Background/Objectives: Andersson lesions are uncommon discovertebral complications of ankylosing spondylitis and may be overlooked when a new mechanical pain pattern is attributed to chronic inflammatory disease. This report describes the diagnostic reasoning, operative decision-making, and short-term outcome of a severe lumbar lesion with three-column instability. Methods: A 38-year-old woman with a 15-year history of ankylosing spondylitis presented with progressive low-back pain, left L5 radiculopathy, and marked functional decline without major trauma. Radiographs, computed tomography, and magnetic resonance imaging were used to characterize the lesion and assess spinal instability and neural compression. A Cawley type E Andersson lesion at L4/5 with severe three-column instability was diagnosed. The patient underwent single-stage posterior–anterior circumferential reconstruction consisting of L4/5 decompression, long-segment L2-S2 fixation, anterior debridement, and tricortical iliac crest grafting. Results: A postoperative gastrocnemius intramuscular venous thrombosis was detected and treated with anticoagulation. At 3 months, the activity-related visual analog scale score improved from 8 to 1, the Oswestry Disability Index improved from 68% to 18%, independent ambulation was restored, and imaging showed stable instrumentation with progressive osseous bridging. At the latest 9-month telephone follow-up, the patient reported maintained pain relief, mobility, and daily function; no new imaging was available. Conclusions: A change from inflammatory to mechanical pain or the development of neurologic findings in ankylosing spondylitis should prompt targeted imaging for an Andersson lesion. In carefully selected patients with severe three-column instability and substantial anterior column deficiency, circumferential reconstruction may provide effective short-term stabilization and functional recovery. Because radiographic follow-up was limited to 3 months, definitive fusion and long-term construct durability cannot be established.

Background/Objectives: Dysplastic spondylolisthesis is associated with developmental abnormalities of the lumbosacral junction, but the contribution of extremely sagittal L5-S1 facet orientation in an adolescent with an intact pars interarticularis remains incompletely characterized. We aimed to describe this unusual quantitative morphologic phenotype and discuss its possible biomechanical implications. Methods: Clinical records and serial imaging of a 14-year-old girl with symptomatic low-grade L5-S1 dysplastic spondylolisthesis were reviewed. Standing radiographs, flexion-extension radiographs, computed tomography (CT), magnetic resonance imaging (MRI), quantitative spinopelvic parameters, intraoperative findings, treatment, and 12-month follow-up were assessed. Results: The patient had approximately one year of progressively worsening low back pain and occasional transient right-leg numbness despite more than 8 months of lumbosacral bracing and more than 6 months of supervised physical therapy. Standing radiographs showed Meyerding grade I L5-S1 anterolisthesis and mild thoracolumbar scoliosis. Quantitative spinopelvic assessment showed a pelvic incidence (PI) of 76.63°, pelvic tilt of 22.21°, sacral slope of 54.42°, lumbar lordosis of 59.5°, sagittal vertical axis of 2.67 cm, and a Dubousset lumbosacral angle of 109°. CT demonstrated an intact pars interarticularis, bilateral facet subluxation, and L5-S1 facet angles of 6° on the left and 3° on the right relative to the sagittal plane. Flexion-extension radiographs showed measurable L5-S1 angular motion from −6.7° to +3.5°, while MRI showed no major central canal stenosis, focal disc herniation, or definite neural compression. Posterior reduction and L5-S1 fusion were performed after individualized counseling, and facet subluxation was confirmed intraoperatively. At 6 and 12 months, the patient remained free of recurrent pain or numbness and radiographs showed maintained lumbosacral alignment. Conclusions: Quantitatively extreme L5-S1 facet sagittalization accompanied by bilateral facet subluxation, an intact pars, and high-PI low-grade spondylolisthesis represents a distinctive morphologic phenotype. This configuration is biomechanically compatible with reduced resistance to anterior shear, although a single case cannot establish independent causality or justify a general indication for early surgery. Comparative quantitative imaging and longitudinal studies are required.