We're not necessarily saying that that can't be an option. But we can say that making a new crown has some advantages.

1) You state that your existing crown is a porcelain metal one (we're assuming you mean porcelain-fused-to-metal or PFM).

To access the interior of your tooth your dentist has had to drill a hole through its porcelain layer. That makes the porcelain inherently unstable and more prone to fracture. Probably in most cases this never becomes an issue but the potential is there.

While only cosmetic in nature, we'll also state that it's unlikely that the color of the filling placed will exactly match your crown. White filling material (dental composite) has some degree of translucency, which means that it will display some of the color of adjacent materials. In this case, it will have a grey tint due to the adjacent metal layer underneath the crown's porcelain surface.

2) The most important reason why a new fully intact crown makes the better choice is because of the seal it is able to create over the tooth (see our discussion above about coronal leakage).

When a filling is used to plug the hole, the quality of the seal it creates for the tooth is difficult to evaluate and monitor.

Probably the best thing that the dentist can do in this case is extend the filling down inside the tooth as far as they can (even down into the openings of the actual root canals). This way the filling's seal is as long as possible, thus creating the most substantial barrier possible.

If the seal is compromised, since your crown has a metal component that covers over the tooth there is no way for the dentist to monitor what's going on inside (like to check for decay) via x-rays.

All and all, predictability of its seal over the tooth is the main advantage that a new crown offers over just plugging the hole in the existing one with a filling.

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