Artificial joints are implantable prostheses that replace diseased or damaged joints and must have adequate wear resistance, mechanical properties and oxidation resistance in addition to biocompatibility requirements. UHMWPE is widely used as a material for artificial joint replacement due to its excellent physical and chemical properties.
There are various methods of modifying the material to improve the overall performance of UHMWPE artificial joint implants, including irradiation cross-linking, heat treatment, incorporation of antioxidants, etc. The latest development is to improve the mechanical properties of artificial joint implants by modulating the flow field to induce the formation of self-reinforced structures.
The development and application of artificial joint technology has been one of the most important achievements in the field of orthopaedic research in the 20th century, providing relief for many patients suffering from osteoarthritis, rheumatoid arthritis, etc.
Total joint arthroplasty is currently the best clinical treatment option for severe joint damage and joint necrosis, and consists mainly of total hip replacement and total knee replacement.
In Europe, 800,000 patients are reported to require total hip replacement each year, and in the USA, approximately 550,000 people receive an artificial joint replacement each year, while the actual number of cases requiring artificial joint replacement far exceeds this figure and is increasing by 7% to 8% each year.
In China, there are approximately 30 million people who require artificial joint replacement.
Due to the complex function of human joints, especially the ability to move in several directions and to withstand certain compression, tension, folding and flexion, the performance of artificial joint materials is very demanding, such as good physical and mechanical properties, high chemical stability, good biocompatibility and so on.
Ultrahigh Melecular Weight Polyethylene (UHMWPE) is an engineering plastic with a relative molecular mass of more than 1.5 million and a composite structure with highly ordered sheet crystals embedded in a random amorphous zone. The structure gives UHMWPE its unique properties.
Its high molecular chain density, moderate crystallinity, very high molecular weight and composite structure give UHMWPE unique properties such as impact resistance, abrasion resistance, low coefficient of friction, chemical resistance, low temperature resistance, stress cracking resistance, low water absorption, biocompatibility and self-lubricating properties that are unmatched by other engineering plastics. It is currently the most important polymer material for artificial joints.
Forming and performance of HMWPE artificial joints
UHMWPE was first used in artificial joints by Charnley in 1962 and is now used as a padding material for acetabular components in hip joints and knee joints, and is often used in the human body in conjunction with cobalt-chromium alloys.
The hip joint consists of a joint head and a joint socket, which connects the femoral head to the joint socket of the pelvis. A total hip replacement consists of 3 parts.
(i) replacement of the hip socket (acetabulum) with a UHMWPE socket.
(ii) Replacement of the fractured femoral head with a metal joint head.
(iii) insertion of a metal rod into the femoral stem to increase the stability of the artificial joint.
The hip socket (acetabulum) itself is usually made of a UHMWPE hemisphere and can be implanted directly into the pelvis; UHMWPE has been used as a hip socket material for over 30 years, but any material has a life expectancy and UHMWPE joints are at risk of failure when implanted into the body.
The stresses of fixation between the spherical head of the femur and the hip socket can lead to wear or tear of the plastic material, which in severe cases can lead to hip repair surgery after 10 to 20 years.
The artificial total knee joint consists of a femoral prosthesis, a tibial prosthesis and a patellar prosthesis, made of a metal femoral condyle, a tibial buttress and a tibial pad and patellar prosthesis made of UHMWPE.
The UHMWPE artificial knee joint is also subject to wear and tear of the hip joint, as the tibial and thigh bone elements can wear through over time in the body, leading to loosening of the joint.
Advances in UHMWPE research in artificial joints
Irradiated cross-linking of UHMWPE artificial joints
Although total joint replacement, which is widely used to relieve joint pain and improve joint function, is one of the most effective and mature procedures, UHMWPE artificial joints are prone to wear and tear due to the large loads and reciprocal movements against hard materials such as stainless steel, titanium alloy and ceramics during long-term use. Clinical studies have shown that artificial joints implanted in the human body will gradually fail due to wear and tear after 10 to 15 years, with nearly 30% of patients requiring revision surgery within 10 years.
