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Hip News

High Cross-Linked Polyethylene Liners in Hip Replacements May Experience Fatigue Damage

Patients who undergo total hip arthroplasty (hip replacements) can be given one of several types of manufactured hips or hip parts. One commonly used type of hip is promoted by the manufacturers as having a wear rate reduction of 40 percent to 100 percent; these are lined with cross-linked polyethylene, a type of resin or plastic.

As these types of replacements have been used in real-life, doctors have found that this strength against fatigue break-down may have a trade off in the mechanical trait of the joint. They have found that the joints may not be as able to bear weight and last as long as other types. The researchers in the study examined 4 separate acetabular, or "cup sockets" that were used as hip replacements in 2 separate patients.

Both patients were women, one was 98 kilograms, with a body mass index of 34, and the other was 91 kg, with a body mass index of 32. The first woman had the first arthroplasty of the right hip when she was 52 years old because she had progressive arthritis after having had a fracture and repair of the hip. After receiving the first hip, with the plastic lining in the socket, despite it working, the woman did experience several hip dislocations. About 10 months after the replacement, she began to have difficulty with the dislocations and there was clicking when she put weight on her hip. One month later, the patient underwent a second arthroplasty with a larger shell (54 mm vs the original 48 mm). After 14 months, there were changes in the joint detected by x-ray. By 18 months, the patient was, once again, experiencing dislocations of the hip. Yet another replacement was performed 23 months after the first one had been done.

The second patient had her initial arthroplasty performed because she had a valgus arthritic dysplastic right hip. Seven months after the surgery, she experienced hip clicking when she put weight on the hip. She had revision surgery with a slightly larger shell (56 mm versus 52 mm). She again began noticing dislocations after 22 months and had to undergo another replacement.

When the researchers examined the hip replacements that were removed, they found in the first patient that the first liner was cracked and the second one had fragments near the top. In the second patient's replacements, the researchers found that the second replacement was also fragmented near the top.

The area of the fragmentation was consistent with damage from stress, the researchers said. However, "the importance of the moderate wear seen in the articular surface of the liners is not clear," they added. It could be due to the plastic deformation after implantation. Because the researchers used fairly large implants, the lining was thinner than it may be in others. This may have played a role in the outcomes.

The authors conclude that although they have these findings, further research needs to be done on the prevalence of the failure.


Stephen S. Tower, MD. Rim Cracking of the Cross-Linked Longevity Polyethylene Acetabular Liner After Total Hip Arthroplasty. In The Journal of Bone and Joint Surgery. October 2007. Vol. 89A. No. 10. pp. 2212-2217

10/25/2007

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