Clasp-knife deformity

Last revised by Yuranga Weerakkody on 15 Feb 2018

Clasp-knife deformity is relatively common congenital anomaly found at the lumbosacral junction.

Terminology

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 2, also known as spina bifida engagement syndrome.

Pathology

It is characterised 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 the vertebra above (usually L5) 1-2.

History and etymology

The term of clasp-knife deformity was coined by G.W. Henry in 1958 as an analogy to the blade of a pocket (clasp) knife folding into its handle 3,4.

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