ABSTRACT
The lateral image demonstrates anterior slip of L5 on S1 by about 25 per cent of the vertebral width, otherwise termed spondylolisthesis or anterolisthesis. The degree of slip is graded in steps of 25 per cent (i.e. <25 per cent grade 1, 25-50 per cent grade 2, etc.). A lucency is seen through the pars interarticularis – the portion of the neural arch that connects the superior and inferior articular facets, suggesting a bony defect. This is otherwise known as spondylolysis. On oblique radiographs, the posterior elements form the appearance of a Scottie dog and a break in the pars interarticularis has the appearance of a collar around the neck.
