POLYETHYLENE FOR TOTAL JOINTS
CONTENTS
The cross-linking of polyethylene
The clinical efficacy of cross-linked cups
Polyethylene doped with E-vitamin
Material
Ultra- High Molecular Weight PolyEthylene (UHMWPE) is the current material of choice for use as a bearing surface in total joint prostheses.
Total joints with components made of this material can function for more than twenty years if they are well designed and well implanted.
However, there is a growing body of evidence showing that wear particles worn off the surface of UHMWPE component may trigger destruction (osteolysis) of the skeleton around the total joint and eventually cause a failure of the whole total joint.
Thus, the material scientists and surgeon work feverishly to improve the characteristics of the material
Biomet's adverizement for Hylamer TM PolyEthylene in 1991 the product failed later
Polymerization
Polyethylene is an organic compound formed by long repeating chains of a single substance: the molecules of the gaseous substance ethylene.
PICTURE
of polymerization of polyethylene
(brown large balls are carbon atoms, small blue balls are hydrogen atoms)
click on the icon for a full size image
In a polymerization process the single ethylene molecule, called mer, is added to another ethylene mer. When many such ethylene meres are added together, they form an extremely long, chained molecule called generally polymer (Literary "polymer" = many meres). In the case of ethylene the final product of many ethylene molecules chained together is accordingly called polyethylene.
The mechanical properties of polyethylene improve slowly with rising molecular weight of the product. A dramatic change in mechanical properties, however, appears when molecular weight of the polyethylene molecule exceeds one million. This appears when more than 35 000 ethylene groups are added together. Such product is called Ultra- High Molecular Weight PolyEthylene. The molecular weight of the UHMWPE currently used in total joint components varies between 4 to 6 millions. Every such UHMWPE molecule is composed of 160 to 215 000 ethylene groups.
The standard UHMWPE with this molecular weight is an extremely tough and yet flexible material. To date it has proven to be the best polymer material for use in total joints.
Wear of polyethylene
Despite its superior mechanical performance, the standard UHMWPE is not a perfect material. It is subject to fatigue failure and it produces to many wear particles. It can also absorb small amounts of fluids or retain small amounts of air in the microscopically small pores (about 0,1 volume percent). This is a small amount but it may cause great deterioration of the material during the long years in the patient’s body..
First now the scientist and surgeons are searching for a UHMWPE material with better wear resistance, although the UHMWPE has been used since the 1960’s,. The reason for the lack of interest in the past has been low demand for the surgical grade UHMWPE The industry estimated in 1993 that only 0.1% of the world’s annual production of this material has been used for production of total joints. Current estimates are difficult to obtain, but available figures show that the figure is still lower than 2 %.
The main problem of the UHMWPE is wear. The wear of currently used polyethylene is usually not large enough to disturb the mechanical function of the polyethylene cups and knee components.
The main problem is production of war particles from the UHMWPE surfaces. The wear particles enhance inflammatory reaction and eventually lead to development of osteolysis (bone dissolving disease).
All new developments of UHMWPE have as a goal production of a material that will be resistant to wear - that will produce only few wear particles
The recently introduced new UHMWPE materials include:
The cross-linked product
New methods of sterilization of the product.
The cross-linking of the polyethylene.
The cross-linked UHMWPE has been introduced in clinical practice in 1998, although it was known since 1960’s that irradiation of UHMWPE increases its wear resistance. In the process called cross-linking, the irradiation "glues" the long polyethylene molecules to keep together.
There is also a chemical method of crosslinking the UHMWPE, used for commercial polyethylene products on large scale. This much more simple and cheaper method of crosslinking is not used for crosslinking of "medical grades" polyethylene for economical reasons. The industry does not wish to incur the costs of new comprehensive tests to prove the biological tolerance of chemically crosslinked polyethylene.
For polyethylene crosslinked by irradiation such tests are not necessary - all currently used polyethylene has been sterilized by irradiation which according to current laws suffices as proof that irradiation used for crosslinking does not change the biological tolerance of the crosslinked product (irradiated perhaps 5 times more). This is one of the many examples of curious mix of laws and economy that rules the development of total joints.
PICTURE
of cross-linking of the UHMWPE
The mechanism of cross-linking is the following: The irradiation knocks an electron out of the Hydrogen atom. The Hydrogen atoms then wander out of the polyethylene molecule as free radicals (blue balls with red hats in the picture). This happens simultaneously on several places in several UHMWPE; the carbon atoms that are lacking theirs Hydrogen neighbors keep "free hands" out to find a new neighbor.
