tissue where the mechanical demands are greatest. Third, improve the material properties of bone tissue such that the bone is stronger at a tissue-level. From a biomechanical standpoint, an ideal drug to cure bone fragility would improve strength and decrease brittleness.