The 'transition' is from the sacrum to the lumbar spine. When I hear 'transitional segment' it usually refers to either having an extra or one less lumbar vertebra. It gets very confusing because we see the 2 lowest lumbar vertebrae as L4 and L5, but the numbering becomes ambiguous.
My guess is that there is a defomity of the vertebrae that is causing the scoliosis. I had a client with a wedge shaped S1 that started his spine upward at a VERY significant angle. In order to keep your head above your body, there must be a compensatory curve back towards center... hence the big S shape that we see as scoliosis.
For scoliosis and complex deformity reconstruction surgery, I often recomment Fabien Bitan in New York and Serena Hu at UCSF. They both have extensive experience. Bitan is a past president of the pediatric scoliosiis society.
I had a 17 y-old client with a deformed transitional segment that caused scoliosis. He reconstruction surgery by Dr. Hu was amazing. I wrote about it here:
Dr. Serena Hu – spinal reconstruction surgeon extraordinaire!
Good luck!!!
Mark
PS.... welcome to the forum!