Wednesday, September 19, 2012

Ceramic-on-metal hip bearings show less wear, friction than all-metal hip bearings

Early clinical and laboratory test results of ceramic-on-metal hip replacements demonstrated decreased wear and friction compared to metal-on-metal hip replacements, according to British investigators.

In conducting in vitro and in vivo studies, John Fisher, PhD, and colleagues found that the differential hardness, smoother surfaces, superior lubrication and corrosive wear reduction of ceramic-on-metal (COM) hip replacements resulted in significantly decreased wear and friction compared to metal-on-metal (MOM).

The investigators also found that COM bearings avoid the stripe wear seen with ceramic-on-ceramic (COC) and MOM hip replacements, and “COM bearings reduced wear and metal ion levels by at least 10-fold compared to MOM bearings,” Fisher told Orthopaedics Today International.
He and his colleagues won the Otto Aufranc Award for this study at the American Association of Hip and Knee Surgeons Specialty Day meeting, where Fisher presented the study results.

Study methods

In an initial 2001 trial, Fisher and colleagues reported that COM hip replacements reduced wear 100-fold compared to MOM bearings.
John Fisher, PhD
John Fisher
In this investigation, they compared MOM and COC hip articulations to the 28-mm Biolox Forte and 28-mm and 36-mm Biolox Delta [all DePuy International Ltd.] COM hip articulations.

“Laboratory study methods included hip joint simulator testing [and] measurements of friction wear and metal ion levels,” Fisher said. “Clinical study methods included a randomized prospective clinical trial comparing the COM bearing to MOM bearings and measurements of metal ion levels from patients’ blood at 6 months postoperatively.”

Lower friction with COM

Investigators tested friction by using a 28-mm head in a pendulum simulator and applying a ± 25° flexion-extension motion, a 2kN peak load and a 100 N swing-phase load in different lubricants. They found a significantly lower friction factor of 0.05 in both COM bearings when in 25% serum, compared to 0.12 in MOM bearings in 25% serum.

In MOM implants, friction decreased with increasing serum concentration, as shown when investigators increased serum levels from 25% to 100% in the lubricating fluid.
In COC bearings and COM bearings, however, friction increased with an increase in serum, although the friction levels were already low, according to Fisher. He said this occurrence suggests that the proteins acted as solid-phase lubricants in MOM bearings.

Wear rate less in COM

“COM bearings provide reduced ion levels compared to MOM bearings and greater design flexibility and reduced risk of component fracture compared to COC bearings.”
— John Fisher, PhD
To determine wear, investigators performed hip simulator tests up to 3 million cycles under standard conditions with 25% serum for both 28-mm and 36-mm heads in each of the three material combinations.

Unlike the MOM couples, the COM and COC combinations showed no initial bedding inwear. The COM articulations showed a markedly lower mean wear rate of 0.1 mm3 per million cycles compared to 1 mm3 for the MOM bearings.

In other experiments, investigators simulated the adverse events of microseparation and edge loading. Again, they found a similar reduction in mean wear rates from 1.5 mm3/Mc for MOM to 0.23 mm3/Mc for the 28-mm Biolox Forte COM bearing.

The researchers also measured metal-ion levels in the lubricating fluid from the simulators at various stages and showed persistently low levels of cobalt, chromium and molybdenum ions in the COM bearings compared with the MOM bearings.

Ion levels reduced over 50%

In the short-term clinical trial, Fisher and colleagues evaluated the 28-mm Biolox Delta COM hip replacement and compared it to similar MOM, COC and ceramic-on-polyethylene articulations. They measured metal ion levels in 31 patient blood samples at 6 months and compared these levels to preop measures using high-resolution Inductively Coupled Plasma Mass Spectrometry.
The results demonstrated a mean reduction of more than 50% for both cobalt and chromium in the patients with COM hips compared to the MOM bearings.

“COM bearings provide reduced ion levels compared to MOM bearings and greater design flexibility and reduced risk of component fracture compared to COC bearings,” Fisher said.
Longer-term follow-up studies are in progress, he noted.

Orthopaedics Today International Consulting Editor David L. Hamblen, PhD, FRCS, contributed to this article.

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