Panelists present advantages and drawbacks of various implants
Though tremendous advancements have been made in developing alternative bearing surfaces for joint replacement implants, problems persist even with the newest technology.
Crosslinking has dramatically decreased wear in both metal and polyethylene prostheses, but wear continues to produce small particles that appear to be more biologically active.
Although ceramic-on-ceramic bearings generate less debris, squeaking continues to be an issue with these components.
During a joint symposium, members of the AAOS and the Orthopaedic Research Society discussed “Current controversies in bearing surface science.” Joshua J. Jacobs, MD, served as moderator and was joined by Stuart B. Goodman, MD, PhD; Clare Rimnac, PhD; William L. Walter, MBBS, PhD; Markus Wimmer, PhD; and Steven Kurtz, PhD.
Polyethylene: Number 1 choice for THR
“Highly crosslinked polyethylene is really the mainstay of total hip replacement (THR) in the United States,” explained Dr. Goodman.
The wear characteristics of bearings made with highly crosslinked polyethylene differ by manufacturer and are determined by “total radiation dose, postirradiation thermal/physical processing, and the sterilization process,” he said.
Nonetheless, highly crosslinked polyethylene demonstrates significantly decreased wear when compared to conventional polyethylene.
For example, in a prospective, randomized study, wear rates in THR patients with conventional polyethylene were compared to those in THR patients with highly crosslinked polyethylene.
After 10 years, “we see a substantial decrease in the mean wear rate, which is linear wear,” he explained. But, he noted, rim cracking has been seen in cups placed in a suboptimal, vertical position.
Although highly crosslinked polyethylene is used less frequently for total knee replacements (TKR), in a laboratory setting it has been shown to decrease wear by approximately 94 percent, including a substantial decrease in pitting and delamination.
“But with the good goes some potential problems. Several researchers have noted that the wear particles that are generated are smaller and could be more biologically active,” said Dr. Goodman.
Additionally, both sudden fracture and fatigue crack propagation resistance are reduced.
Taking polyethylene to the next level
Dr. Rimnac addressed the predicament that material scientists face when attempting to optimize wear resistance, strength, and fracture resistance simultaneously.
“A modification that improves a material’s wear resistance generally reduces its ductility and can lead to a material that is more brittle and less resistant to fracture under both sudden fracture and fatigue or cyclic loading conditions,” she explained.
But second-generation highly crosslinked polyethylene bearings are attempting to address some of the problems found with the first generation.
“We now have the opportunity not only to optimize wear resistance in the second generation of highly crosslinked polyethylene implants,” Dr. Rimnac explained, “but also to further improve oxidation and fracture resistance. It’s too early, however, to know the clinical performance of second-generation polyethylene implants because their introduction into clinical use is relatively recent.”
Dr. Goodman also outlined the benefits and disadvantages of using metal-on-highly crosslinked polyethylene bearings.
“These bearings are biocompatible and have high wear resistance. We can use larger femoral heads and this has been shown, at least in the lab, to possibly allow greater activity levels.
“The production of smaller particles, however, has been associated with the highly crosslinked bearing surface. Conventional—not highly cross-linked—polyethylene is well suited for the elderly patient. The metal-on-highly crosslinked polyethylene implant can also be more expensive,” Dr. Goodman explained.
Metal-on-metal: A very mixed bag
“Several outcome studies demonstrate that metal-on-metal (MoM) bearings are very functional and efficacious,” Dr. Goodman said.
One study of 83 MoM THRs in 73 patients found no revisions or loosening of components after an average of 7.3 years follow-up.
At 10 years, survivorship was 100 percent.
In comparison to polyethylene, however, MoM bearings can produce a much larger number of small “nanoparticles,” according to Dr. Goodman.
In addition, multiple reports of osteolysis and aseptic loosening as well as a possible association with hypersensitivity reactions have been found. In a review of 19 patients with MoM THR, 7 patients had osteolysis, even though all of the components were well fixed.
