## 2 Materials and Methods  
The use of customized 3D-printed implants in the present study was approved by the Institutional Review Board (Prot. BACINOCUSTOM (2014)--CE AVEC ID: EM468/2021_47/2014/Oss/IOR_EM1) for studies involving humans. This study was conducted in accordance with the Declaration of Helsinki. All of the patients gave written informed consent for the procedure. Patients with pelvis bone tumors were operated at the Rizzoli Orthopedic Institute. They all underwent massive hip reconstruction, including a type 2 tumor resection (Figure 1), as per [PERSON]'s classification of pelvic resections [55]. They all shared the need for the complete reconstruction of the hip joint. A total of 17 patients were randomly divided into two groups; one patient was excluded for low-resolution post-operation CT scans (slice thickness > 2 mm) which prevented the 3D-model reconstruction for subsequent analysis. This is because the CT scans for that patient were carried out using settings for clinical assessment at the 6 month follow-up rather than for morphological analysis. Future scans intended for morphological analysis must adhere to the required scan settings. Ultimately, eight patients (female/male: 4/4; age: 32.2 \(\pm\) 8.0; Body Mass Index:22.0 \(\pm\) 3.0 kg/m\({}^{2}\)) had received a customized 3D-printed implant, and the other eight patients (female/male: 3/5; age: 42.8 \(\pm\) 9.8; Body Mass Index: 21.0 \(\pm\) 3.2 kg/m\({}^{2}\)) received bone grafts in combination with standard off-the-shelf acetabular and femoral components (CLS stem/acetabular component, Zimmer GmbH, Winterthur, Switzerland). The second group represents the conventional procedure for these clinical cases, and thus serves as a benchmark for 3D-printed custom implants. Both the custom-made 3D-printed implant, as well as the relevant surgical instruments, are in line with the European Directive requirements 93/42/EEC [56]. The implants were manufactured by Waldemar Link GmbH and Co. KG, Hamburg, Germany, using a medical grade titanium-aluminum-vanadium alloy (Ti6 Al4V), which is in conformation with \"The International Organization for Standardization\" (ISO) 5832/3 and \"American Society for Testing and Materials\" F136 [57; 58]. The custom implants were made using electron beam melting, an innovative 3D-printing method. This follows an iterative process which first starts by creating a vacuum in the chamber, then spreads a layer of the chosen metal powder on the printer bed. The electron beam is then aimed at the desired locations to bombard the electrons with the metal, which generates extreme heat high enough to melt and fuse the metal powder. The print bed is lowered and another fresh layer of metal is spread on the surface. This is repeated until the part is complete. Electron beam sintering uses a vacuum to eliminate any particles that would bombard the electrons and thus deviate them from the target location in the powder. Titanium was chosen since recent advancements in 3D-printing technologies have enabled it to be used as a bio-compatible metal [22] and because it is found to promote osseointegration [59] better than cobalt-chromium [60]. The 3D-printed implants were based on 3D digital models designed for the specific 3D anatomical models of patient pelvises, obtained from a pre-op CT. Additionally, a 700 micron lattice was made in areas in direct contact with bone that is intended for fusion. The lattice creates a perfect trabecular root, thus bolstering secondary osseointegration as well as nearing the mechanical strength of the surrounding bone. Patients then underwent another CT scan at their 6 month follow-up (Figure 2). Those scans, stored in the form of Digital Imaging and Communications in Medicine files, were segmented using Mimics Innovation Suite (Materialise NV, Leuven, Belgium) to form the 3D anatomical models. No inflammatory reactions or other side effects were observed in this follow-up period after surgery.