Bone strength refers to the capacity of skeletal tissue to withstand external forces without fracturing. The force required to break a bone is not a single fixed number, but a range influenced by numerous factors.
The Science of Bone Strength
Bones possess a remarkable structural design that provides both rigidity and flexibility, allowing them to resist various stresses. This dual nature arises from their composite material makeup. The organic component, primarily collagen, provides elasticity and tensile strength, enabling bones to bend slightly without breaking. The inorganic component, mainly calcium phosphate minerals, contributes compressive strength and hardness, making bones resistant to crushing forces.
Two primary types of bone tissue contribute to skeletal integrity. Cortical bone, or compact bone, forms the dense outer layer of most bones and is resistant to bending and torsion. Cancellous bone, also known as spongy or trabecular bone, is found at the ends of long bones and within vertebrae, providing a lighter, shock-absorbing structure.
Forces Required to Fracture Bones
The pressure needed to fracture a bone varies based on its type, size, and how force is applied. For instance, the human femur, a large long bone, can withstand substantial compressive forces, potentially requiring 1,700 to 2,000 pounds per square inch (PSI) to fracture axially. Smaller bones, such as ribs, may fracture with less force, sometimes as low as 100 to 200 PSI under direct impact.
Vertebrae, which bear the body’s weight, are designed to withstand significant axial compression, but can fracture under pressures around 1,000 PSI depending on the specific vertebra and bone health. The type of force also plays a role. Bones are stronger in compression than in tension. Tensile forces, which pull the bone apart, may cause fractures at lower PSI values compared to compressive forces. Shear forces, acting parallel to the bone’s surface, and torsional forces, involving twisting, can also lead to fractures at varying thresholds.
Factors Influencing Bone Fracture Thresholds
Several variables influence the pressure a bone can withstand before fracturing. Age is a factor, as children’s bones tend to be more flexible and often bend or buckle rather than cleanly break. In contrast, older adults often experience decreased bone density, particularly due to conditions like osteoporosis, making their bones more brittle and susceptible to fracture from less force. For instance, a fall that might only bruise a younger person could cause a hip fracture in an elderly individual.
Nutritional status also plays a role in bone strength. Deficiencies in essential minerals like calcium and vitamins such as Vitamin D can compromise bone mineralization, leading to weaker bones that fracture more easily. Certain medical conditions, including some endocrine disorders or long-term use of specific medications like corticosteroids, can also weaken bone structure over time.
The direction and rate of force application are equally important. A sudden, sharp impact (high-rate force) is more likely to cause a fracture than a slow, gradual increase in pressure, even if the peak force is similar, because the bone has less time to deform and dissipate the energy.
What Happens When a Bone Breaks
A bone fracture represents a physical disruption in the continuity of the bone when the applied mechanical force exceeds its structural integrity. The nature of the break can vary widely depending on the force’s magnitude and direction.
Fractures are broadly classified by their characteristics. A complete fracture means the bone is broken into two or more separate pieces, while an incomplete fracture, such as a hairline crack, does not fully span the bone. Fractures can also be simple, where the bone breaks in one place, or comminuted, involving multiple fragments. The physical outcome is a loss of structural support and often significant pain, swelling, and inability to use the affected limb or body part.