Biomechanical Validation of Additively Manufactured Polymeric Femora

INTRODUCTION: Fourth generation composite bone models (Sawbones®) are biomechanically validated for testing loading conditions on normal human bone. However, for patients with altered anatomy, such as primary or metastatic bone neoplasms, accurate biomechanical models are unavailable. Rapid prototyping technologies, including additive manufacturing, can offer a solution if properly harnessed. The aim of this study was to develop and validate a 3-dimensional (3D) printed model of a normal human femur using polylactic acid (PLA) that can be adapted to variant anatomy. METHODS: A literature review was conducted to identify the material properties of human femur bone. Standard dog-bone shaped tensile and compressive testing specimens were created with consistent printing parameters to evaluate the anisotropic mechanical properties of 3D printed PLA. After reviewing the differences in material properties, it was determined that incorporating geometric variation in the models was necessary to accurately replicate the biomechanical behavior of bone. A digital model of a normal human femur was obtained from Sawbones and prepared for 3D printing with wall thicknesses ranging from 0.4mm to 2.4mm and an infill density of 20%. A total of 12 models were printed, and standard three-point bending tests were performed, with measurements recorded to evaluate the mechanical response. The data obtained from the mechanical testing was analyzed to determine the starting point for future print parameter modification. RESULTS: According to the literature review, values for material properties of the anatomic human femur are commonly reported as: flexural modulus of 11-20 gigapascals (GPa), flexural strength of 140-220 megapascals (MPa), ultimate tensile strength of 130 MPa, and ultimate compressive strength of 205 MPa. Testing of the dog-bone specimens yielded the following material properties: elastic modulus of 2.10 ± 0.03 GPa, ultimate tensile strength of 52.65 ± 1.90 MPa, and ultimate compressive strength of 57.70 ± 3.23 MPa. Testing of the 12 printed models produced a flexural modulus ranging from 7.46 to 18.35 GPa and a flexural strength of 342.4 to 832.7 MPa. CONCLUSIONS: Our working hypothesis was that an optimized PLA-printed femoral diaphysis could model the biomechanical properties seen in actual human bone. The flexural modulus of the printed models replicated values reported in the literature for anatomic human femora, and the flexural strength above that seen in bone can be modulated down with geometric adjustments. These results serve as proof of principle that a 3D printed femur is a viable option for biomechanical studies. However, given the difference in material properties between human bone and PLA, geometric modifications such as changes to simulated cortical thickness and infill density are necessary to achieve model validity. Moving forward, further fine-tuning of the print parameters will lead to an optimized biomechanical response. The models will then be validated against Sawbones models. The full scope of this project includes adapting the geometry to patients with altered anatomy. Additional research will further expose the potential of 3D printing technology in both clinical and educational settings, such as preoperative planning, intraoperative reference, and the facilitation of resident and student education in orthopedics.