STRESS ANALYSIS OF REMOVABLE PARTIAL DENTURES OF DISTAL EXTENSION CASES FABRICATED OF TWO RECENT AESTHETIC CAD/CAM PROSTHETIC MATERIALS VERSUS CONVENTIONALLY MANUFACTURED METALLIC RPDs

Recently, there are different types of esthetic materials used for RPDs framework fabrication some of which are applicable for CAD/CAM system. Distal extension base removable partial dentures exhibit composite type of support, which leads to movement of the denture base under load application and transmission of high amount of stresses to the supporting structures.
Aim: The current study was conducted to analyze the stress transmitted to the abutment teeth and the residual ridge by three types of RPDs of different materials; Conventionally manufactured metallic Co-Cr RPDs, and two thermoplastic RPD materials; Acetal (Polyoxymethylene; POM) and a modified PEEK (Poly-Ether-Ether-Ketone) material (BioHPP; Biocompatible high performance polymer) distal extension RPD frameworks both manufactured by computer aided design and computer aided manufacturing (CAD/CAM) system as two esthetic prosthetic materials.
Methods: Strain gauge technology was used to assess micro-strains induced to the denture supporting structures for the three fabricated RPDs groups. Master casts of ten patients with mandibular bilateral distal extension (the remaining teeth extending from left 1st premolar to right 1st premolar) were used so that each cast was duplicated into 4 casts; three of these casts were used for manufacturing of acetal and BioHPP frameworks by CAD/CAM system, and conventional Co-Cr RPD frameworks. The fourth cast was duplicated to obtain acrylic cast with movable acrylic abutment teeth over which the three RPDs (two metal-free and one metallic RPDs) of each case were seated and tested. The microstrain of the supporting structures was recorded on bilateral and unilateral loading.
Results: On bilateral loading: Mean value of microstrains recorded at the abutments was -63.75 and -18.75 at the ridge for Co-Cr group; For acetal group, it was -91.25 at the abutments and 18.94 at the ridge; For BioHPP group, microstrain was -106 at abutments, while it was 37.15 at the ridges. On unilateral loading: For Co-Cr group, microstrains recorded at right abutment was -52, while it was -6 at left abutment, and at right ridge it was -16.5 and -2 at left ridge; For acetal group, microstrains recorded at right abutment was -176 and -10 at left abutment, and at right ridge it was 17 and 4.5 at left ridge; For BioHPP group, microstrains recorded at right abutment was -284, while it was 10.5 at left abutment, and at right ridge it was 50.5 and 6.5 at left ridge.
Conclusion: Co-Cr RPDs group showed the least stresses transmitted to the supporting tissues followed by Acetal RPDs, while BioHPP RPDs group showed the highest mean values of microstrain induced to the abutment teeth and the residual ridge.