Beware of Problems with Knee Replacements

Knee replacements have been around long enough now that their track record is clear. Ninety per cent of these implants last 15 to 20 years without problems. Patients experience a pain free return to normal function. Reports of patient satisfaction are high. But that doesn't mean problems can't or don't occur -- they can and they do.

Three of the most common (and biggest) problems include osteolysis (bone resorption), delamination (plastic covering joint surface wears away), and fracture (of the implant or the bone supporting the implant). In this instructional course lecture from the American Academy of Orthopaedic Surgeons, the problem of osteolysis is addressed.

What really causes osteolysis after a knee replacement? Why does this complicating factor result in implant failure? And how should the problem be handled? These are the three main topics discussed in this course lecture. Let's take a look at each one.

First, the main reason osteolysis occurs is because tiny particles flake off from the polyethylene (plastic) portion of the implant. The body responds to this debris as it would any "foreign invader." It sets up an inflammatory response to destroy the particles. In the process, bone is destroyed as well.

Studies show the debris can be made up of different sized flakes of material. The smaller pieces are the ones that seem to trigger the inflammation. Larger particles aren't as likely to start a foreign-body cellular response, but instead delaminate or wear away the smooth surface of the implant. Either way, the end-result can be osteolysis and implant failure.

The implant itself can be a source of problems. Different design features have advantages but also disadvantages. For example, efforts have been made to shape the implant so that it conforms better to the bone. Implants with small contact areas have less surface area to spread the force and load placed on the knee. The increased stress on the implant also increases the risk of wear and osteolysis.

A second design feature is the placement of holes in the baseplate of the implant. The holes actually gave particles of debris a way to migrate or move into the joint. Screws used to hold the tibial portion of the implant in place did the same thing.

The locking mechanism on the implant is another design feature that is intended to hold the implant steady. But even the tiniest movement or slippage of that locking feature and the backside of the tibial implant starts to wear. Over time, the finish on the surface of the implant wears away resulting in delamination, greater stress on the implant, and resulting implant failure.

Not only is the implant itself a potential source of problems, but the way the implant is manufactured has actually been identified as a factor. Off-the-shelf implants that are machined to finish them (rather than compression molded) are more likely to have surface irregularities that lead to delamination.

And one final manufacturing factor that affects how well implants hold up is the method of sterilization used. Of course the sterilization process is important and must be done but the way in which it is done can contribute to oxidation (breakdown) of the polyethylene. When that happens, the same results occur: debris formation, bone osteolysis, and implant failure.

Scientists are studying ways to prevent this oxidation. Additives such as vitamin E have been added to the polyethylene in an effort to maintain the stable properties of the plastic. The hope is that by stabilizing the plastic portion of the implant, there will be less uneven load and better resistance to wear and tear.

Let's shift our focus now to the management of osteolysis. The first thing to know is that patients must be monitored so that the start of any osteolytic process can be seen and dealt with. Small areas of osteolysis don't require intervention but just observation over time. If the problem starts to get worse, efforts must be made to put a stop to the progression.

Right now, there isn't a "best known" treatment to halt the progression of osteolysis. Medications such as antiinflammatories and bisphosphonates (prevent bone resorption) are the first thing to try. Studies are really needed to see if this approach is effective. With large bone lesions, the polyethylene liner is removed and replaced.

Surgeons are advised to make sure the polyethylene liner (original and/or replacement) isn't old (i.e., hasn't been on the shelf a long time) and that the liner is smooth with no cracks or damage to the surface. It is also necessary to check the locking mechanism to make sure it is working properly. If the bone lesion is just too big and continuing to get larger, the surgeon may have to replace the entire implant.

Every effort is made to prevent bone loss from the osteolysis and during the revision surgery. X-rays, CT scans, and/or MRIs help give the surgeon additional information about the location, size, and extent of the osteolysis. Bone grafting may be necessary to fill in the holes and stimulate bone growth and repair. The course instructor offers fellow surgeons ideas for surgically treating complex reconstructions required for osteolytic lesions.

Reference: Giles R. Scuderi, MD. Complications After Total Knee Arthroplasty. How to Manage Patients with Osteolysis. In The Journal of Bone and Joint Surgery. November 16, 2011. Vol. 93-A. No. 22. Pp. 2127-2135.

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