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Surgeons have many choices for hip surface replacements

Panel at BOA addressed the advancements and challenges of this technology.

Issue: November 2004

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There are many choices for hip surface replacements on the market, and there are several factors that should be considered when selecting a device. The science behind metal-on-metal bearings and surface replacements was discussed at a British Orthopaedic Association Annual Congress in Manchester, England, and data and specifications on seven companies’ devices were presented in an instructional session.

“The fact is, surface replacement is here,” said Martyn L. Porter, FRCS, of Wrightington Hospital in Lancashire, England, who co-moderated the session. “There are a variety of surface replacements, and some of the issues are quite complex in terms of engineering and clinical [concerns].” He pointed out that about 8% of hip replacements are surface replacements, and that number could grow to as much as 20% or 25%.

Seven physicians and scientists spoke about products from DePuy, Wright Cremascoli Ortho, Smith & Nephew, Biomet Merck, Corin Group, Zimmer and Finsbury Orthopaedics. John Fisher, BSc, PhD, of the University of Leeds in England, began the session with an overview of the science of metal-on-metal bearings and surface replacements.

Wear rates

One of the main concerns with surface replacements, Fisher said, is that both long- and short-term wear releases metal ions (usually cobalt-chromium) into the patient’s body. “We do know that high concentrations of metal particles and ions can be toxic to cells, and there have been isolated reports of adverse T-cell hypersensitivity reactions to a few patients,” he said. “They can also be genotoxic, but we don’t know the clinical impact of that, and the reports of cancer risk are not yet proven.”

Wear rates have been shown to be anywhere between 0.1 mm³ and 1 mm³ per million cycles, depending on a variety of factors including carbon content of the alloy and design and geometry of the device. Fisher also said that the steady state, long-term wear rate is “dependent on lubrication regime, which is also dependent on the design, diameter, radial clearance, loading regime and kinematics.”

A high carbon content in the alloy has been shown in several studies to produce lower wear than low carbon content alloys, and Fisher said little difference has been found between wrought and cast alloys. “The bottom line in terms of materials is that we should use high-carbon content alloys,” he said.

Fisher said that as the size of surface replacement components has increased, the radial clearance between the head and the cup has increased as well, creating more bedding-in wear and a thinner lubrication film.

“If we want to improve the design of surface replacements then we need to move high radial clearances to low radial clearances to improve the lubrication regime,” Fisher said. “But metal-on-metal are lubrication-sensitive bearings. Yes, we can use lubrication to reduce wear, but what happens in adverse conditions? Clinical conditions are highly variable. Positions, geometry and force patterns are different in different patients.” He also said that loading regime and surgical methods could affect the short- and long-term wear.

Improvements in design

Presentations in support of the various companies’ surface replacements indicated that most companies have moved toward lower radial clearance designs, with approximately 50 mm as a generally accepted size for the bearings.

Wear and the amount of ions released into the body have been dramatically decreased in newer generation of devices. DePuy’s Articular Surface Replacement (ASR), for example, is in its fourth generation, and according to Andrew G. Cobb, FRCS, of Epsom General Hospital in Surrey, England, the new version has an 80% reduction in wear vs. the third generation ASR. “It will take the ASR 21 years to produce the volume of [ions] that the third generation produced during the bedding-in period,” Cobb said.

Companies are also offering more options in terms of sizes of the head and cup, choices of acetabular fixation, etc. Wright Cremascoli, for example, offers bearing sizes increasing in 2-mm increments from 36 mm to 56 mm, as well as two cup sizes and cemented or uncemented acetabular fixation.

“There still remain concerns amongst you about metal particles, ion release … genotoxicity and hypersensitivity,” Fisher said. “I believe we all have a responsibility to continue to try [to improve] the tribology and the design of these surface replacement bearings to further reduce the metal particle and ion levels that are delivered to the patient.”

For more information:

• Porter ML, Fisher J. Which surface replacement? Instructional session at the British Orthopaedic Association Annual Congress. Sept. 15-17, 2004. Manchester, England.