What’s the cause of this lumbar CT abnormality?
Vol.17, No.09, October 2009

This 39-year-old man complained of slowly progressive ongoing low back pain. No specific traumatic event was identified. He was otherwise healthy with normal lab values.

The lumbar CT scan abnormality as identified by the arrow most likely corresponds to:

a) an acute L4 body fracture
b) a lumbar chordoma
c) L4 body spondylolysis
d) vacuum phenomenon in the facet joint
e) benign Tarlov cyst of the dura


The CT scan abnormality involving the L4 vertebral body most likely corresponds to: c) L4 body spondylolysis

The lucent defect, as identified by the arrow in the CT image shown, corresponds to a bony defect in the pars interarticularis portion of the posterior elements of the L4 vertebral body (see also diagram 1). It was present on both sides (bilateral; not shown). Note is also made of mild anterior slippage of the L4 body upon the lower L5 body, known as Grade I spondylolisthesis. This has resulted from the posterior element defects.

The exact origin of the spondylolysis defects is up for debate. Many suggest they result from repetitive microtrauma, leading to stress fractures and eventually to full pars fractures. Others portend that the defects may be congenital, or at least result from congenital susceptibility to their development, with weakness in the pars regions of those affected bones at birth. Perhaps a combination of the two is the most likely answer. Certainly such defects can also result from acute high impact trauma, although that is not the case in the vast majority of cases discovered, including the patient presented. Spondylolysis and spondylolisthesis are the two most common causes of back pain in children, present in up to 6% of individuals by age 6. It’s also commonly identified as a source of low back pain in adolescents and young adults. The most common site is the L5 body. Many, if not most, cases of spondylolysis, though, are asymptomatic, discovered incidentally on lumbar or abdominal CT scans or MRIs performed for other reasons.

The majority of patients with spondylolysis can be treated conservatively. Such measures may include back muscle and core abdominal muscle training, posture improvement, and, in more acute or severe cases, back braces. The dilemma arises for young active athletes who would not want to scale back their activities to allow for adequate healing (if such a process — healing — can occur in these defects; even that occurrence is debated). Some patients may actually require open spinal stabilization surgery to prevent further spondylolisthesis and reduce local pain development.

There are no destructive features present on the CT image shown, which would rule out the presence of a chordoma. A chordoma is a rare malignant neoplasm arising from cellular remnants of the notochord, most commonly developing in the clivus or in the sacrum. The margins of the defect are well corticated and reveal a thin line of ossification, consistent with a long-standing lesion (thus ruling out acute L4 body fracture). Vacuum phenomenon refers to air escaping from solution into the joints (often seen in lumbar discs and facet joints), usually associated with degenerative changes. The air would be markedly black on the images and would have to reside within the facet joint for option d) to be true. There is no facet joint air present in the image shown. A benign Tarlov cyst refers to fluid-filled pockets of dural ectasia, commonly seen as cyst-like structures in the sacral region. These benign lesions can cause adjacent bone remodelling from pressure effect. They are typically asymptomatic and picked up incidentally on scans performed for other reasons. These are not a viable option in this case.


  1. Wilms G, et al. American Journal of Neuroradiology 2009;30(4):674-80.
  2. Wong C-H, et al. Spine 2003;28:E347-50.
  3. Ulmer JL, et al. American Journal of Roentgenology 1997;169:233-9.

Spondylolysis diagram from website: http://www.eorthopod.com/public/patient_education/6572/lumbar_spondylolysis.html


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Clinical challenge image
Figure 1: Sagittal CT through the lumbar spine revealing an additional lucent line in the posterior elements of the L4 vertebral body (black arrow).
Clinical challenge image
Diagram 1: The black line indicates the pars interarticularis defect in the L5 vertebral body in this case. Compare this with the sagittal CT image of the L4 body on the patient presented.
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