Causes and Mechanisms
It is typically caused by stress fracture of the bone, and is especially common in adolescents who overtrain in activities. It has been proposed that the pars interarticularis is especially vulnerable when the spine is in an extended position, and a force suddenly presses the vertebrae together, such as when landing on one's feet after a hop. This pressure acts like a nut-cracker on the pars interarticularis and can fracture it in susceptible individuals.
Spondylolysis (spon-dee-low-lye-sis) also runs in families and is more prevalent in some populations, suggesting a hereditary component such as a tendency toward thin vertebral bone.
Although this condition can be caused by repetitive trauma done to the lumbar spine or strenuous sports such as football or gymnastics, anatomy also plays a major role. According to research done in 2007, genetic make up of the lumbar spine has much to do with the occurrence of spondylolysis in certain individuals. A study done on 115 male skeletons with L5 spondylolysis was proof that a slight tweek in the anatomy of the spine can increase the likelihood of spondylolysis. In those with spondylolysis, many times the inter-facet region of vertebra L4 is more trapezoidal in shape with a larger width than that of a normal vertebra. This also changes the iner-facet height, making them shorter and narrower. With this defect in the spine, a normal load applied to the spine will be greater compared to those without it. This is due to reduced surface area and torsional range of motion during twists and bends.
Spondylolysis is the most common cause of spondylolisthesis (spon-dee-low-lis-thee-sis) in pediatric patients with a classic presentation that of a female gymnast. In the older population, degenerative disc disease commonly leads to spondylolisthesis without spondylolysis; in these instances, the spinal canal narrows because the spino-laminar arch at one level slides forward on the lower level effectively flattening the canal. The hereditary factor mentioned above is quite notable, since the frequency of spondylolisthesis among the Inuit peoples is 30–50%; however, there is controversy about this, as can be seen on the wiki page for spondylolisthesis: "(This statistic is controversial, however, because further anthropological studies have shown that the occurrence of spondylolysis in Inuit people living in Western-style communities is within the normal variancy at 7%. These people are hereditarily linked to the study group showing ~40%. It is theorized that the nomadic Inuit have a higher incidence of spondylolysis due to trauma acquired as infants by being carried in an amauti. While in an amauti, the baby is put into compressive extension with each step taken by the mother. Also, all other studies have shown a normal variance of spondylolysis within cultures of 6-10%, further suggesting that spondylolysis is an environmental abnormality and that a hereditary tie is very unlikely).
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