Empa Swiss Federal Laboratories for Materials Science and Technology, Duebendorf, Switzerland

Natural joints can wear out due to daily stress and body movement, so you would expect that an artificial joint replacement would be the answer. However, even the best artificial joints can undergo wear, and wear particles can trigger unwanted immune reactions, making it necessary to replace the joint.

Fig. 1 – Three intervertebral disc implants. On the right is the uncoated implant, in the middle is the DLC-coated implant with the unsatisfactory bonding agent and the corresponding corrosion, and on the left is the stable DLC-coated implant optimized by Empa.
This is normally a standard procedure that can be repeated up to three times with most implants. But, since bone material is lost each time an implant is explanted, the new joint has to replace more bone and is, therefore, larger. However, in the case of intervertebral discs, this is virtually impossible. They are too close to spinal nerves and tissue structures that could be damaged by another operation.

Dangerous Discs

A team of researchers at the Empa Swiss Federal Laboratories for Materials Science and Technology, Duebendorf, Switzerland, explains that up until now, intervertebral discs have not been replaced by mobile joints, but by socalled cages, a kind of place holder that both supports and allows the adjacent vertebrae to grow and fuse together. However, this causes stiffening at the point where the disc had provided adequate freedom of movement. Over the years, this stiffening can result in the adjacent discs also having to be reinforced due to the increased stress on them. Mobile intervertebral disc implants could reduce this problem. However, many products currently available carry the risk of triggering allergies or rejection reactions due to material abrasion.

Fig. 2 – Shown is the simulator, which tested the Empa joint implants for five years.
Initial attempts to increase the lifespan of artificial joints were made by various manufacturers in the past using a super-hard coating made of DLC (diamond-like carbon), with disastrous consequences. Approximately 80 percent of DLC-coated hip joints failed within just eight years.

Researchers at Empa’s Laboratory for Nanoscale Materials Science investigated this problem and found that the implant failure did not originate from the coating itself, but was caused by the corrosion behavior of the bonding agent between the DLC layer and the metal body. This layer was made of silicon, which corroded over the years, causing it to flake, and this subsequently led to increased abrasion and bone loss.

Search for a New Bonding Agent

Their goal was to find a bonding agent that does not corrode and that lasts a lifetime in the body. The researchers searched laboriously, saying that they tried half the periodic table. The winner? Tantalum was their choice to be used as the bonding agent. A tantalum coating was tested in a mobile disc implant and the researchers simulated 100 million cycles, or about 100 years of movement in a specially designed joint simulator. The small intervertebral disc implant held out, remaining fully operational with no abrasion or corrosion, they said. They plan to test the new bonding agent in combination with DLC coatings for other joints.