The planar images show a mild focal area of increased activity in the posterior process of the L5 vertebra bilaterally. Less common levels are L4 and L3. When the defects are bilateral, spondylolisthesis may subsequently occur.
Spondylolisthesis occurs in roughly two-thirds of adults with bilateral pars defects.
The etiology of spondylolysis is not fully known. However, common theories include that this may result from fatigue fracture related to repetitive trauma. The pars interarticularis may be hypoplastic making it prone to fractures. Pars defects are associated with athletes involved in contact sports. This suggests that hereditary factors play a role in the pathogenesis as well as traumatic factors.