Challenges of Developing 3D Printed Trabecular Implants

Ti-6Al-4V trabecular implants including lattice structures to enhance mechanical and biological properties can be made to show similar mechanical responses to human bone and have been used with success for more than 10 years. However, there are some limitations and challenges in the designing, product development, and production stages of these high-tech medical goods.   Given my work at Btech, a manufacturer and reseller in the medical 3D printing space, I want to share our experiences and strategies to overcome in-house medical device production challenges.

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Figure 1. TraBCups belonging trabecular implant product tree of Btech called TraBtech 

Design

3D Printed Trabecular Implants
Figure 2. Spinal cage models with varying lattice strut thickness designed in nTop. 

One of the most formidable barriers while developing trabecular implants is the design stage.

First, a design software that does not support documentation and traceability of a new medical device with trabecular osseointegration lattice structures cause lower volume of production and limited amount of people who can benefit from the 3D printed orthopaedic devices(Cho, 2021). Designing complex features and lattices, developing workflows that ensure traceability and applying the trademarked lattices to a whole portfolio are major design challenges related to software for developing additive manufactured trabecular implants. 

Trabecular Implants
Figure 3. TraBCup model designed in Materialise 3-matic.

Additionally, the database containing detailed information about mechanical and biological properties of human bones (Wang et al., 2016) is not yet available. Designing accurate and reliable trabecular structures needs a database of properties of bones for different age and gender groups at different locations. 

Additionally, there is a need for topology optimization algorithms to deal with multiple length scales at the same time. It should be mentioned that constraints and limitations of current AM technologies, such as the critical angle of the overhanging structure and the difficulty in removing the supporting structure, should be involved in newly developed topology optimization algorithms so that the “optimized” designs could actually be fabricated by AM. 

Finally, the trabecular orthopaedic device production processes must involve orthopaedic surgeons, radiologists, engineers, and implant companies as a team. However, design software is created by engineers to use by engineers. There is a need for integrated platforms to produce multi-disciplined design algorithms for utilisation of additive manufacturing technology among all related branches such as 3-matic and nTopology (Figure 2,3).

Regulations

The lack of regulations and relevant laws of 3D printed medical devices is a potential risk for developing additively manufactured trabecular implants (Gao et al., 2018). 

Initially, there is a need for regulatory guidance on multiple subjects that include the following: 

  • Design control (design input, process, and output), 
  • Prior additive manufacturing stage, raw materials used in additive manufacturing stage, 
  • Post-processing stage to ensure stability, cleaning, machining, biocompatibility after sterilization at the post-additive manufacturing stage. 

Although FDA released the final version of guidance document “Technical Considerations for Additive Manufactured Medical Devices” to determine the exact rules in a proper and safe way at the end of 2017, the release of new European standards in early 2020 produced a lot of uncertainties and caused many companies to reduce their activities in Europe.

Cost

The software, hardware, skilled human resources, and printing materials lead to higher cost of additive manufacturing compared to traditional production processes(Javaid & Haleem, 2018). 

3D printed trabecular implants demand high cost in design, production, and post process stages.It is also well known that efficiency, which is related to capacity use, stacking, process parameter optimization and part orientation, is an important point to save costs. This requires technology and process knowledge. Otherwise, manufacturers need to deal with high failure rate, long computation time, and capacity loss due to unoptimized files and process parameters and lack of mechanical properties. All of these will add additional units to part costs.

Clinical Data 

3D Printed Trabecular Implants
Figure 4. SEM Images regarding cell adhesion and proliferation over lattice structures on implant

Long term clinical results of additive manufactured trabecular implants are not yet sufficient similar to any other new surgical technique(Wong & Scheinemann, 2018). 

Even though there are an increasing number of clinical trials and long-term follow ups, there is a common perception that medical additive manufacturing is limited with anatomical models used for planning surgery, and not for implantation]. Also due to a lack of systematic understanding, surgeons often focus more on immediate operational success of the implant than the need of clinical follow ups.    Long-term evaluations for trabecular titanium implants are not only needed to establish 3D implant technology, but also to increase the awareness among surgeons and enhance clinical outcomes. In addition, topologically optimized porous implants should be evaluated in vivo in the manners of biocompatibility and material performance.

Porous Structures

3D Printed Trabecular Implants
Figure 5. nTop Unit Lattice Cell Library

Trabecular metal porous structures minimize stress shielding and promote bone in-growth for implant fixation. (Stress shielding refers to the reduction in bone density as a result of removal of typical stress from the bone by an implant. This is because by Wolff’s law, bone in a healthy person or animal will remodel in response to the loads it is placed under.) However, there are some limitations to obtain this kind of structure in implant surfaces.

