A phenomenon in which an implant far stiffer than bone carries most of the load, the mechanical stimulus to the surrounding bone falls, and the bone is resorbed and thins.
Japanese name: 応力遮蔽
Stress shielding appears in 5 practice questions across 1 topic, including Biomaterials.
Field: Biomaterials
When bone and an implant are joined and carry a load together, both strain by nearly the same amount, so the stress is shared in proportion to Young's modulus. The Young's modulus of cortical bone is about 10–30 GPa, whereas stainless steel is about 200 GPa, Co-Cr alloys about 210 GPa and Ti-6Al-4V about 110 GPa. When a high-modulus metal carries most of the load, the stress on the surrounding bone becomes smaller than it should be. From the bone's point of view the load is shielded, so the phenomenon is called stress shielding. It is a problem with the stems of artificial hip joints and with plates that fix fractures.
Bone changes the balance of formation and resorption according to the mechanical stimulus it receives, becoming thicker where it is used heavily and thinner where it is not (Wolff's law). Where stress shielding reduces the stimulus, resorption outpaces formation, bone density falls and the bone thins. As a result, loosening develops between implant and bone, or the bone becomes prone to fracture after the implant is removed. The weakening of bone during long bed rest or in weightlessness has the same mechanism. In artificial hip joints, bone loss is often said to be most noticeable near the upper end of the femur.
The basic remedy is to bring the stiffness of the implant closer to that of bone. On the materials side, β titanium alloys with a Young's modulus of about 55–80 GPa and porous metals whose modulus is lowered by pores have been developed. The Young's modulus of a porous body with open pores falls roughly with the square of the relative density, and bone growing into the pores also strengthens fixation. On the shape side, stems are made thinner and shorter, or designed to transfer load at a position close to the bone. However, lowering the stiffness too far risks implant deformation and fatigue failure, so it is decided in balance with strength.
Answer: B. The implant carries most of the load and the stress on the surrounding bone falls, so the bone is resorbed and thins
When bone and an implant carry a load together, both strain by nearly the same amount, so the one with the larger Young's modulus carries most of the stress. The Young's modulus of cortical bone is about 10–30 GPa, but Co-Cr alloys at about 210 GPa and Ti-6Al-4V at about 110 GPa are far higher, so the stress on the bone becomes smaller than it should be. Bone changes the balance of formation and resorption according to the mechanical stimulus it receives (Wolff's law), so where the stimulus falls, bone is resorbed and thins, which causes implant loosening and fractures around the implant. Excessive stress on the bone is the opposite situation. Metal ion release due to damage to the passive film is a separate problem, such as fretting corrosion.
Polymers often have a Young's modulus too far below that of bone, or cannot withstand the load in terms of strength, creep and fatigue, so on their own they are unsuitable for sites that carry large loads. Composites whose modulus is brought close to that of bone, such as carbon-fiber-reinforced PEEK, are used in spinal fixation devices and similar applications.
No, the opposite. The implant takes over the load, so the stimulus to the bone falls, and the bone is resorbed and weakens. It is the opposite problem to bone failing under excessive stress.
Clinically, changes in bone mass around the implant are followed with X-ray images and bone density measurements. At the design stage, the stress distribution in bone and implant is calculated by the finite element method, and the drop in bone stress compared with the case without an implant is evaluated.
And even a material with no toxic elements is not compatible with an application if it is so stiff that it causes stress shielding, if its wear debris causes inflammation, or if it forms thrombi in contact with blood.
Which statement correctly describes the idea of biocompatib…The implant takes over about 85% of the load that the bone alone used to carry, so the stimulus to the bone falls sharply and stress shielding occurs.
A rod of cortical bone (Young's modulus 20 GPa) and a rod o…β alloys have a Young's modulus of about 55–80 GPa, lower than that of the α+β alloy Ti-6Al-4V (about 110 GPa) and closer to that of bone (about 10–30 GPa), so they reduce stress shielding.
What is the main reason why β titanium alloys such as the T…Open pores also serve as a scaffold for bone to grow into, so porous metals are one way to reduce stress shielding.
Estimate the Young's modulus of porous titanium with open p…Young's modulus Passivity β titanium alloy Biocompatibility Biodegradable polymer Bone cement Hydroxyapatite Osseointegration Superelasticity