Finite Element Analysis and Transiliac-Transsacral Screw Fixation for Posterior Pelvic Ring with Sacrum Dysplasia

Posterior pelvic ring sacroiliac screws are preferred by clinicians for their good biomechanical performance. However, there are few studies on mechanical analysis and intraoperative screw insertion of the dysplastic sacrum and sacroiliac screw. This study investigated the biomechanical performance of oblique sacroiliac screws (OSS) in S1 combined with transiliac-transsacral screws (TTSs) in S2 for pelvic fracture or sacroiliac dislocation with dysplastic sacrum and evaluated the safety of screw placement assisted by the navigation template.

Six models were established, including one OSS fixation in the S2 segment, one transverse sacroiliac screw (TSS) fixation in the S2 segment, one TTS fixation in the S2 segment, one OSS fixation in the S1 and S2 segments, one OSS fixation in the S1 segment and one TSS fixation in the S2 segment, one OSS fixation in the S1 segment and one TTS fixation in the S2 segment. Then, finite element analysis (FEA) was performed. Twelve dysplastic sacrum patients with pelvis fracture or sacroiliac dislocation underwent OSS insertion in the S1 combined with TTS insertion in the S2 under the assistance of the patient-specific locked navigation template. Grading and Matta scores were evaluated after surgery.

In the one-screw fixation group, the vertical displacements of the sacrum surface of S2 OSS, S2 TSS and S2 TTS were 1.23, 1.42, and 1.22 mm, respectively, and the maximum stress of screw were 139.45 MPa, 144.81 MPa, 126.14 MPa, respectively. In the two-screw fixation group, the vertical displacements of the sacrum surface of the S1 OSS + S2 OSS, S1 OSS + S2 TSS and S1 OSS + S2 TTS were 0.91, 1.06, and 0.75 mm, respectively, and the maximum stress of screw were 149.26 MPa, 167.13 Pa, 136.76 MPa, respectively. Clinically, a total of 12 TTS and OSS were inserted under the assistance of navigation templates, with a surgical time of 55 ± 7.69 min, bleeding of 57.5 ± 18.15 ml and radiation times of 14.5 ± 4.95. One of the TTS and one of the OSS were grade 1, and the other screws were grade 0. The Matta scores of nine patients were excellent, and three patents were good.

OSS in the S1 combined with TTS in the S2 had the best mechanical stability in six models, and it is safe for screw insertion assisted by the patient-specific locked navigation template.