Double-Arm Clasp with Anterior Placed Occlusal Rest Retained Mandibular Distal Extension Removable Partial Denture - Stress Analysis and Alveolar Bone Height Changes of Abutments.

Aim: study the effect of double-arm clasps as retainer units for bilateral mandibular distal extension removable partial denture regard: Study of stress analysis applied on the abutments (in-vitro) and abutment alveolar bone height changes (in-vivo).The retainer clasps includes: Reverse circlet clasp and Modified Half-and-Half clasp assembly. Materials and methods: In-vitro study: Model RPD distal extension with three frameworks of different retainer units for stress analysis. In-vivo study: In a randomized controlled clinical trial, fifteen patients with completely edentulous maxilla and remaining mandibular eight anterior teeth including 1st premolar. They were randomly divided to three groups. All patients received maxillary complete denture. Group I: where distal extension RPD designed with Reverse circlet clasp. Group II: where distal extension RPD designed with modified halfandhalf clasp assembly without distal occlusal rest and retentive clasp arm arising from anterior occlusal rest. Group III: where distal extension RPD designed with modified Half and Half clasp assembly without distal occlusal rest and modified retentive clasp arm arising from proximal guiding plate. The abutment tooth was evaluated radio graphically by digital periapical radiographs after insertion, after 6 months and after 12months after denture insertion. Results: Group I (Reverse circlet clasp) significant increase in bone resorption of abutment compared to those in Group II and Group III (modified half-and-half clasp assembly). From stress point of view, in bilateral loading group I applied more stress to abutment compared to group II and III. Conclusion: Group II &III modified half-and-half clasp assemblies (stress releasing clasp assembly) superior to reverse circlet clasp regard to abutment alveolar bone resorption of distal extension cases. Reverse circlet clasp (Group I) not purely stress releasing by disengagement regarding double arm clasp utilize for this purpose. From stress transmitted point of view (bilateral loading), modified half-and-half clasp assembly (group II) exhibit less stresses transmitted to the abutment than exhibit in group III as a result obtained in this study (in-vitro) that minimize abutment alveolar bone resorption compared to group I to group II& III. This research recommends the application of a new stress releasing clasp designs for distal extension RPD (modified halfand-half clasp assemblies as in group II & III). These assemblies should be added to retainer units of stress releasing action by disengagement.