Evaluation of hybrid prostheses with different anterior cantilever length placed on different designed implants in atrophic maxilla by finite element stress analysis

Aim: The biomechanical effects of All-on-Four concept in the complete edentulous atrophic maxilla are still unclear. The aim of the present study is to determine the limits of the anterior cantilever length of the prosthesis in patients to be treated with this concept and to investigate the biomechanical effect of the use of angle correction implants (ACI) in the All-on-Four concept by finite element stress analysis (FEA) method. Materials and Methods: 6 models were created for 2 different implant designs and 3 different cantilever lengths in the 3-D modeling program. In group 1, two standard implants (SI) were placed 30o tilted with 30° multi-unit abutments in the atrophic maxilla posterior region, also two anterior SI were placed 17° tilted with 17° multi-unit abutments in anterior region. In the second study group, two 24° angle correcting implants (ACI) in the posterior region and two 12° ACI in the anterior region were placed in accordance with the implant angles with 4 straight multi-unit abutments in the same jaw model. Additionally, three different prostheses were modeled with anterior cantilever (AC) / antero-posterior distance (AP) ratios of 1/4, 1/2, and 3/4. Maximum anterior bite force was applied at an interincisal angle around the cingulum of the anterior teeth and analyzed by the FEA. Results: The stresses occured on implants, surrounding bone and prosthetic framework increased with the increase of AC distance. The tensile stress on the posterior trabecular bone in the SI group exceeded the strength limit of the bone when the AC / AP ratio was 3/4. The stress values were lower in the ACI groups than SI groups. Also, the stresses occured on the implants, the surrounding bone and the prosthetic screws were more homogeneously distributed in the ACI group than in the SI group. Conclusions: Within the limitation of this study, In patients who will be treated with the All-on-Four concept using standard implants, care should be taken that the AK / AP ratio of the prostheses is less than 3/4 or the AK distance is less than 12 mm. The use of angled dental implants in the All-on-Four concept seems to be a promising option with better results in terms of biomechanics, and needs to be investigated with in-vitro tests and long-term clinical studies.