1. Bone density. The compressive and tensile strength of bone is proportional to the density. The porosity of cortical bone is 3-30%. Cancellous bone can have a porosity of >90%. Cancellous bone has a lower ultimate strength, is 10% as stiff, but 500% as ductile as cortical bone.
2. Patient age. Bone is more ductile in the young, leading to the characteristic greenstick and plastic deformation types of failure. With increasing age, as well as osteoporosis, bone becomes more brittle. The modulus of elasticity reduces by 1.5% per annum. In the elderly, bone may be osteoporotic — cortical bone is resorbed endosteally, widening the intramedullary canal. The bony trabeculae of cancellous bone become thinner. The resultant reduction in bone density weakens the bone and the ultimate strength reduces by 5-7% per decade.
3. Bone geometry
• The longer a bone, the greater the potential bending moment.
• The larger the cross-sectional area, the greater the stress to failure.
• The area moment of inertia measures the amount of bone in a cross-section and the distance from the neutral axis. The more bone further from the axis, the stronger and stiffer the bone. In a cylinder the relationship is to the power four.
4. Wolff's law (1892). This states that bone is laid down in response to stress and resorbed where stress is absent. Consequently there can be localized areas of reduced density (after plate removal due to stress shielding) or systemic reduction after prolonged recumbency.
5. Stress raisers. A cortical defect occupying 20% of the diameter, caused for example by a tumour or screw removal, can reduce the strength of a bone by up to 60%. Screw defects heal over in 8 weeks.
6. Muscle function. Muscle protects bone. An example is the skier's 'boot top' fracture of the tibia - contraction of the triceps surae, resisting ankle flexion, compresses the posterior tibial cortex. As bone is weaker in tension than in compression, the fracture starts anteriorly.
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