The main requirements for surgical materials are corrosion resistance and strength. Biological inertness is often sacrificed for the sake of the latter, moreover, the mechanical compatibility of the implant and body structures were taken into account only in the last 20 years: the elasticity of most alloys is higher than that of bone and cartilage (230-110 GPa versus 25 GPa and 1-0.2 GPA), which leads to an uneven spreading of loads and deformations (maximal at the attachment points of the implant to the bone). Therefore, it is better to use alloys with a low elasticity (Ti-Ta systems with an elasticity of 80–70 GPa), or to form various cuts and bends on the implant, reducing its rigidity. However, the strength of most metals decreases with rigidity, except for titanium nickelide.
Robust and cheap surgical steel is the main material for the production of surgical instruments, but, unfortunately, it has the property of magnetization, so prostheses cannot be made of it. Magnetic alloys are a contraindication to MRI, therefore, prosthetics made of titanium are preferable.
Metal osteosynthesis is used to treat fractures, and titanium plates are great as fixers and immobilizers. But steel products are susceptible to corrosion under the influence of the body's fluids and the phenomenon of galvanization - immobilizers are destroyed very quickly, the inflammatory processes cause a strong pain syndrome due to the active interaction of Fe ions.
Nickel-titanium alloys has superelastic and shape memory effects. Titanium nickelide is also ideal in its compatibility with human tissues, but it is not widely used yet because of the complexity of production.
In single countries (Russia is among them) nickelide can be produced on an industrial scale. Unification of structures has not yet been carried out, so the implants are made individually in small quantities.
The rules for the production of implants are as follows: clamps made of titanium nickelide, of course, can not replace immobilizers, plates and transpedicular devices of alloys; immobilizers must mechanically match the strength of the tissue that is held together (U-shaped locks has mechanical behavior closest to the bone, and the cartilaginous and ligamentous structures correspond to the looped structures). But temporary osteosynthesis with titanium is extremely convenient to conduct, and it is also an excellent material for prosthetics.
The difference in stiffness in vertebrology is eliminated through following ways: clamps for the arms or spinous processes of vertebrae reduces displacement to 70-90% of a healthy segment. Stabilization of the spine with metal has been used in medicine since the beginning of the twentieth century. Using clamps with different sizes of compression and stiffness, changing the place of installation ensure fixation of the spine with the required stability. Connective tissue is formed at the site of removed structures within 3-4 weeks, changing the mechanical strength of vertebral segment, therefore the volume of the corresponding segment displacements is reduced to 40 - 60%. As a result, the load acting on the fixative as tissue regenerates gradually decreases. In many cases (with the exception of multiple compression fractures of the vertebral bodies), it is possible to refuse an external immobilizer immediately after the operation.
If the joint is severely deformed or destroyed, then the endoprosthesis will be the best solution. Hip, shoulder, elbow, knee, and ankle endoprosthetics is a difficult, because a part of a non-natural joint is removed and then replaced with an implant. Since endoprostheses are mechanisms, requirements for their strength, elasticity and congruence are increased, and bio inert tantalum and titanium here come at an opportune moment. The titanium alloy Ti-6A1-4V has a particularly high mechanical strength, having torsion-axial characteristics that are extremely close to those of the bone. Alloys Ti-5AI-2,5Fe and Ti-6AI-17 Niobium do not contain toxic vanadium and inelastic, and Ti-Ta30 is similar in its properties to thermal expansion to metal ceramics.
Tantalum-zirconium alloys have a trabecular surface structure that accelerates osseous integration - the buildup of living bone tissue on the implant surface.
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