Imaging description

The pars interarticularis (or pars) is a short segment of the vertebra located between the superior and inferior facets of the articular process. Spondylolysis, also commonly referred to as a pars defect, is a unilateral or bilateral osseous defect in the pars interarticularis and is most common at the L5 vertebral body level (Figure 25.1) [1]. Pars defects usually result from dysplastic pars at birth exposed to chronic repetitive stress.

The radiographic appearance of pars defects varies with the age of the lesion.

Acute traumatic pars defects occurring in a non-dysplastic vertebral level are rare and result from high-energy trauma [2]. They are usually hyperextension injuries, and can be missed on plain radiographs [3]. Findings that suggest an acute injury include irregular bony edges, lack of soft tissue calcification, and associated fractures (Figure 25.1).

In contrast, chronic injuries typically have smooth, rounded, and corticated edges (Figures 25.2 and 25.3). Fibrocartilaginous material will develop in the gap, subsequently replaced by hypertrophic bone (Gill’s nodules) [4]. Other imaging signs of a chronic unilateral spondylolysis include deviation of the spinous process and sclerosis of the contralateral pedicle [5]. Collimated lateral radiographs of the region of concern can help in questionable cases, but unless the beam is tangential to the defect, it may be missed [1, 4]. A five-view radiographic series (which includes 45 degree obliques) has a 96.5% sensitivity for the detection of pars defects [4].