Dental implants have enabled people who are missing or have lost teeth to have either a secure removable appliance(s) or permanently fixed replacement teeth rather than wearing dentures as many of their parents and grandparents did.
Titanium dental implants were first introduced nearly 40 years ago and have become fairly widely used particularly in the last couple of decades. However, with use, demand has risen from both patients and dentists for non-metallic dental implants to replace missing teeth roots for a variety or reasons including aesthetics and health concerns.
To this end, research has been feverish, however, the mouth provides a complex and challenging environment for any implanted material. Criteria include:
- The ability to withstand the cyclical compressive and shearing stresses induced during chewing
- A constantly wet environment
- Frequent changes in temperature and pH
- Resistance to surface abrasion
- Stability so that the material does not degrade with age appreciably
- Resistance to the actions of the microbial biofilms that form on implanted materials
- Exposure to a variety of chemicals (ie: foods, drinks, mouthwashes, toothpastes, etc)
- That the material not be so stiff that it stresses bone or fractures, but not so flexible that it provides inadequate support ie: that it matches the physical properties of bone and tooth as closely as possible
- That the material be radio-opaque (so that it shows on x-rays)
- That is has good aesthetics
- That bacteria and food does not unduly accumulate upon the material and
- Most importantly, the requirement that the material is biocompatible!
To this end, the spotlight over the last decade or so has fallen upon zirconium which is a grey-white transition metal or metalloid in its pure form. It belongs to the same chemical group in the periodic table as titanium which has an atomic weight of 22 with zirconium sitting directly below it with an atomic weight of 40. Zirconium also resembles titanium in its pure metallic form and the name is derived from the Arabic words zar and gun which translate as meaning ‘gold’.
It is mined as zirconium silicate ore which is also known as zircon and which is found in countries such as India, Thailand, Australia and Sri Lanka. The metal zirconium is very resistant to corrosion and for this reason is often used as an alloying material.
As zirconium dioxide (ZrO2) or zirconia it is classified as a ceramic and can exist in different lattice forms when heated to high temperatures in a process known as transformation toughening. The element yttrium is added to stabilise these crystalline forms.
In one of the lattice structures, zirconium dioxide forms what is called (incorrectly) cubic zirconia – a clear synthetic gemstone used to make diamond-like jewellery. The other tetragonal crystalline form produces an opaque white zirconia which is very hard, tough and chemically resistant. This form is used in laboratory equipment, dental restorations, dental implants, ceramic knives and surgical blades, the brake pads of sports cars and as a refractory material in jet engines and the space shuttle. This form is much closer in composition to natural teeth than titanium.
A history of ceramic dental implants
Yttria stabilised zirconia has been successfully used in medical applications such as orthopaedic hip prostheses since the 1980s. And the first zirconium dental implants (Bioceram) were first introduced in the early 1980s and although these osseointegrated, the fact that they were a one-piece design meant that they had limited applications. Various improvements in chemical and surface treatments over the years have improved osseous healing.
Approximately 20 years ago, Professor Schulte from Tübingen, Germany developed implants made of aluminium oxides but these suffered quite high rates of loss and were discontinued. Then approximately 10 years ago, Professor Sandhaus from Geneva, Switzerland developed ceramic implants made of zirconium oxide.
The Paracelsus Klinik in Switzerland which is a centre for holistic medical and dental treatment started using zirconia implants ten years ago. They monitor their patients and found their results with this first generation of zirconia implants to be disappointing. However, in 2005 they started using a new generation of zirconium implants (Z-Look3-Implantat) with much more success. And since 2009 they have been using the Ziraldent and two-piece implant system, Zeramex.
Some manufacturers such as the Swedish Nobel Biocare, Swiss Z-Systems and Spanish CeraRoot come from a dental implant background, whereas others such as the US companies Zimmer and Biomet, and the Korean Acucera have a history of manufacturing orthopaedic appliances. Several zirconia dental implant systems have recently been approved for use by the FDA in the USA.
The first set of surgical instruments made from zirconium was not developed and deployed until the second half of 2005. This development means that the patient need have no contact with metals during the procedure and reduces the potential for inflammatory or immune reactions in those sensitised to metals.
Continued in Problems With Titanium Implants