The opposite "free hands" in two molecules meet and cross-link one UHMWPE molecule to another on several places. The degree of cross-linking increases with the dose of absorbed irradiation.
If you like it, you may compare the cross-linking process to cooling of a plate of warm spaghetti.
PICTURE:
cross-linking the spaghetti
The plate of warm spaghetti looks like a couple of conventional UHMWPE molecules. They are movable; "scratch with a fork" will easily move spaghetti strings out of plate.
Now put the spaghetti plate to cool. The cool spaghetti coalesce into one unmovable mass, you cannot remove individual strings with a fork. The cold thus cross-linked the spaghetti to a "wear resistant mass".
The wear resistance of irradiated material in a hip simulator is dependent on the irradiation dose:
4 Mrad – the maximal radiation dose used for sterilization of the material produces 80% reduction of wear in the irradiated UHMWPE
10 Mrad – the maximal dose used for cross-linking of the UHMWPE by some manufacturers causes 95% reduction of wear in the irradiated UHMWPE.
Further reduction of the last 5% of wear is then difficult to achieve.
Initial research studied done in the 1970's studied the changes of the mechanical characteristics in the irradiated UHMWPE immediately after irradiation. These old reports showed that UHMWPE may a absorb up to 100Mrad, before it becomes brittle and unusable for mechanical applications.
Modern studies, however, demonstrated that irradiated UHMWPE ages much quicker than the conventional product, even if the irradiated UHMWPE is maintained in oxygen-free atmosphere. Studies demonstrated that after 5.5 years, the UHMWPE irradiated with only 2.5 Mrad (sterilization dose) becomes brittle, even if irradiated in oxygen-free atmosphere.(Deng 2001).
This kind of behavior, if it realizes in cups made from the highly cross-linked UHMWPE, would be detrimental.
Oxidation damage
The irradiation has deleterious effects on the UHMWPE capacity to withstand oxidation. The damage by oxidation depends on the radiation dose. Already the dose used for the sterilization of the material makes the UHMWPE susceptible to oxidation damage
The mechanism of this damage is the following: Irradiation of UHMWPE produces not only cross-links, but also free Hydrogen atoms. These free atoms (free radicals) combine with oxygen and both together are shredding the very long UHMWPE molecule into shorter slices. Already small concentration of free radicals in combination with oxygen make great damage, as appears from this example.
An UHMWPE with a molecular weight of 4, 000 000 molecular weight consists of 575 000 bonds between individual carbon atoms. It suffices if the oxidation severs only 25 of these bonds. One long UHMWPE molecule then changes into several molecules of "high-density polyethylene", a product with relatively low molecular weight (< 1 million) and high wear rates.
The practical solution to the problem how to stem the oxidation of HMWPE is simple. You may prohibit the contact of the cross-linked UHMWPE with oxygen (in the air) or chase away the free radical scoundrels from it.
Theoretically you may prohibit the contact between irradiated, cross-linked UHMWPE cup and oxygen by packing the cup component in airtight box, with atmosphere of nitrogen. Not feasible in practice, because when you take the cup out of the inert atmosphere in the airtight package and place it into the the body, it would be bathed in body fluids containing solved oxygen.
So the more secure route is chasing out the scoundrels –the free hydrogen atoms - to find a new place. This is best done by reheating the irradiated product to melting point. All molecules begin to move and the freely wandering scoundrels will find a new place in the crystal lattice somewhere between them.
This is the way the industry uses today.
The modern highly cross-linked UHMWPE is
first cross-linked by irradiation (with doses between 5 to 10 Mrad),
then heated close to melting point.
the component made from this highly cross-linked UHMWPE is then finally
packaged without access of air.
But these products, wrapped in airtight packages must be sterilized, made free of bacteria, before they can be put in the patient’s body. And here come the second improvement of the modern UHMWPE implants:
Sterilization with gas plasma.
Historically, the polyethylene implants, as other parts of total joints, have been sterilized with massive doses of gamma rays (up to 4 Mrad). This dose is sufficient to cause further cross-linking of the UHMWPE with following oxidation damage. Sterilization by heat is not possible; heating up the final product will change its structure.
