Finite Element Study Of Different Fixation Methods For Sacroiliac Joint Dislocation And Sacral Fracture

Sacroiliac joint dislocation and sacral fracture are complex pelvic injuries,which directly affect people’s daily activities.There is no uniform treatment plan for the above two types of injuries.Therefore,it is extremely necessary to compare the biomechanical differences between the two types of pelvic injuries treated by different fixation methods using finite element method.According to the anatomical structure of human lumbar spine and pelvis,Mimics,Geomagic Studio and Hypermesh were applied to construct the complete finite model of lumbar spine(L4-L5)and pelvis,based on the CT(Computed Tomography)result of a healthy female(in 165 cm height,55 kg weight and 33 years old),including the end plate,nucleus pulposus,fibrous ring,cartilage,symphysis pubis,as well as ligaments attached to lumber spine and pelvis.The finite element fixation models for two types of sacroiliac joint dislocation(Tile B & Tile C)and two types of sacral fracture(Type U & Type H)were constructed on this basis.Four fixation modes were proposed for sacroiliac joint dislocation as followings:(1)Fixed by one S1 sacroiliac screw(S1-1);(2)Fixed by one S2 sacroiliac screw(S2-1);(3)Fixed by one S1 sacroiliac screw and one S2 sacroiliac screw(S1-S2);(4)Fixed by two S1 sacroiliac screws(S1-2).Three fixation models for sacral fracture included:(1)two penetrate screws penetrating S1 and S2 respectively(S1S2);(2)one L4-L5 pedicle screw+one wing of ilium screw(L4L5+IS);(3)one L4-L5 pedicle screw+one S1 penetrate screw+one wing of ilium screw(L4L5+S1+IS).Comparisons were made of biomechanical differences among different loading methods in different operation modes as mentioned above(anterior flexion,posterior extension,axial rotation,and axial lateral bending).The finite element calculation has shown that the displacement values generated on the surface of sacrum for the treatment of Tile B sacroiliac joint dislocation fixed by S1-1,S1-2 and S1-S2 were within the normal limits(less than the displacement values of normal pelvis),indicating that S1-1 sacroiliac screw can satisfy the fixation requirement for Tile B sacroiliac joint dislocation.However,the displacement values generated on the surface of sacrum for the treatment of Tile B sacroiliac joint dislocation were only within the normal limits when being fixed by S1-2 and S1-S2.Based on the comprehensive consideration of displacement,angular displacement and other indicators,S1-S2 was better than S1-2,with a higher stability.Therefore,treatment for Tile C sacroiliac joint dislocation shall give priority to S1-S2.In the treatment of U-type sacral fracture,for the separation values of fracture gap and the displacement values generated on the surface of sacrum,L4L5+S1+IS fixation was less than the other two fixation modes;for the maximum stresses,L4L5+S1+IS and S1S2 were not significantly different,and less than L4L5+IS.For the treatment of Type H sacral fracture,L4L5+S1+IS was better than the other two fixation modes,given the separation values of fracture gap,maximum stresses and the displacement values generated on the surface of sacrum and other indicators comprehensively.Thus,treatment for sacral fracture shall give priority to L4L5+S1+IS.