Ethan Clarke
Knee arthroplasty, or total knee replacement, is a surgical procedure that involves resurfacing a knee joint damaged by arthritis or injury. The surgery typically involves capping the ends of the bones that form the knee joint with metal and plastic parts, along with the kneecap. The plastic component used in knee arthroplasty is often polyethylene, a commonly used plastic with the chemical composition (C2H4)n. This plastic has been widely used in knee arthroplasty since the mid-20th century, with newer variations being developed over time. While polyethylene wear can cause issues that may require revision surgery, advancements in prosthetic design and surgical techniques have led to a 95% endurance rate for knee prostheses over 15 years.
| Characteristics | Values |
|---|---|
| What is TKA? | Total Knee Arthroplasty (TKA) is a surgical procedure to resurface a knee damaged by arthritis. |
| What materials are used in TKA? | Metal and plastic parts are used to cap the ends of the bones that form the knee joint, along with the kneecap. |
| What type of plastic is used? | Polyethylene (PE) is a commonly used plastic in knee arthroplasty. |
| How is PE made? | PE is made by irradiating Ultra High Molecular Weight Polyethylene (UHMWPE) with gamma or electron beams to break the carbon-hydrogen chains, producing free radicals that facilitate polymer degradation and form cross-links. |
| Are there different types of PE? | Yes, there are different types of PE such as highly cross-linked polyethylene (HXLPE) and UHMWPE. |
| What are the concerns with using HXLPE? | The risks of using HXLPE include tibial post fracture, disruption of the locking mechanism, and liner fracture, which can lead to increased wear and osteolysis. |
| What is the history of PE? | The history of PE dates back to 1898 when German scientist Hans von Pechman prepared it while heating diazomethane. However, it was not suitable for industrial use due to instability. In 1933, Eric Fawcett and Reginald Gibson synthesized the first industrially useful PE by applying high pressure to ethylene and benzaldehyde. |
| When did PE start being used in TKA? | PE has been used extensively in knee arthroplasty since the mid-20th century, and progress in manufacturing has led to newer polyethylenes with improved properties. |
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What You'll Learn
- Polyethylene is a commonly used plastic in knee arthroplasty
- Ultra-high-molecular-weight polyethylene (UHMWPE) is a subset of polyethylene
- Metal and plastic parts are used to cap the ends of the bones
- The patellar component may be plastic and metal-backed
- Knee arthroplasty surgery typically takes one to two hours
Polyethylene is a commonly used plastic in knee arthroplasty
Polyethylene (PE) has been used extensively in knee arthroplasty since the mid-20th century. It is a commonly used plastic, with millions of tonnes produced annually. The generic chemical composition of PE is denoted by the formula (C2H4)n. The value of ‘n’ can differ depending on the molecular size.
Ultra-high-molecular-weight polyethylene (UHMWPE) is a subset of polyethylene with extremely long chains and a molecular weight between 2 and 6 million units. It has a strong resistance to abrasion and high impact strength. It is biocompatible, provides a low-friction articulating surface, and can be manufactured relatively easily. However, there are concerns about its in vivo use, including the negative effects of PE wear particles on joint tissues and implant longevity.
To increase the longevity of arthroplasty and improve wear resistance, newer versions of PE have been designed with different wear properties. Highly cross-linked polyethylene (HXLPE) has been used in total hip arthroplasty with excellent outcomes; however, its use in total knee arthroplasty (TKA) remains conflicting. While simulation studies have shown decreased wear and oxidation rates with HXLPE compared to conventional polyethylene (CPE), short- to mid-term clinical studies have not demonstrated a significant difference between the two. The risks of using HXLPE in knee arthroplasty include tibial post fracture, disruption of the locking mechanism, and liner fracture, which can lead to increased wear and osteolysis.
The manufacturing processes of PE directly affect the material characteristics and include the irradiation dose, types of post-irradiation thermal processing, and endpoint sterilization. Compression-moulded polyethylene, for example, has been shown to exhibit less wear and may be preferable for clinical use.
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Ultra-high-molecular-weight polyethylene (UHMWPE) is a subset of polyethylene
Total knee arthroplasty (TKA) is the most common joint arthroplasty performed in the United States, with polyethylene being a commonly used plastic with millions of tonnes produced annually. Polyethylene (PE) has been extensively used in knee arthroplasty since the mid-20th century.
UHMWPE is synthesized from its monomer ethylene, which is bonded together to form the base polyethylene product. These molecules are several orders of magnitude longer than those of high-density polyethylene (HDPE) due to a synthesis process based on metallocene catalysts. UHMWPE molecules typically have 100,000 to 250,000 monomer units per molecule, compared to 700 to 1,800 monomers in HDPE.
UHMWPE is processed by various methods such as compression moulding, ram extrusion, gel spinning, and sintering. The end product is a fibre with a high degree of molecular orientation, resulting in exceptional tensile strength. UHMWPE is easy to machine and has found increasing industrial applications due to its resistance to wear and impact.
In the context of biomedical applications, UHMWPE is an engineering polymer that has been used in total joint arthroplasty in orthopedic and spine implants since the 1960s. Its high wear resistance, ductility, and biocompatibility make it a promising material for durable implants in patients.