More cellular structures such as randomized meshes need to be created in order to simulate the bone structure taking design and production limitations into considerations (Figure 5,6)(Chen et al., 2020). 

3D Printed Trabecular Implants
Figure 6. Variety of 3D Printed Lattice Samples 

Also, the dynamic static boundaries of most porotic structures are not certain still, and analysis should be done in designs and prototypes. In addition, to choose the best performance, cellular structures that are manufactured by additive manufacturing should be put together and compared with different applications by using finite element methods and biomechanical tests including compression, torsion and fatigue. Additionally, lattice structures that have varied strut thickness and pore size called gradient structures need to be considered under static and dynamic analysis of the region where the trabecular titanium implant is taking place.

Finally, additive manufacturing limitations and accuracy problems need to be considered while designing porous structures.

Compressive and fatigue strength of the strongest cubic lattice (which is typically produced with EBM technology) are currently lower than human cortical tibia and femur bones(Sing et al., 2016). This might be an issue for the implementation regions that require strength. As mentioned above, gradient structures carry a huge role in solving this problem. 

Furthermore, the tensile property of porous structures has not been reported due to their rough strut surface, internal defects and incompletely decomposed α′-martensitic microstructure. (Due to the changing temperature, phase transformations called martensitic transformations happen in Ti-6Al-4V alloy. During this phenomena, intermediate, metastable forms α’ and α’’ are composed. After fast cooling processes, these metastable forms cannot transform to α phase completely and behave as internal defects in final product which affect the mechanical behaviour of the implant.)  Compression ductility is also limited. (In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stress before failure.) Post processing such as heat treatment may improve ductility. It should be mentioned that lack of ductility is a major issue for wider applications of EBM lattices. In addition, there is a lower limit for porosity (at least 50%) to enhance osteo-inductive potential of the trabecular implants.

Producibility

There are many ongoing research activities on the fabrication of cellular lattice structures by SLM and EBM. They focus on the dimensional accuracy of the fabricated structures, mechanical properties, and biocompatibility of these structures. Key challenges have also been identified in these areas, such as the powder adhesions to the struts and the difficulty in removal of the unmelted powder within the structures.

EBM fabricated lattices are hard to apply on porous metal implants because these implants have complex structure and the strut thicknesses are usually lower than 1  mm.High fatigue performance requires much more smoother surfaces than the surfaces with reduced roughness by chemical processes such as chemical etching with HF and HNO3. It is also important for the lattice nodes which are the most stress concentrated areas.

3D Printed Trabecular Implants
Figure 7. Specimens produced with EBM and SLM

However, the nodes manufactured by EBM are not smooth and contain unmelted powder particles. Thus, lattice specific design and fabrication parameters should be developed to achieve desired quality. The pores and fusion defects are very common for EBM process. Thus, the relative density range of EBM manufactured Ti-6Al-4V is between 99.4-99.8%. Due to the nature of electron beam melting process, gaps could be formed between fused titanim powders especially on the points that manufacturing parameters are changed (eg. From a layer of solid part to a layer of lattice part). However, this porosity created by EBM process existentially may lead to more osseo-integrative surfaces and fusion defects could be minimized with careful control of EBM process thanks to comprehensive understanding of the EBM fabrication.

3D Printed Trabecular Implants
Figure 8. Production planning with Magics Build Processor

Researchers stated that 150-250 µm of pore size is optimal for new tissue formation requirements. This range may increase to 300-400 µm when angiogenesis and bony ingrowth are considered. However, the minimum cavity size achieved with EBM for Ti-6Al4V lattice is about 450 µm. Thus, there is a need for chemical post process steps to increase new bone tissue formation for biomimetic purposes. 

One potential risk of manufacturing small cavities by EBM is how to remove the unmelted powder particles from them in a lattice. These particles may have to stay in the lattice and then risk entering the patient’s body. 

SLM

SLM manufactured trabecular porous trabecular implants have great interest due the variety of applications that they can be used for(Maconachie et al., 2019). However, some challenges remain, such as accounting for the manufacturing defects from the fabrication process, accurately predicting the behaviour of lattice structures, implementation of functionally graded lattice structures, and better characterisation of fatigue behaviour. (Functionally graded lattice structures (FGLS) are a unique type of lattice structure where the density of the designed structure is optimally distributed, thus enhancing the performance of the lattice structure in comparison with the uniform density distribution one.)

