Durasul® Highly Crosslinked Polyethylene
A breakthrough in long-lasting joint replacement.
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A leading problem in hip and knee replacement is the wearing down of the implant surface over time. Continuous motion between the metal and plastic polyethylene components can cause small particles to be removed from the surface of the polyethylene. Also, the polyethylene may degrade, or age, over time. Durasul polyethylene was developed to address the problems of wear and aging. Durasul polyethylene, when used in conjunction with clinically successful hip and knee replacement systems from Zimmer, offers patients significant promise of long-term durability. Durasul was developed in conjunction with renowned orthopedic surgeon William H. Harris, MD, at Massachusetts General Hospital (MGH) and the Massachusetts Institute of Technology (MIT). Durasul has demonstrated improved wear performance over conventional polyethylene in laboratory testing.* *The results of in vitro wear tests have not been shown to correlate with clinical wear mechanisms.
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Durasul polyethylene’s unique wear-resistance is achieved through a patented electron-beam radiation process. Irradiation of the material creates crosslinks in the material's molecular structure. The polyethylene is then subjected to an increased temperature for a precise amount of time to remove free radicals. This step is the key to Durasul's resistance to aging.
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Wear and Tear
Joint implants may typically last less than 15 years before having to be replaced due to wear. Replacement surgeries, called "revisions," are often very difficult procedures. Wear occurs as the the implant’s metal component rubs against the polyethylene component. This can wear away the polyethylene over time, causing small wear particles to break off in the body. The body’s immune system rejects this foreign debris, attacking it much like it would attack an infection. |
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Unfortunately, since the polyethylene debris typically settles around
the site of the implant, the immune system may attack the surrounding
bone tissue. This is known as osteolysis – literally, "bone death". As
the patient loses bone tissue in his or her hip, the implant may become
loose and no longer function properly. Many orthopedic surgeons identify
osteolysis as the number one cause of hip implant failure.
In knee implants, the polyethylene may crack, pit and “delaminate” (flake off). This may also cause microscopic particles to break off, leading to osteolysis. Wear can lead to implant failure. Patients not only face another bout of major surgery to replace the implant – they also have lost valuable bone tissue. With each revision surgery, the patient becomes more dependent on the implant. |
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Background on Polyethylene
To understand Durasul, it helps to know a little about the structure of polyethylene, a type of plastic that is used in a wide variety of applications, from industrial to medical. A molecule of polyethylene is a long chain of carbon atoms, with two hydrogen atoms attached to each carbon. Polyethylene with significantly long chains of these atoms is referred to as Ultra-High Molecular Weight Polyethylene (UHMWPE). |
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UHMWPE exhibits greater toughness and wear resistance than other types
of polyethylene. In fact, it is used in bulletproof vests and as a
replacement for ice in ice skating rinks. UHMWPE is also commonly used
in hip and knee implants.
Unfortunately, UHMWPE can degrade over time through a process known as oxidation. Oxidation causes aging, a change in a material's properties over time. Oxidation can occur when an atom or group of atoms in the polyethylene chain have at least one unpaired electron, known as a free radical. Molecules of oxygen in the body can bond to the site of the "missing" electron. In this oxidized state, the material may become brittle and wear at a higher rate. |
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Wear and Aging Durasul polyethylene's resistance to wear and aging is achieved through a patented manufacturing process. First, the polyethylene is energized with an electron beam. This forms crosslinks in the material. |
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| Crosslinks are simply molecular connections between chains of polyethylene molecules. Crosslinking produces a complex three-dimensional structure in the material, making the polyethylene chains more difficult to separate, and increasing wear-resistance. (The green bars in the illustration below represent crosslinks.) |
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| After crosslinking, the polyethylene is melt-annealed. This allows the polyethylene chains to become mobile and rearrange within the material. Any remaining free radicals combine with other free radicals, forming additional crosslinks. The elimination of free radicals gives Durasul its resistance to oxidation, a form of aging that may lead to embrittlement and increases wear. |
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Large Diameter Hip Replacement System Addresses Two Biggest
Challenges in Hip Replacement: Wear and Stability
Durasul acetabular inserts revolutionized the industry by outperforming conventional inserts in extensive laboratory wear testing. This same Durasul technology also allows the use of larger femoral heads (the "ball" of the ball-and-socket joint). Tests using the larger heads with Durasul confirm the earlier excellent wear results. Durasul’s excellent durability properties enable the use of much thinner inserts than are possible with conventional polyethylene. Thinner inserts allow for larger diameter heads . This increased diameter provides greater freedom of movement to patients and further reduces the possibility of dislocation. |
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"The wear resistance of Durasul is remarkable. That alone makes its introduction a major advance in total hip surgery," said William H. Harris, MD, world-renowned orthopedic surgeon and director of the research laboratory at Massachusetts General Hospital (MGH). "Now, to add to that, the striking advantages of the use of the larger diameter femoral heads will dramatically increase the benefits to each patient." |
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