Clasp-knife deformity

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

Clasp-knife syndrome, is one of many causes of low back pain. It occurs 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.

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 called spina bifida engagement syndrome.

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.

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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Article information

rID: 22826
Sections: Pathology, Syndromes
Tag: cases
Synonyms or Alternate Spellings:
  • Clasp knife deformity
  • Clasp-knife syndrome
  • Spina bifida engagement syndrome
  • Knife clasp deformity

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