Porcelain Butt Margin
Posted on 07/14/2011
Doctors who prefer a metal based restoration
- such as a PFM - over the all ceramic counterpart - are often frustrated by
the gray line that appears at the margin over time. This is particularly
relevant to non-precious or semi-precious metal alloys. High gold content
alloys - especially Captek - will typically show a more yellowish margin. One
technique to avoid this graying at the margin is to create a "Porcelain
Butt Joint" margin. A butt joint is a shoulder preparation with a complete
right angle (90 degree) shoulder. This technique allows for the lab to build
porcelain to the edge of the preparation as porcelain requires a
"platform" to support it. There are a number of common preparation
mistakes which often results is porcelain fracture at the margin. We highlight
five key items below:
(1) The right angle preparation must be created with a FLAT END BUR such as the
Brassler 558 and FINISHED with a FLAT END diamond such as the Brassler 846. A
round end or tapered bur will not create the flat right angled preparation
needed for a butt joint.
(2) The shoulder must be carried through the proximal embrasures. If it is not,
the metal margin will show in the proximals as laboratory tech cannot build
porcelain on a chamfer or feather edge prep. It is recommended that the
shoulder be extended two-thirds facially of the tooth.
(3) The preparation must be flat and cannot have bumps - especially common on
the mesial and distal shoulders. If bumps are not smoothed out, porcelain will
not sit cleanly and is likely to fracture.
(4) The edge of the prep should be smooth WITHOUT A LIP. A lip is often created
when using a narrow or round end bur. A fragile lip will often fracture. The
edge of the prep should be smoothed off with an instrument such as a flat end
diamond.
(5) For anterior cases, it is critical that the margin is extended by 0.5 to
1.0mm into the sulcus (sub-gingival). This will allow for correct crown
contours and closed gingival embrasures.
We thank our friends at Bay View Dental Lab in Virginia for providing some of
the information for this post.
- such as a PFM - over the all ceramic counterpart - are often frustrated by
the gray line that appears at the margin over time. This is particularly
relevant to non-precious or semi-precious metal alloys. High gold content
alloys - especially Captek - will typically show a more yellowish margin. One
technique to avoid this graying at the margin is to create a "Porcelain
Butt Joint" margin. A butt joint is a shoulder preparation with a complete
right angle (90 degree) shoulder. This technique allows for the lab to build
porcelain to the edge of the preparation as porcelain requires a
"platform" to support it. There are a number of common preparation
mistakes which often results is porcelain fracture at the margin. We highlight
five key items below:
(1) The right angle preparation must be created with a FLAT END BUR such as the
Brassler 558 and FINISHED with a FLAT END diamond such as the Brassler 846. A
round end or tapered bur will not create the flat right angled preparation
needed for a butt joint.
(2) The shoulder must be carried through the proximal embrasures. If it is not,
the metal margin will show in the proximals as laboratory tech cannot build
porcelain on a chamfer or feather edge prep. It is recommended that the
shoulder be extended two-thirds facially of the tooth.
(3) The preparation must be flat and cannot have bumps - especially common on
the mesial and distal shoulders. If bumps are not smoothed out, porcelain will
not sit cleanly and is likely to fracture.
(4) The edge of the prep should be smooth WITHOUT A LIP. A lip is often created
when using a narrow or round end bur. A fragile lip will often fracture. The
edge of the prep should be smoothed off with an instrument such as a flat end
diamond.
(5) For anterior cases, it is critical that the margin is extended by 0.5 to
1.0mm into the sulcus (sub-gingival). This will allow for correct crown
contours and closed gingival embrasures.
We thank our friends at Bay View Dental Lab in Virginia for providing some of
the information for this post.
