There has been some concern that the plastic component parts won't hold up under the wear and tear of today's active seniors (and younger patients).

Metal implants have always shown superior strength in laboratory studies. There's less stress on the bone where the metal comes in contact with bone.

Earlier use of the all-poly design was reported to loosen easily (compared to metal) or collapse into the bone.

Researchers are taking a second look at the all-poly model these days because metal backed implants have a downside. There's quite a bit of wear on the "backside" where the implant rests on the bone. Severe breakdown of the bone can occur.

The results of a recent study using the all-poly tibial component in younger adults (less than 60 years old) had a low overall failure rate (1.8 percent). The study was considered "intermediate" as the patients were followed from two to 11 years. Long-term studies (20 years or more) are still needed.

For now it looks like the all-poly is a safe bet for the first 10 years at least.