TCF4 – In 2007, deletions of or point mutations in this gene, which is located on 18q, were identified as the cause of Pitt-Hopkins disease. This is the first gene that has been definitively shown to directly cause a clinical phenotype when deleted. If a deletion includes the TCF4 gene (located at 52,889,562-52,946,887), features of Pitt-Hopkins may be present, including abnormal corpus callosum; short neck; small penis; accessory and wide-spaced nipples; broad or clubbed fingers; and sacral dimple. Those with deletions inclusive of TCF4 have a significantly more severe cognitive phenotype. TSHZ1 - Point mutations and deletions of this gene, located on 18q, are linked with congenital aural atresia  Individuals with deletions inclusive of this gene have a 78% chance of having aural atresia. Critical regions – Recent research has narrowed the critical regions for four features of the distal 18q- phenotype down to a small segment of distal 18q, although the precise genes responsible for those features remain to be identified. Haplolethal Regions - There are two regions on chromosome 18 that has never been found to be deleted. They are located between the centromere and 22,826,284 bp (18q11.2) and between 43,832,732 and 45,297,446 bp (18q21.1). It is hypothesized that there are genes in these regions that are lethal when deleted. Nomenclature
The phrase “ring 18” refers to the shape that the normally linear chromosome assumes when one tip of the chromosome joins the other. A ring-shaped chromosome is the result.
(5)
Associated radiological findings include a vacuum phenomenon (in the nucleus pulposis of the adjacent intervertebral disc), reduction of disc height with corresponding loss of the disc space, marginal sclerosis of the adjacent vertebral bodies, osteophyte formation and apophyseal joint instability. With a retrolisthesis there is always a less than ideal positioning of spinal segments. There is also always a reduced anterior to posterior dimension of the spinal canal compared to the way it is supposed to be. The greater the posterior displacement, the more significant it is for producing a dysfunctional spinal cord or even a cauda equina syndrome. Joint stability
Joint stability is easily evaluated by the use of flexion and extension lateral x-ray views of the spine. A summary of part of the DRE tables (6) give a guide as to the implications of the joint instability. If either translation or angular change is determined from flexion to extension to the degree shown in the table below, then Category IV instability is present. See also Joint stability.Translation is a gliding motion where one bone of a joint glides over its neighbour. Management
References

6. Hadley, Lee A. (MD), (1973) “Anatomicoroentgenographic Studies of the Spine”. 390.7. Cocchiarella L., Andersson, G. “American Medical Association Guides to the Evaluation of Permanent Impairment”, 5th  edition, Tables 15- 3, 15-4, 15-5.8. Giles, L.G.F. ; Muller R.; and Winter G.J. (2006) “Lumbosacral disc bulge or protrusion suggested by lateral lumbosacral plain x-ray film – preliminary results.” Journal of Bone and Joint Surgery. British Volume, Vol 88-B, Issue SUPP_III, 450.