The pars interarticularis is the bony bridge in the vertebral neural arch connecting the superior and inferior articular processes, the narrow isthmus between the facet joints at each level. It is most susceptible to stress fracture at L5, where it resists the shear force between the L5 superior facets (pulling posteriorly with the spine) and the L5 inferior facets (fixed to S1). A bilateral pars defect allows the vertebral body to slip anteriorly (isthmic spondylolisthesis).
| Articulations |
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|---|---|
| Landmarks |
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| Ossification | Ossifies from the neural arch centres in fetal life; defects here are usually acquired stress fractures rather than failure of ossification. |
Spondylolysis (isolated pars stress fracture) is the most common cause of low back pain in young athletes, particularly in sports involving repeated hyperextension (gymnastics, cricket fast bowling, rowing). Single photon emission CT (SPECT) identifies acute stress reactions before plain radiograph changes. Oblique lumbar radiographs show the Scottie dog sign, with the pars defect appearing as a collar around the dog's neck. Acute pars fractures in young athletes are treated with a rigid brace for 6 months. Bilateral pars defects with greater than grade II spondylolisthesis require surgical fusion.
Repetitive hyperextension loading in young athletes fractures the pars interarticularis at L5, producing activity-related low back pain reproduced by the single-leg hyperextension test; SPECT or MRI identifies early bone stress reactions before radiograph changes, managed by activity modification and bracing for acute lesions.
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