Augmentation of DMLS Biomimetic Dental Implants with Weight-Bearing Strut to Balance of Biologic and Mechanical Demands: From Bench to Animal

Abstract

A mismatch of elastic modulus values could result in undesirable bone resorption around the dental implant. The objective of this study was to optimize direct metal laser sintering (DMLS)-manufactured Ti₆Al₄V dental implants' design, minimize elastic mismatch, allow for maximal bone ingrowth, and improve long-term fixation of the implant. In this study, DMLS dental implants with different morphological characteristics were fabricated. Three-point bending, torsional, and stability tests were performed to compare the mechanical properties of different designs. Improvement of the weaker design was attempted by augmentation with a longitudinal 3D-printed strut. The osseointegrative properties were evaluated. The results showed that the increase in porosity decreased the mechanical properties, while augmentation with a longitudinal weight-bearing strut can improve mechanical strength. Maximal alkaline phosphatase gene expression of MG63 cells attained on 60% porosity Ti₆Al₄V discs. In vivo experiments showed good incorporation of bone into the porous scaffolds of the DMLS dental implant, resulting in a higher pull-out strength. In summary, we introduced a new design concept by augmenting the implant with a longitudinal weight-bearing strut to achieve the ideal combination of high strength and low elastic modulus; our results showed that there is a chance to reach the balance of both biologic and mechanical demands.

Keywords: 3D-printing; Ti6Al4V; biomimetic; direct metal laser sintering; porous titanium.

Figure 1

Gross morphology of the biomimetic direct metal laser sintering (DMLS) Ti₆Al₄V dental implants. (A) External characterization: the computer-assisted-design (CAD) of the external configurations of the seven different dental implants. (B) Scanning electron microscopy (SEM) images. SEM images showed no inter-layer difference when probed on the external portion of the implants. Although the surfaces of the implants were very rough, the struts were well formed and continuous. Bar = 100 µm. (C) Biomechanical parameters (3-point bending test) of the biomimetic Ti₆Al₄V dental implants. The non-porous design had superior biomechanical profiles compared to the porous designs. (n = 7 for each design. * indicates significant differences when compared to #6 dental implant; ** indicates significant differences when compared to #4 dental implant).

Figure 2

(A) Torsional and (B) stability tests of the biomimetic DMLS Ti₆Al₄V dental implants (n = 10). During torsional testing, the highest mean maximal torque value was obtained for the non-porous design. Throughout the entirety of the torsional experiments, type #4 dental implants (non-porous) did not break. During stability testing, the mean screw-in or screw-out torque was significantly higher for the non-porous design, while no significant differences existed in the pull-out strength among the three designs tested. (* indicates significant differences when compared to the #6 dental implant; ** indicates significant differences when compared to the #4 dental implant).

Figure 3

Morphological characteristics and biomechanical parameters of the augmented biomimetic DMLS Ti₆Al₄V dental implants (n = 10 for each design). The weaker type #6 dental implant (300–500 µm, 55% porosity) was augmented with three different designs of longitudinal printed strut to optimize its biomechanical characteristics. The resulting augmented dental implants (type #6-A, #6-B, and #6-C) all exhibited superior biomechanical profiles compared to the original design (#6). Approximately a 2.3 times increase in both peak load and maximal stress were observed for type #6-C implant. (* indicates significant differences when compared to the #6 dental implant; ** indicates significant differences when compared to the #4 dental implant).

Figure 4

MG63 cells differentiation on DMLS Ti₆Al₄V discs: in vitro biocompatibility analysis. (A) MG63 cells attached, proliferated and differentiated well on both non-porous and porous DMLS Ti₆Al₄V discs. The MG63 cell clusters attached on the surface of discs and grew into the pores. Representative results of 10 independent experiments are shown. (Length of scale bar: as described) (B) Comparisons of alkaline phosphatase (ALP) mRNA gene expressions or (C) relative ALP activity after 14 days of culturing MG63 cells in control medium (non-differentiation), osteogenic (differentiation) medium, or on Ti₆Al₄V discs of various (0, 20, 40, 60 or 80%) porosities (n = 10). The ALP gene expression was significantly up-regulated in the titanium samples, which attained maximal expression at 60% porosity.

Figure 5

Biocompatibility and biomechanical parameters of the biomimetic DMLS Ti₆Al₄V dental implants in animal experiments (n = 6 in each group). (A) Micro-computed tomography analysis showed good incorporation of bone onto the threads of type #4 non-porous dental implant and into the porous scaffold structures of the type #6 dental implant. (B) Mechanical tests showed significantly higher pull-out strength (2.4 folds) for the type #6 porous dental implants at both 6 and 12 weeks. (** indicates significant differences when compared to #4 non-porous dental implant).