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Total hip replacement surgery is a highly common procedure today, with nearly 200,000 Americans receiving implants annually. Vital to understanding the breadth of implant ability is being able to accurately evaluate wear and tear. The longer an implant can be sustained, the greater the success, and with more people receiving implants at younger ages, the orthopedics industry is working to advance implants to a lifetime capacity. A local study is currently in development to analyze the wear and mobility of hip implants. InMotion Musculoskeletal Institute partnered with Campbell Clinic and Baptist Rehabilitation-Germantown to develop an RSA lab, one of only a few of its kind in the U.S.

RSA stands for radiostereometric analysis, which is a special technique of taking two x-rays from different directions at the same time, essentially giving a three dimensional perspective, or "stereo" representation from a two dimensional image. This technique allows orthopedic surgeons to precisely measure (within micrometers) how the body and implant are interacting, and how much position change has occurred since implantation of the prosthesis.

First, to measure implant position, a surgeon inserts markers – tiny tantalum beads about the size of a poppy seed – into the bone surrounding the implant. These beads become stably integrated into the bone and are used as references to detect any change in position. After the markers are inserted, two x-ray tubes (fixed or mobile) are used simultaneously within an UmRSA® calibration cage, which is used for calculating the 3D positions of the tantalum markers. Finally the data is digitally collected and analyzed by scientists using comprehensive software.

Ruxandra Marinescu, senior scientist and manager of the biomechanics laboratory at InMotion explained that RSA takes the task of predicting any migration or loosening of an implant to a level of acuity that was impossible with traditional x-rays. Moreover, previous studies have shown RSA to be accurate to 2/10ths of a millimeter of motion.

"I think that's why there is strong interest from the clinical community," said Marinescu. "RSA can reduce the need for a revision of an implant."

InMotion purchased the RSA equipment from RSA Biomedical™. Equipment includes the tantalum marker beads, marker injectors, calibration cages and UmRSA® 6.0 software. The prospective trial is designed to detect the difference between two types of commonly used, low-wear polyethylene hip implants. The team is looking to include 150 to 200 patients in the study. RSA measurements will be taken immediately post-op and in stages at six months, one year and two years. Researchers are hoping to extend it to five years as well.

"We want to see if improvements in polyethylene in the lab translate to the clinical setting and to the patients," said orthopedic surgeon and principal investigator in the trial, John Crockarell, MD. "The goal is to reduce wear in implants."

Determining wear and movement more precisely will allow physicians and researchers to gauge what is working and how an implant can possibly be improved. If the implant maintains proper positioning and doesn't loosen, even in younger patients, "the implant will last a lifetime," Crockarell said.

"The data shows a linear, steady state of wear," added Crockarell. He also noted that if they can keep up with study patients at five years out, "We can see how it will wear throughout their life."

Campbell Clinic, as the clinical partner, will implant the markers as hip replacement candidates are recruited for the trial. The radiography lab is housed at Baptist Germantown where patients will receive the x-rays while the InMotion team looks on. Measurements are then digitally transmitted to InMotion where researchers can analyze the data. The x-rays detail two reference points, on the bone and on the implant, comparing the femoral head to the ball and socket.

As explained by RSA Biomedical literature, advanced mathematical algorithms are used to obtain the 2D position of the marker center with high precision and are not disturbed by nearby metal objects. The software then reconstructs the 3D positions of the markers, calculating the different segments within the skeleton and prosthesis.

The research team at InMotion benefited from the experience of the director of Harvard Medical School's clinical RSA lab, Henrik Malchau, MD, PhD. Malchau is on the InMotion scientific advisory the impending study. InMotion conducts all their research with a multidisciplinary team of committee and provided training to Marinescu and other engineers on analyzing the RSA data and conducting biomedical engineers, scientists, nurses and interns. In fact, current medical students and residents who heard about the RSA lab have inquired about internships for the study.

"There are very few RSA labs in the South and the only other clinical RSA lab is in Boston," said Marinescu. "So much goes into making this happen. We're very excited about (the RSA lab). I really believe in this method." The success of RSA has been proven in Europe for years and it is becoming a promising diagnostic tool in the United States to benefit current and future hip replacement patients. In the mean time, local candidates undergoing total hip replacement surgery may have the opportunity to contribute to the advancement of hip prosthetics. With the new lab, RSA equipment and software in place, and training underway, the study is expected to begin this fall.

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