Depending on the size of the cavity and the state of the tooth, several restoration techniques are possible. The choice of material depends on how much tooth has been lost and on whether the remaining walls need protecting.
Early decay
For a small cavity of this kind, composites (resin-based materials) are a perfect fit. They look good but are a temporary answer (limited strength and sealing). Their advantage is that they contain no metal.
Amalgam would also do the job, but for several reasons (unattractive, contains mercury, a metal that causes corrosion) it is used less and less nowadays, and we stopped placing it a long time ago.
A larger cavity
For a cavity of this size, amalgams and composites cannot be relied on to last. Over the years both materials crack, the join with the tooth widens and lets bacteria through, and decay starts again.
They also cannot protect fragile walls. These fillings have to be redone again and again, each time bigger, until the tooth eventually has to be root-treated and crowned.
An inlay, made in the laboratory, has a far more precise shape, protects the remaining walls better and allows the tooth to stay alive. Made of ceramic or resin, it blends in with the natural tooth.
Decay reaching the nerve
A root-treated tooth is extremely fragile: partly because so much of it has been lost, and partly because the dentine, no longer supplied with blood, becomes brittle. The tooth also tends to darken.
Fractures of root-treated teeth are, along with receding gums, the leading cause of extraction. These teeth must be protected by a crown.
In some cases they are first rebuilt using either a metal post and core or a dental post. These modern prosthetic techniques reproduce the natural colour of the tooth and preserve, or even improve on, its original shape.
