Development and Mechanical Characterization of Polycaprolactone-Hydroxyapatite Spine Cages for Transforaminal Lumbar Interbody Fusion.

Introduction: Degenerative lumbar spondylolisthesis is a pathological condition characterized by disc degeneration, facet joint arthritis, and disruption in the lumbar spine's stabilizing structures, for which surgical intervention becomes required when conservative therapeutic measures prove ineffective. In such cases, a transforaminal lumbar interbody fusion (TLIF) is performed with Polyetheretherketone (PEEK) and titanium spine cages. However, these materials present complications from elastic modulus mismatch with bone and a lack of osteointegration, respectively [2]. Furthermore, the universal spine cage’s lack of patient specificity has resulted in cage subsidence in up to 35% of cases [3]. This study focuses on a Polycaprolactone-Hydroxyapatite (PCL-HA) blend bio-ink, which has been reported to promote osteogenesis, coupled with novel imaging techniques, to develop patient-specific spine cages with optimal mechanical properties.

Materials and

Methods: Spinal Cage

Design: Spine cages with varying infill (Figure 1) were printed under established protocols [5] with a PCL:HA ratio of 80:20 to ensure slow degradation rates in spinal fusion [4] and for optimal mechanical properties [5]. Mechanical Testing: Spine cage designs were subjected to mechanical testing to determine compressive loading properties. Samples were tested on a servo-hydraulic mechanical test frame (Instron 8874, Instron) with a 25kN load cell. Samples (n=10) were tested in unconfined compression, with custom aluminum platens connected to a universal joint to ensure normal application of forces. Samples were loaded at a rate of 1 mm/min until the onset of plastic deformation to determine stiffness (N/mm; slope of the linear region of the force-displacement curve) and yield force (N; force at 0.2% offset strain relative to the linear region). Cuboids of known geometry (n=12) were tested to determine the elastic modulus (MPa; slope of the linear region of the stress-strain curve) of the PCL-HA material. All mechanical properties were calculated using custom MATLAB scripts (MATLAB 2022a, MATLAB). Results for stiffness and yield force were evaluated using one-way ANOVA (GraphPad Prism 10, GraphPad) to determine the effect of scaffold design on mechanical properties. Patient-specific Modeling: DICOM medical imaging files of the target vertebrae were converted to STL files with open-source splicing software. The cage surfaces were modeled in Onshape to match the endplates of the patient's vertebrae, and the finalized design was bio-printed with the bio-ink proposed above.

Results, Conclusions, and Discussions:

Results: The 3 mm grid spinal cage presented the highest stiffness (4420 +/- 186.9 N/mm) compared to the 4 mm grid (3514 +/- 538.9 N/mm, p < 0.0001) and the 4 mm ZigZag (3149 +/- 222.9 N/mm, p < 0.0001) (Figure 2A). Similarly, the 3 mm grid resulted in the highest yield force (2672 +/- 164.2 N) compared to the 4mm-grid (1885 +/- 386.1 N, p < 0.0001) and the 4mm-ZigZag (1681 +/- 94.52 N, p < 0.0001) (Figure 2B). No significant differences were observed between the 4 mm grid and 4 mm ZigZag designs for stiffness or yield force. The modulus of the printed PCL-HA material was determined to be 255.9 +/- 34.35 MPa. The endplates of the target lumbar vertebrae were successfully modeled and printed onto the spine cages (Figure 3).

Discussion: The average yield force of the 3 mm grid is more than 1000 N greater than the forces used to model in vivo loading of the anterior column [3], and its stiffness value is closest to optimized TLIF cages [3]. Additionally, the modulus of PCL-HA printed material falls within the reported range of human trabecular bone (120-450 MPa) [3], making it a more optimal scaffold. However, the strength of the 3 mm grid design is likely due more to the fact that this design had a higher infill percentage and less due to the overall impact of the geometrical pattern. Furthermore, in the body, the scaffold undergoes point loading due to the flat surfaces on the top and bottom of the scaffold, increasing the likelihood of cage subsidence. Custom-fit surfaces on the spine cage minimize this source of failure while retaining loading characteristics.

Conclusion: This spine cage is an improvement on the current TLIF cage design. The osteogenic properties of hydroxyapatite allow for fully synthetic cages that forgo the need for bone grafts. Furthermore, due to the custom-fit nature of the design, the proposed cage design limits the risk of cage subsidence. Further work will aim to vascularize these spine cages.

Acknowledgements (Optional):

References:
1) Mobbs RJ et al. J Spine Surg. 2015.
2) Adl Amini et al. J Spine Surg. 2022
3) Fang Xie, et al. (2022). BMC Musculoskelet Disord, 2022.
4) Kang, H, et al. J Biomech Eng, 2013.
5) Campos T, et al. ASBMR 2023, Vancouver, Canada.
6) Morgan, E. F., et al. Annu Rev Biomed Eng. 2018