Well, the short answer is probably because they are too long. They also might be too far out.
It’s important to remember that when you’re restoring maxillary anterior teeth, unless you give the laboratory very specific guidelines on where you want the maxillary incisal edge, their tendency is to make the teeth too long and to make them too far labially.The other thing that we have the tendency to do on maxillary anterior teeth is under-reduce, which sets up a lot of these problems.
One of the things that is extremely important to remember is that if all the lab has to go by is a nice model of the prepared teeth with good margins, they are simply guessing at where the maxillary incisal edges will be in space. We have to determine these things in the mouth and then give very specific communication to the lab on where that’s going to be, and then have some verification or protocols to make sure they do it.
Determination, communication, and verification are the names of the game.
That’s why at the academy, we begin our treatment planning with the 2D and 3D checklist to end up with a diagnostic wax up where the maxillary incisal edges should be fairly close. Now with the Dawson Diagnostic Wizard, completing 2D and 3D checklist and communicating the results to the lab is even easier.
But as good as a wax up is, it’s never going to be right on.
That’s why we’re going to go through a specific protocol to fine tune the provisional restorations and dial in that maxillary incisal edge so that we have optimum esthetics, phonetics, and function. Once that’s done, an impression of the provisional restoration will be made and the lab will cross-mount the temp models with the models.
Of course, different protocols can be used by the laboratory. For years, we’ve used a custom incisal guide table and a silicone putty labial matrix of the provisional restoration so they can check these contours. And now today, the lab can scan the provisional model, and then literally place a virtual copy of the provisional right over the scanned model that allows the lab to either print or mill the restorations so that the three dimensional lingual contour and the maxillary incisal edge is exactly the same.
The key thing to remember is that we don’t want something as important as the length of the maxillary incisors left up to guesswork from the laboratory.
These are things we as dentists have to figure out and determine thoroughly with the provisional restorations. Then and only then can we have the predictability we’re shooting for with our permanent restorations.