Biomechanical comparisons of two sacroiliac screws with different lengths in the treatment of central sacral fractures in a finite element model / 中华创伤杂志

ABSTRACT

Objective To compare the biomechanical characteristics of lengthened sacroiliac screw and long sacroiliac screw fixated in different ways for the treatment of central vertical sacral fractures to provide reference for clinical practice.Methods A finite element model with three dimensions of Tile type C pelvic ring injury (central type Denis Ⅲ fracture of sacrum) was produced.The central sacral fractures were fixated with lengthened sacroiliac screw or long sacroiliac screw in 7 types of models respectively as follows.(1) One long sacroiliac screw was fixated in S1 segment from the right (C1);(2) One long sacroiliac screw was fixated in S2 segment from the right (C2);(3)Two long sacroiliac screws were fixated in S1 and S2 segments respectively from the right (C12);(4) One long sacroiliac screw was fixated in S1 segment from the right and one long sacroiliac screw was fixated in S2 segment from the left (C12hybrid);(5) One lengthened sacroiliac screw was fixated in S1 segment from the right (J1);(6) One lengthened sacroiliac screw was fixated in S2 segment from the right (J2);(7)Two lengthened sacroiliac screws were fixated in S1 and S2 segments respectively from the right (J12).The shift and angle displacement of the central superior surface of sacrum in the case of standing on both feet were measured and compared.The maximum Von Mises stresses in different positions were measured.And various mechanical indices (including stresses of various parts and stress distribution of screws and pelvis,etc) were also quantified and compared.Results The stability of the posterior ring-screw complex was optimal when the sacrum was fixated with S1 & S2 lengthened sacroiliac screws.The maximum Von Mises stresses of screw were affected by sacral segments,namely,S1 ＞ S2 ＞ S1& S2.The maximum Von Mises stress of screws in S1 and S2 segments were similar.The maximum stress of the lengthened sacroiliac screw was lower than that of the long sacroiliac screw.The maximum Von Mises stress of bilateral sacroiliac joints in the lengthened sacroiliac screw fixation model was similar to that of normal pelvis.The maximum stress in the left sacroiliac joint of the long sacroiliac screw fixation model was larger than that of the normal pelvis.The maximum Von Mises stress of screw-bone interface of the long sacroiliac screw fixation model was larger than that of the lengthened sacroiliac screw fixation model.The screw stress distribution of the lengthened sacroiliac screw was superior to that of the long sacroiliac screw,and the screw stress distribution of the double segment fixation was superior to that of the single segment fixation.There was relatively less difference between the pelvic stress distribution of double segment fixation model and that of the normal pelvic model.Conclusion The fixation of S1 & S2 lengthened sacroiliac screws utilized in central sacral fractures of Tile C pelvic ring injury results in a more stable posterior pelvic ringscrew complex,more dispersed screw stress and better pelvic stress distribution which is closed to normal,and can also reduce the risk of internal fixation failure as well as lower back pain.