Statement of problem: Studies have shown the efficacy of mandibular overdentures retained by a single implant for oral rehabilitation of edentulous patients. The primary complication of these overdentures has been reported to be their high incidence of midline fracture. The addition of a reinforcing framework to the acrylic denture base can prevent fractures and ensure the durability of overdenture treatment. Purpose: The aim was to compare crestal bone loss and prosthetic maintenance events of single implant-retained mandibular overdentures (SIMO) reinforced by Polyetherether ketone (PEEK) and metallic frameworks after a two-year follow-up period. Materials and methods: Twenty-two edentulous patients received SIMOs containing reinforcing frameworks. The resin patterns of the frameworks were printed using CAD/CAM. The printed castable resin frameworks were processed using either PEEK by injection molding technique (PEEK group) or cobalt-chromium metal by conventional casting techniques (Metal group). Crestal bone loss was evaluated using digital radiography and prosthetic maintenance events were analyzed. The follow-up period was two years after overdenture loading. Results: There was a slight increase in crestal bone loss in the Metal group compared to the PEEK group, but the difference was statistically insignificant. There was no overdenture fracture, crack, or abutment fracture in either group. No significant differences in prosthodontic maintenance events between the two groups were noted. Conclusions: Based on the limitations of this study, it is concluded that the crestal bone loss and prosthetic maintenance results of SIMO reinforced with a PEEK framework were comparable to those of metal reinforcement.