“These were painful hip replacements that had an effusion and prominent perivascular lymphocytic cuffing,” he said.
Other studies have shown that MoM implants lead to a substantial increase in ion levels of cobalt and chrome, which could have implications for younger patients. Long-term follow-up studies are needed to measure the impact of this reaction.
The formation of pseudotumors in patients with MoM hip resurfacings has also been recently reported in the literature (Fig. 1). Though the incidence was found to be approximately 1 percent after 5 years, the complications are particularly serious because “one sees necrotic connective tissue with macrophages. The pseudotumors are extremely difficult to deal with because they compromise the muscle and the surrounding bone and tissue,” stated Dr. Goodman (Fig. 2).
The sound and furor over CoC implants
Ceramic-on-ceramic (CoC) bearing surfaces seem to work well for younger, more active patients, according to Dr. Goodman. Although expensive, they offer biocompatibility, very low friction, and high wear resistance.
“Wear on CoC heads is 10 times less than on MoM heads and 100 times less than the older polyethylenes. The new crosslinked polyethylenes promise a massive reduction in wear that may bring them closer to ceramics,” said Dr. Walter.
A recent retrospective study of 101 patients (132 hips) who received CoC implants from 1997 to 2004 found more than 70 percent survivorship at 15 years, “but there were a number of revisions—not necessarily due to the bearing surface but to the acetabular fixation,” according to Dr. Goodman.
Other studies also support the use of CoC bearings in active, young patients. Unfortunately, the incidence of squeaking or noise may be higher with CoC bearings.
According to Dr. Walter, patients whose hips squeak tend to be younger, taller, and heavier “indicating greater mechanical demands on the hip.
“If a patient has a squeaking hip, you should first get a computed tomography scan because it could indicate a more serious problem such as a fractured rim,” he advised.
“Under severe conditions, such as edge loading or metal transfer onto the head, the fluid film lubrication may break down. That, in turn, may cause a dramatic increase in the coefficient of friction—providing energy that may be modified as sound,” he explained.
In his analysis of the acoustics of squeaking hips, Dr. Walter found several apparent causes, including the material used for the implants and its thickness. These two factors could cause different frequencies and amplifications.
“CoC is a technically demanding operation where the components really need to be placed optimally,” agreed Dr. Goodman.
“The tapers need to be clean, perfectly dry, and undamaged,” Dr. Walter explained. “The position of the component is critical to avoid impingement of the neck against the rim.”
Which would you choose?
At the conclusion of the symposium, the panel members were asked which bearing surface each would choose if having a hip replacement.
Dr. Walter opted for a ceramic-on-ceramic implant. Dr. Wimmer, however, said he would be very concerned about using the ceramic-on-ceramic or ceramic-on-polyethylene implant “without knowing more about the design details.”
Dr. Kurtz would prefer to have ceramic-on-polyethylene.
Dr. Rimnac would choose metal-on-polyethylene.
“Looking at the past and what’s available right now for hip replacement,” said Dr. Goodman, “I would still have metal-on-highly crosslinked polyethylene. But what you use is not as important as how the device is inserted and who does the surgery. Perioperative care is important, too.”
Annie Hayashi is the senior science writer for AAOS Now. She can be reached at firstname.lastname@example.org
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The following conflicts of interest were disclosed by the panelists. Dr. Goodman: Biomedica; Biomedical Therapeutics; Zimmer and MTF. Dr. Rimnac: DePuy, A Johnson & Johnson Company; Musculoskeletal Transplant Foundation; Zimmer; Stryker; Biomet and Acumed, LLC. Dr. Kurtz: Medtronic; Synthes; Johnson & Johnson; Stryker; DePuy spine; Archus Orthopaedics; Zimmer; Biomet; DePuy, A Johnson & Johnson Company; Encore Medical; Exactech, Inc; Medtronic and Kyphon, Inc. Dr. Wimmer did not have any disclosures to report. Disclosure information was not available for Dr. Walter.
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