Influence of mandibular superstructure shape on implant stresses during simulated posterior biting.

STATEMENT OF PROBLEM Theoretical considerations on the ideal implant-supported prosthetic superstructure shape lack the effect of complex mandibular deformation patterns during function. PURPOSE This study compared implant abutment stresses for idealized superstructures with different cross-sectional shapes and material properties during a simulated, complex biting task. MATERIAL AND METHODS A simplified and idealized 3-dimensional finite element computer model was built, which consisted of a sectioned mandible rehabilitated with 5 titanium implants and an attached superstructure beam composed of metal alloy and acrylic resin. The model was submitted to loads mimicking simultaneous bending and (to a lesser degree) torsion of the mandibular corpus during a bilateral posterior bite. Maximum and minimum principal stresses were calculated at implant abutment sites for each of 6 beam cross sections of the prosthetic superstructure and 2 types of materials. RESULTS Predicted implant stresses varied significantly between implant sites for different superstructure shapes. The lowest principal stresses were obtained by using a superstructure with a rectangular-shaped beam oriented vertically. Contrary to former theoretical considerations, the ideal "I-beam" superstructure cross section did not yield the lowest stresses. Superstructure materials with a lower modulus of elasticity seem to not only increase implant abutment stresses overall but also slightly reduce the tensile stresses on the most anterior implants. CONCLUSION Simulated implant abutment stresses may be significantly affected by the shape of the prosthetic superstructure, by diverse mandibular loading conditions, and to a lesser extent, by the prosthetic material properties.