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Alternative bearing materials seek improved function, longer survivorship
A hybrid implant, with a ceramic head and metal acetabular cup, may offer the benefits of both materials. Clinical trials are under way.
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Ultra-high molecular weight polyethylene has been used as a bearing material in hip prostheses for many years, and though effective, the material has some drawbacks. As the material wears, it produces polyethylene debris, which in turn causes a biological response leading to osteolysis.
Using a smaller diameter head, such as a 28-mm vs. a 36-mm head, can decrease the amount of particulate debris produced by traditional polyethylene bearings. However, reducing the head size affects function. And with patients living longer and more active lifestyles, function is becoming an increasingly important consideration for both patients and surgeons, according to John Fisher, PhD, a professor at the Institute of Medical and Biological Engineering at the University of Leeds, England.
“Patients today essentially want to do more activities. They want better function, better range of motion, and that requires a larger head size,” Fisher told Orthopedics Today. “With conventional polyethylene, there is an inherent design contradiction. If you increase the head size, you increase the wear, and so you reduce the lifetime.”
Bearing options
Because of this, researchers have been studying new alternative bearing materials to develop implants that maximize function while reducing wear and particle debris production, thus extending the lifetime of the implant. Their efforts have led to several new bearing materials, including highly crosslinked polyethylene, alumina ceramic-on-ceramic and cobalt chrome metal-on-metal.
Fisher and colleagues have conducted countless laboratory tests to evaluate the comparative performance of alternative bearings. Such tests have included wear simulations, wear particle analyses and cell culture studies.
Highly crosslinked polyethylene is ultra-high molecular weight polyethylene that has undergone an irradiation crosslinking process. This process results in a four- to six-times lower polyethylene wear rate and improved implant survivorship.
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However, there are still some caveats to using the material. “It is still polyethylene, and as you increase the head size, you still increase the wear,” Fisher said. “You can improve the overall function and stability with the larger head size. But you do that at the cost of increasing the wear.”
The other drawback: The particles produced by highly crosslinked polyethylene as the material wears are about twice as reactive as conventional polyethylene particle debris.
“You might have less wear. But the particles you produce per unit volume are more reactive. So the benefit of the reduced wear is not as great as you would expect,” he said.
He explained that the biological reactivity of polyethylene depends on the size distribution of its particles. Highly crosslinked polyethylene particles have a smaller proportion of larger particles and a larger proportion of smaller particles in the more reactive size range. “So by crosslinking, you shift the distribution of the particle size,” he said.
“With younger patients now, you are looking for something like a tenfold improvement [in wear] compared with conventional polyethylene,” Fisher added.
“It is an improvement. And of course, as an improvement it is appropriate for less demanding patients. But those younger patients, in their lifetime, will typically walk 10 times as many steps as an older patient would have 10 years ago. So a tenfold increase in performance is at least what we are looking for. And the highly crosslinked polyethylene doesn’t really quite get you there,” he said.
Alumina ceramic bearings
Alumina ceramic is one material that addresses the particulate debris issue. Ceramic is a very hard material and has a fiftyfold reduction in wear compared to conventional polyethylene.
However, it too has some inherent design limitations. Because the material is rather brittle, a ceramic implant needs a thick acetabular cup to prevent breakage and cracking. This limits head size, which limits function.
“Ceramic gives you very low wear, and it also has particles that are of low reactivity as well,” Fisher said. “But the practical design constraints limit, at the moment, how far you can go in terms of increasing the head size.” Fisher said the maximum head size is 36 mm.
