## 4 Discussion
The highest stresses concentrate in the areas of the framework with the lowest possibility of movement, that is the areas of support, either non-rigid in the CONVENTIONAL RPD model, provided by the direct retainers, or rigid in the IARPD PM and IARPD MO models, provided by the ball attachment-matrix contact. In the framework of the CONVENTIONAL RPD model more than 25% of the equivalent von Mises stress values were superior to 1 MPa, and values over 20 MPa rising to 200 MPa spread through the direct retainers (clasps and occlusal rests), minor connectors and major connector. This distribution of von Mises stresses in the CONVENTIONAL RPD model is consistent with the areas where most of the technical problems are detected in the re-examination of denture-wearing patients, which include loss of retention of the clasps due to permanent deformation, as well as their fracture, and deformation of the minor and major connectors. The correspondence between these two findings is particularly perceptible in the clasps, which are associated with a large fraction of all technical failures and, simultaneously, present high tensile stresses, namely in the transition from the non-flexible to the flexible portion of the clasp. The clasps may lose retention force, because of multiple deflections, and present gradual loss of elasticity [43]. Experimental studies point to permanent deformation or fracture of chromium-cobalt specimens with at least 0.5 mm deflection after a limited number of cycles, ranging from 20,000 to 30,000 [44; 45; 46]. In these specimens, fracture location is in close relation to the areas with the highest tensile stress distribution [46] and is concordant with the results obtained in the present study for the CONVENTIONAL RPD model. The installation of implants in the edentulous areas to promote support and retention to the RPD resulted in a 3 and 6-fold reduction in the mean equivalent stress in the frameworks of the IARPD PM and IARPD MO models, respectively. Modelling of these implant-assisted cases was conducted on no other design modification than the addition of the matrix to the framework, which led to transferal of stress from the direct retainers and minor connectors to the matrix, where the connection to the implant was modelled as rigid. This stress distribution across the framework differs from that reported by [PERSON] [47] because of the different geometries studied. Those authors did not incorporate the attachment matrix within the framework therefore, the stress distribution resembled that of a conventional removable partial denture, where in the minor connectors remain the most stressed areas. Nevertheless, clinical studies on implant-assisted removable partial dentures are consistent with our results, reporting a reduction of prosthodonic events in the retainers and simultaneously greater need for maintenance associated with the implant-related components, the patrix and matrix, where the finite element analysis  
Figure 10: Representation of the IARPD MOLARS model as a simple supported beam with a non-rigid support in the abutment tooth and a rigid support in the implant. (**a**) Springs represent the vertical resistance promoted by the soft tissues and the occlusal load is distributed exclusively over the replacement teeth. (**b**) Schematic representation of the pattern of vertical displacement occurring in each of the extension bases of an implant-assisted Kennedy Class I RPD with distal support as a simple supported beam Euler–Bernoulli problem. revealed higher stress accumulation [48; 49]. In a retrospective study with a mean follow-up of 8 years (ranging from 3 to 16 years) of patients rehabilitated with implant-assisted removable partial dentures to replace three teeth at both mandible sides, [PERSON] et al. [50] compared a cohort of patients with implants placed anteriorly, in the position of the premolars, and a cohort of patients with implants placed posteriorly, in the position of the second molars.