Therefore, to extend the life of artificial joint implants and to avoid the risk and cost of secondary surgery, there is an urgent need to solve the problem of wear-resistant artificial joint materials. In order to effectively improve the wear resistance of joint implants, many researchers have found that the use of irradiation cross-linking is highly effective.
Irradiation cross-linking of UHMWPE involves exposing UHMWPE to high doses of γ-rays or electron beam radiation, which causes the decomposition of UHMWPE molecular chains to form C and H radicals, which then combine to form cross-linked dots through the free radicals on the different molecular chains, these cross-linked dots reduce the movement of the molecular chains and thus improve the wear resistance of UHMWPE implants. The wear resistance of UHMWPE implants after irradiation cross-linking has been investigated by many scholars.
The wear rate of UHMWPE joint implants decreases rapidly with increasing irradiation dose, reaching a plateau at 100 kGy.
Advances in UHMWPE research in the field of artificial joints
Antioxidation of UHMWPE artificial joints
While irradiation crosslinking substantially improves the wear resistance of UHMWPE joint implants, some of the free radicals generated by irradiation are trapped in the crystal regions of UHMWPE, where the molecular chains are regularly arranged into the lattice and have little to no motility.
Over time, the free radicals trapped in the crystal zone will undergo a series of oxidative degradation reactions with oxygen, causing deterioration in the mechanical properties of the joint implant and eventually leading to oxidative embrittlement.
Due to the poor flow properties of UHMWPE, press sintering is often used. Press sintering allows UHMWPE to remain at high temperatures for too long, and oxidative degradation, chain breaking reactions, formation of double bonds, free radicals, etc. may occur.
Therefore, the elimination of free radicals left after irradiation crosslinking and during processing and forming is another key task to improve the wear resistance of joint implants. It has been found that there are two main methods for the effective elimination of free radicals, namely heat treatment and the addition of antioxidants.
Advances in UHMWPE research in the field of artificial joints
Self-reinforcement of UHMWPE artificial joints
It has been noticed that with the increasing number of young orthopaedic patients, the mechanical properties of UHMWPE joint implants have become more stringent, therefore, improving the mechanical properties of UHMWPE joint implants is a very
The need to improve the mechanical properties of UHMWPE joint implants is therefore an urgent and important research issue.
Based on biocompatibility and compatibility with the UHMWPE interface, a self-reinforced method has been developed to improve the mechanical properties of joint implants.
It was found that the processability of UHMWPE could be improved by using LMWPE, which has good flow properties, and then a shear flow field was applied to the UHMWPE/LMWPE blend (by means of a modified injection moulding machine – vibration injection moulding) to induce self-reinforced structures. The shear flow field is applied in the UHMWPE / LMWPE blend (via a modified injection moulding machine – vibratory injection moulding) to induce self-reinforced structure formation.
The interlocking shish-kebab structure is regulated in the LMWPE phase of the blended system to enhance the mechanical properties of the artificial joint implant.
Advances in UHMWPE research in the field of artificial joints
Irradiation cross-linking, heat treatment and the addition of antioxidants have effectively improved the wear resistance of UHMWPE, and the formation of self-reinforced structures induced by the regulated external field (flow field) has significantly improved the mechanical strength of the implant.
Although the overall performance of UHMWPE artificial joint implants has been significantly improved, the demand for artificial joints is increasing with the ageing of the world’s population, the increase in high-energy injuries and the impact of diet and environmental factors on the body, with a huge potential market.
Recent clinical studies have shown that aseptic loosening and insufficient mechanical strength of prostheses after artificial joint replacement are still problems that need to be solved and more academics need to be involved in this work.
At the same time, research and development of new materials with good biocompatibility, fatigue resistance, wear resistance and high strength is also a new direction for the development of artificial joint materials.
