Isthmic Spondylolisthesis

Isthmic spondylolisthesis is forward slippage of one vertebra over the one below, caused by a defect (fracture or elongation) of the pars interarticularis — the narrow bridge of bone connecting the superior and inferior articular processes. When both pars are fractured, the posterior restraint fails and the vertebral body slides forward relative to the sacrum or next lower vertebra. The L5–S1 level accounts for approximately 75% of cases; L4–L5 is the next most common. The underlying pars defect, called spondylolysis, develops most often in adolescent athletes — gymnasts, football linemen, wrestlers — due to repetitive hyperextension loading. A pars stress fracture may heal with rest, or it may progress to bilateral defects and eventually listhesis. Adults may present decades later when disc degeneration at the slipped level causes nerve root compression or spinal stenosis. Management depends on the grade of slip (Meyerding I–IV), symptoms, and neurological status. Mild slips in adolescents often respond to activity modification and physical therapy. Adults with significant radiculopathy, neurogenic claudication, or slip progression typically require surgical decompression and fusion with pedicle screw fixation to restore alignment and stability.