Transitional segment (transitional vertebral segment): a congenital anamoly of a vertebra in which it develops characteristics of the adjoining structure or region, e.g lumbosacral, cervicothoracic. The clinical significance of this lies in its aberrant motion characteristics; gross postural effects on the super incumbent spinal column or pseudoarthrosis between the enlarged transverse processes and either the sacrum or ilia.


a transitional segment in which the first sacral segment becomes like an additional lumbar vertebra articulating with the second sacral segment.


1. Incomplete separation and differentiation of the fifth lumbar vertebra   (L5) such that it takes on characteristics of a sacral vertebra

2. When transverse processes of the fifth lumbar are atypically   Large,  causing pseudoarthrosis with the sacrum and/or ilia referred to as batwing deformity if bilateral.