Tile classification of pelvic fractures based on forces acting on pelvis. A, Type B1: External rotation or anteroposterior compression through left femur (arrows) disrupts symphysis, pelvis, and anterior sacroiliac ligament until ilium impinges against posterior aspect of sacrum. If force stops at this level, partial stability of pelvis is maintained by interosseous sacroiliac ligaments. B, Type B2-1: Lateral compression (internal rotation) force implodes hemipelvis. Rami may fracture anteriorly, and posterior impaction of sacrum may occur, with some disruption of posterior structures, but partial stability is maintained by intact pelvic floor and compression of sacrum. C, Type C: Shearing (translational) force disrupts symphysis, pelvic floor, and posterior structures, rendering hemipelvis completely unstable.