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Metal and plastic parts are used to cap the ends of the bones
Knee replacement surgery, also known as knee arthroplasty or total knee replacement, involves capping the ends of the bones that form the knee joint with metal and plastic parts, along with the kneecap. This procedure is commonly recommended for individuals with severe arthritis or significant knee injuries. The metal and plastic components work together to replicate the natural function, shape, and size of the knee joint.
The metal used in knee arthroplasty is typically contoured to resemble the femoral condyles and trochlea. On the other hand, the plastic component, known as polyethylene, is often used in the patellar component and as a spacer between the metal parts. Polyethylene (PE) has been a staple in knee arthroplasty since the mid-20th century, with ongoing advancements in material manufacturing leading to newer polyethylenes with enhanced properties.
Ultra-high molecular weight polyethylene (UHMWPE) is a type of polyethylene with extremely long chains and a high molecular weight. By irradiating UHMWPE with gamma or electron beams, highly cross-linked polyethylene (HXLPE) can be produced. This process involves breaking the carbon-hydrogen chains to generate free radicals, which facilitate polymer degradation and the formation of cross-links. While an increase in radiation dose improves wear resistance, it may compromise other mechanical properties, such as tensile strength.
The metal and plastic parts in knee arthroplasty work in conjunction to provide stability, range of motion, and a smooth, cushioned surface for the knee joint. The metal components provide the necessary strength and support, while the plastic parts, like the patellar component and spacers, facilitate smooth movement and shock absorption. This combination of materials aims to replicate the function of natural bone and cartilage, providing a durable and functional solution for damaged knees.
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The patellar component may be plastic and metal-backed
Total knee arthroplasty (TKA) is the most common joint arthroplasty performed in the United States, with an estimated 672,000 TKAs performed in 2009. The patellar component of a TKA may be plastic and metal-backed. Polyethylene, a commonly used plastic, has been used extensively in knee arthroplasty since the mid-20th century. The generic chemical composition of polyethylene is denoted by the formula (C2H4)n. The value of 'n' can differ depending on the molecular size.
The use of polyethylene in TKA has been associated with a high rate of failure, which has subsequently led to a decrease in its use. One type of failure of a metal-backed patella occurred when the polyethylene separated from the metal backing, causing wear and requiring revision. This type of failure occurred in 16 knees of 14 patients, with an average time to failure of 14 months. Another study reported a single failure out of 29 isolated patellar revisions after metal-backed patellar failure, with radiographic evidence of femoral osteolysis.
Despite the failures, isolated patellar component revisions after metal-backed patellar failure have shown satisfactory results, ranging from 78% to 100%. The use of highly cross-linked polyethylene (HXLPE) in knee arthroplasty has been studied, but the results are limited and slightly conflicting. HXLPE has shown improved wear rates in hip arthroplasty, but the benefits have not been as significant in knee arthroplasty. The risks of using HXLPE in knee arthroplasty include tibial post fracture, disruption of the locking mechanism, and liner fracture, which can lead to increased wear and osteolysis.
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Knee arthroplasty surgery typically takes one to two hours
Knee arthroplasty, or knee replacement surgery, is a procedure that typically takes one to two hours. During this time, the surgeon will remove damaged parts of the natural knee joint and replace them with an artificial joint, or prosthesis. This prosthesis is made of metal and plastic and is designed to replicate the shape, size, and function of a natural knee joint.
The metal components are used to cap the ends of the bones that form the knee joint, along with the kneecap. The plastic, specifically polyethylene, is used to recreate the smooth cushion of the cartilage that was damaged or removed. Polyethylene has been used extensively in knee arthroplasty since the mid-20th century, with newer polyethylenes being developed over the last few decades to improve wear rates.
In addition to the use of metal and plastic, the surgeon may also use surgical cement to attach the prosthesis to the bone. This is known as a cemented prosthesis and is the most common type of artificial knee prosthesis. Alternatively, an uncemented prosthesis attaches to the bone with a porous surface that allows the bone to grow and attach to the prosthesis. Sometimes, a combination of the two types of prostheses is used to replace the knee.
The decision to perform a total or partial knee replacement depends on the extent of the damage. A total knee replacement involves replacing all three areas of the knee joint: the inside (medial), outside (lateral), and under the kneecap (patellofemoral). On the other hand, a partial knee replacement is performed when only one or two areas of the knee joint are damaged, which is more common in younger adults who have experienced an injury or trauma.
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Frequently asked questions
Yes, TKA or total knee arthroplasty involves the use of plastic and metal parts to cap the ends of the bones that form the knee joint.
Polyethylene (PE) is a commonly used plastic in TKA. Ultra-high-molecular-weight polyethylene (UHMWPE) is a subset of polyethylene with extremely long chains and a molecular weight between 2 and 6 million units.
The plastic is used to create a smooth cushion that replaces the damaged cartilage in the knee joint.
Clinical studies indicate the safety of at least two types of highly cross-linked polyethylene (HXLPE) at 2 and 5 years. However, there are concerns about the use of thick tibial inserts and posterior stabilized components made of HXLPE.
Plastic, in combination with metal, is used in TKA to replicate the shape, size, and function of a natural knee joint, providing a smooth surface for movement and reducing pain and stiffness.
