Rehabilitation of the atrophic mandible with short implants in different positions: A finite elements study

The aim of this study was to analyze whether the use of inclined short implants without lower transcortical involvement (test model - SI), thus preserving the mandibular lower cortical bone, could optimize stress distribution.

Six identical atrophic mandible models were created featuring 8mm of height at the symphysis. Two study factors were evaluated: implant length and angulation. Implant length was represented either by short implants (7mm) with preservation of the mandibular lower cortical bone or standard implants (9mm) with a bicortical approach and 3 possible implant positioning configurations: 4 distally-inclined implants at 45° (experimental model), all-on-four, 4 vertical implants. All tridimensional (3D) models were analyzed using the Finite Element Method (FEM) and the Ansys Workbench software.

The maximum stress on the bone at the cervical region of the implants in the experimental model was 132MPa and transcortical involvement with implant inclination yielded higher values (171MPa). Regarding von Mises stress on the retaining screw of the prosthesis, 61MPa was recorded for the experimental model while upright implants had the highest values (223MPa). At the acrylic base, 4MPa was recorded for the experimental model whereas models with upright implants showed the highest stress values (11MPa).

Rehabilitation of severely resorbed mandibles with 4 short implants placed distally at 45°, without lower transcortical involvement, were biomechanically more favorable, generating lower stress peaks, than the models with short implants on an all-on-four, or on an upright configuration, with or without lower transcortical involvement.