One of the most commonly performed dental procedures, but perhaps the most misunderstood, is the common "filling". It seems simple, right? There's a hole in a tooth, fill it up with stuff. Just like right before your rent inspection, when you fill up that hole your mate Steve made in the wall at the party last weekend. Teeth are tricky creatures though, and the mouth is an incredibly hostile environment in which to get materials to stick and last. There's a lot that goes into preparing a tooth to be filled, and selecting the right material.
For starters, let's talk like dentists. We don't call them "fillings" when we're amongst ourselves. It implies that a hole is simply being filled up. The term "restoration" is more widely accepted, as the aim is to restore the form, function, health, and aesthetics of a broken down tooth. A tooth can need a restoration for many reasons, such as decay, breakage, developmental defects... the list goes on.
Preparation
The first step is to prepare the tooth for restoration, (the drilling bit). If there's decay, it has to be removed. It's a little like rust in that aspect. You can paint over your rusty car door, but eventually it will just continue to rust. All decay must be physically removed from a tooth before restoring. Once all the decay has been removed, the dentist then looks at the shape of the remaining cavity to determine if a restoration will stay put or not. A lot of the time additional healthy tooth structure has to be removed in order to make sure the restoration won't fall out, or that the remaining bits of tooth won't break under pressure.
Working under water
One of the biggest challenges is that the mouth is a wet place. Every tried to glue two wet things together? It doesn't work well. Where moisture from saliva and bleeding gums is a problem, (I'm looking at you, non-flossers), the dentist must choose a material that will be somewhat tolerant of some wetness while it's being placed. If the dentist is able to use a rubber dam in your mouth it makes the process a lot cleaner and dryer, meaning there's a wider option of materials which can be used effectively.
A hostile environment
The average force of the human bite between the molars is in the region of 700N. That's how much force it would take to hold a 70kg person off the ground. It's not small! This means that the materials used to restore teeth have to be extremely strong, hard wearing, resistant to acid and therefore decay, look pretty, feel nice, and bring about world peace. Ok maybe that last one is a stretch, but we expect a lot out of our restorative materials. There still isn't a magic bullet on the market, so your dentist has to weigh up the risks and benefits of each available material for your body, your mouth, and then the individual tooth. Sometimes there's even different areas on a single tooth that require different restorative materials. It's a complex decision.
Teeth are not rocks
Teeth are living structures, full of dynamic fluid movement and living tissue within the pulp at the centre. This fluid makes bonding a restoration to tooth structure challenging. A very specific set of processes have to be carried out in order to make it stick. We also need to consider the poor old pulp. It's probably already slightly inflamed because of what's already happened to the tooth, (decay, fracture etc). Now we're going to paint a bunch of different chemicals on the tooth and slap a foreign material right in it, millimetres away from the pulp itself. Any material which will further irritate the pulp may not be desirable, and we need to balance this potential for irritation with all the other properties we want from a material.
The final result should be a restoration that looks pleasing if it shows in your smile, feels smooth to your tongue, fits nicely into the bite in all jaw movements, and minimises the risk of breakages and recurrences of decay.
Restoring a tooth well is far more complicated than just "filling a hole".