## 5 Conclusions  
Three-dimensional printing technology is gaining popularity in different orthopedic fields. Three-dimensional models are successfully used for pre-operative planning in spine surgery, oncology, trauma (especially acetabular fracture) and hip replacement/revision. In all these cases, 3D models were reported to help surgeons to better visualize anatomy, fracture morphology and to simulate the reconstruction. Three-dimensional printing technology is also involved in development of PSI, which can be used for hip or knee replacement.  
However, there is no literature regarding the application of 3D-printed models in pre-operative planning for complex primary or revision TKA. In these cases, 3D-printed models may be useful to better visualize the deformity, to accurately plan implant size and positioning and to simulate the surgery in complex cases. Furthermore, especially in complex revision TKA, a 3D model may help surgeons to better visualize bone losses, and to evaluate the compatibility between different systems used, such as augments, cones, sleeves and stems. Some manufacturers can provide surgeons also with further information regarding bone thickness, which may be useful to pre-operatively know which zones are at risk of fracture during surgery.  
In conclusion, 3D-printed models may be a useful tool in pre-operative setting of complex primary and revision TKA, also considering the relative low cost compared to other technologies, but it cannot replace a careful clinical and radiological evaluation. However, more studies will be useful to better evaluate clinical advantages of this relatively new technology.  
Conceptualization: F.R. and R.R.; Methodology: F.R. and U.C.; Investigation: F.R. and U.C.; Writing--original draft preparation: F.R. and D.E.B.; Writing--review and editing: F.D., M.B. and D.E.B.; Formal analysis: R.R., M.B. and F.D.; Supervision: R.R. and D.E.B.; Project administration roles: R.R. and D.E.B. All authors have read and agreed to the published version of the manuscript.  
No funding for this manuscript has been requested.  
This is a narrative review, so no IRB approval is needed. However, it includes two clinical cases. Considering that patients underwent the same pre-operative evaluation to all other patients (including a CT scan for 3D printed model), and the only difference was that a 3D printed model of the involved knee has been produced, the Institutional Board consider this study \"exempt from IRB approval\".  
All the patients included in the study gave verbal and written consent to the surgery and to use images of it. No further consent was needed.  
Data available on request due to restrictions eg privacy or ethical. The data presented in this study are available on request from the corresponding author. The data are not publicly available due to ethical reasons.  
[PERSON], MD, is paid teaching consultant for Arthrex, Zimmer-Biomet, Lima, Medacta, Smith and Nephew, DePuy. [PERSON], MD, is a paid teaching consultant for Arthrex and Zimmer-Biomet, received editorial royalties for Elsevier and Springer and he is part of the editorial board \"The Knee\" journal, Elsevier. [PERSON] is a Teaching consultant and marketing consultant for Johnson & Johnson. The other authors have no financial or no-financial conflict of interest.