acific Coast Society of Orthodontists Bulletin Summer 2018 - 40

Pearls of the Pacific
Special Section

Fabricating clear retainers for patients whose
teeth have severe undercuts can be tricky. If
undercuts are not blocked out prior to retainer
fabrication, the retainer can be quite difficult to
seat and even more difficult to remove. Furthermore, crown restorations on teeth with severe
undercuts are subject to undue pressures, as the
plastic engages the undercuts each time the
retainer is removed; this often results in the
unseating of crown restorations.
A common way to address this issue if taking
final impressions with alginate is to block out the
undercuts with wax on a stone model prior to
retainer fabrication. If, however, final intraoral
scans are being taken for retainer fabrication,
manipulate the gingival margins using your scan
data software so that undercuts are eliminated.
This is often accomplished digitally by raising the
simulated gingival tissue high enough to cover
any existing undercuts. As a result, a retainer fabricated from this adjusted model will not engage
any of the undercuts. 

Tooth #28 and tooth #29 are porcelain crown
restorations. Notice the differences in occlusal
anatomy when compared to that of tooth #20
and tooth #21; mandibular left premolars are
unrestored.

➡
➡

Dr. Ronald Chung
Los Angeles, CA

Digitally Blocking Out Undercuts
for Clear Retainer Fabrication

➡
➡

RETENTION

➡
➡

Red arrows showing existing undercuts on
mandibular right premolars restored with crowns

Gingival margins raised by 2.5mm so that
undercuts are covered. A retainer fabricated from
this model will not engage the existing undercuts.

40

PCSO Bulletin  Summer 2018



Table of Contents for the Digital Edition of acific Coast Society of Orthodontists Bulletin Summer 2018

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acific Coast Society of Orthodontists Bulletin Summer 2018 - 40
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acific Coast Society of Orthodontists Bulletin Summer 2018 - 50
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acific Coast Society of Orthodontists Bulletin Summer 2018 - 67
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