Designing dental implants
Do you have a hip replacement or a dental implant? Or know someone that does? And do you know what they are made from, or even how they’re made? This week we’re exploring the helpful materials that keep us healthy. First, open wide! Emma Hildyard has been to see Nick Williams the dentist. Plus, Materials Engineer Zoe Laughlin from the Institute of Making at University College London.
Emma - Most dentists advise us to brush our teeth for two minutes twice a day. But the average adult only devotes around 70 seconds of their time to cleaning their teeth. Poor dental hygiene can result in tooth decay and gum disease which, if untreated, can result in tooth loss. I wondered what dentists are currently using to replace lost teeth. And what better way than to go to a local dentist surgery here in Cambridge to find out!
Nick - Hi Emma, welcome to Devonshire House, I’m Nick Williams. Would you like to come in and have a seat?
Emma - Whilst Nick cranked up the chair so that I wasn't lying down, I asked him to explain what a dental implant actually is.
Nick - Dental implants are the best way we have of replacing natural teeth. Historically, people would have had a missing tooth and you would have to prepare the teeth on either side to make way for a bridge which is quite destructive. The good thing about a dental implant is that it’s standalone so you don't have to destroy the natural tooth structure on either side.
Emma - Nick showed me an example of a dental implant, which looked like a normal white tooth that was attached to a metal screw at its base. Initially, the metal screw is placed into the mouth on its own and the crown is then attached at a later date. Very reluctantly I asked how the metal screw was secured in the mouth.
Nick - Where the missing tooth is, you drill a small hole to receive the implant, you put the implant in - similar to how you would screw into a piece of wood. A standard implant is around four millimeters in diameter, so you prepare a site that's three and a half millimeters, so the implant is very slightly bigger than the site you prepare it for. You put it in and a slow torque, there's a very slight amount of bone compression but you don't want too much, just so that it's stable enough to be gripped for that initial healing phase.
Emma - Nick then explained how your body responds to the implant being screwed into your mouth and how we use its response to make sure that it’s secure enough to not wiggle about.
Nick - When you break a bone for example, you get a blood clot into that area and over time that clot turns into new bone. You get a similar process for the site that's prepared for the implant. You get the blood, growing onto the implant and that then slowly turns into bone, Normally we’ll wait anywhere between six weeks to three months. The bone is then hard enough to take an impression of the implant fixture and get a crown made specifically to go on top.
Emma - The metal screw is made from a titanium alloy that has been designed specifically to be put in the human body. I asked Zoey Laughlin, a materials engineer from University College London why titanium is a good metal to use in medical implants.
Zoe - It's great to be used in the body because it's essentially described as bio compatible in that the body doesn't reject it. Imagine if you were to get a splinter or a bit of shaving of metal lodged in you through an injury, the body slowly works it out, it slowly rejects that foreign body. But it doesn't do that with titanium so it’s safe to implant it into the body and the body doesn't try to somehow eject it. It's also really strong and lightweight. If you're making a kind of hip joint, you don't suddenly double the weight of your right hand side by implanting it in.
Emma - Titanium is very corrosion resistant too, meaning that it won't begin to form rust like iron would. As a result of their unique properties, titanium alloys are used in a variety of medical implants ranging from teeth to hip and knee implants. These implants are often complicated shapes, like the screw. These require a special process of machining called CNC milling. These machines use rotary cutters to remove material from a bulk block of material.
Zoe - They'll have a computer generated CAD file that is the bespoke particular shape they want and then you would mill the titanium out of that shape. That often can lead to quite a lot of wastage because, you can imagine, you mill it out of a solid block of it. Lots of that material then gets lost. It can occasionally make things weaker when you mill them because you create lots of little tiny stresses and strains in the material that can later fracture. There's work now being done to look at 3D printing with titanium. This is a technique that essentially starts with titanium powder in a box. I'm slightly simplifying it, but imagine you've got a vat of powdered metal and you fire a laser beam at it and the metal will melt and fuse where the laser beam is focused. You go blast blast blast blast and you run the laser over a layer of the powder and it fuses together. Then you deposit another little layer of powder like you're sort of sieving flour onto the surface, then you blast it again with the laser and you just do that thousands of times and you start to build up these fused 3D printed titanium objects.
Emma - Once the screw has been made and implanted, what about the crown? The bit that is attached on the top of the screw that looks like a tooth. What’s that made from, and how is that made?
Zoe - That can be an entirely ceramic object that starts life as a little cube. They take a scan of your mouth and the tooth that they want to copy let’s say, and they can make a virtual model of what tooth shape they want and then they send it to a tiny CNS mill that carves out this perfect little tooth that can be made in a whole range of tones of white and beige to match exactly your teeth shade. So that it's really a very unobtrusive and unobvious tooth implant.
Emma - Well there you have it. It's nice to have an insight into the materials that help us chew. It's no excuse for not brushing your teeth for two minutes though.