**5. Conclusions**

In this review of 3D printed acetabular implants, we compared and contrasted 3D printing with conventional manufacturing. We defined the clinical and engineering rationale of 3D printed acetabular cups, summarized the key variables involved in the manufacturing process, and created a classification of the applications. Whilst early clinical outcomes related to 3D printed cups have been promising, in-depth robust investigations are needed.

3D printing has already revolutionized the way customized orthopaedic implants are produced and the same may happen for o ff-the-shelf implants. Complex porous structures to achieve enhanced fixation with bone and the printing of personalized shapes are the two main advantages of 3D printing over conventional manufacture for customized implants. Enhanced porous structures and design flexibility is the main rationale for the use of o ff-the-shelf implants.

However, the importance of standardization of all the steps involved in the production process before implantation in the human body is clear. Both the International Organization for Standardization (ISO) and the American Society for Testing and Material (ASTM) have released guidelines related to terminology, design, process, materials and test methods [25,31,110], but significant gaps remain with respect to the interplay of various parameters in the properties of the final part and how to investigate the potential presence of defects.

In-depth analysis of the properties of these implants is highly suggested in order to avoid unexpected wide-scale failures as occurred with metal-on-metal implants [10,11]. A better understanding of the link between the microstructure, processing and properties of 3D printed components is necessary, considering the complex thermal cycles and the distinctive layer-over-layer building experienced with this technology, which are issues uniquely associated with the use of 3D printer technology [6].

Although it is still unclear if this new manufacturing technology is completely suitable for orthopaedic implants, further analysis of both pristine and retrieved components, together with long-term clinical outcomes and surveillance of new implants placed on the market, will help the transition to 3D printing to be managed safely.

**Author Contributions:** Conceptualization, L.D., H.H., A.D.L., J.H., A.H.; writing—original draft preparation, L.D.; writing—review and editing, H.H., A.D.L., J.H., A.H.

**Acknowledgments:** This review was supported by The Maurice Hatter Foundation, the RNOH Charity, the Rosetrees Trust and the Stoneygate Trust and the National Institute for Health Research University College London Hospitals Biomedical Research Centre.

**Conflicts of Interest:** The authors declare no conflict of interest.
