61 - 3D-Printed Biomimetic Cervical Vertebral Body for Spinal Arthroplasty Testing

Introduction: Cervical disc arthroplasty is a promising, motion-sparing, alternative to fusion surgeries. However, disc replacements have failed to gain widespread acceptance. Preclinical studies have not been successful in predicting performance of these devices and biomechanical studies have yet to replicate clinical failure mechanisms in vitro. The paucity of cervical spine biomechanical testing may be due to the lack of biofidelic cervical vertebral body models. Composite models have become commonplace in the preclinical assessment of joint replacements; however, there are no comparable models for the cervical spine. The goal of this study was to create a tunable, customizable model of a cervical vertebra to assess cervical device performance.

Methods: Using additive manufacturing software, rectangular prints with three lattice structures, gyroid, triangle, and zig-zag, and a range of in-fill densities, 5% to 100%, were 3D-printed (Figure 1). Each specimen was tested for coronal compressive and shear strength using a custom apparatus on a servo-hydraulic load (Figure 2). One print pattern was selected based on similarity to cervical cancellous bone and strength was evaluated in the sagittal plane. The compressive and shear strengths, along with the elastic modulus, was calculated for all combinations in the coronal and sagittal planes. Eleven vertebral regions of unique density, based on CT scan densities, were converted to strength using reference equations for cervical strength, identified through previous literature. Each bone density was converted to strength and subsequently correlated to the lattice structure and in-fill density with the desired material properties. Finally, a complete cervical vertebra model was 3D-printed.

Results: Materials testing identified a linear relationship between in-fill densities and strength for all lattice structures (Figure 3). The axial compressive strength of the gyroid specimens ranged from 1.5MPa at 10% infill to 31.3MPa at 100% infill and the triangle structure ranged from 2.7MPa at 10% infill to 58.4MPa at 100% infill. The gyroid specimens were tested in the coronal plane, as well, and compressive strength was found to be about 20% less than the compressive strength in the axial plane. Triangle and zig-zag specimens buckled at failure while gyroid specimens were crushed, similar to the failure pattern observed in cancellous bone. Based on these results, a cervical vertebra model was made with eleven different in-fill densities ranging from 33% gyroid to 84% triangle (Figure 4).

Discussion: This study shows the successful use of additive manufacturing techniques and materials to recreate material properties of cancellous bone in the vertebral body. As the cervical spine has varying compressive strengths from the lateral edges to the central portions and around the pedicles, a completely customizable model with varying strengths may be more clinically relevant for biomechanical testing. The iterations from the current study are necessary to ensure a close match to the cancellous bone material. The findings from the present study introduced a framework for using additive manufacturing to create a tunable, customizable biomimetic model of a cervical vertebra. This model may provide a useful tool for a more comprehensive and efficient approach to the preclinical evaluation of cervical disc replacements.