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Plastics are a gold standard in medicine

Plastic built into your body? Maybe it sounds crazy, but when it comes to joint replacements, it’s not crazy at all, as articular inserts are made from polyethylene to ensure the movement of artificial joints. Of course, it is not the same type of polymer from which supermarket plastic bags are made, but rather ultra-high molecular weight polyethylene (UHMWPE) with a fine structure.

Plastics are a gold standard in medicine
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Polymers are not used to make whole prostheses. Attachment of an implant to the bone is ensured by using a metal anchor made mainly of titanium or metal alloys. The metal component holds onto the bone thanks to a specially modified surface and shape. Non-friction of the joint surface is ensured by an articulation insert made of UHMWPE. It is used to make hip, knee, elbow, shoulder and ankle joint replacements.

Why this type of polymer? "It has a much lower coefficient of friction than Teflon, which was unsuccessful during experiments of joint replacements in the 1950s. It is totally biocompatible, so the human body accepts it without any problem and it is not oxidized. It's a “gold standard” used around the world. There simply is no other material with similar properties that would be approved by all the world's authorities," explains Professor David Pokorny of the Orthopaedic Clinic, 1st Faculty of Medicine, Charles University in Prague, who has been researching the material for joint replacements.

We want the joint to last for fifty years

Ultra-high-molecular polyethylene was first used in 1962, but replacements back then lasted only ten to fifteen years in patients’ bodies. Today, materials lasting thirty or more years are already in use. 

"By modifying the polyethylene molecules, we achieve much better properties and, in particular, a longer life of the implants so that replacing them is not necessary. Joint inserts from newly developed material could last for 50 years or more," explained Professor Pokorny, who at the end of 2016, together with his team, received the Technology Agency of the Czech Republic award [ED. NOTE: WHAT? 1) IT’S THE “TECHNOLOGY AGENCY”, NOT “TECHNOLOGICAL AGENCY”. 2) RECEIVED…SOME KIND OF AWARD PRESENTED BY THE AGENCY, I SUPPOSE (HENCE “OF THE”)], precisely for their research of ultra-high-molecular polyethylene.

Professor Pokorny focused on the development of an environment that simulates the human body during testing. "The noble UHMWPE polymer can be modified by heating, irradiation and other methods. But when we adjust the material, we don’t know what it’s doing in the patient's body in the next ten or twenty years. That's why we've developed an environment simulating the human body, where we place the material, letting it age for ten years, even though it’s actually been there for only three months. Then we take the material out, test it and examine how it has changed," says Professor Pokorny, describing his team's contribution. In the second part of the study, scientists investigated what admixtures could be added to polyethylene to make it last longer.

The problem of the life span of artificial joints lies in the fact that, in the case of the original materials, when parts of a joint rub against each other,  they release microparticles that cause inflammation in the body and activate cells that break down the bone. All of this causes the patient to experience pain and leads gradually to the detachment of the implant. Researchers are therefore looking for a material that would wear as little as possible without losing other necessary properties. Molecules of high-molecular-weight polyethylene are modified using various technologies to achieve chaining and thus extend their longevity.

To replace hip joints, all domestic and foreign companies are now delivering joint inserts made of modern, crosslinked polyethylene with very high durability. Scientists are seeking even better materials.

"We are now working on the development of the next generation of polyethylene, which has a stabilised structure with different antioxidants. It can provide even greater resistance to polymer oxidation, which also takes place in the patient’s body, thus reducing the quality of the implant," says Professor Pokorny.