OBJECTIVETo compare the stability of sacroiliac screws fixation for the treatment of bilateral vertical sacral fractures to provide reference for clinic application.

METHODSA finite element model of Tile C pelvic ring injury (bilateral type Denis II fracture of sacrum) was produced. The bilateral sacral fractures were fixed with sacroiliac screws in 4 types of models respectively: two bidirectional sacroiliac screws fixation in the S₁ segment, two bidirectional sacroiliac screws fixation in the S₂ segment, one sacroiliac screw fixation in the S₁ segment and one sacroiliac screw fixation in the S₂ segment, two bidirectional sacroiliac screws fixation in S₁ and S₂ segments respectively. By the ABAQUS 6.9.1 software, in the case of standing on both feet, 600 N vertical load was imitated to be imposed to the superior surface of the sacrum and downward translation and backward angle displacement of the middle part of the sacral superior surface and everted angle displacement of the top of iliac bones were extracted for analysis. The stability of sacroiliac screws fixation was compared according to the principle of the better stability the smaller displacement.

RESULTSThe stability of 2 bidirectional sacroiliac screws fixation in S₁ and S₂ segments respectively was markedly superior to that of 2 bidirectional sacroiliac screws fixation in S₁ or S₂ segment and was also markedly superior to that of one sacroiliac screw fixation in S₁ segment and one sacroiliac screw fixation in S₂ segment. The vertical and everted stability (the downward translation: 0.531 mm; the everted angle displacement: 0.156° (left side), 0.163° (right side)) of sacroiliac screws fixation in two bidirectional sacroiliac screws fixation in the S₂ segment was superior to that of two bidirectional sacroiliac screws fixation in the S₁ segment (the downward translation: 0.673 mm; the everted angle displacement: 0.200° (left side), 0.232° (right side)). The rotational stability of two bidirectional sacroiliac screws fixation in the S₁ segment (the backward angle displacement: 0.269°) was superior to that of two bidirectional sacroiliac screws fixation in the S₂ segment (the backward angle displacement: 0.287°). Moreover, the rotational stability of one sacroiliac screw fixation in the S₁ segment and one sacroiliac screw fixation in the S₂ segment was inferior to that of two bidirectional sacroiliac screws fixation in the S₁ segment or two bidirectional sacroiliac screws fixation in the S₂ segment, and the vertical and everted stability of one sacroiliac screw fixation in the S₁ segment and one sacroiliac screw fixation in the S₂ segment was between that of two bidirectional sacroiliac screws fixation in the S₁ segment and two bidirectional sacroiliac screws fixation in the S₂ segment.

CONCLUSIONSTwo bidirectional sacroiliac screws fixation in S₁ and S₂ segments respectively is recommended to be utilized for fixing bilateral sacral fractures of Tile C pelvic ring injury as far as possible. It is suggested to choose sacral segments in which sacroiliac screws fixed according to vertical, rotational and everted stability degree of sacral fractures.