## 3 Results  
### Applications of 3D-Printed Models  
Three-dimensionally-printed models have been extensively used in different orthopedic fields. [PERSON] et al. summarized the application of 3D-printed models in shoulder surgery, particularly for Total Shoulder Arthroplasty (TSA), either by creating a 3D model to allow a more precise pre-operative planning, especially in case of severe glenoid deformities, or to create a Patient-Specific Instrumentation (PSI) in more complex cases [10]. Several studies reported good results and possible implant positioning improvement with both PSI and pre-operative planning with 3D-printed models, especially in complex TSA [11; 12]. Other authors evaluated the utility of 3D-printed models in both shoulder instability and traumatological surgery, concluding that these models may help surgeons to make an accurate pre-operative planning and to simulate the surgery, leading to better outcomes [13; 14; 15; 16]. Some researchers described 3D-printed models' utility in spine surgery, especially for correction of deformities (scoliosis) or complex fractures. This technology may help surgeons with both pre-operative planning, surgical simulations, doctor-patient communication, development of intra-operative PSI and even implantable devices. In particular, 3D technology's efficacy in improving screw placement was evaluated and confirmed by different studies [8; 17; 18]. Three-dimensionally-printed models can also be successfully applied to complex traumatology, in which pre-operative planning and good visualization of fracture morphology is mandatory and not always easy with standard images modalities, such as X-rays or CT scans. Good outcomes are described using 3D-printed models in pre-operative planning for complex proximal tibial plateau fractures, with better visualization of fracture's morphology, decreased operative time, intraoperative blood loss and fluoroscopy time [19; 20; 21; 22; 23]. [PERSON] et al. recently published their data on 50 patients with hip or knee fracture who underwent a THA/TKA, concluding that 3D-printed models can help surgeons in pre-operative planning as well as in improving accuracy and safety [24]. However, most of the studies on trauma surgery and 3D-printed models focused on acetabular fractures, confirming that 3D-printed models help in better defining morphology of acetabular fractures, leading to reduced operative time, blood loss and intra-operative use of fluoroscopy. However, there is no consensus that these advantages can lead to better functional outcomes [25; 26; 27]. A recent systematic review and meta-analysis evaluated the effect of 3D-printed models on pre-operative planning and outcomes in orthopedic trauma surgery. The authors included 17 studies with 922 patients with different fracture sites and confirmed in all cases a reduction in operative time, blood loss and fluoroscopy use when pre-operative planning with 3D-printed models was performed [28]. Many researchers focused on the utility of 3D technology in total hip arthroplasty (THA) or revision THA. In particular, THA in dysplastic, traumatic or post-septic cases can be challenging for the surgeon due to altered anatomy. In these cases, 3D-printed models may help to identify pelvic structure, acetabular deformity, bone defects and to plan some surgical needs such as ideal extent of reaming and possible use of augments [29; 30; 31; 32]. Several researchers also evaluated the outcomes of PSI in complex primary or revision hip replacement cases, with good results. However, this technology implies high costs, and it should be reserved to only difficult cases [33; 34]. Few papers have evaluated 3D-printed models for pre-operative planning in knee surgery, mostly describing the outcomes using a 3D-printed model wedge to improve alignment accuracy in Opening Wedge High Tibial Osteotomy (OWHTO). [PERSON] et al.