gms | German Medical Science

Objectives: The combination of simulation software and latest 3D manufacturing techniques for medical grade titanium alloys opens a new way for patients undergoing surgical treatments. One example is the reconstruction of large bone structures leading to patient specific (PS) geometries. Although, surgeons and patients rely on established surgical interventions, new approaches are sorely needed to even improve the surgical outcome. For this reason the presented study was conducted using software products together with experimental 3D loading tests, to guarantee the technical success of highly reliable PS implants. As a first application a fibula-reconstructed mandible was chosen, using PS osteosynthesis plates. The plates were designed based on CT (computed tomography) datasets and will be tested using an enhanced multiaxial 3D endurance test system.

Methods: Improved osteosynthesis plates were created by a three step approach: A) Based on CT data (fibula and mandible and its virtual reconstruction results) the shape of the osteosynthesis plates were modeled and optimized by a combination of the finite-element (FE) method together with a topology optimization routine. B) State of the art formless 3D manufacturing techniques for medical grade 5 titanium alloy (3D printing by electron beam melting and 5-axial CNC-milling) were used to realize the software-generated shapes. C) Multiaxial endurance testing using a custom made 3D loading system that simulates chewing cycles until failure occurs. The 3D loading case was adapted from a biomechanical model, based on the mandibular muscle forces. Two resulting force vectors - one for each cheek - were calculated from the mastication muscle system. In brief: chewing forces of 320 N/healthy cheek (static) and 160 N/healthy cheek (cyclic loading, >1.0E+06 cycles) will be applied. The orientation of the force vectors were optimized in a way that the plane of the mandibular teeth meet the maxilla in an orthogonal way (4° anterior and 0.5° lateral with respect to the mandibular angle).

Results and Conclusion: Reaching the end of the first two steps of designing and manufacturing the PS osteosynthesis plates, the mechanical endurance testing is now on its way. The recently extended multiaxial endurance test system is capable to test two reconstructed mandibles in parallel and the test frequency has been increased from 2 to a maximum frequency of 10 Hz. However, in primary tests we identified a frequency of 5 Hz to be an ideal frequency for both the electron beam melted and the CNC-milled plates. As the test stand is made of non-corrosive components (anodized aluminum and stainless steel), it provides a safe way to clean all components after the testing of anatomical specimen.

The research project "TOPOS - Topology Optimized Osteosynthesis" is funded by the Bayerische Forschungsstiftung (BFS), grant no. AZ-1019-12.