## 4 Discussion  
A 3D-printed knee model may add different advantages to standard digital templating in both primary and revision TKA. First, it allows for direct visualization of the knee, including bone defect and neurovascular structures, which can be reproduced with the 3D printing. This feature may turn out to be extremely useful in certain primary setting, such as polomyelitis sequale. Furthermore, surgeons can decide to print the entire lower limb, to better visualize extra-articular deformities in three dimensions, including rotational deformities, which may be difficult to be visualized with standard imaging techniques. The real dimensions of the knee can also be directly experienced with the 3D model, allowing the surgeon for a precise implant sizing. Lastly, the surgeon can simulate implant positioning in the 3D model pre-operatively, with a good accuracy compared to the surgery. All these advantages were also previously described for complex primary THA [29]. However, in both primary and revision TKA, 3D printed models cannot replace a careful clinical evaluation and complete radiological examination, which are mandatory and give different information, such as degree of constraint needed, need for extensive approach, coronal and sagittal alignment and many others in both primary and revision TKA. In the revision setting, 3D printing can add more information to the standard pre-operative planning. For example, the bone and implant can be printed separately for a better  
Figure 10: X-rays one year after surgery. (**A**) Long-leg weight bearing view; (**B**) lateral view; (**C**) antero-posterior view, (**D**) patellar view. evaluation of bone losses. Furthermore, compared to CT scans, 3D-printed models have no artifacts due to images manipulation, allowing again for a better bone loss evaluation. As well as in primary cases, the 3D-printed model can be used to \"simulate\" the surgery, and it allows a better visualization of the relationship between stem, augments, and cones or sleeves. In some implants, the internal diameter of cones is independent from the size, and it can match only a few stems' diameter. If augments are added in the epiphyseal zone, the relationship between tibial component and cone or sleeve changes causing, in some cases, an incompatibility between presence of both augments, stems and cones/ sleeves. Furthermore, if an offset stem is used, it may not fit into the cone because the diameter is bigger due to the offset. All these aspects can be only partially evaluated on digital templating, while a 3D-printed model may help in visualizing them. Finally, as previously described, some 3D printing companies also allow for a bone thickness evaluation. This information cannot be fully obtained from CT scan due to artifacts, and it may be useful in pre-operative setting, especially if a cone or sleeve must be implanted, because it may conflict with the anterior, posterior, medial or lateral cortex, with subsequent risk of intra-operative fracture. Three-dimensionally-printed models are pretty \"low cost\" compared to other technologies possibly used in primary or revision TKA, such as PSI: a model costs between EUR500 and EUR1000, depending on its complexity. Obviously, PSI and other similar technologies have different advantages compared to 3D-printed models, but the latter seems to have a good cost-effectiveness in pre-operative planning of complex cases. It must be mentioned that manufacturers involved in 3D-printed models are studying new solutions, including bone quality evaluation through Bone Mineral Density (BMD). Some of them can define bone \"quantity\", intended as bone thickness, as shown in the cases, and reproduce it with a colored scale. This feature can add new information, and the surgeon may be able to know pre-operatively where the risk of intra-operative fractures is very high. This study has different limitations. First, it is a narrative review of the available literature on this emerging technology.