Even though optimization of processing parameters developed to reduce the defects occurred in production of lattice structures, errors in specific degree are inherent to SLM manufacturing. While geometric irregularities and surface roughness defects might be solved with a post machining process, increasing complexity of lattice structures make things harder to solve. It is important to use a predictive approach such as finite element analysis prior to fabrication to account for these defects.

Some simulation models have been developed that use reduced order elements such as beam elements. These models are computationally efficient but struggle to account for the complex interactions between struts at their intersections which significantly affect mechanical behaviour. Continuum element models that more precisely capture the behaviour of strut interactions have also been developed. These models more accurately represent as-fabricated geometries, but due to the dense meshes necessary to accurately capture deformation and the high number of degrees of freedom necessary to do this, they are very computationally expensive.

3D Printed Trabecular Implants
Figure 9. Respectively; Unit lattice structure SEM images after production (A) and Unit lattice structure SEM images with various post-processing techniques (B-C)

After a decade of work in medical additive manufacturing, clinical experience from past patient specific implants, and collaborations with industry AM leaders, the latest project at Btech is TraBtech Hip Project. TraBtech Hip Cup is CE certified at the end of 2020 and currently in the commercialization stage. Btech is going to apply for a 510K submission to FDA soon and looking for business partners to grow globally.

References:

Chen, H., Han, Q., Wang, C., Liu, Y., Chen, B., & Wang, J. (2020). Porous Scaffold Design for Additive Manufacturing in Orthopedics: A Review. Frontiers in Bioengineering and Biotechnology, 8. https://doi.org/10.3389/fbioe.2020.00609

Cho, C. (2021). The New Way of Healthcare with Additive Manufacturing. The New Way of Healthcare with Additive Manufacturing. https://www.medtechintelligence.com/column/the-new-way-of-healthcare-with-additive-manufacturing/

Gao, C., Wang, C., Jin, H., Wang, Z., Li, Z., Shi, C., Leng, Y., Yang, F., Liu, H., & Wang, J. (2018). Additive manufacturing technique-designed metallic porous implants for clinical application in orthopedics. RSC Advances, 8(44), 25210–25227. https://doi.org/10.1039/c8ra04815k

Javaid, M., & Haleem, A. (2018). Additive manufacturing applications in orthopaedics: A review. Journal of Clinical Orthopaedics and Trauma, 9(3), 202–206. https://doi.org/10.1016/j.jcot.2018.04.008

Maconachie, T., Leary, M., Lozanovski, B., Zhang, X., Qian, M., Faruque, O., & Brandt, M. (2019). SLM lattice structures: Properties, performance, applications and challenges. Materials and Design, 183, 108137. https://doi.org/10.1016/j.matdes.2019.108137

Sing, S. L., An, J., Yeong, W. Y., & Wiria, F. E. (2016). Laser and electron-beam powder-bed additive manufacturing of metallic implants: A review on processes, materials and designs. Journal of Orthopaedic Research, 34(3), 369–385. https://doi.org/10.1002/jor.23075

Wang, X., Xu, S., Zhou, S., Xu, W., Leary, M., Choong, P., Qian, M., Brandt, M., & Xie, Y. M. (2016). Topological design and additive manufacturing of porous metals for bone scaffolds and orthopaedic implants: A review. Biomaterials, 83, 127–141. https://doi.org/10.1016/j.biomaterials.2016.01.012

Wong, K. C., & Scheinemann, P. (2018). Additive manufactured metallic implants for orthopaedic applications. Science China Materials, 61(4), 440–454. https://doi.org/10.1007/s40843-017-9243-9

About the Author:

Kuntay Aktaş

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Kuntay Aktaş (M) is an expert in additive manufacturing applications, medical 3D Printing and AM technology implementations.He is cofounder of BTech Innovation and currently working as CEO of the company. During the years at BTech Innovation, he has had the privilege of working with global companies (USA, UK, Germany, Sweden, France, Belgium, Italy, Latvia, Netherland, Canada) and technology experts as representative, consultant and partner all around the World. He has ability to manage multidisciplinary projects and to navigate complex challenges and always been an integral part of BTech team who succeed many global case studies, awards and projects and became leading engineering company in additive manufacturing industry. He has been selected as “Fortune 40 under 40” in 2019 and 2020 by Fortune Magazine. He has a mechanical engineer BSc degree and Bioegnineering MSc degree.

M. Onur Demirak

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M. Onur Demirak is a biomedical engineer with a background in additive manufacturing and biomechanics. He is currently with Btech Innovation developing patient-spesific and standart implants additively manufactured with metal alloys and high performance polymers.

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