There are two separate problems that can occur when noted “l5 did not fuse that it is in two pieces instead of one”. This could be a spina bifida occulta where the back arch of the vertebra did not fuse leaving a slightly open channel or an isthmic spondylolisthesis where fractures have occurred in the side of the vertebra.
A spina bifida occulta is a non-problem and an isthmic spondylolisthesis needs to be looked after.
Dr. Corenman