## 3 Results
### Clinical Case: Complex Revision TKA  
A female patient, 66 years-old, slightly overweight (BMI 27 kg/m\({}^{2}\)) and without significant comorbidities, underwent right and left revision TKA for aseptic loosening with a rotating hinge implant. A couple of years later, she developed progressive left knee pain, in which an Endomodell(r) (Waldemar Link GmbH, Hamburg, Germany) rotating hinge was implanted. The pain was mainly located at the tibial side. At the clinical evaluation, there was one single anterior scar, no signs of infection, ROM was 0-100\({}^{\circ}\), tibial pain was evocated during palpation and the knee was stable. PJI was firstly ruled out with negative lab test and negative white blood cells (WBC) and polymorphoucleate (PMN%) count in the synovial liquid. Standard X-rays demonstrated a varus lower limb alignment, tibial aseptic loosening, previous augments on the tibial side, full stem cementation, acceptable patellar height and a huge bone loss was expected, especially on the tibial side. A CT scan was also performed, and bone loss was confirmed on the tibial side, even if it was difficult to evaluate due to the presence of multiple artifacts. Figure 6 shows pre-operative X-rays and CT scan. Pre-operative planning was performed as usual using a digital template, including implant (size and position), stem (size, type, and position), augments and cones for both bone loss filling and implant fixation. The predictable difficulties in this re-revision TKA were removal of the implant (rotating hinge), removal of the cement (full stem cementation), bone loss treatment, obtaining a good implant fixation considering predictable poor bone quality, and restoring tibial joint line and correct patellar height. Considering all these difficulties, a 3D-printed model was obtained from the CT scan (Medics(r), Turin, Italy). The surgeon chose to print implant and cement together in the 3D model of the tibial component, and to isolate bone loss in the tibia, which was the main concern. Figure 7 shows the 3D-printed model. Figure 6: Pre-operative X-rays of the re-revision TKA case. (**A**) Long-leg weight bearing view; (**B**) antero-posterior view; (**C**) lateral view; (**D**) patellar view; (**E**) coronal view of a computer tomography (CT) scan demonstrating tibial bone loss (red circle); (**F**) coronal view of a CT scan demonstrating femoral artifacts (red circle); (**G**) axial view of a CT scan demonstrating tibial artifacts (red circle). In the provided document, different information was collected, such as femoral to tibial gap (26 mm); femoral component lateral positioning (9.3\({}^{\circ}\) flexion); mediolateral (ML) and antero-posterior (AP) diameter for the femoral side, both with and without cement, to better evaluate implant size (ML 67.6 mm, AP medial 47.5 mm, AP lateral 49.6 mm); ML and AP diameter for the tibial side (respectively, 72.2 mm and 54.13 mm); and intramedullary canal diameter both on femoral and tibial side (respectively, distal diameter 16.2 mm \(\times\) 20.1 mm and 15.7 mm \(\times\) 17.8 mm, proximal diameter 19.5 mm \(\times\) 28.4 mm and 26.4 mm \(\times\) 27.6 mm). All these features are shown in Figure 8.  
Figure 7: Three-dimensionally-printed model of the case. (**A**) Image by the company (grey bone, blue implant, light blue cement); (**B**) 3D model femoral and tibial component; (**C**) 3D femoral model with implant separated from bone-cement; (**D**) 3D tibial model with implant separated from bone without cement; (**E**) 3D femoral model showing bone (white) and cement (blue); (**F**) 3D tibial model showing tibial bone loss. Similar to the primary TKA 3D-printed model, in this case, the manufacturer provided a bone thickness evaluation (minimum thickness = red, maximum thickness = green). On the femoral size, the bone was very thin in the anterior and posterior femoral cortex (about 0.86 mm), while distal femoral c  
Figure 8: Data provided with the 3D-printed model.