3D printed plates based on generative design biomechanically outperform manual digital fitting and conventional systems printed in photopolymers in bridging mandibular bone defects of critical size in dogs

Abstract

Conventional plate osteosynthesis of critical-sized bone defects in canine mandibles can fail to restore former functionality and stability due to adaption limits. Three-dimensional (3D) printed patient-specific implants are becoming increasingly popular as these can be customized to avoid critical structures, achieve perfect alignment to individual bone contours, and may provide better stability. Using a 3D surface model for the mandible, four plate designs were created and evaluated for their properties to stabilize a defined 30 mm critical-size bone defect. Design-1 was manually designed, and further shape optimized using Autodesk ^® Fusion 360 (ADF360) and finite element analysis (FE) to generate Design-2. Design-4 was created with the generative design (GD) function from ADF360 using preplaced screw terminals and loading conditions as boundaries. A 12-hole reconstruction titanium locking plate (LP) (2.4/3.0 mm) was also tested, which was scanned, converted to a STL file and 3D printed (Design-3). Each design was 3D printed from a photopolymer resin (VPW) and a photopolymer resin in combination with a thermoplastic elastomer (VPWT) and loaded in cantilever bending using a customized servo-hydraulic mechanical testing system; n = 5 repetitions each. No material defects pre- or post-failure testing were found in the printed mandibles and screws. Plate fractures were most often observed in similar locations, depending on the design. Design-4 has 2.8-3.6 times ultimate strength compared to other plates, even though only 40% more volume was used. Maximum load capacities did not differ significantly from those of the other three designs. All plate types, except D3, were 35% stronger when made of VPW, compared to VPWT. VPWT D3 plates were only 6% stronger. Generative design is faster and easier to handle than optimizing manually designed plates using FE to create customized implants with maximum load-bearing capacity and minimum material requirements. Although guidelines for selecting appropriate outcomes and subsequent refinements to the optimized design are still needed, this may represent a straightforward approach to implementing additive manufacturing in individualized surgical care. The aim of this work is to analyze different design techniques, which can later be used for the development of implants made of biocompatible materials.

Keywords: additive manufacturing; autodesk fusion 360; biomechanical evaluation; canine; critical size; customized endoprosthesis; jaw; osteosynthesis.

Figure 1

Workflow showing the process of printing a 3D model of a right canine mandible with a simulated 30mm defect and the development and printing of four different plate designs.

Figure 2

(A) Section analysis of D4 to show the thickness of the midsection of the plate. (B) Cross section of D4.

Figure 3

Servo-hydraulic testing system. (A) Custom 3D printed guide for correct positioning of the mandible. (B) Photograph of a fixated mandible-plate-construct in cantilever bending. (C) Illustration of cantilever bending of a mandible fixated at the ramus and canine teeth in the testing system with applied force perpendicular to the body of the mandible.

Figure 4

Force-displacement curves for plate designs D1–4 (A–D) comparing rigid (VPW) and combination of rigid + rubbery (VPW+Tango) 3D printing materials. Yield points (circles) are shown in respective axis cutouts of start point linear variable interrelation; peak force/failure points are displayed as rhomb. mP_p, mean peak force point; mP_y, mean point of yield.

Figure 5

Mechanical properties of plate designs D1–4 comparing rigid (VPW) and combination of rigid + rubbery (VPW+Tango) 3D printing materials. (A) Failure modes representative for different plate designs. Note bending deformation in D1 being different to breakage in other designs. (B) Displacement at maximum load, (C) displacement at yield point (YP), and (D) force load at YP; ordinary two-way ANOVA, Tukey's multiple comparisons. (E) Maximum force load at point of failure; two-tailed Mann-Whitney-U test (VPW D1–3 vs 4, VPW D4 vs VPW+T D4) and ordinary two-way ANOVA with Tukey's multiple comparisons for all other variable combinations. (F, G) Material stiffness prior and after YP; ordinary two-way ANOVA, Tukey's multiple comparisons. (B–G) n = 5 replicates/design and material; bars represent mean, error bars represent ± SD.