The forward slippage does NOT always occur. This non-slipped pars defect is called a "Spondylolysis" and is almost always a precursor to the actual forward slippage.
The first theory proposed a failure of ossification during embryonic development, leading to a pars interarticularis defect at birth
The second theory demonstrated that the pars defect began to appear around age six and became progressively more common till age 16. After age 16, the incidence fell and rarely developed after adolescence
It is currently thought that the defect develops from small stress fractures that fail to heal and form a chronic nonunion.
Is a true congenital spondylolisthesis that occurs because of malformation of the lumbosacral junction with small, incompetent facet joints.
Very rare, but tends to progress rapidly
Often associated with more severe neurological deficits.
Is the most commonly found type of spondylolisthesis in people under 50 years of age.
It is believed that "biomechanical stress," such as repetitive mechanical strain from heavy work or sports, causes a fatigue fracture within the pars interarticularis.
This type of Isthmic spondylolisthesis is characterized by a elongated pars without separation.
It is believed that the elongation occurs secondary to "repeated, minor trabecular stress fractures of the pars." Each time these possible sub-acute stress fractures occur and heal, the vertebral body is displaced farther and farther forward. Eventually, the pars may fail to heal and result as a full pars defect.
These types of spondylolisthesis' are extremely rare and result from an acute pars fracture, often as result of traumatic lumbar hyperextension injury
This is the most common form of spondylolisthesis in patients over 50 years of age and rarely occurs in those under 50
There is no fracture or elongation of the pars interarticularis and the neural arch is intact. In contrast, patients with isthmic spondyolisthesis almost universally have widening of the central spinal canal at the level of the slip. This narrowing of the canal in degenerative spondylolisthesis has been termed the "napkin ring effect...