Wear and Cement

fit together perfectly – increasingly rapid. The plastic particles (polyethylene wear) are biologically active and result in local inflammation. The inflammation then consequently attacks the surrounding tissue and destroys it over time. The bone is “eaten” causing gigantic holes in some parts and the artificial joint loses its hold. This in turn, results in renewed massive pain.

Cement (methyl methacrylate) is not just dead material but also very rough and brittle. In addition, when cement hardens it releases considerable heat, sometimes in excess of 80° Celsius. This causes death of the overheated surrounding biological material. Over the years, cement develops cracks and, at some point, even breaks apart completely. When the inflammatory mechanisms triggered by the polyethylene abrasion are added in, the loosening process proceeds much faster.

Polyethylene particle surrounded by an inflammatory cells.

So why did some papers, e.g. by Callaghan et al in 2000 (also Kavanagh, Wroblewski, Older, etc.), show that the probability of reoperation for a cemented joint is only 77% after 25 years?

The answer is very simple: Patients who had surgery 25 years ago were already relatively old at that time, i.e. older than 70 in most cases. In addition, they were barely able to be physically active before surgery due to their ailment and after surgery, their mobility reduced even further. As a result, the implanted joints experienced only little load and wear.

For years, most of these patients had already been in a home for senior citizens. During the follow-up exam after 25 years it wasn’t even checked whether they were still able to walk. The records only show that the patient had no further hip surgery until that point. Because of this, most of these cement studies are completely useless and cannot be applied to today’s situation.

If we look at the results of cemented shafts and plastic cups in young, active patients, they are dramatically worse and sometimes don’t even last for 10 years! Why is that?

Very simple: Young patients and even some of today’s older patients are much more active than previous generations. As early as 1998, the American Tom Schmalzried published in JBJS that the average patient receiving a hip prosthesis in San Francisco produces about one million loading cycles (steps) per year. In 2002, only four years later, the same group published that the patients were producing an average of as many as two million loading cycles – twice as much after just four years. Today, active patients even take four to five million steps per year. But why is this so important?

The original plastic hip sockets were designed for about 10 million loading cycles. If a patient takes one million steps a year, it can be assumed that the plastic socket will last about 10 years. With two million steps per year, the lifespan decreases to five years and goes down further with even more cycles. Even though the plastic was improved somewhat, the plastic sockets were simply not suitable for young, active patients since they were never developed for that kind of stress.