Biomechanics of 3-implant-supported and 4-implant-supported mandibular screw-retained prostheses: A 3D finite element analysis study

Statement of problem. The number of implants required for the rehabilitation of completely edentulous mandibles has been controversial. The use of a greater number of implants can produce favorable biomechanical outcomes. However, this will lead to high costs and may require complex surgical procedures. Therefore, the minimum number of implants that can produce desirable outcomes should be used. Purpose. The purpose of this 3D finite element study was to compare the biomechanics of mandibular 3-implant-supported to 4-implant-supported prostheses. The opposing occlusion was a maxillary complete denture or natural dentition. Material and methods. Two finite element analysis mandibular anatomic models were created. Implants were virtually placed in the mandibular lateral incisor and second premolar region bilaterally in the 4-implant-supported prosthesis model. For the 3-implant-supported model, they were placed in the midline and bilaterally in the second premolar region. Screw-retained polymethyl methacrylate prostheses were designed. Reverse engineering was used to convert standard tessellation language files into computer-aided design solid models. Vertical and oblique loading was applied twice: simulating an opposing maxillary complete denture and a natural dentition. Von Mises stresses and equivalent strains generated in the peri-implant bone, implants' von Mises stresses and the maximum vertical displacement of the prosthesis were recorded. Results. All recorded outcomes reported higher values for the 3-implant-supported prosthesis compared with the 4-implant-supported models for both applied loads. When opposed by a maxillary complete denture, maximum strain values for the 3-implant-supported (2.3X10(3) mu epsilon) and 4-implant-supported (1.6X10(3) mu epsilon) models were less than the different threshold limits for the bone resorption reported (3x10(3) , 3.6x10(3) , 6.6x10(3) mu epsilon). When opposed by a maxillary natural dentition, maximum strain values for the 3-implant-supported (4.10x10(3) mu epsilon) and 4-implant-supported (3.88x10(3) mu epsilon) models were less than the highest reported threshold limit for bone resorption (6.6x10(3) mu epsilon) in contrast with other reported threshold limits (3x10(3) , 3.6x10(3) mu epsilon). In both designs irrespective of the magnitude and direction of loading, the maximum recorded von Mises stresses of the implants (126 MPa) and denture displacement (3.24x10(2) mu m) were less than titanium's yield strength of (960 to 1180 MPa) and the displacement values (5.2x10(3) to 8.8x10(3) mu m) reported in the literature. Conclusions. When opposed by a complete denture, recorded biomechanical outcomes for the 3- and 4-implant-supported designs were within physiologic limits. When opposed by a natural dentition, the von Mises stresses of the implants and denture displacement values for both designs were within a favorable mechanical range, whereas peri-implant stresses and strain exceeded most reported physiologic tolerance levels of bone except for the 6.6x10(3) mu epsilon threshold limit for the bone resorption reported.