News Release

Bone Cement Used In Total Hip Replacements Slowly Degrades In Body Over Time, According To UCSF Study

Peer-Reviewed Publication

University of California - San Francisco

A new study led by UC San Francisco researchers has found that acrylic bone cement, often used to attach total hip replacements to bone, degrades in the body over time and this, says researchers, may contribute to the loosening in cemented total hip replacements.

"This is the first study that shows there is a chemical breakdown in the structure of the cement," said Michael Ries, MD, lead investigator of the study, UCSF associate professor of orthopaedic surgery, and orthopaedic surgeon at the UCSF Arthroplasty Center, part of UCSF Stanford Health Care. "The findings can be used to improve the longevity of cemented total hip replacements."

Ries and research colleagues presented study findings on February 4 at the annual meeting of the American Academy of Orthopaedic Surgeons in Anaheim, CA.

A total hip replacement is a successful treatment for arthritis in the hip, said Ries. If severe arthritis is treated without surgery, often the pain can be so great that walking is no longer possible and a wheelchair is the only option for mobility, he added.

In a total hip replacement, the hip joint is replaced with a metal ball attached to a metal stem fitted into the upper portion of the thigh bone and a plastic socket is implanted into the pelvis, replacing the damaged socket. Approximately 50 to 60 percent of total hip replacements use cement to anchor the ball and socket into the bone, according to Ries.

One possible complication of a total hip replacement, he said, is the metal ball and plastic socket may become loose, resulting in pain. If loosening is significant, a revision of the joint replacement may be needed. About 90 percent of hip replacements last 10 years and 80 percent last 20 years. In the current study, Ries and research colleagues studied the chemical structure of bone cement used in hip replacements -- polymethylmethacrylate (PMMA) -- after it had been in the body to determine the effects of the body's environment on PMMA.

The researchers studied two brands of cement, Simplex and Palicose with gentamycin. They retrieved 11 samples of Simplex cement and 11 samples Palicose with gentamycin cement from 22 patients at the time of revision total hip replacement. Time of implantation for the Simplex samples ranged from one to 23 and for the Palicose with gentamycin samples ranged from one to sixteen years. Using a separation technique called gel permeation chromatography (GPC), researchers determined the molecular weight of the samples retrieved. Molecular weight of unimplanted Simplex and Palicose with gentamycin were determined and used as a control.

Study findings show both brands of bone cements decreased in molecular weight with time after implantation. However, different brands degraded at different rates.

The molecular weight of Simplex cement decreased 30.4 percent after 23 years inside the body, said Ries. Similarly there was a decrease (10.5 percent) in molecular weight of Palicose with gentamycin cement after 16 years.

Factors such as enzymatic activity, local pH, mechanical loading, porosity, or other characteristics of the cement may also influence the degrading of PMMA inside the body, said Ries. Further studies to determine mechanisms by which implanted PMMA degrades may help to identify factors to improve the life of cemented total hip arthroplasty.

Co-authors of the study are Ben Pelletier, undergraduate student, Kerry Hughs, undergraduate student, and Lisa Pruitt, PhD, associate professor, department of mechanical engineering, University of California, Berkeley; Berkeley; Oystein Hovik, MD, attending orthopaedic surgeon, Lovisenberg Diakonale Sykehus, Oslo, Norway; Finnur Snorrason, MD, PhD, attending orthopaedic surgeon, Buskerud Sentralskehus Dronniggt, Drammen, Norway; and Susan Muller, PhD, associate professor, department of chemical engineering, University of California, Berkeley;

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