Improving Implants for Hip OA
A better understanding of artificial joint materials and individuals’ responses to them is key to improving hip replacement success.
When osteoarthritis (OA) damages a hip to the point that every movement causes pain, replacing the joint with a prosthesis can restore the ability to function pain-free. But for many people – particularly younger, more active ones – an artificial joint is not a permanent fix. Joint replacements can fail over time, often necessitating further, more difficult surgery. By better understanding what causes hip implants to fail – or alternatively what makes others last – researchers are gaining new understandings that may improve implant longevity and make them an appropriate and lasting option for more people.
While much of the research focuses on materials and design of the implants themselves, researchers are also examining individuals’ responses to implants, says Joshua Jacobs, MD, professor and chairman of orthopaedic surgery at Rush University Medical Center in Chicago. This could lead not only to improvements in design and materials but potentially to tests that could indicate the best implant for an individual before surgery and the use of agents to prevent implant problems after surgery.
If you anticipate a hip replacement, or revision, in your future, here’s how these two areas of research may make a difference.
Focusing on the Prosthesis
When doctors want to know what makes implants work – or not – the best place to look is at the implants themselves. “When prostheses are removed during revision surgery, they can be analyzed by a variety of analytic techniques to try to understand the reasons they may have failed,” says Dr. Jacobs. Similarly, successful implants that are removed post-mortem give unique information about the factors associated with success.
“We have learned a good deal about the way implants perform in the human body in a way we can’t otherwise by actually examining retrieved devices,” he says.
In one recent study using retrieved joints, published in the journal Science, Dr. Jacobs and his colleagues found that a layer consisting in part of a graphic carbon – a solid lubricant with industrial applications – forms naturally on the articulating surfaces of metal-on-metal joint prostheses in the body. While earlier research had shown that a lubricating layer forms on metallic joints as the result of friction, this was the first to discover the make-up of that layer, a discovery that could lead to ways to design better implant surface, says Dr. Jacobs. “For example, is there a way we can design alloys or design surfaces so that layer can reliably form thereby causing less debris release form wear and corrosion?”
A separate study on retrieved joints by researchers at New York’s Hospital for Special Surgery used technology called scanning electron microscopy to find new clues about the damage that occurs to failed joints. The study, which examined 46 retrieved metal-on-metal total hip prostheses from 44 patients, found that 98 percent of the cups (the portion of the implant that fits into the pelvis) and the and 93 percent of the heads (the ball at the end of the femur) showed moderate to severe scratching, while moderate to severe pitting was found in 43 percent of the cups and 67 percent of the heads. They also identified areas near the cups and head that had lost their sheen, indicating wear from friction.
“The analysis, using one of the largest collections of retrieved metal-on-metal implants, shows previously-unidentified patterns of wear,” says Douglas Padgett, MD, chief of the Adult Reconstruction and Joint Replacement Division and chief of the Hip Service at Hospital for Special Surgery.
The researchers are now using a technique called high resolution laser profiling to quantify damage in hopes of finding clues to the mechanism behind the damage, Dr. Padgett says.
Looking at patterns of wear and tear on retrieved devices not only allows scientists to better understand causes of damage, but can also enable them to develop better testing modalities for new prosthetic designs.
“For example, to make sure that in our preclinical hip simulations – that is typically where you have a machine that mimics the motion of a hip or knee and you try to see the patterns of wear and tear – you can reproduce the patterns of wear that you see in the retrieved devices,” says. Dr. Jacobs. “Then you have a model that you can use to test out a variety of things – new implants, new materials, adverse conditions where the implant might be functioning like malpositioning, etc.”
Focusing on the Patient
Regardless of the implant’s design or materials it is inevitable that some amount of debris will be released from the implant over time, says Dr. Jacobs. For some people that will elicit a response that ultimately results in prosthesis failure.
Understanding the body’s response to an implant can help by predicting who is most likely to respond to a certain implant material and finding ways to prevent or stop the response in susceptible individuals.
If doctors can understand the local cell responses to implant debris and the molecular pathways involved in implant failure, they will have a potential targets for therapy, meaning a drug that could be delivered either locally or systemically to stop block the process and keep implants secure.
Dr. Jacobs and other researchers suspect the response will be different for different people. Researchers are looking for genetic factors or hypersensitivities to some debris that would cause some individuals to be more reactive than others. “Some studies have shown that certain individuals, because of their genetic make-up may be more reactive to certain types of implants and that is an intriguing finding that researchers can follow up on, ” he says.
Although there are currently no tests that predict implant reactions, Dr. Jacobs suspects that in the future there may be a panel or battery of tests – looking at one’s genome, individual cell response or sensitivity to certain materials – that will guide doctors in selecting the best implant for a particular patient or the enable to stop or prevent a reaction if there are indications a patient is at risk of one.
“For example, if you know a patient is at high risk, there may be some kind of pharmacologic agent that you can use intermittently to block the localized cell response,” says Dr. Jacobs. “That is the sort of vision many of us have as we look into the future.”