Additively manufactured spinal implants mimic the mechanical properties of bone

Medical device production is a prevalent industry in Ireland and advanced manufacturing research company, Irish Manufacturing Research (IMR), is working with other organisations to investigate what can be achieved with 3D printed medical devices.

IMR designed and manufactured a series of representative spinal implants for the project using software from advanced manufacturing software company nTopology and then produced them on Renishaw’s RenAM 500M metal AM system.

nTopology is a generative design company based in New York and has developed a next generation design engineering software for advanced manufacturing. This software platform enables the creation of complex, performance-driven designs that exploit the capabilities of additive manufacturing (AM). These advanced designs can be created in minutes instead of hours or days. nTopology’s software enables engineering workflows, manufacturing processes and knowledge to be captured within the software to enable users to create custom workflows to meet their exact requirements.

Spinal implants are needed to restore intervertebral height in patients with a range of medical conditions including degenerative disc disease, herniated disc, spondylolisthesis, spinal stenosis and osteoporosis. Conventional manufacturing techniques are unable to produce spinal implants with a lattice structure, which offers a high surface area to encourage migration of osteoblasts into the implant and the ability to optimise the mechanical properties of a porous volume to meet the required loading conditions.

IMR identified AM as a suitable method to manufacture lattice structures optimised for osseointegration. However, the most suitable design software and AM machines had to be identified.

“The capabilities of AM hardware have developed fast and are now outpacing traditional design tool capabilities,” explains Duann Scott, nTopology’s director of business development and partnerships. “To allow a smooth AM workflow from design to manufacture, hardware and design software need to communicate effectively. Easy translation from design software to an AM machine is especially important when producing spinal implants, because intermediary stages and information transfer provide opportunities for errors and inconsistencies to occur.”

Renishaw, IMR and nTopology joined forces to produce spinal implants that incorporate lattice structures using AM. First, IMR created a design envelope to identify the unique opportunities presented by AM to improve patient outcomes. nTopology then provided the software needed to design the complex geometry of the spinal implants and Renishaw’s RenAM 500M machine was used to manufacture the implants using AM.

IMR conducted extensive research to identify the appropriate dimensions for the specified use case and the loading conditions the implants must withstand in day-to-day life and in extreme situations such as running or jumping. This data was combined with the known material properties of bone in patients with diseases known to create the need for a spinal implant. The three companies then worked together to design the mechanical characteristics of the device, which are largely a function of the geometry of the unit cells used in the lattice structure, to achieve mechanical properties closer to those of human bone and to optimise the porous lattice for osseointegration.

Once the design parameters for the implants were decided, IMR produced the design files using nTop Platform. nTopology and Renishaw worked together closely to ensure their products were compatible, so that a design could be seamlessly translated from the nTop Platform to the RenAM 500M.

Prototypes were then produced from grade 23 titanium (Ti 6Al-4V ELI) and IMR performed a series of tests to demonstrate that the device met the most relevant elements of the standard specifications required for FDA approval. Chemical properties were tested to ensure they met ASTM F136 and ASTM F3302, the standard specification for wrought grade 23 titanium to be used in orthopaedic implants and the standard specification for the AM of titanium alloys by powder bed fusion respectively. The mechanical properties of the porous structure were characterised per ISO 13314, a test method used to determine the compressive properties and failure mode of a porous metallic material. Finally, testing ensured compliance with ASTM 1104 and ASTM 1147, standard test methods to demonstrate that porous structures do not delaminate from the solid faces.

“To demonstrate proof of concept, we built witness coupons on the RenAM 500M build plate and performed destructive testing of the coupons,” explains Sean McConnell, senior research engineer at IMR. “We did this to ascertain the chemical, metallurgical, and mechanical properties of the implants.”

This proof of concept study demonstrated that AM can be used to produce spinal implants with characteristics that are not possible to achieve with conventional manufacturing methods. The RenAM 500M was used for manufacturing both the prototypes and the final implants, so the procedure did not have to be translated for different machines. This streamlined workflow will result in considerable cost and time savings.

“Two years ago, AM in IMR was non-existent,” explained McConnell. “Thanks to all the support we have received from Renishaw developing spinal implants as well as working on other projects, we have upskilled our staff and are now well established in the AM space.

“Renishaw worked tirelessly with us on improving the AM process for producing the spinal implants,” continues McConnell. “Together, we designed a set of experiments that yield the most appropriate parameter settings for the product. As a result, we reduced the amount of post-processing required on key features by a factor of ten.”

As well as demonstrating the ability of AM to produce spinal implants, this study shows that the entire workflow from design to AM can be rapid and smooth, with the need for just two technologies — design software and AM hardware.

“The spinal implants project allowed us to develop our understanding of medical device production and the capabilities of AM machines,” explains Scott. “This enabled us to develop our design software so that it can be used to drive the use of advanced manufacturing technology for medical devices.

“There was a considerable amount of trial and error involved in adapting the software to align with the requirements of the RenAM 500M,” adds Scott. “Projects like this usually take years, but the collaboration between nTopology, Renishaw and IMR meant that we could complete the study in a matter of months.”