Biomechanics of osteoporotic fractures.

The objective of this review article is to present biomechanics concepts and data relevant to osteoporotic fractures. Fractures are mechanical events that occur when the applied load exceeds the fracture load (bone strength); both loading and strength must be considered to understand fracture risk. Hip fractures are almost always due to a fall, but only 5% of falls result in fracture. Hip fracture risk is greatest for a sideways fall that impacts on the greater trochanter. The loading events that cause vertebral fractures are poorly understood but include falls and heavy lifting. Activities that involve forward flexion and lifting generate the largest forces on the spine. Factors that affect bone strength include bone size (geometry) and bone density (vBMD). Men have larger bones at all ages compared to women and this is the main factor in the gender difference in whole-bone strength. Both men and women lose trabecular bone density and thus bone strength with aging, which is the main reason for the age-related loss of bone strength at the hip and spine, although dramatic decreases in the toughness of cortical bone may also contribute to osteoporotic fragility. The factor of risk (applied force/fracture force) is a useful concept for considering both the injury and the disease component of osteoporotic fractures. Within this article, I will review data on age-related changes in factors that affect fracture risk. Advanced engineering concepts will not be presented, but a familiarity with basic mechanical principles is assumed.