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Left convex lumbar scoliosis caused my a malformation of the 1st sacral vertebra (butterfly vertebra). The left smaller "wing" of the butterfly forms a nearthrosis with the os sacrum. Instead of a normal intervertebral disc, there is an X-shaped synchondrosis in the coronal plane, bordered by the 5th lumbar, the right and left hemivertbrae of the butterfly S1, and the 2nd sacral vertebra.