spondylolysis or pars defect

Spondylolysis is the rendering incompetent the bony ring, or arch, at the back of the segment that notches the facet joint surfaces together. The bone is usually broken by the spine taking savage impact up through the back of the spine instead of through the brilliant impact-defraying mechanism of the disc-and-vertebral-body at the front. This injury is a type of stress fracture of the spine. 

Spondylolysis is the weakening of the bony catch at the back of the spine by breakage of by the pulling the bone, like stretching hard toffee 

Most commonly, the bone is broken by landing heavily on the heel (or bottom) with the spine in the arched position. Stress fractures such as this are common with fast bowlers in cricket who land heavily and repeatedly on one heel with the lower back extended. The Inuit population haver a very high incidence of spondylolysis and spondylolisthesis (see below) from falling on their bottoms on the ice.  

Bilateral spondylolysis makes the vertebra less stable to slip forward, out of line with the other segments, in the condition called spondylolisthesis

The bony break itself is known as a pars break. It happens through the neck of bone that supports the lower opposing surface of the facet joint. The vertebra is rendered less stable if a breakage in the neural arch occurs on both sides. If the vertebra starts to slip forward as a consequence of the bony catch being weakened bilaterally, the condition is then termed spondylolisthesis.

There are four grades (I-IV) of spondylolisthesis, depending on how far forward the upper vertebra has crept forward off the one below. In some rare instances, the spine falls right off the front of the sacrum at the lumbo-sacral level. This is called spondyloptosis, the fifth grade (V) of this spine disorder. 

The Grades I-V of spondylolisthesis and spondyloptosis

In my many years of practicing, I have only seen one case of sponyloptosis. Alarming as it may look, this spine disorder is unlikely to be the result of a traumatic mishap. It is more likely to be caused by congenital malformation, known as a pars defect, of the neural arch and the facet joints (spinal bifida) in combination with degenerative changes.

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