The inherent mechanical flaws of the pars interarticularis in combination with the high-stress loads seen in the lower lumbar region render this region prone to stress fractures.Īdditionally, there has been a strong association reported with spina bifida occulta. Furthermore, the pars in the lower lumbar vertebra characteristically have uneven trabeculation and cortication. Mechanical factors include the physically narrow structure of the pars interarticularis as compared to other regions of the vertebrae. The weakness of the pars region is multifactorial, with a hereditary and an acquired mechanical component. The pars interarticularis is most susceptible to chronic axial loading injury because it is a weak point in the vertebrae, and this region bears the highest stress load in extension/flexion. As discussed, although generally thought to be the result of chronic repetitive stress to the pars region, these injuries can also occur due to a single acute overload injury. Additionally, this theory is supported by the progression of unilateral pars defects into bilateral pars defects with age, again suggesting repetitive axial loading over time, both leading to the initial injury as well as disease progression. This theory garners support from the fact that, as noted below in epidemiology, the research observed zero cases of pars defects in 500 newborns and zero cases of pars defects in 143 non-ambulatory patients, suggesting this pathology develops as a result of repetitive axial loading over time. Currently, the most accepted theory is repetitive mechanical stress, specifically lumbar extension and rotation, which results in overuse or stress fracture to the pars interarticularis. Depending on the time of presentation and degree of injury, most cases of pars defects respond well to conservative treatment and relative rest from sport. As such, the diagnosis of a pars interarticularis defect confirmation is only with radiographic support. Although this history is typical, there is a broad differential diagnosis that might explain these symptoms. The second common presentation is an adolescent athlete involved in a sport requiring repetitive lumbar loading in extension and rotation, presenting with acute or insidious onset low back pain that is aggravated by continued lumbar loading. Two common clinical presentations of a pars defect include the imaging of an asymptomatic adolescent or adult in whom there is the incidental discovery of a pars defect. Grading of spondylolisthesis is included below in “staging.” Spondylolisthesis can be graded based upon the percent degree of displacement of one vertebral body compared to the other. In cases of bilateral pars interarticularis defects, there is the potential for anterior or posterior spondylolisthesis (the slipping of one vertebral body relative to the adjacent segment). Though history can be suggestive, especially in the case of young athletes involved in higher-risk sport (see below), diagnosis is made radiographically by the presence of fracture through the pars interarticularis. This injury occurs almost exclusively in the lower lumbar region, most often at L5. The definition of pars interarticularis defect is a unilateral or bilateral overuse or fatigue stress fracture involving the pars interarticularis of the posterior vertebral arch. Anatomically, one can describe the pars as the region between two, one superior and one inferior, zygapophyseal joints. The pars interarticularis (pars) lies between the superior and inferior articular process bilaterally at each vertebral level. Pars interarticularis defect (otherwise referred to as spondylolysis) represents a common cause of axial back pain in adolescents, especially in the case of young athletes.
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