Design and 3D Printing of Novel Titanium Spine Rods with Lower Flexural Modulus & Stiffness Profile with Optimised Imaging Compatibility

Introduction: Many of the cancer patients(~10%) develop metastatic spine disease(MSD) at some point of their lifetime. Titanium(Ti6Al4V) is currently the ‘gold standard’ material for spine implants. However, titanium is still not the optimal implant material for use in metastatic spine tumour surgery(MSTS) due to various considerations. Standard solid titanium rods(SSR) have high flexural modulus(110GPa)/stiffness that can lead to stress-shielding and eventually result in delayed implant loosening/construct failure at implant-bone interface. Titanium implants, even though have improved compatibility with CT/MR imaging compared to stainless steel, still generates artefacts, hindering early detection of tumour recurrence and postoperative complications. Artefacts generated by SSR interfere with RT planning/tissue segmentation and dose delivery. New/emerging implant material such as polyether-ether-ketone(PEEK) and carbon-fiber reinforced-PEEK(CFR-PEEK) have been explored for use in MSTS due to their lower modulus of elasticity(~17 GPa). These implants exhibit improved imaging and RT-compatibility. However, rods manufactured using PEEK-based materials are not contourable and are manufactured with pre-determined curvatures and lengths, limiting their operative applications.

Aim: To manufacture and test novel design titanium spine rods with lower flexural modulus/stiffness vs SSR using 3D printing technology. Additionally, we also aimed to reduce the generation of artefacts during CT and MR imaging observed in SSR.

Methods: Novel spine rod designs with/without internal cellular structures were created using nTopology software(nTopology, Inc. USA). Dimensions of rods included diameter-5.5mm, shell thickness-0.5mm and length-100mm. Rods were designed based finite element analysis(FEA) results. The rods were then 3D printed using selective laser melting(SLM) method. Three-point bending test was performed to assess mechanical performance of 3D printed rods, while a French bender was used to assess intraoperative rod contourability and haptic feedback was documented. Furthermore, 3D printed spine rods were tested for compatibility and artefact generation(qualitatively assessment) utilising CT & MR imaging.

Results: Different spine rod designs generated includes shell, voronoi, gyroid, diamond, weaire-phelan, kelvin, and star. Three-point bending tests showed 3D printed rods had lower flexural modulus ranging from 2-25% compared to SSR. Shell rods exhibited the highest in flexural modulus of 25%(~77.4GPa) and star rod exhibited lowest reduction in flexural modulus of 2%(100.8GPa). Gyroid rod exhibited an increase in flexural modulus by 8%(~111.8GPa). 3D printed rod shoed reducintion in stiffness raging from 40-59%. Shell rod displayed highest reduction in stiffness of 59% (179.9N/mm) and gyroid had least reduction in stiffness of 40%(~259.2N/mm). Haptic feedback rod bending test showed that shell, voronoi, and diamond rods demonstrated lesser difficulty in bending compared to other rods, however, gyroid exhibited bending difficulty similar to SSR. All 3D printed rods demonstrated improved CT and MR imaging compatibility with reduced artefacts vs SSR.

Conclusion: By utilising novel design approach, we succesfully genrated a spine rod design portfolio with lower flexural modulus/stiffness profile and CT/MR imaging properites was established for potential use in MSTS/other conditions such as osteoporosis. Thus, exploration of new rod designs in surgical application could enhance the treatment outcome and improve quality of life for the patients.