Quality control of additively manufactured medical implants with CT

The main objective of the EMPIR-project MetAMMI is to provide a comprehensive basis to enable the safe use of medical implants manufactured by additive manufacturing (AM). 6 national research institutes, 6 academic partners, 3 industry partners as well as 2 clinical partners are part of the consortium (see http://projects.lne.eu/jrp-metammi/).

AM offers an effective solution for the medical sector, as it enables the production of patient-customized implants with complex geometry. However, this promising manufacturing technology did not yet reach a satisfactory level of confidence expected by notified bodies and the medical sector. This hinders the acceptance of medical implants produced by AM.

In order to increase the level of confidence of AM technologies, several AM production processes for medical implants were assessed. The evaluation is based, for example, on the geometric deviations and the surface quality of the manufactured implants. Medical objects made of different materials (polymers, metal and ceramics) produced using several manufacturing techniques and parameters were used in the study.

For the evaluation, X-ray computed tomography (CT) was used, being able to acquire the complete inner and outer geometry of the implant in a non-destructive way. Further relevant aspects such as material defects, porosity and typical error present in the assessed AM technology (e.g. cracks) were included in the analysis.

A cylindrical object with a lattice-structure based design was used in the investigation, as it promotes bone growth inside of the implant. This structure was manufactured by several project partners with several AM-processes and different materials; and measured with CT at PTB. The measurement results of the implants with the best manufacturing parameters for the respective manufacturing technology and materials are shown in Fig. 1. In the slice pictures, the green contour represents the nominal geometry and the white contour represents the actual geometry.

It can be seen that the ceramic implant made of TCP (Tricalcium Phosphate) produced with the AM process LCM (Lithography-based Ceramic Manufacturing) presents a good match of the outer geometry with a smooth surface. However, LCM delivers smaller inner structures than designed, see Fig. 1(a), (b) and (c). On the other hand, the AM-process SLM (selective laser melting) produces implants made of Cobalt-Chromium (CoCr) with a better match of the outer and inner geometry, but with high surface roughness, see Fig. 1(d), (e) and (f). BJ (Binder Jetting) is a potential solution for the AM of polymers. However, the BJ technology delivers objects made of PMMA (Poly(Methyl-Methacrylate)) with rough surface and with high geometrical deviations, see Fig. 1(g), (h) and (i). Additionally, implants made of Dental SG resin were produced by SLA (Stereo-Lithography vat photopolymerization). The latter technology delivers the best results regarding overall geometry match with smooth surface and low porosity, see Fig. 1(j), (k) and (l). Thus, it can be concluded that the quality of the implant highly depends on the material, manufacturing technology and the manufacturing parameters. Additionally, several implants with different manufacturing parameters were produced. From that, it was possible to conclude that the manufacturing parameters play an important role for the implant quality.


Fig 1. Exemplary CT scans implants made of different materials using AM technologies: (a), (b) and (c) Lithography-based ceramic manufacturing for TCP; (d), (e) and (f) Selective laser melting for CoCr; (g), (h) and (i) Binder jetting for PMMA; and (j), (k) and (l) Stereolithography vat photopolymerization for dental SG resin. In the slice pictures, the green contour represents the nominal geometry and the white contour represents the actual geometry.