Stresses Induced By RPI-clasp On Supporting Structures With Vertical Loading

Objectives: As the preservation of what's remaining is considered more essential than restoration of what's missing, the effect of incorporation of the commonly used RPI-clasp in distal-extension removable partial dentures (RPDs) on supporting structures needs to be more clarified. This study aimed to investigate stress distribution of RPI elements (proximal plate, mesial occlusal rest and retentive clasp arm) under tissue-ward and tissue-away vertical loading conditions. Stresses were analyzed using three-dimensional finite element method to reveal the most stressed areas beneath these elements in class II Kennedy RPDs. Methods: Classically-designed bilateral RPD with RPI clasp on second premolar of edentulous side, embrasure clasp on first and second molars as well as cingulum rest as indirect retainer on canine of intact side was produced. Application of vertical load (100N) was applied on distally-extended denture base in both tissue-ward and tissue-away directions; resultant stresses beneath RPI elements were evaluated. Results: Tissue-away loading resulted in stresses; the highest of which were detected beneath the proximal plate that might acted as a separate fulcrum within the elements of RPI, followed by the mesial rest then the minor connector that acted as a bracing unit. The least stresses were detected at the retentive tip. Tissue-ward loading resulted in stresses, the highest of which were detected beneath the mesial rest. Minimal stresses were observed beneath the proximal plate. Almost zero stresses were detected around the retentive arm. Conclusion: Within the limitations of this study, the RPI clasp induced stresses on the supporting structures in both tissue-ward and tissue-away loading conditions. As was expected, tissue-ward loading resulted in maximal stresses beneath the mesial occlusal rest. However, the maximal stresses during tissue-away loading were unexpectedly observed beneath the proximal plate that may lead to recommending further studies regarding its optimal length and area of contact with the abutment tooth.