Biomechanical evaluation of individual 3D-printed vertebrae

Background Personalized 3D-printed bone models are becoming increasingly popular in clinical care. Common applications include the visualization of idiopathic deformities or complex joint fractures. Functionalizing such printed replicas in terms of individual mechanical properties holds great potential for clinical training and research but is challenging due to the complexity of the bone structure. This study aims at developing a parametrizable structure as a substitute for spongious bone in order by simplifying 3D reconstruction and printing. Methods 43 vertebrae from 6 body donors aged 86.8 ± 7.8 years were examined. Each spine underwent a clinical computed tomography scan. Cylindrical samples (Ø6 x 12 mm) were randomly taken from the left or right side of the vertebral body using a core drill in the superior-inferior direction. Specific software was used for determining the volumetric Hounsfield-Units of the spongious bone in each vertebral hemisphere. In parallel, a parametric hexagonal grid structure was designed using engineering software. All rods within the lattice have a variable length L and a fixed diameter of t = 0.4 mm. By varying the ratio t/L, six different porosities were defined. For each of these, five cylindrical lattice samples (diameter/length = 1/2) from two different synthetic resins were manufactured using the stereolithography printing process. All samples were mechanically characterized by uniaxial compressive testing. Curve fitting based on power functions (y = ax ^b ) allowed the determination of correlations between mechanical parameters and Hounsfield-Units (bone) as well as the lattice parameter t/L (3D-printed lattice). Finally, three vertebrae with varying bone quality were printed with their respected parameterized lattice and evaluated by comparing the axial screw pullout forces of the human and the respective printed bones. Results There is a significant correlation between the mechanical properties of the bone specimens and the determined Hounsfield-Units. Furthermore, the mechanical properties of the lattice can be excellently described by the ratio t/L. The printed vertebrae showed pull-out forces similar to those of osteoporotic bone. Conlusion The mechanical behavior of vertebral human spongious bone can be well reproduced by a 3D-printed generic lattice structure. Patient-specific bone models can be generated by integrating the parameterizable lattice structure into the specific bone contours. These models can help in improving patient care, for instance by enabling highly realistic surgical approaches for particularly complex anatomies.