Clasp-knife deformity is relatively common anomaly found at the lumbosacral junction and characterized by a large midline defect of the posterior arch of the sacrum (usually S1/S2 and occasionally S3) along with hypertrophy (or overgrowth) of the spinous process of vertebra above (usually L5).

折刀畸形是较常见的腰骶部异常；特点是骶骨后弓的中线的大块缺损（通常S1/S2和偶尔是S3）伴其上椎体（通常是腰5）棘突肥大（或过度生长）。

When a Clasp-knife deformity is accompanied by pain on extension secondary to protrusion of the enlarged spinous process (knife blade) into the sacral spinal canal, it is called Clasp-knife syndrome.