What does it mean to have biphasic scoliosis?

This is not a formal. This term "biphasic" is not a normal term when used to describe a scoliosis. There are several different types of scoliosis classified on their causes (idiopathic, congenital, neuromuscular) & then we describe scoliosis based on its location and appearance on radiographs or x-ray (thoracic, thorcolumbar, lumbar, cervical, - single curve, double curve, major curve, minor curve, kyphoscoliosis, etc.
Scoliosis. Scoliosis is defined as curving of the spinethe term "biphasic scoliosis" is not a term that I am familiar with however what they could have meant is that you have two curvatures of your spine (thus having scoliosis) in opposite directions (one area of your spine curving to the left and the other curving to the right in the shape of an s).