Poster AA7 - 3D Printed Femoral Diaphysis for Biomechanical Testing

Introduction: The human femur is a perennial focal point of biomechanical studies. The historical gold standard for such studies has been human cadaveric specimens. Cadavers require special handling and preservation techniques, are extremely costly to purchase and ship, and invoke ethical, religious, and cultural controversy. Furthermore, substantial inter-specimen variability as well as over-representation of elderly cohorts challenges the scientific validity of their continued use for biomechanical analyses. Mechanically realistic synthetic femur models have emerged as a widely accepted surrogate for biomechanical testing of the femur since 1987, with current fourth-generation models demonstrating reproducible and realistic mechanical properties. These models cost almost a third of their cadaver counterparts, but price does remain a prohibitive factor for many investigators. While these models are available in various sizes and densities (i.e. osteoporotic models), they cannot model more advanced pathologies, such as bone tumors and post-traumatic or congenital deformities. Recent advancements in three-dimensional (3D) printing technologies have enabled the rapid creation of customized synthetic femurs with specific geometric and material properties that can be used in biomechanical testing. The current research aims to develop and validate a biomechanical 3D printed femoral diaphysis model created using an inexpensive FDM printer with poly-lactic acid (PLA) as an affordable and readily available substrate. In this study, the print variables of interest are wall layer number and infill density. Our primary hypotheses are that the PLA-derived 3D-printed femoral diaphysis specimens subjected to three-point bending can emulate the flexural modulus and flexural strength of a normal human femur.

Materials and

Methods: PLA was printed on an enthusiast-level fused deposition modeling printer. A virtual composite femur model for biomechanical testing was purchased. Layer height (0.2mm) and infill pattern (Gyroid) were held constant. A 200mm central diaphyseal segment was printed in batches with initial infill of 100% with 6 external layers. An iterative process of 3-point bending specimens in consistent orientation to failure and adjusting infill percentage and external layer number was performed. Once target flexural modulus and flexural strength were achieved, 10 copies were printed at the respective settings for additional 3-point bending testing for validation. Each specimen was loaded until fracture. Flexural modulus was measured as the slope of each flexural stress-strain curve in the elastic region. The flexural strength was selected at the curve’s yield point using the 0.2% strain offset method. All results were pooled and normal data distribution was assumed. Each data point was compared to the target values for the normal human femur using a Student T-test with significance set at p-value < 0.05. Linear regression analysis was performed to observe the relation between printing parameters and flexural properties.

Results, Conclusions, and Discussions:

Results:
The 10 3D-printed PLA diaphyseal femur specimens with 5% infill density with 2 wall layers with a resolution of 200μm depicted accurate flexural strength of 184.8 MPa ±8.18 MPa which did not statistically differ from historical values of human femur (p = 0.856). Similarly, 10 PLA diaphyseal femurs with 20% infill density and 6 wall layers depicted a flexural modulus of 18.54 GPa ±0.543 GPa, which also did not statistically differ from historical values of human femur (p = 0.934, Table 1).

DISCUSSION AND

Conclusion:
This study evaluated the biomechanical response of 3D-printed femoral diaphysis models using PLA to 3-point bending. With 6 external layers and 20% infill, the flexural modulus was well within the established range of normal human femurs. With 2 external layers at 5% infill, the flexural strength was similarly representative of historical data for human femurs. In both cases, the values obtained were consistent and reproducible, as evidenced by their respective low standard deviations across specimens. To the best of our knowledge, this is the first study of its kind to establish acceptable printing parameters for a cheap thermoplastic for the purposes of biomechanical testing of femoral diaphyseal bone. Continued investigation has potential to expand access of biomechanical bone models in a cost-effective manner and allow patient-specific pre-operative surgical construct analysis. Future studies will focus on validating these results across multiple printer and filament sources, validating metaphyseal/cancellous bone, expanding testing conditions to torsion and compression, and ultimately modeling patient-specific, pathologically altered bone.

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