We can learn a lot from some of the famous bridges around us. Notice the top of the page? It's the Humber Bridge - the longest single span suspension bridge in the world when it was built. The north tower was built on the bank, straight onto good, solid rock, but the southern tower had to be built in the river. You can't put a monster structure like this on mud, so a caisson had to be built and the structure dug down into the rock.
First rule to learn from engineers - If you put a bridge on poor quality foundations, it will fail!
Ever seen that piece of film with the bridge swinging like a hammock with a car on top?
The Tacoma Narrows Bridge Collapsed in 1940. Ultimately, it collapsed because the aerodynamics showed the structure wasn't stiff enough and the wind twisted it until it fell apart.
Second rule - make your bridge construction such that it will take the forces applied to it.
Let's apply this to teeth!
Anything that replaces a missing part of the body, for instance a leg, is called a prosthesis. If you can take the part off or out, it's called a "removable prosthesis". If it's permanently attached, it's called a "fixed prosthesis". A dental bridge is a structure, fixed permanently to either natural teeth, or to the top of dental implants. On this logic, a bridge is a "fixed prosthesis".
Dental fixed prostheses come in various types -
- Fixed - Fixed. The bridge is fixed rigidly at each end. Usually made of porcelain bonded onto a metal structure. Sometimes all metal. The cutting edge is all white materials, but the dental materials industry has a nasty habit of testing something "in vitro" (in a laboratory) for two years and saying it's great. Show us the results after ten years "in vivo" (in real life) and we'll believe them. Pardon the cynicism! Think of a good old fashioned stone bridge.
- Fixed - Moveable. A joint is placed in the bridge to allow the two parts to move relative to each other. It's like the sliding expansion joints (the bit you drive over and it goes bang-bang under the wheels) on a road bridge to stop it buckling when it gets hot.
- Cantilever. The bridge hangs off one end only. Like holding a bucket of water at arms length. It takes a lot of force to hold it - and it tries to pull you over. It'll do the same to a tooth if the set up isn't right - push down on the pontic and the tooth will fall over.
- Maryland. A Maryland bridge is stuck onto the tooth or teeth next to a space using wings on the back of the adjoining teeth, usually made of a Nickel/Chrome alloy with added Beryllium. The Beryllium is etched out with acid and makes tiny pockets in the metal. The bridge is cemented using what is effectively sloppy white filling material. The cement sticks to the tooth by creating roughness of the tooth surface by etching it with acid. It then it sticks to the metal wing by running into microscopic roughness caused by removing the Beryllium and by shotblasting the surface using aluminium oxide grit. We do this inside something that looks like an Astronauts helmet - Health and Safety!
Some Marylands are made of tooth coloured materials. From our experience they are less reliable (see above under cynicism!). We've had some superb successes, but we've had failures as well! And we don't like failures! And laboratories NEVER say it's their fault!
So we're very careful where we use them. We've been making Maryland bridges since they arrived 30 years ago, so we have a "fair amount" of experience in them. Carefully chosen sites, with carefully chosen construction, and correct cementation and they're superb. ack to top
© Hesslewood Lodge Dental Practice, 16 Nov 2015