The finishing line of the preparation (or the so called "The preparation margin") is the final margin that separates between the prepared and the unprepared tooth structure.
This line should be smoothly continuous from one surface to another;
otherwise, it will interfere with the seating of the crown if it is poorly done. The margin between the prepared and unprepared tooth structure is a very critical area as most failures start from this margin.
Types of finishing line according to its design or configuration
The following designs for finishing line could be used depending on the type of
the crown restoration:
► Knife edge (also named "feather end")
► Chamfer
► Heavy chamfer
► Shoulder
► Radial shoulder
► Shoulder with bevel
Chamfer finishing line
It is a well-defined finishing line somewhat like knife edge finishing line except that the cut is made deeper. It forms a 130-160° cavo-surface line angle.
A roundend tapered fissure bur is used to obtain this preparation margin. It provides adequate space at the cervical region so can make the contour of the crown restoration within the contour of natural tooth without overcontouring of the final restoration.
However, since the restoration margin obtained with this type of finishing line is thick, so it is unburnishable.
VIDEO: FIXED PROSTHODONTICS: All-Ceramic crown preparation for e.max and Zirconia - Dr. Richard Stevenson
This type of finishing line is indicated for areas to be covered by metal only as the knife edge finishing line, so it is mainly used for:
1. Full Metal Crown (All the surfaces).
2. The lingual and proximal surfaces of full veneer crown, three- quarter crown and post crown.
