Lecture on endoprosthetics
How are endoprostheses arranged
Materials scientist Fyodor Senatov tells us what the endoprosthesis consists of and how its components are made.
Why do people need endoprosthetics
First, injuries. For example, athletes constantly need treatment for knees and hip joints. Secondly, there are congenital defects of the joints. Thirdly, age-related changes.
When a person breaks a bone, osteoblasts, the cells that build it, and osteoclasts, the cells that destroy it, begin to work. This is a constant regeneration process. There are fewer cells in cartilage tissue than in bone, so regeneration is limited. In case of extensive damage to the joint or its wear and tear, it is often necessary to resort to total endoprosthetics.
Components of the endoprosthesis
Hip and knee replacement surgeries are in demand, the shoulder and even the fingers are also changing. But the most popular are hip joints.
The hip joint endoprosthesis is a rather complex structure. The femoral component is inserted into the femoral canal. From it comes the neck, where the ball — head of the endoprosthesis is located, which rests on the acetabular component — this is what is in the pelvic bone and along which there is friction.
The leg of the joint endoprosthesis is usually made of different metals, these are titanium alloys or cobalt-chromium. The materials must be both durable and wear-resistant, because the leg enters the femur and must be tightly fixed. The material of the leg should have the same characteristics as the natural bone.
If the material of the joint endoprosthesis leg is too rigid, then uneven micro-stresses occur. They accumulate on a material having a larger elastic modulus, for example, on titanium. The surrounding bone remains unloaded. It changes its structure and porosity, becomes brittle. Probably the development of cracks, bone damage and loosening of the leg of the prosthesis inside the bone.
From the leg of the endoprosthesis there is a head on which there are a neck and a ball. The neck can be of different lengths and allows you to optimally adjust the tension of the muscles. The ball located on the neck is made of cobalt-chromium alloy or ceramic. The material should be as wear-resistant as possible, because wear particles are formed during friction. Experimentally, conditions have already been selected in which the surface of the ball is not perfectly smooth, but has small roughness for its optimal tribological behavior.
The last part of the endoprosthesis is the insert along which the ball is rubbing. Previously, it was made of metals, but clinical trials have shown that metal wear particles are toxic and can lead to osteolysis. Macrophages trigger the body's reactions leading to changes in the bone, it is damaged around the endoprosthesis and destroyed. In recent decades, the inserts of the joint endoprosthesis are made of polymer or ceramic. The main advantage of ceramics is high wear resistance, and polymers have high impact strength and cheapness.
The most popular material for endoprostheses
The most popular polymer is ultrahigh molecular weight polyethylene. It's almost like polyethylene from bags, only with long macromolecules. The molecular weight in this polymer is about 10 million grams per mole. This is a colossal amount, so the mechanical properties of such polyethylene are much better than those of a package. At the same time, it is modified for optimal use in medicine.
Polyethylene is constantly being improved, but it is inferior to ceramics in wear resistance. The wear of the ceramic-ceramic pair is about 0.01 millimeters per year. The wear of polyethylene on metal or ceramics is less than 0.1 per year. Due to the fact that the material wears out, it is necessary to resort to revision endoprosthetics, which is approximately 10% of operations. If a young athlete has a prosthesis, then after 15 years an update is necessary, even if the prosthesis is made of a sufficiently good material.
Manufacture of the acetabular component
The acetabular component of the prosthesis experiences the main friction. The use of ultra-high molecular weight polyethylene is effective here: it has a fairly good wear resistance relative to other polymer materials. It is used in the lining of dump trucks, even in the form of a coating on an all-weather rink.
This material is produced in two ways. The first method of making cups is thermal pressing. Powder is poured into the preform, pressure is applied and a high temperature of about 200 degrees. The material acquires the desired shape — cups. The problem of the method is in time: only one cup can be made in one mold.
The second and fastest method that is used everywhere is extrusion. This is plunger extrusion. The powder is poured into the reactor, heated and crushed by a piston. A melted rod comes out through the hole, which is then ground to the shape of a cup. Such cups are distributed all over the world: about a million operations are performed per year.
Polymer sterilization
Three methods of sterilization of cups: low-temperature plasma, ethylene oxide, gamma sterilization. But the latter has a problem. Gamma quanta have a sufficiently high energy: the polymer is destroyed, the molecules are torn, and the material deteriorates. It begins to oxidize, which leads to deterioration of mechanical characteristics and its structure. Therefore, we have to look for the optimum between high-quality sterilization and material damage.
There are two main strategies for improving polymer performance. The first is to avoid oxidation. This can be done if you introduce antioxidants, for example, vitamin E. It avoids oxidation after sterilization and during operation inside the body.
The second strategy is to increase the wear resistance of the material to extend its service life. This can be done by chemical or physical crosslinking of macromolecules. The chemical method is the elimination of hydrogens and the crosslinking of individual bonds. The physical method is bombardment with gamma quanta or high—energy electrons. This is called super-stitched polyethylene, which is the main type of material for creating endoprostheses of joints.
Portal "Eternal youth" http://vechnayamolodost.ru