Finite element analysis of the angle range in trans-inferior alveolar nerve implantation at the mandibular second molar

Abstract

Background: Trans- inferior alveolar nerve (IAN) implantation technique was wildly used while the potential appropriate angle range in which the residual alveolar bone can bear the stress without absorption are currently unclear. This study aimed to evaluate the stress distribution pattern of the interface between bone and implant by finite element analysis (FEA) to determine the appropriate range of the implant tilt angle.

Methods: Cone beam computed tomography (CBCT) images of 120 patients with missing mandibular second molars and vertical bone height < 9 mm in the edentulous area were selected. The distances from the mandibular nerve canal to the buccal cortex, the lingual cortex and the alveolar ridge crest were measured by using a combination of software. The angular ranges of the buccal-lingual inclination of simulated trans-IAN implants were measured and three-dimensional finite element models were constructed in the mandibular second molar area according to the differences of the inclination angles. A vertical load (200N) was then applied to analyze the biomechanical conditions of the implant-bone interface during median occlusion.

Results: The distance at the second molar from the nerve canal to the buccal cortex, lingual cortex and alveolar crest were 6.861 ± 1.194 mm, 2.843 ± 0.933 mm and 7.944 ± 0.77 mm. Trans-IAN implantation was feasible in 73.33% of patients. The minimum angle and maximum angles of the buccal-lingual inclination of the simulated implant were 19.135 ± 6.721° and 39.282 ± 6.581°. When a vertical static load of 200N was applied, the tensile stress in cortical bone gradually increased with the increase of the implant tilt angle. When the inclination angle reached 30°, the tensile stress (105.9 MPa) exceeded the yield strength (104 MPa) of cortical bone. Compared with the conventional implants, the stress peak value of the vertical ultra-short implant in cortical bone was greater than the stress peak value of the conventional implants at 10°(79.81 MPa) and 20°(82.83 MPa) and was smaller than the stress of the implant at 30°(105.9 MPa) and 40°(107.8 MPa). Therefore, when the bone mass allows, conventional-length implants should be selected whenever possible, and an operative range of the trans-IAN implantation in the mandibular second molar could be retained with an inclination angle of < 30°.

Conclusions: The mandibular nerve canal at the mandibular second molar was obviously biased to the lingual side, which ensured sufficient bone mass at the buccal side. In most patients with severe mandibular atrophy, it was possible to maintain a safe distance from the nerve canal with conventional-length implants via the trans-IAN implantation technique.

Keywords: Finite element analysis; Implant tilt angle; Mandibular atrophy; Mandibular second molar; Trans- inferior alveolar nerve implants.

Fig. 1

Cone beam computed tomography. A Measurement of the distance from the mandibular nerve canal (red circle) to the buccal cortex (blue arrow), the lingual cortex (green arrow) and the alveolar crest (yellow arrow). B and C Measurement of the minimum (B) and maximum (C) angles of buccal-lingual inclination of the virtual implant

Fig. 2

Model for the finite element analysis. A The right lower posterior dental bone mass comprising the mandibular first molar. B The mandible with cortical bone thickness of approximately 2 mm trimmed by multi-software to include only the mandibular second molar. C The two implant types evaluated in this study, left: ultra-short implant (4.0 × 6-mm), right: conventional implant (4.1 × 10-mm). D A portion of the second molar region was excised as the final mandibular model and the implant and prosthetic crown were installed

Fig. 3

The nephogram (A) and graph (B) of the von Mises stresses for all configurations under vertical loading. A The von Mises stress concentrations on implant components in all configurations. The conventional implants with inclination angles of 0°, 10°, 20°, 30° or 40° respectively from left to right and top to bottom. B Stress curve of the implants under different inclination angles

Fig. 4

Left: Magnified view of tensile (maximum principal) stress of peri-implant cortical bone for short implant axial implantation and conventional implant tilt 10° 20° 30° 40°implantation from top to bottom; Right: Magnified view of compressive (minimum principal) stress of peri-implant cortical bone for short implant axial implantation and conventional implant tilt 10° 20° 30° 40° implantation from top to bottom

Fig. 5

Maximum principal stress and minimum principal stress for all configurations under vertical loading