Bone tissue is a composite material designed to resist failure under immense mechanical loading, a property that can be quantified using principles of physics and materials science. The question of how many pounds per square inch (PSI) are required to cause a fracture seeks a specific, quantitative measurement of bone failure.
Defining the Mechanics of Bone Stress
Mechanical stress is defined as the force applied over a specific area, and pounds per square inch (PSI) is a common unit for this measurement. One pound of force exerted on a single square inch of material equals one PSI. The fracture threshold is not a single number because bone reacts differently to various mechanical stresses.
In a laboratory setting, researchers test bone by applying three primary types of force that mimic real-world injury. Compression involves pushing the material together, such as the force running down the length of the femur when standing. Tension is the opposite force, pulling the material apart, which occurs when a bone is bent or twisted. Shear force involves pushing one part of the material in one direction and an adjacent part in the opposite direction, causing a sliding or slicing motion.
Bone Strength Thresholds (Measured in PSI)
The amount of PSI a bone can withstand depends significantly on the type of bone tissue being measured. Cortical bone, which forms the dense, hard outer layer of most bones, is substantially stronger and more resistant to mechanical failure. Trabecular bone, the spongy, porous tissue found inside the ends of long bones and in the vertebrae, is much less dense and therefore yields at lower stress levels.
For a healthy, average adult, the compressive strength of the cortical bone in a long bone like the femur is exceptionally high, often exceeding 20,000 to 29,000 PSI. Tensile strength, the force required to pull the bone apart, is lower, often measured around 19,000 PSI for cortical bone. Bone is typically weakest when subjected to shear forces, where the failure threshold can be around 7,500 PSI.
Because these values are so large, biomechanists often use Megapascals (MPa), where 1 MPa equals approximately 145 PSI, making the cortical bone’s compressive strength roughly 200 MPa. In contrast, the porous structure of trabecular bone makes it much weaker, with its strength values often being two orders of magnitude smaller than cortical bone. Trabecular bone failure occurs at stress levels much closer to the general range of 1,000 to 2,000 PSI that is sometimes cited for typical bone fracture. The difference in density between the two bone types accounts for the wide range of fracture thresholds across the human skeleton.
Biological and Structural Variables Affecting Fracture
The mechanical failure thresholds stated in PSI are representative averages for healthy tissue, but they are not fixed values. Bone mineral density (BMD) is a primary determinant, as the loss of mineral content, such as in osteoporosis, weakens the internal structure of the tissue. A decrease in BMD means less material is available to bear the load, consequently lowering the effective PSI required for a fracture to occur.
The geometry and size of the bone also play a role in fracture resistance. Bones with a larger cross-sectional area, such as the shaft of the femur, are better able to distribute force over a greater surface. The thickness of the cortical shell is particularly important, providing greater structural resistance to bending and torsional forces.
Age is another significant factor, as bone strength peaks in early adulthood and progressively declines afterward due to age-related loss of collagen and mineral. Certain health conditions, including hormonal imbalances, nutritional deficiencies, and habits like smoking, can compromise the tissue’s quality and structure. These internal variables mean that a force harmless to one individual’s bone may easily exceed the fracture threshold of another’s.
Force Generation in Injury Scenarios
High-energy trauma, such as a motor vehicle accident, generates massive and sudden forces that easily exceed the compressive strength of the femur. Rapid deceleration creates high-magnitude compressive or shear forces delivered instantly, overwhelming the bone’s capacity to absorb energy.
In contrast, low-energy trauma, like a simple fall, often results in fractures at lower overall force levels, particularly in bones weakened by age or disease. A person falling onto their hip can generate a bending force that results in a tensile fracture of the femoral neck. In this scenario, the concentration of force over a small area and the direction of the stress overcomes the bone’s strength, not necessarily an extremely high PSI value.
Rotational force, or torque, is particularly damaging and often results in spiral fractures of the long bones. This twisting motion creates a complex combination of shear and tension stresses that exploit the bone’s lower resistance to these forces. Even a small bone, such as a rib, can be fractured with relatively low total force, sometimes requiring only a few hundred pounds, which translates to a low PSI if distributed widely.