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The long-term durability of using a cementless cup implant in repeated hip replacement surgery may be greater than previously thought, according to a new study.

Of the 138 hips studied by Rush University Medical Center orthopedic surgeons during the last 20 years, 95 percent of the implants stayed intact.

In the late 1980s, Rush researchers developed one of the first implants that did not require the cement then standard in hip replacement implants. Because of the cup shape as well as the textured treatment of the material, patients’ bones could grow into the implanted device without cement.

In a study reported in the February Journal of Bone and Joint Surgery, doctors found that 21 of 77 hips still being monitored in the study needed to have revision surgery.

When original device parts fail, patients often undergo revision surgeries to replace them, according to the American Academy of Orthopaedic Surgeons Web site. Of the 21 implants on hips needing revision, only one implant was found to be loose, the study found.

Hip replacement surgery, formally known as total hip arthroplasty, consists of three parts that are surgically implanted in the hip: a cup that replaces the hip socket, a ball that replaces the fractured head or tip of the thigh bone and a metal stem attached to the thigh bone to add stability to the implant, according to the National Institutes of Health Web site.

A first-generation version of the device was originally implanted in 138 hips in 132 patients as late as the late 1980s at Rush. Since then, Rush continues to study the success of the implant in 79 of the patients it is still following.

Three studies analyzing the condition of the hip replacements in these patients have been released in the past two decades. The average age of patients included in the 2009 study is 50, and their ages range from 20 to 79.

While it was one of the first devices of its kind, the cementless cup is now a standard option.

“I use this procedure on 99 percent of my primary hip replacement patients now, and 95 percent of my revision patients,” said Dr. Craig Della Valle, an investigator on the study and orthopedic surgeon at Rush. “In our research, we look not only at the past, but closely follow the patients that we are operating on today.”

About 231,000 people in the United States had total hip replacement surgery in 2006, according to the latest figures available from the Centers for Disease Control and Prevention’s National Hospital Discharge Survey.

According to Della Valle, one of the key innovations the team made in the implant was adjusting materials used to make the implant.

“For the initial cohort, we upgraded the standard materials used on the shell to use highly cross-linked polyethylene,” he said. Cross-linking is a process during which the plastic used in the shell is exposed to a short burst of radiation, which increases its resistance to wear.

“This study is important,” said Dr. Michael Moran, an orthopedic surgeon at Advocate Christ Medical Center in Oak Lawn, who uses the cementless cup device, as do many hospitals throughout the nation. “We have learned through painful experience that new technologies are not always better. That’s why we need to look at long-term studies like these, so we can do the best job for our patients.”

But Della Valle said the group recognized that despite the device’s success, there is still work to be done. “We look at the failure mechanism, where the surface is wearing over time. There is work to be done to improve the wear-resistance of weight-bearing surfaces on the shell.”

Moran said he sees this in his patients as well. “Other companies and researchers have tried lots of different materials, but they each have their own costs,’” Moran said.

Infection was another issue Della Valle said needed further work. A common problem in any surgery, infection is especially prevalent in hip replacement due to the chipping off of particles from the implant, which can cause infection.