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• • progression of deformity before or after surgery
  • pseudoarthrosis (fusion failure)
  • anatomic compensation changes of the sacrum as the slip progress - the sacrum may become more vertically oriented to accommodate the slip of the upper vertebra
  • lumbar index - this quantifies the shape that L5 takes as a higher slip grade develops. It is used mostly to assess the risk of slip progression in children.

If the diagnosis is not clear from X-ray or if there is a need to see specific areas of the spine more clearly, other imaging modalities may be used including:

• Single-photon emission computed tomography (SPECT) - this is a bone scan that uses a radioactive isotope called technetium (Tc99-m) to view the spine and helps in early identification of stress injuries to the pars interarticularis. SPECT helps in localization of defects and fracture-healing potential. If a bone scan is positive, it indicates that slippage may still be ongoing which can impact therapy. If it is negative, the slippage is no longer acute and the potential for benefit from certain therapies such as fusion surgery is reduced.
• Computerized axial tomography (CAT scan) - very effective in the diagnosis of spondylolysis, disk degeneration, facet joint changes, stenosis, and presence or absence of bony structures around healed microfractures of the pars as well as cracks, unhealed microfractures, and the degree of severity of spondylolytic defects.

• Magnetic resonance imaging (MRI) - this is not as effective as CAT in visualizing bony detail but has the advantage of avoiding use of radiation. MRI is effective in visualizing:

  • soft tissue structures (nerves and disks between vertebrae) and their relationship to the vertebra
  • if adjacent disks have suffered damage (such as "wear and tear") because of the slippage
  • signs of edema (swelling) around the spondylolytic defect
  • nerve root compression and the presence of spinal stenosis

CAT scan and MRI are performed if there is a need to see bone damage more clearly than is visualized on an X-ray or if there is a need to determine the presence of nerve compression.