Spondylolysis refers to failure of fusion of the pars interarticularis, most often at the lower lumbar spine. This is usually a congenital defect, although it may occasionally be post-traumatic. The process may affect one or both sides of the spine. Oblique radiographs are best for the detection of this abnormality.

A	Oblique radiograph of the lumbar spine. This shows a pars defect.
B	Opposite oblique radiograph of the lumbar spine. This shows another pars defect at L5-S1.
C	Coned radiograph of the L5-S1 region. The abnormalities are difficult to appreciate.
D	Axial CT. This image confirms bilateral pars defects at L5-S1.

Links to online textbooks:

• Wheeless' Orthopedics: discussion of pars defects

Links to online cases:

• Uhrad.com
• Washington University

References:

• Saifuddin A, White J, Tucker S, Taylor BA: Orientation of lumbar pars defects: implications for radiological detection and surgical management J Bone Joint Surg Br. 1998 Mar;80(2):208-11.           [See related articles]
• Harvey CJ, Richenberg JL, Saifuddin A, Wolman RL: The radiological investigation of lumbar spondylolysis Clin Radiol. 1998 Oct;53(10):723-8.           [See related articles]