We do a lot of hip and knee replacements every day, and often patients are curious about what materials the joint prosthesis is made of. In short, the common materials for joint prostheses are rare alloys (titanium alloys, cobalt-chromium alloys), polyethylene and ceramics, and bone cement “polymethyl methacrylate” is used to fix the prostheses.
If these scientific words make you feel boring, let us put it another way: “Your joint prosthesis and J-20 fighter use the same material!” “Ceramics can also be used to make joint prostheses!” “Your The joint prosthesis parts and the plastic bag containing the fritters are made of polyethylene! “Don’t be surprised, these are facts. Let’s introduce the materials of the joint prosthesis in detail below.
Osteoarthroplasty is common in the hip and knee joints. For the convenience of description, we divide the joint prosthesis into the main part, the friction interface and the bone-prosthesis interface.
Among various metals, the mechanical parameters of titanium alloys are relatively close to human bones, and they have good mechanical compatibility with bones, neither because they are too soft to support, nor because they are too hard to crush bones. Titanium alloy has a dense oxide layer on the surface, which is difficult to be corroded in the human body, has good biocompatibility, and rarely has a rejection reaction. At present, no clear toxic effect has been found. Although there are various excellent properties that make titanium alloys an ideal material for orthopedic plants, titanium alloys are difficult to smelt and more difficult to process, which is one of the reasons why joint prostheses are more expensive.
2. Friction interface
The friction interface is the core of the function of joint movement. If the friction interface is made of metal, such as the metal femoral head of the hip joint prosthesis and the femoral condyle of the knee prosthesis, cobalt-chromium or cobalt-chromium-molybdenum alloy is used. The most outstanding characteristic of this type of alloy is wear resistance. Cobalt is used in the industry to make carbide tools, and molybdenum is an indispensable metal material for tank armor. Alloys made of strong and strong alloys are wear-resistant and impact-resistant. Using these alloys to make the friction interface of a joint prosthesis is beneficial to prolong the life of the prosthesis and reduce the generation of harmful wear debris.
In order to cushion and wear-resistant, a plastic spacer needs to be placed between the tibia and femoral prosthesis of the knee joint, and the plastic pad can also be selected for the acetabular part of the hip prosthesis. Polyethylene is an ancient and common plastic that is found in everyday life, from washbasins to plastic bags.
Joint prostheses have to be implanted in the human body and will not be damaged for many years. Is it difficult to use such low-end plastic? In fact, in history, humans have also tried more advanced plastics, such as polytetrafluoroethylene (Teflon), but their performance is not ideal. After years of screening in the waves, the polyethylene has become the most resistant to wear and impact. Excellent choice. The polyethylene used in joint prostheses is slightly different from the one used to make plastic washbasins. In the 1960s, people increased the molecular size of ordinary polyethylene to produce “ultra-high molecular weight polyethylene”, which resulted in better wear resistance and lower coefficient of friction.
Since the 90′s, through chemical reactions and even high-energy rays, supplemented by fine heat treatment, polyethylene molecules have “pulled hands” with each other to form highly cross-linked polyethylene, further increasing wear resistance. Therefore, the polyethylene used in joint prostheses and the polyethylene commonly used in life, except for their names and similarities, are actually not at the same level.
Ceramics can indeed be used to make artificial joints. Moreover, ceramics are used in the most advanced hip joint prostheses. The ceramic material used for joint prostheses is not the kind of bowl used in your home. Ceramics in life are hard materials formed by clay sintering at high temperature; ceramics for joint prostheses are made of high-purity alumina and zirconia, and the sintering temperature is higher and controlled more strictly.
What are alumina and zirconia?
Natural zirconia crystals are called zirconia. You don’t know what zirconia is, so you must have heard of the natural crystals of alumina: sapphire and ruby—both gems with high hardness. Ceramics made from such excellent materials and special processes are both wear-resistant and hard. Ceramic-made hip prostheses wear only 5 microns (200,000 to one meter) per year, and are durable, making them the best choice for young patients.
Porcelain bowls are easy to shatter, so people invented enamel bowls, and the research and development of joint prostheses is the same. Early ceramic hip joint prostheses were prone to fragmentation. This problem was not solved. People sintered a layer of ceramic on the metal surface of the prosthesis and became a “black crystal” prosthesis. This not only retains the abrasion resistance of ceramics, but also takes advantage of the metal not to be broken. In fact, the fourth-generation ceramic prosthesis has rarely cracked, and the black crystal prosthesis and the ceramic prosthesis have their advantages. There is no clear distinction between good and bad.
3. Bone-prosthesis interface
The prosthesis needs to be fixed on the bone. For most knee prostheses and a few hip joint prostheses, a layer of “bone cement” needs to be applied between the prosthesis and the bone, just like the mortar used for masonry. The gap of the prosthesis to achieve the fixation of the prosthesis. “Bone cement” is a polymeric material whose chemical name is “polymethyl methacrylate”.
“Polymethyl methacrylate” also has a more popular name-”plexiglass”, which is another common material in life, the one used to make billboards and seals on bus stops. Ordinary plexiglass is chemically modified to make it more suitable for orthopedic applications, and it becomes bone cement. Bone cement has stable chemical properties and good biocompatibility. It has been used in orthopedic surgery for more than 60 years.
Of course, in the hip joint, this layer of cement is not needed in most cases, and replaced by some coating materials, including hydroxyapatite, tiny titanium beads or sandblasted rough surfaces and trabecular coating. And so on, are all to achieve biological healing between bones and metal prostheses, that is, grow together.
to sum up
The above are commonly used materials for joint prostheses. A joint prosthesis may seem inconspicuous, but behind it is the fusion of technologies in many fields such as metallurgy, chemical engineering, and machining. It is a condensed experience of decades of artificial joint surgery. Its complexity and exquisiteness are breathtaking.
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Post time: Feb-11-2020