Prevention of distal extension cantilever fracture in mandibular overdentures

• Published in: Journal of dentistry Vol. 43; no. 9; pp. 1140 - 1147
• Main Authors: Quirynen, Thomas, Quirynen, Marc, Duyck, Joke
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01.09.2015; Elsevier Limited
• Subjects: Bar design; Bar extension fracture; CAD; Computer aided design; Conflicts of interest; Dental Prosthesis Design - methods; Dental Restoration Failure; Dental Stress Analysis - methods; Dentistry; Denture, Overlay; Design analysis; Economics; Finite Element Analysis; Finite element method; Fracture mechanics; Guidelines; Humans; Implant retained overdentures; Implant suprastructures; Laboratories; Mathematical analysis; Mechanical failures; Metallurgical considerations; Patient satisfaction; Prostheses; Prosthetic complications; Strength; Transplants & implants; Implant suprastructures; Metallurgical considerations; Mechanical failures; Prosthetic complications; Implant retained overdentures; Bar extension fracture; Bar design
• ISSN: 0300-5712; 1879-176X
• DOI: 10.1016/j.jdent.2015.06.007

Fractures of distal bar extensions, supporting a mandibular overdenture, do occur with significant functional and economic consequences for the patient. This study therefore aims to evaluate the effect of different bar cross-sectional shapes and surfaces, bar extension lengths and the placement of a support rib under the distal bar extension on fracture resistance. The 2nd moment area and static strength were calculated for 11 frequently used bar designs using finite element analysis (FEA). For two specific designs (Ackermann round Ø 1.8mm and Dolder-Y macro, the former with and without a support rib) additional physical static and fatigue strength tests were included. The FEA static strength data corresponded well to the 2nd moment area (a similar ranking when maximum allowed force was considered). The application of a rib support (Ackermann Ø 1.8mm) and limitations of the bar extension length (6mm for the Ackermann Ø 1.8mm, 8mm for the Dolder-Y macro) allowed the bars to exceed 5×106 cycles of 120 and 250N, respectively, before fracture. The region of highest stresses in FEA corresponded well with the locations of the fractures observed in static- and fatigue-testing. With some simple guidelines/modifications, the number of bar extension fractures can be reduced significantly. This 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.