The purpose of this in-vitro study was to compare the retention of circlet (E) clasps and back action clasps against three different abutment surface materials (enamel, composite, CAD/CAM ceramic crown) during long-term simulation of attachment and detachment, and to measure the loss of retention and wear after 16,000 simulated cycles of attachment-detachment. Forty-eight models were constructed by placing either an upper first premolar or a metal die inside a metal rectangular block. Models were divided according to the abutment teeth into three groups. Group GI consisted of 16 unrestored human premolars with sound enamel. Group GII had 16 premolars re-contoured buccally using composite resin. Group GIII had 16 metal dies (duplicated from a human premolar) covered by CAD/CAM all-ceramic crowns. On the models, E-circlet (E) and back-action (B) clasps were constructed to engage the model's teeth. Removal and insertion cycling of clasps was carried out for 250, 500, 1,000, 2,000, 4,000, 8,000, and 16, 000 cycles by using a chewing simulator. The retention force of each clasp was measured before cycling and at different intervals by using universal testing machine. An acrylic replica was made for each abutment retention surface before and after cycling. Each replica was examined by SEM, and the wear areas were measured. The data was analyzed statistically using 1-way ANOVA, 2-way ANOVA, and Mann-Whitney tests. No significant differences in retention of either clasp were found between the three abutment material surfaces. However, there was a significant decrease in retention force of the circlet (E) clasp between 1,000 and 2,000 cycles but not the back action clasp. There were no significant differences in retention loss after 16,000 cycles (P≥0.05) of both clasps (E, B) on the three abutment materials (enamel, composite resin, CAD/CAM ceramic crown). There were significant differences among the wear areas of the abutment surface of the six subgroups (P≤0.05). Within the limitations of this study, the following conclusions were drawn: 1) The back action clasp maintains its retention force for a longer period than the circlet (E) clasp. 2) Composite resin contouring of teeth seems to be a viable technique for creating retention for the RPD clasps because there was no significant difference between the 3 abutment materials regarding their retention forces at different intervals (P0.05). 3) The difference in design between circlet E-clasps and back action clasps had no significant effect on the loss of retention force after 16,000 cycles. 4) Using different abutment surfaces for clasp retention had no significant effect on the amount of retention loss after 16,000 cycles. 5) The composite resin contoured teeth showed more wear than the enamel and ceramic by the action of E and back action clasps. However, E-clasps caused more wear on the abutment materials than back action clasps.

https://macau.uni-kiel.de/servlets/MCRFileNodeServlet/dissertation_derivate_00005931/Diss-_M.Helal-final-mk_(submited).pdf

Effects of Long‐Term Simulated RPD Clasp Attachment/Detachment on Retention Loss and Wear for Two Clasp Types and Three Abutment Material Surfaces

Abrasion of enamel and composite resin by removable partial denture clasps.

Historically, invasive restorative procedures have been required to create undercuts for removable partial denture retainers where natural undercuts are lacking. Resin-bonding procedures have allowed the development of conservative, minimally invasive methods for creating undercuts. A technique using a partialcoverage porcelain laminate is described.

Use of a partial-coverage porcelain laminate to enhance clasp retention

In cases where there are no suitable natural undercuts on the teeth which can be used to provide clasp retention for a partial denture artificial undercuts can be created by the application of a composite filling material. An in-vitro investigation was undertaken to assess the surface integrity of three different composite restorative materials following repeated cycles of movement of cobalt-chromium I-bar clasps over artificial undercuts created using the composite materials. A laser reflection confocal microscope was used to quantify surface changes in the composite specimens. Simulation of the equivalent of 5 years of clinical usage showed that a composite resin with the smallest particle size exhibited the least surface wear.

The abrasion of dental composite by cobalt-chromium clasps.

C ustom-fitted methyl methacrylate resin ocular prostheses were developed at the U.S. Naval Dental School in 1944 because of a scarcity of imported stock glass ocular prostheses. ’ Since that time acrylic resin has become the standard material for use in ocular prostheses, whether custom made or manufactured. Several articles have appeared in the prosthodontic literature that describe various methods of fabrication of custom ocular prostheses.“’ Superior results for ocular prostheses are obtained with the modified impression method.8 This method virtually eliminates the presence of irritating dead spaces in the defect and is far superior to attempting to adjust and modify the shape of a manufactured ocular prosthesis to fit the defect. Ocular prostheses have not usually been custom made for incorporation into an orbital prosthesis because the marginal shape is not critical and extra time is involved in custom molding. There are several advantages to developing a simplified method for custom molding ocular portions of orbital prostheses. First, esthetics could be customized. Second, a relatively large supply of manufactured eyes would not have to be kept in stock. Third, the scleral curvature of the prosthesis could be decreased.5 Frequently when the inner and outer canthi of an orbital prosthesis are carved to the proper dimensions, the curvature of the globe is found to be too severe. The scleral surface of the ocular part becomes too deep in relation to the canthi because the curvature is too great (Fig. 1). The purpose of this article is to describe the fabrication of a reusable metal mold that simplifies the fabrication of custom ocular portions of orbital prosthe-

Lauciello Fr

Papers

Fabricated retention for removable partial dentures.

Fabrication of cobalt-chromium dies for custom tooth molding.