In these specimens the anterior part consists of the vertebral body carrying the pedicles and the transverse and superior articular processes, whereas the posterior segment is composed of the lamina and the spinous and inferior articular processes. This condition is occasionally present, either unilaterally or bilaterally, in the other lumbar vertebrae and even in the thoracic vertebrae. In the second type, the fifth lumbar shows a tendency, on one or both sides, to conform to the type of the first sacral vertebra, with which it may become fused. However, there may only be four lumbar vertebrae. Failure of the union of the halves of the vertebral arch and defective laminae, in this and other regions of the column, are found in spina bifida.
Lumbosacral facets are variable.
Observations on the diameters of human vertebrae in different regions. Armstrong, J.R., Crisp, E.J., Freedman, B., Gillespie, H.W., Golding, F.C., Jackson, H. and K. Lloyd. The articular facets in relation to low-back pain and sciatic radiation. J. Bone Joint Surg. The anatomical features and functional significance of lumbar transitional vertebra.