The only remaining way is sterilization of the polyethylene components by gas.
Sterilization with ethylene oxide gas
The ethylene oxide gas kills confidently all bacteria, but the gas must be removed after the sterilization by airing. This process exposes the product to air for long periods. So that even if the definitive package is airtight, and the atmosphere inside the package are inert gases, there may still be air entrapped inside the microscopic pores of the UHMWPE components.
Sterilization with gas plasma
The last sterilization technique is sterilization by gas in plasmatic (ionized) form, so called "gas plasma". The gases used for this technique (hydrogen peroxide,e.g.) are easy to remove from the product, because they act only on the surface of the component. This technique does not expose the final product for the effect of air oxygen and does not enhance the temperature during the sterilization above 50 degrees C.
This sterilization technique thus does not influence the mechanical characteristics of the component.
It is used by many manufacturers.
The efficacy of the new cross-link UHMWPE to withstand wear
Measurement of the wear in the laboratory
Measurement of wear of UHMWPE in the laboratory may seems straightforward. Weight the cup before you put it into the hip simulator, test it and then remove it and weight it again. The loss of weight between the two measurements is the wear.
In practice, however, the tested cups must be lubricated (water, blood serum). The UHMWPE swells, absorb liquids, and gains weight. So that the scientist developed clever methods to compensate for the weight gain.
According to the test in laboratory on hip simulators, the new highly cross-linked UHMWPE has several times lower wear rate than the conventional UHMWPE. One manufacturer (Sulzer, Duracon) even says that his product has no measurable wear at all at testing in laboratory. Sulzer data 2000.
More objective laboratory data (Heisel 2003) show that wear volume (after 5 millions cycles in simulator) was 30 cu mm for an acetabular cup made of cross-linked and then remelted UHMWPE, whereas the conventional UHMWPE wore 260 cu mm.
Thus, in the laboratory experiment, the cup component made from conventional UHMWPE wore eight times more than the cross-linked product.
Wear of the cross-linked cups in the body:
Measurement of the wear in the body
Picture: measurement of the wear on X-ray pictures.
Click on the icon for a full size image
It is theoretically simple (upper row). Directly after the operation the ball lies in the center of the cup. Several years after the operation the femoral ball drills a pathway through the polyethylene. It is then placed eccentrically in the shell. Measure the distance between the center of the ball and the surface of the shell on the X-ray pictures taken directly after the operation and measure this distance at the second picture taken at the follow-up years after surgery. The difference between these measurements is the wear.
In practice the measurement is not so simple on modern metal- backed cups. (lower row - schematic pictures). A typical example of "a lamb in the mist" picture.
Both metal backing and femoral ball are radio-opaque. Directly on the postoperative pictures one can distinguish the center of the femoral ball and the surface of the metal backing, and measure the distances.
On later postoperative picture, when the ball move deeper in the cup, the contours of the ball components are blurred by the superposed shadow of the metal backing. It may be then difficult to distinguish the boundaries of these two components.
The surgeons then use different templates that they put on the x-ray pictures. With the help of these templates they can distinguish the boundaries of the two components better. There is still discussion how precise is the radiological measurement of the cup wear. One extreme opinion says that "None of the studies of wear of the polyethylene cup components fulfills the demands on scientific design and accuracy" (Lewis 2000).
Recently appeared first reports that measured the wear of cross-linked cups on X-ray pictures of the patients' hips. The authors of these reports compared the wear rates of cross-linked cups with the wear rate of cups made from conventional UHMWPE, implanted in other patients
First published reports demonstrate that in the patient's body the wear resistance of the new highly cross-linked UHMWPE cups is lower than in the hip simulators. In the three reports, the wear rate of cross-linked cups improved with about 50% compared with cups made from conventional UHMWPE in two cups (Stryker and DePuy); for the Durasul cups (Sulzer) there was not observed any significant improvement. These reports are in contrast not only with the laboratory tests but also with data on the Sulzer's Website. (Heisel 2003)
Table
Improvement of wear rates of cups made from cross-linked UHMWPE compared with cups made from conventional UHMWPE.
