The development of osseointegration by Dr. Per-Ingvar Branemark and colleagues in Sweden in the 1960's led to some of modern dentistry's greatest advancements. Osseointegration is defined as: "the formation of a direct interface between an implant and bone, without intervening soft tissue." For quite some time now, we have been able to replace lost teeth and their supporting bone with totally artificial roots and crowns. A spectacular secondary advantage of this process is that dental implants give the body a 'reason' to maintain the bone around the artificial tooth; when teeth are missing, the bone slowly melts away over the years, since, rather like a TV repairman or switchboard operator, it's no longer needed.
A remarkable dental implant success story is reported in the 1/2012 issue of Nobel Biocare News. Dr. Branemark's second patient, a Mr. Johansson, has maintained 11 dental implants and their associated restorations since 1967. 44 years of complete success- he has had oral implants longer than anyone else on earth! Dr. Branemark's first patient, Mr. Larsson, passed away in 2006; his implants were in place and fully functional since 1965.
These gentlemen were treated with complex restorations that replaced all their teeth. That is still the most notable use of dental implants, but far more common today is single tooth replacement. We might lose a tooth due to deep recurrent cavities, fractures, trauma, and other reasons. I myself had two missing teeth (upper second premolars) simply because they didn't form when I was a lad. The universe doesn't seem to give a warranty on defects like this, so I rode out my primary teeth as long as possible. Once they were getting weak, I had Dr. Thomas Kohler extract them, raise my sinuses with bone grafts, and place two dental implants. Then, my partner in practice, Dr. Barry Sukoneck, restored them with artificial crowns that feel so natural that I never think about the differences between them and my own teeth.
But what are dental implants made of, and what are the components that go into them?
Here, from the remarkable DDSGP app for the iPad, http://ddsgp.com/DDSGP/Home.html, is a concise pic of the components of a dental implant restoration, with images that reflect Nobel Biocare products:
The fixture is the part that is surgically placed in the jaw. Fixtures are made of titanium. There are many reasons for this, but primarily it's a classic example of a Goldilocks Problem. You know- not too hot, not too cold. When titanium is forged into a root-form dental implant, as soon as it cools after casting it forms a thin oxide layer on the surface, where it's exposed to air. It turns out that titanium oxide is just reactive enough that bone cells love it, and grow right up against it, happily forming a union with no inflamed fibrous tissue in between. Gold and other noble metals don't oxidize enough, or at all, so they would just sit there, too inert to attract bone cells in this way. Other metals oxidize too much. All I have to do is say "iron rusts" and you get the idea.
It is worth noting that teeth have ligaments between them and the surrounding jawbone. No matter how tight they are, natural teeth always move just a little. Dental implant fixtures have a rigid attachment to the bone and do not move or wiggle at all. (I can't detect the difference by feel in my own mouth however; I can only see this when I press on a patient's teeth, with good illumination.)
There are probably dozens of ways to attach a functional and esthetic tooth to an implant fixture. For reasons of stability and retrievability, this usually involves an abutment that is held in place with a set screw. Abutments are usually titanium as well, but they can be white zirconia. Sometimes they are separate from the overlying crown, or artificial tooth, which is usually made with porcelain. (Crowns can be porcelain alone, porcelain and metal, or porcelain and zirconia. Or all zirconia. Are you getting the idea that there are some complicated choices to be made in implant dentistry? You're right!)
The crowns can be cemented over the bolted-in abutment. If that set screw ever loosens, though, it's difficult to reach it again to tighten or replace it. Possible, but difficult. So, the crowns can be screwed in. This way increases cost though, and the set screws for the crowns are very small and more prone to loosening than the larger abutment set screws.
We have thus come to prefer an arrangement, where possible, where the crown is pre-fused onto the abutment by the lab, and it has a screw access channel that we fill with composite resin material when we place the implant restoration. This way, it's easy to regain access to the screw if it ever loosens, but the whole component stack is kept simple and no metal has to show.
Here's how they come out when everything is put together:
Not only do we regain a lost tooth- as I indicated, the bone around that titanium has a reason to be there, and it tends to stay as long as the patient has good oral hygiene and the restoration is designed well. Without an implant, it melts away significantly, even if a bridge or partial denture is used to replace a tooth. Crowns based on dental implants can be made to look spectacular. Nowadays we use biological principles to strive to make the gums look natural as well. I don't want to say that dental implants are better than natural teeth, but they can't get cavities and are normally both beautiful and strong.