Metal-on-metal
Conventional polyethylene implants (Blue) were initially used with a relatively small head size of 22 mm. Surgeons later began using larger diameter heads to improve function. This led to increased wear and earlier implant failure, which is indicated by the arrow moving up and to the left. Highly crosslinked polyethylene (Orange) resulted in reduced wear and an increased lifetime over conventional polyethylene. However, the issue of increased wear with increased head sizes remains, which again affects implant survivorship. Alumina ceramic-on-ceramic bearings (Purple) offer much less wear compared to conventional and crosslinked polyethylene. And because the material is much harder than polyethylene, increasing implant head size has less of an effect on wear rates and implant survivorship. However, the acetabular cup must be sufficiently thick to prevent cracking and breaking, which limits the head size that can be used to 36 mm or less. This in turn limits potential function. Cobalt chrome metal-on-metal bearings (Green) are not as hard as alumina ceramics and thus produce more wear particles. But metal-on-metal bearings still have significantly lower wear rates than polyethylene, and there is no concern about osteolysis. Also, increasing the head size can actually improve lubrication, reduce wear and improve potential function. However, elevated metal ion levels remain a concern as some patients can develop hypersensitivity reactions. A new hybrid implant that uses alumina ceramic bearings and a cobalt chrome acetabular cup (Red) is in clinical trials by DePuy Orthopaedics. The new implant may allow for larger head sizes and improved function similar to metal-on-metal implants while resulting in wear rates similar to ceramic on ceramic implants. Clinical trials are currently evaluating 28-mm and 36-mm diameter heads. But Fisher notes the implant’s metal cup will allow the use of a much larger head. Source: Fisher J |
A cobalt chrome metal-on-metal surface offers the potential for substantially larger head sizes without the risk of cracking. Additionally, it offers significantly lower wear rates than conventional polyethylene, and wear rates further decrease as head size increases, according to Fisher.
“You can use [metal-on-metal bearings] in the form of quite thin shells in the acetabulum, which allows you to go up to very big head sizes ... of about 50 mm to 60 mm, which you cannot do with polyethylene or ceramic,” Fisher said.
“The other thing that characterizes metal-on-metal, it is what we call a lubrication-sensitive bearing,” he said. “It benefits from fluid lubrication to a degree, and that means that ... if you increase the head size you actually get a reduction in wear as opposed to a polyethylene bearing, where if you increase the head size you increase the wear.”
However, metal-on-metal also has its drawbacks, Fisher noted. The metal wear particles release metal ions, which can lead patients to develop elevated metal ion levels. And for a small set of patients, these elevated ion levels can lead to hypersensitivity reactions similar to allergic reactions from metal, Fisher said.
“It’s actually quite difficult to predict who the patients are that might react. So there is an emphasis on recognizing the potential sensitivity to metal ions and trying to reduce metal wear as much as possible,” he said.
“There’s a bit of a risk. For an individual patient, it’s probably less than 1%. But you cannot identify which patients that would be,” he added.
Other options
With polyethylene and ceramic bearings, there is no risk of increased metal ion levels or potential for subsequent hypersensitivity reactions, Fisher said.
“I guess what you have is with polyethylene, there is a reasonably predictable response. You know that if you have a high level of activity and a long lifetime, then there will be a good chance of getting polyethylene osteolysis. But for patients who are not too demanding, that’s a good option,” Fisher said.
“Then, for much more younger and active patients — the more demanding patients — you have the ceramic, despite the limitation on the head size the occasional risk of fracture,” he said.
But the final bearing combination Fisher discussed is a hybrid device that DePuy Orthopaedics is currently testing clinically. “It’s a combination of ceramic-on-metal bearing – a ceramic head and a metal acetabular cup, which allows you to have a thin metal cup for potentially larger head sizes,” he said.
“The other advantage is that you do not have the risk of fracture or chipping of the cup, allowing more design flexibility,” he added. “But the real advantage is that experiments show a massive reduction in metal wear ... and metal ion levels.”
Fisher and colleagues at his center invented and patented the new device and licensed it to DePuy. Currently, the clinical trials are evaluating head sizes up to 36 mm. But designs can potentially go bigger than that. “There are no constraints on going bigger like there are with ceramic-on-ceramic,” Fisher noted.
Additionally, because it combines ceramic and metal components, increasing the head size may reduce wear further.
For more information:
- Fisher J. Tribology of new alternative bearings. Presented at the American Association of Hip and Knee Surgeons Subspecialty Day Meeting, American Academy of Orthopaedic Surgeons 73rd Annual Meeting. March 22-26, 2006. Chicago.