PARS DEFECTS
The pars interarticularis is a narrow area of bone located between the superior and inferior articular processes
The normal pars comprises an anterior and posterior cortex with a thin medullary column between
The anterior cortex is usually a little thicker than the posterior
Two patterns are seen
• A horizontal defect within the thinner part of the pars
• A vertical defect at the junction of the pedicle and pars
Both are likely stress related but there is some question whether the horizontal type may also be related to congenital posterior element defects
These are more generally referred to as pars defects
The vertical type more often called pars stress fractures
Look for
• Loss of normal T1 marrow signal in the narrow pars and adjacent pedicle
• A continuous column of normal marrow signal is a reliable indicator
• 512 matrix is useful for accurate depiction
• Bone oedema in the pars on STIR
• Bone oedema in the pedicle on STIR
CLASSIFICATION
• 0 Normal
• 0a No marrow oedema. Bony sclerosis/cortical thickening
• 1 Pars stress No Fracture
• 2 Anterior cortex fracture
• 3 Anterior and posterior cortex fractured
• 4 No oedema but established bone defect
EVOLUTION AND HEALING (CT often more helpful)
• A Medullary sclerosis presumed callus around incomplete fracture
• B Medullary sclerosis around complete fracture
• C Medullary sclerosis around complete fracture with corticated margin*
• D Medullary sclerosis no/healed fracture
*sometimes designated a grade 4 in the previous classification
NOTES
• May need CT to differentiate 1 and 2
• T1 VIBE proposed as alternative to CT
• A could be progressing or healing - may need FU studies
• Repetitive healing/fracture cycles can be difficult to sort out
• SPECT CT may assist with these patients, but carries a high radiation burden