Pelvic ring injuries are uncommon but serious injuries. Percutaneous sacro-iliac screw fixation (SSF) is the standard treatment for posterior stabilization of pelvic fractures. Compression forces of the SSF might cause deformity of the sacrum and the pelvic ring. The aim of this radio-volumetric study is to evaluate the morphometry of the sacrum and pelvic ring in SSF for posterior pelvic fractures. (1) Methods: We conducted a radio-volumetric study measuring the bony sacral volume before and after SSF for a pelvic fracture based on a three-dimensional reconstruction of the pre- and postoperative computed tomography scan of 19 patients with a C-type pelvic fracture. In addition to the bony sacral volume, we assessed the pelvic deformity and the load bearing axis. We compared the results of patients without anterior stabilization (Group A) to patients who had additional ORIF of the anterior pelvic ring. (2) Results: Median age of the patients was 41.2 years (±17.8). All patients received percutaneous SSF with partially threaded 7.3 mm screws. The sacral volume decreased from 202.9 to 194.3 cm3in group A (non-operative treatment anterior,n= 10) and an increase of sacral volume from 229.8 to 250.4 cm3in group B (anterior ORIF;n= 9). Evaluation of the pelvic deformity also reflected this trend by a decrease of the ipsilateral load-bearing angle in group A (37.0° to 36.4°) and an increase of this angle in group B (36.3 to 39.9°). (3) Conclusions: Bony sacral volume and pelvic deformity after sacro-iliac screw fixation in pelvic fractures depend on treatment of the anterior pelvic ring. Reduction and fixation of the anterior fracture shows an increase of the bony sacral volume and the load bearing angle leading to a closer to normal reconstruction of the pelvic anatomy.