Manufacturer/Product
Irradiation dose
Wear rate reduced with (Heisel 2003)
Sulzer's own data/ mg wear Stryker/ Crossfire
10 Mrad 54 % 6,4 mg DePuy/ Marathon 5Mrad 68,2% 5,2 mg Sulzer/ Durasul 9,5 Mrad 10 % *) not measurable
For all these reports, one must know that during the first one to two years (the observation period of the recent reports) after surgery the ball component makes place for itself within the cup component. The polyethylene flows (creeps) away, and only partially is also worn away. During this early period, it is usually impossible to distinguish between creep of the UHMWPE and wear. The scientists speak poetically about "bedding-in period".
Moreover, studies also demonstrated that irradiation of the UHMWPE diminishes significantly the creep resistance of the product.
Obviously, it is difficult to know whether the progress of the femoral ball inside the polyethylene cup is caused by the softness of the cup, that creeps to sides leaving more place for the ball
or whether the ball literary drills its way through the cup.
In the first case no polyethylene debris are being produced, in the second case the ball produces many small polyethylene particles which may cause osteolysis.
The scientists and surgeons thus turn to observation of damage on the joint surface of the cup components made from highly cross-linked UHMWPE.
If the new cross-linked product would be more wear resistant, then there would be less scratch markings on the surface of cross-linked products compared with conventional product
Direct observation of the surface of the retrieved cups ( cups that were some years in the patient body and then were retrieved for some failure of the prosthesis -yes, there have been also failures) could not distinguish more surface damage on the cups made of the conventional product as compared with the highly cross-linked product (Huo 2003).
Shelf-life
The UHMWPE component ages, especially if it is placed on the shelf in wait to be placed in the patient's body. If the package is not airtight, and if the cup component is placed in the air atmosphere, the UHMWPE will succumb to the corrosive influence of the air's oxygen.
It is thus mandatory that you will not be operated on with total joints whose UHMWPE components were manufactured several years ago. This can happen in hospitals with low operation volume.
Ask your surgeon how old / what is the fabrication date / is the device that will be placed in your body
Polyethylene liner failure
Fracture of the modular polyethylene liner of the acetabulum is caused by excessive wear. See the following pictures:
Click here for the picture Usually the failure starts by dislodgement of the polyethylene liner from its metallic sleeve. ( 1)The ball component then moves up toward the upper rim of the polyethylene liner. The stress on the liner is asymmetric and high. The high localized stress causes initially increased wear. (2)The asymmetrically placed ball thus wears successively out a hole in the polyethylene liner. (3) Eventually, the increased stress overrides the stress resistance of the polyethylene material and the liner breaks.
The liner must be exchanged. The metallic sleeve may be still well attached to the skeleton, even if the liner is totally destructed. The opinions differ as to whether whole cup, inclusive of the well fixed sleeve should be exchanged or whether it suffices with a "small " surgery - exchange of the broken liner only. (Lie 2007)
Polyethylene doped with E vitamin
| Click for picture of the cup |
To
prevent
oxidation
damage,
the
scientists
are
now
studying
the
polyethylene
that
is
doped
with
the
"natural"
antioxidant
-
the
E-
vitamin. Biomet introduced recently this product (E-poly TM) for clinical study together with the Massachussets General Hospital (Biomechanical Laboratory there). It is rumored that Zimmer worked on like product but abandoned it recently. |
| Cup made from the E-poly TM (Biomet) |
subchapter under preparation (Oct 2007)
Allergy to polyethylene
as yet there are no confirmed reports on allergy to the polyethylene component of the artificial joints.
The wear particles of polyethylene produced by prosthetic wear are too big to provoke immunologic reaction. Ingested in the macrophages, these particles provoke instead an osteolytic reaction.
It is, however, theoretically possible that patient's protein substances that once adhered to the prosthetic polyethylene became changed by this adherence. Such changed protein substances may act as allergens and evoke immunologic reaction.
For more information see:
www.sulzerorthoeu.ch/technology/research/news/page-4e.html
this website is from 2000 and some information there is in need of replenishment with fresh data
www.uhmwpe.org - actual information, yet much selected in some areas.
Deng M, Shalaby SW.: J Biomed Mater Res 2001; 54: 428-35
Li S, Burstein AH.: J Bone Joint Surg-Am 1994; 76-A: 1080- 89
Heisel C. et al.: J Bone Joint Surg-Am 2003; 85-Am: 1366- 79
Huo LC.: J Bone Joint Surg-Am 2003; 85-Am: 1852-64
Lewis G.: J Bone Joint Surg-Am 2000; 82-A: 297-300
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