| Discussion: |
A pars interarticularis defect (also called spondylolysis), which consists of an interruption of the vertebral arch at the bony bridge that holds together the superior and inferior articular processes (pars interarticularis). The defect is usually seen at L5 (82%) or L4 (15%), and rarely at L3 (1%). It may be unilateral but it is most commonly bilateral. If it is complete and bilateral a virtual separation ensues in the vertebra between the body, (with pedicles, transverse processes and superior articular process) posteriorly. With time, a sliding anterior movement of the vertebra (spondylolisthesis) is produced and the defect may become symptomatic by compression on neural structures. In up to 80% of cases at diagnosis an associated spondylolisthesis is observed. The pathogenesis is controversial. According to the most widely accepted theory the defect is in fact a fracture resulting from repeated pauci or asymptomatic microtrauma. This theory is substantiated by the consideration that the pars interarticularis at L5 level is the structure most exposed to mechanical constraint at the lumbosacral junction and is sheared during extension by the inferior facets of L4 and the superior facet of S1. Furthermore, the fracture line is often associated with the formation of a callus with irregular and sclerotic margins similar to that of healing fractures.
On imaging with X-rays the finding is often visible in the lateral projection but it is best depicted in the oblique projections in which the upper and lower articular processes are degaged. An image of little dog (the "Scottie dog" sign) is produced in this projection by the structures of the vertebral arch in which the pars interarticularis represents the neck.
On conventional 3 or 4 mm axial CT scan images the representation of the defect is often somewhat disapppointing and in many cases it is suggested by the associated presence of the callus and by the elongation of the vertebral canal rather than directly visualized. This depends on the fact that plane of acquisition on conventional studies is usually that of an intervertebral space and at L5-S1 level this approaches perpendicularity to the pars interarticularis. Reconstruction on thin acquisition sections or an opposite obliquity of acquisition plane are needed best to reproduce the defect, as well as complete exploration of two adjacent vertebrae.
Contribution of MR in this setting is limited.