The best tooth preparations

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Welcome to our series of information files designed to help
us as a laboratory improve quality.

As you read this  it's easy to see that working with SBO provides the best
value service. Our mission is to work with you to build your practice in to the
best it can be, these emails are an integral part of that mission.

Lack of occlusal room

The following image shows a regular posterior situation in a cross section looking
from the mesial.

When the lower tooth is prepared it looks like this.

This looks as though we have enough clearance, if shims were to be used a 1mm shim
could pull through nicely...plenty of room then?

If we then put the full contour crown back, something immediate becomes obvious...
...there has been no reduction in the central fossa area. This means the resultant
crown will be flat and have a very unatural occlusal morphology. The occlusal carving
on this type of crowns are often called 'chicken scratches'!
In order to have good morphology on a crown we need equal space around the whole
tooth, in the mouth this may look like a lot of space, but it will allow us to
replace the occlusal anatomy in a natural way.

The image above shows the problem on an actual case, the occlusal height in the
central fossa is limited. Whilst we have enough clearance to make the case free
of the bite, we don't have enough to produce a detailed and natural anatomy.

The buccal planes

A natural tooth has 3 bussal planes shown by the red, blue and green lines in this

If a natural buccal surface is required on the finished crown, we need adequate
space along these 3 planes. Usually the green and red planes are prepared well,
however the blue plane is often not prepared, giving the following result.

Right at the incisal edge the restoration will be thin. This will result in 1 of
2 compromises:

1. The colour is good, but in order to do this the ceramic needs to be a minimum
of 1.2mm, so the contour becomes compromised

2. The contour is good, but because the ceramic is so thin the colour becomes compromised

The following image shows an actual case.

This case is a 3 unit bridge, the left lateral (closest) is prepared according to
the 3 buccal planes, however the right central (furthest) is missing the incisal
plane reduction. This tooth will therefore be compromised.

Sharp angles

We often have cases with very sharp angles on the preparation. This can cause this
area to wear very easily on the model and lead to a crown not seating well. We are
of course able to silver plate dies to prevent this, but in the main this adds an
unacceptable level of cost to the finished crown.

The preparation on the UL5 in the photograph shows the mesial palatal angle as being
very sharp. This looks as though it is a drag, but it was confirmed that this is
not the case.
These sharp areas also create weak areas in metal free restorations and can be a
contributing factor is fractures, smooth preparations are even more essential when
metal free is required