The issue of longevity of the prosthesis may not be as important when looking at a hip replacement for a 78-year-old patient with low activity demands. However, it is a critical consideration for the 53-year-old who still wants to be active in recreational sports and has over thirty years of life expectancy remaining.
So, getting back to the original question regarding how long joint replacements can be expected to last, there are three issues we need to address:
- First, why do hip replacements “wear out”?
- Second, what are we as members of the medical community (and medical device manufacturers) doing to maximize hip replacement longevity.
- And finally, the original question of how long can a patient hope for a hip replacement to hold up.
Why Do Hip Replacements Wear Out?
First, hip replacements can fail for several reasons that have nothing to do with wear of the prosthesis. Infection, early loosening, and improper implantation are three reasons for early failure of total hips that are independent of wear. In the long term, however, the most common reason for failure of the prosthesis is bearing surface wear.
A total hip is essentially a ball-and-socket construct. The ball and socket are typically anchored to the underlying bone by a press-fit and bone ingrowth mechanism. The components are “press-fit” into the bone and within several months the bone has grown into the porous coating of the component, essentially incorporating the component into the bone itself. Once this occurs, the chance of spontaneous loosening of the hip replacement is very low. The bearing surfaces of the hip replacement are the surfaces where the motion occurs. Typically the “ball” is made of cobalt-chromium or ceramic. The lining of the cup is made of either cobalt-chromium (metal), ceramic, or polyethylene (plastic).
Recently, there have been many highly publicized failures of metal-on-metal hip replacements. In response to this, the vast majority of total hips being implanted today are utilizing a polyethylene liner for the cup. It is the wear characteristics of this polyethylene (“poly” or plastic) that can be critical in determining the longevity of the hip replacement. If the poly wears quickly, the chance of failure increases significantly. Wear particles from the poly can result in bone resorption (loss) around the hip replacement (due to the body’s response to the wear particles). Also, if the poly wears through completely then the hip can become unstable and/or very painful. So, in summary, the characteristics of the poly can be critical in the long term survival of the hip replacement.
How Do We Improve the Longevity of Hip Replacements?
So, what are we doing to improve the longevity of hip replacements? In my opinion, the most important long-term issue that orthopedic surgeons and medical device companies have to conquer is improving the wear characteristics of the polyethylene (the plastic liner of the cup). Getting initial stability of the components through bone ingrowth is a problem that has mostly been solved.
The challenge is making the hip replacement last and the main issue here (as stated above) is poly wear. If you think about what a hip replacement must stand up against (literally millions of cycles of movement over the years), it is very clear that the poly must be very high quality with low wear characteristics. An analogous situation may be a car tire. The challenge would be to create a car tire that lasts a million miles instead of the standard fifty thousand. That is quite a challenge!
So what are we doing about it? Companies are actively looking at many options of manipulating the poly in order to improve longevity. Ten to fifteen years ago, the development of cross-linked poly (with cross-linked chemical bonds) significantly improved wear characteristics. Recently, companies have sought to continue to improve this process.
A novel idea that has the potential to significantly improve the longevity of the polyethylene is the introduction of Vitamin E. Vitamin E is an antioxidant that can help stabilize the poly over time and prevent wear. Several companies have come out with versions of Vitamin E infused poly in the last several years. The first of these companies was Biomet, which introduced E1 poly to the market. The lab data on this poly is excellent and offers the potential of markedly improving the performance of total hip replacements.
How Long Should Patients Hope for Hip Replacements to Last?
Finally, the question of how long should patients hope for hip replacements to last. For this, I have to offer the disclaimer that no one can perfectly predict the future twenty to thirty years down the road. But I do think that we can look at current clinical and laboratory studies to make predictions about the longevity of current hip replacements.
First, one of the key determinants of the lifespan on the hip replacement is the activity demands placed on it. So, an active runner is going to have a higher risk of wear than a more sedentary individual.
That being said, here are my thoughts and predictions based on current clinical and lab data. I feel that the hip replacements currently being implanted COULD potentially last 30 years or more, even in moderately active individuals (i.e., someone who plays golf, doubles tennis, and lives a moderately active lifestyle). Obviously this depends on correct implantation by the surgeon and results can vary patient to patient. But if one looks at several recent studies, there is a very optimistic picture for younger patients who may need hip replacement.