A 28-year-old is diagnosed with isthmic spondylolisthesis (Meyerding Grade II) of L5-S1 with persistent radiculopathy after 6 weeks of conservative treatment. The pars defect in isthmic spondylolisthesis occurs at which anatomical structure?
- A Pedicle of the vertebra
- B Pars interarticularis (isthmus) — narrow bridge between superior and inferior articular processes ✓
- C Lamina of the vertebra
- D Superior articular process of the subjacent vertebra
Correct answer: B. Pars interarticularis (isthmus) — narrow bridge between superior and inferior articular processes
Explanation
The pars interarticularis (isthmus) is the narrow bony bridge connecting the superior articular process, lamina, and inferior articular process of a vertebra. Stress fracture through this structure (spondylolysis) allows the anterior elements (vertebral body + superior articular processes + transverse processes) to slip forward relative to the posterior elements, producing isthmic spondylolisthesis. L5-S1 is the most common level. Meyerding Grade II = 25-50% anterior slip. Bilateral pars defects are required for forward slippage to occur.
Reference: Maheshwari Essential Orthopaedics, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.