Finite element analysis of retrograde superior ramus screw of pubis for the treament of pelvic anterior ring fracture.

Background: Retrograde superior ramus screw of pubis (SRSP) is a new kind of pelvic minimally invasive internal fixation apparatus developed by our team. The purpose of this study was to analyze the biomechanical stability of this new minimally invasive pelvic internal fixation device, and to provide this new device with theoretical basis for clinical application.

Methods: The Tile C1.3 pelvic fracture model was established. The posterior ring was fixed in the same way with two sacroiliac screws. And the anterior ring was fixed with SRSP, reconstruction plate, minimal invasive subcutaneous internal fixator (INFIX) and hollow screw respectively, to establish the finite element model of fracture-internal fixation. Finite element analysis was used to analyze the deformation and Von Mises(V-M) stress distribution of different kind of fixation under three kinds of stress conditions: vertical self-weight load, anterior-posterior(A-P) compression and lateral compression.

Results: Among the four-kind fixation models, all the maximum displacement of fracture site were significantly less than 2 cm, and the maximum V-M stress of internal fixation was lower than the yield stress of titanium metal (1050 MPa). The maximum displacement and V-M stress of total model/internal fixation in INFIX group were higher than those in the others under three stress conditions except for two cases, which were the maximum displacement of total model in SRSP group (0.26266 mm) under A-P compression and the maximum displacement of internal fixation in SRSP group (0.32588 mm) under lateral compression. The values of total model/internal fixation displacement and V-M stress distribution in SRSP group were similar to those of reconstructed plate group and hollow screw group. Furthermore, the stress distribution of SRSP group was more uniform from the stress nephogram.

Conclusion: All four kinds of internal fixation can effectively repair Tile C1.3 pelvic fractures. Also fracture-fixation pelvis model were basically restore the normal mechanical conduction path, rebuilding overall stability of the pelvic ring with good static mechanical stability. The stress distribution of fracture-internal fixation model in SRSP group was more uniform. Compared with INFIX group, SRSP group was more advantageous in preventing excessive displacement of the fracture site, loosening and deformation of the internal fixation, etc.