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