Finite element analysis (FEA) studies in 2.5-mm round bar design: the effects of bar length and material composition...

The round bar/overdenture prosthesis is commonly used in the restoration of the totally edentulous implant patient. The length of bar span and types of alloys used in clinical cases have raised questions related to beam flexure and its role as a possible etiology of reported clinical failures in cast alloy systems. Three-dimensional finite element analyses were thus conducted on a 2.5-mm round bar for investigation of mechanical performance with respect to failure potential as a function of bar length and bar material property. Specifically, three bar lengths (6 mm, 12 mm, and 18 mm) and three alloy materials were analyzed, representing a clinical range of usage. The ends of each bar were modeled fixed to a 2.5-mm coping which was attached to a 3.8-mm root-form-type implant. The implant was modeled rigidly fixed in a representative block of bone. A 200-N occlusal force was applied to the model, as would be transmitted through an attachment clip, 5 mm in length, for the three respective bar lengths. The results of these analyses suggested possible yielding (or failure) in the 18-mm case. Bar length was found to play a stronger role in the adequacy of the overall design as compared with changing material properties in the range of alloy stiffnesses tested. Factors of safety with respect to static yield strength ranged from 2.82 to 66.46 for the designs evaluated. Fatigue factors of safety ranged from 1.63 to 38.88. A factor of safety of 5 or greater is suggested for the design of round bar systems in order for bar failure to be prevented.