Figuring out why artificial joints fail

"Adults are living longer, and many are maintaining a very active lifestyle, creating the need for long-lasting joint replacements," says Robin Pourzal, PhD, a research scientist in the Department of Orthopedic Surgery at Rush University Medical Center.

Total hip replacements now last between 15 and 25 years in most cases. However, if a reaction occurs, the implant life time can be less than five years.

Further extending the life of artificial knees and hips is a challenge, given that they already are made carefully from metals such as titanium or cobalt-chromium based alloys, and portions are made of polyethylene. Using an advanced technology available nowhere else in the U.S., Rush has begun a retrospective study to analyze implants that fail in the hope of improving future implants and preventing or decreasing the incidence of joint replacement failure.

"If we know why and when the implants fail, the implant can be adjusted before surgically implanting, and surgeons can plan in advance for how and when an implant may fail," Pourzal says.

More than 50,000 do-overs needed a year

In 2011, orthopedic surgeons implanted 306,000 total hip replacements in the U.S. alone, according to the United States Bone and Joint Initiative, a collaborative effort by numerous medical professional societies. That same year, doctors also performed an additional 50,600 revision procedures to replace previously implanted artificial hips. Both numbers increase every year.

"The reasons for revision surgery are varied and can include infection, instability and loosening of the prostheses, and the need for revision can occur within the first few months after implantation," Pourzal says.

Revision surgery for joint replacement failure also may be necessary due to the corrosion or wear on implant surfaces. Implant failure occurs due to adverse reactions in surrounding tissue to the minute debris that results from corrosion and/or implant debris wearing off a non-corrosive joint. In addition, there are various types of adverse local tissue reactions, which often are not examined in detail.

"Currently, there isn't much research data published on this topic, because typically corrosion is not documented as a cause for revision. Awareness of corrosion as a major contributor has been increasing over the last few years," Pourzal says.

Looking back to move forward

Retrospective studies can help yield greater understanding of what leads to failure, whether it's due to design features or patient characteristics, Pourzal explains. He's examining approximately 400 implants that have been retrieved from patients since 2000 that qualify for the study.

Failed implants are collected and cataloged in Rush's Implant Pathology Laboratory. Many of the surgically retrieved implants come from patients who underwent their original procedure elsewhere and came to Rush for their revision surgery.

Additionally, more than 1100 patients have consented to participate in a postmortem retrieval program, allowing for the retrieval of their joint prostheses in surrounding bone and the collection of various organ samples at the time of their death. "The information garnered from these postmortem retrieved components is among some of the most valuable as it allows for the examination of implants that were well functioning," Pourzal says.

"This is a very comprehensive retrospective study that examines all types of implant failure," says Deborah Hall, the manager of the implant lab. "Having the patient information in detail like we have is important in understanding how much material is released into the body and what factors lead to a reaction. It's what differentiates our study from others."

Pourzal uses advanced profiling technology for rapid, high precision measurement of the surfaces of the retrieved artificial hip joints. The analysis reveals scars from wear on the joints and the shape and location of wear patches, providing valuable information about potential reasons for each joint's failure.

"The Ortholux 5 coordinate measuring machine is new technology and currently is the first and only one in the United States. It allows us to quantify how much corrosion and wear products are released into the body," says Joshua Jacobs, MD, chairperson of the Department of Orthopedic Surgery. Jacobs focuses his research studies on clinical performance of orthopedic biomaterials, including implant retrieval analysis.

Thanks to generous philanthropic contributions, Rush was able to purchase the Ortholux 5 from Redlux, a British manufacturer of high-tech measurement equipment, in September 2015.

Understanding failure, and success

By quantifying the amount of material loss due to corrosion, Pourzal and his fellow researchers can build a statistical model to determine which factors affect material loss most significantly. The model takes into account patient factors (weight, age, gender, etc.), implant design alignment factors and material factors.

As a result, researchers at Rush will be able to make recommendations to implant manufacturers and to surgeons about how to prevent the occurrence of corrosion or other complications that led to implant failure.

"The new machine also helps us to examine well-functioning implants that have not failed and figure out how to reproduce it," Pourzal says. "We now can see a correlation between how a surgeon implants the hip, in terms of implant alignment, and the wear of the device. This is all new data that wouldn't be possible without this technology."

In the study, researchers begin by inspecting the implants and categorizing the degrees of corrosion and damage. Then they use the new technology to quantify the damage and its impact on joint function specifically.

The machine has a white light confocal sensor, which allows for analysis of relevant implant surfaces without touching them and therefore potentially affecting the extent of the damage to the joint. It also can analyze an implant in five to 10 minutes. "It would take a researcher several hours to analyze each joint using current technology, and the data wouldn't be as comprehensive," Pourzal says.

Thanks to the availability of the large joint retrieval collection and the new Ortholux 5, Pourzal and Hall expect to achieve relevant results within the next year. "We hope that the result of this research study will allow the orthopedic community to improve implant design and material selection, and that it will contribute to fewer complications and longer-lasting joint replacements," Jacobs says.