## 1 Introduction
In response to these procedures, there are currently several surgical procedures that are used for effective reconstruction. Some of the most common procedures are pure allograft reconstruction [13; 27; 28; 29], allograft-prosthetic composite reconstruction [30; 31], and arthrodeis [19]. While these procedures have their unique advantages, they also come with some inherent limitations. As such, there is no clear procedure that stands out. Because of this, the development of a reconstruction procedure using custom 3D-printed mega-prosthesis has occurred, which already has some good indications for such cases of massive bone loss [32]. Previous studies indicate that 3D-printed custom implants offer an effective strategy for complex pelvic reconstruction after tumor resection, showing acceptable functional outcomes and prosthesis survival rates. One study reported a median Musculoskeletal Tumor Society score of 63.3% and a survival rate of 73.3% at 33.8 months follow-up [22]. A second study found an average Musculoskeletal Tumor Society score of 46.3% and a 5 year survival rate of 85.7% [10]. A third study reported a mean Musculoskeletal Tumor Society score of 72% and a 3 year prosthesis survival rate of 69% [23]. Existing research into the effects of 3D-printed custom implants on hip surgery is mainly focused on clinical and functional criteria, such as Oxford Hip Score, Harris hip score, aseptic loosening, dislocation, formation of cracks, and nerve injuries [9; 33; 34; 35; 36]. However, there seems to be a lack of research on the morphological analysis of custom 3D-printed implants in comparison with conventional procedures. That is why, after an extensive review of the published research, a set of the most revealing morphological parameters of surgical success have been delineated, the first being the hip joint center of rotation, which is defined as the femur head center. While there are published morphological analyses of pelvic reconstructions, such as on the hip center position and its influence on functional outcomes [37; 38], these do not include surgeries with 3D-printed custom implants. However, they do highlight that this position has the most significant impact on kinematics, such as on the range of hip joint motion. Additionally, deviations in the position of the hip center of rotation can impact the bio-mechanics of the pelvis during loading. This disrupts pelvic load transmission and thus generates stress concentrations that can increase the chance of implant mechanical failure in the long run [39]. That is why hip joint center position is taken as the primary parameter in the present analysis. The greater trochanter offset is the distance from the femur head center to the greater trochanter [40]. The greater trochanter is the epiphysis for insertion of the main abductor muscles, gluteus medius and minimus; it extends the femur neck and shaft junction [41; 42; 43]. In addition, the piriformis muscles insert into the tip of the greater trochanter. It is part of the muscle group which activates to perform a hip extension and external rotation [44]. Inaccurate restoration of the greater trochanter offset after surgery, such that it does not match the healthy contra-lateral side, has been found to be a cause of pain in the abductor muscle due to increased tension, required to maintain the pelvis balanced [43], as a cause of increased wear of the implant articulating surface, and as a cause of inferior patient-reported outcomes [45; 46]. Finally, this parameter indicates how satisfactory the lower limb length has been restored [47]. The acetabular inclination angle is the angle measured in the frontal plane between the normal acetabular ridge plane and the longitudinal axis of the patient [48]. An optimal inclination angle causes a better outcome for the functioning of the joint. Maintaining an angle lower than 45 degrees was found to avoid implant wear [49; 50; 51; 52], thus elongating the lifetime of the implant, as well as minimizing joint impingement [53].