A defect of the pars interarticularis (usually L5), with or without spondylolisthesis; in adolescents, MRI shows the stress reaction before a fracture line, and CT shows whether the defect is established or healing.
Orient first
- Adolescent athletes (extension sports) with low back pain.
- Bilateral defects allow anterolisthesis, graded by Meyerding.
- The oblique radiograph "Scottie dog collar" is insensitive and adds dose.
Acquire the study
- Limited thin-section CT through L4 to S1 with sagittal and oblique reformats.
The manoeuvre
- Thin axial and sagittal reformats in bone window: pars defect with sclerotic (chronic) or sharp (acute) margins.
- Unilateral vs bilateral; incomplete defects.
- Slip and the contralateral pars.
What confirms it
- A pars defect on CT or a fracture line on MRI; stress reaction on MRI in the early stage.
What licenses you to exclude it
- Normal pars signal on fat-saturated MRI excludes an active stress reaction.
The classic misread
- Calling a normal pars defect on axial MRI due to slice obliquity.
- Ordering oblique radiographs in adolescents.