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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 optimised by Empa.

Abstract:
Due to the daily stresses and movement in the body, even the best artificial joints wear out; the material undergoes wear, and wear particles can trigger unwanted immune reactions, making it necessary to replace the joint. This is normally a standard procedure that can be repeated up to three times with most implants. As bone material is lost each time an implant is explanted, the new joint has to replace more bone and is therefore larger. 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.

Joint implants without an expiry date

Switzerland | Posted on June 3rd, 2014

Up to now, intervertebral discs have not been replaced by mobile joints, but by so-called cages, a kind of place holder that both supports and allows the adjacent vertebrae to grow and fuse together.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 previously 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.

What makes artificial joints durable?

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% 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 behaviour 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, which led to increased abrasion and, as a result, bone loss. "Our aim was to find a bonding agent which does not corrode and which lasts a lifetime in the body," explains Kerstin Thorwarth.

The first step towards intervertebral discs

This was a laborious task, as the Empa researcher emphasises: "We tried half the periodic table." One was finally found and tantalum was used as the bonding agent. This coating was tested in a so-called total disc replacement - a mobile disc implant. We simulated 100 million cycles, i.e. 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. The new bonding agent is soon also to be used in combination with DLC coatings for other joints. "The intervertebral disc is the most awkward joint in terms of implants. Because tantalum has performed so well, the DLC project can now be applied to other joints," says Thorwarth.

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About Empa
Empa as a Swiss Materials Science and Technology Institution within the ETH domain is part of the Swiss Science-Technology-Education community. It specializes in applied research and development as well as sophisticated services in the field of sustainable materials science and technology. Its core activities are innovative collaboration with industry and public institutions to ensure the safety of humankind and the environment, knowledge propagation and university-level teaching. The Empa Academy disseminates the latest results of our work at events and in publications. The focal points of our activities are: modern materials, their surfaces and interfaces, construction materials and systems, materials and systems that protect the human body and ensure its wellbeing, information, simulation and reliability technology, and mobility, energy and the environment. Approximately 820 employees work in over 30 specialist fields in nationally and internationally funded research programs, partnership-based development projects and interdisciplinary customer-specific service assignments.

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Contacts:
Further information
Dr Kerstin Thorwarth
Nanoscale Materials Science
Tel. +41 58 765 45 47

Dr Roland Hauert
Joining Technologies and Corrosion
Tel. +41 58 765 45 47

Editor/Media Contact
Cornelia Zogg
Communications
Tel. +41 58 765 45 99