The composite resin contoured surfaces showed more wear than the enamel and ceramic surfaces. E-clasps caused more wear on the abutment materials than back-action clasps.

(1) The back-action clasp maintains its retention force for a longer period than the circlet (E) clasp. (2) Composite resin contouring of teeth provided retention comparable to enamel and a ceramic material.

A reduction of the occlusal table width in free-end saddle RPDs resulted in decreased masticatory efficiency 2 months after denture fabrication, but there was no significant difference in muscular activity between narrow and wide teeth. However, after 4 months, the masticatory efficiency of the participants with narrower teeth improved without any changes in muscular activity.

Objective: To evaluate the fitting, flexure strength and clasp retentive force and deformation of RPD frameworks made from cobalt chromium and polyetheretherketone. Materials and Methods: Mandibular cast with a unilateral bounded saddle was fabricated, scaned and transferred to Exocad software. Twenty frameworks were fabricated and divided into two groups, group I for PEEK fabricated by CAD/CAM milling technique and group II for Co-Cr fabricated by conventional lost wax technique from 3-D printed resin pattern. Framework fitness was evaluated by replica technique and weighing by a digital analytical balance. Retention was measured by applying withdrawal force to frameworks by a universal testing machine. Deformation of clasps was evaluated by measuring the distance between 2 reference points on the tips of the retentive and reciprocal arms before and after repeated insertion/removal cycles by a chewing simulator. Biaxial flexure strength was determined using a piston-on-3points technique. Results: PEEK material group showed more adaptation than metal. For retention and deformation, metal group recorded higher retention mean value than PEEK but due to the higher deformation in metal there was no significant difference in retention after one year of use simulation. Metal group recorded higher flexure strength than PEEK. Conclusions: Within the limitations of this study, it could be concluded that milled PEEK frameworks have better tissue surface adaptation, totally acceptable retention forces reaches between 9 and 12N, and flexure strength between 137 and 144MPa. which is valid for clinical use.

Statement of problem The retention qualities of acetal resin clasps are questionable. Purpose To asses the initial retention and after cycling for 1,200 cycles of acetal resin clasps as compared to cobalt-chromium clasps. Materials and methods Extracted maxillary first premolars and molars were collected. Each tooth was embedded in acrylic block up to the cementoenamel junction. On each block, partial denture model consisted of Akers clasp, minor connector, cobalt-chromium horizontal plate representing the denture base, and a vertical arm on the horizontal plate was constructed. Models were divided into four groups (five each). Group I had acetal resin clasps on premolars. Group II had cobalt-chromium clasps on premolars. Group III had acetal resin clasps on molars. Group IV had cobalt-chromium clasps on molars. Akers clasps were designed to utilize 0.01 inch undercut. Each model was connected to the universal testing machine through the vertical arm. The model was placed in artificial saliva and a tensile load was applied to measure the initial retention. Then retention was measured after cycling for 200, 400, 600, 800, 1,000 and 1,200 cycles. One-way analysis of variance followed by Duncan's multiple range test was used to test the effect of groups and cycling on tensile load required for clasp dislodgment. Results Groups I and III showed no change in retention on cycling up to 1,200 cycles. Group I showed significantly higher retention than group III. Groups II and IV showed significant reduction in retention upon cycling more than 800 cycles. In spite of this reduction in retention, it is still higher than that in groups I and III. Conclusion Cobalt-chromium clasps showed higher retention than acetal resin clasps despite the deformation encountered in the former. Acetal resin clasps showed more retention qualities on premolar teeth than that on molar teeth. Clinical implications Acetal resin clasps can be used to retain Kennedy's class III partial denture. However, when used on molar teeth, it is recommended to increase the thickness of the retentive clasp arm or to use a deeper undercut. How to cite this article Mohamed T, Baraka OA, Badawy MM. Comparison between Acetal Resin and Cobalt-Chromium Removable Partial Denture Clasp Retention: An in vitro Study. Int J Prosthodont Restor Dent 2013;3(2):50-56.