The fifth lumbar vertebra is subject to many abnormalities of development which may cause mechanical trouble. The most frequent anomaly is probably in the shape or size of the transverse processes. This will not be considered here.
Next in frequency is a variation in the articular processes, which may vary in size, in shape, or in direction. If the articulation be of the lumbar type, with the long axis of the joint antero-posterior, and the flat facets facing to the side, it is possible for the vertebra to slide forward on the sacrum when the spine is straight or extended, especially if the articular processes be small and the ligaments weak. If the processes be of the dorsal type, and this is by no means rare, flexion of the spine, with violence, may cause the facets to override; more easily, again, if these facets be small and the ligaments be
RYERSON EW. RECURRENT SPONDYLOLISTHESIS, WITH PARALYSIS; BONE-SPLINT TRANSPLANTATION. JAMA. 1915;LXIV(1):24-25. doi:10.1001/jama.1915.02570270026006