1.-Effect of bar design and dimensions on bar extension fracturePrevention of distal extension cantilever fracture in mandibular overdentures. Materials and methods:The 2 nd moment area and static strength were calculated for 11 frequently used bar designs using finite element analysis (FEA). For two specific designs (Ackermann round ∅ 1.8 mm and Dolder Y-macro, the former with and without a support rib) additional physical static and fatigue strength tests were included. Results:The FEA static strength data corresponded well to the 2 nd moment area (a similar ranking when maximum allowed force was considered). The application of a rib support (Ackermann ∅ 1.8 mm) and limitations of the bar extension length (6 mm for the Ackermann ∅ 1.8 mm, 8 mm for the Dolder Y-macro) allowed the bars to exceed 5x10 6 cycles of 120 and 250 N respectively, before fracture. The region of highest stresses in FEA corresponded well with the locations of the fractures observed in static-and fatigue testing. Conclusions:With some simple guidelines/modifications, the number of bar extension fractures can be reduced significantly. Clinical significanceThis study focusses on distal bar extensions which improve the positioning of an implant supported overdenture. By combining laboratory testing and finite element simulations we aim to: (1) explain why fractures occur (dependent on physical characteristics of the bar), and (2) give clinical guidelines on how to prevent such fractures. 2.-Effect of bar design and dimensions on bar extension fracture