Lattice structures for medical implants

Photos: Tangible Solutions Inc.

Many orthopedic implant companies claim their technologies facilitate bone ingrowth (or allow bone growth). Little clinical evidence backs up these claims, other than titanium’s promotion of an environment for bone to grow. Plasma-spray or acid-etched titanium surfaces without substrate porosity, for example, may yield surfaces for initial expulsion resistance (preventing the cage from moving after implantation) and provide surfaces for bone to grow onto, but they yield little long-term ingrowth to provide the best fixation and long-term results.

Industry research has established common pore and strut sizes, however, further challenges must be studied:

• Surface morphology of lattice structures – macro, micro topography
• Lattice type, shape manipulation for optimal initial attachment, long-term proliferation
• Correlation between optimal pore/strut sizes, macro stiffness of structures – preventing stress-shielding, adjacent biological issues
• Manipulation of mechanical properties via computational methods (functional lattice grading, topology optimization)

Many software tools struggle to build the kinds of structures that will lead to the next generation of orthopedic implants. Furthermore, selective laser melting (SLM) machines are becoming more advanced, manufacturing complex structures with strut sizes smaller than 0.100mm (0.004") and features as small as 0.080mm (0.003").

Next-generation software provides tools to harness SLM capabilities, allowing advanced lattice-feature manipulation. Newly developed capabilities allow design engineers to take triply periodic minimal surface (TPMS) lattices and grade them to any ramped structure.

Rather than stop at linear remapping, any geometry representation can be altered in each axis with any complex parametric equation. Complicated models can also be generated via parameters or topology optimization and used to functionally grade structures.

Using the inputs (See Figure 1), the volume lattice can be thickened with the Voronoi lattice input. The amount of thickening can be controlled precisely, given parameterization. Where the Voronoi lattice material exists, the final lattice thickness will be greater. The same principles can be applied when setting up the volume lattice – the cellular point spacing can be ramped via any representation that can be conceived and applied.

Complex lattices can also be transformed from one to another with no definable transition point, potentially limiting single weak shear points. Having the freedom to achieve the strut and pore size that produces biological benefits, and maneuvers the structural elements, allows for complex manipulation of orthotropic properties. This further helps engineers evaluate how bone ingrowth will change the macro stiffness of the construct and how to create lattice elements that provide long-term benefits for biological optimization.

Manipulating the mixed elements is not bound to a linear input. Objects can be mixed using complex structures or primitive models, (See Figure 2) where the same two TPMS lattices are mixed using an oblate spheroid. Where the sphere exists, the cells are a Gyroid TPMS structure, and the design moves from the sphere transitions to a Lidinoid cellular structure with no obvious transition point.

These tools transition complex lattice shapes to external solid features, simulating biological mimicry of bony structure (cortical and trabecular elements) and the seamless organic transition between them. This type of control over the internal and external structures, as well as discrete control of the transitioning elements, can help give implant developers the ability to alter structural properties on a macro and micro level. It also helps them understand how structures alter as bone integrates into them and engineer implants with specific ingrowth factors.

This can be further refined by setting up transition phases from one structural element to another – to define continuous mechanical properties from entirely different structural types.

nTopology https://ntopology.com

Tangible Solutions Inc. https://www.tangible.com