Spondylolysis represents a defect in the pars interarticularis of the vertebra. It may be associated with spondylolisthesis. Spondylolysis is most frequently observed in the lumbar spine, with the fifth lumbar vertebra most commonly affected. The frequency diminishes on proceeding cephalad in the lumbar region. Symptoms and signs may be absent, and back and radicular pain, tenderness, gait abnormality, and neurologic deficits can be observed.
3-7% of vertebral columns reveal at least one area of spondylolysis. The frequency is greatest in athletes, with a male predominance of 2-4:1. Spondylolysis is typically discovered in childhood or early adulthood, and the frequency of the abnormality does not increase after the age of 20 years. Pars interarticularis defects are rare in infants. The frequency rises precipitously between 5-7 years.
Current consensus strongly supports an acquired traumatic lesion originating sometime between infancy and early adult life. Spondylolysis most frequently results from a fatigue fracture occurring after repeated trauma rather than from an acute fracture after a single traumatic episode. The pars interarticularis appears to be the vulnerable point when repetitive stresses act on the vertebral arch. An increase in lumbar lordosis as well as a more vertical attitude of the top of the sacrum may accentuate the stresses placed on this arch. Spondylolysis is virtually non-existent in patients who have never walked. There are differences between spondylolysis and other types of fatigue fractures: Spondylolysis frequently develops at an earlier age than other fatigue fractures; there is a hereditary predisposition; the fluffy periosteal callus formation that commonly is noted at sites of other stress fractures rarely is observed in spondylolysis; it develops after minor trauma; and the defect in the pars interarticularis commonly persists.
Spondylolytic families can be discovered in which over 25% of persons demonstrate a defect in the pars interarticularis. Furthermore, patients have an increased frequency of nearby congenital anomalies of the spine, such as transitional vertebrae and spina bifida. These findings may indicate not that the defect itself is inherited but rather that there is a genetic influence on the strength of the bone of the pars interarticularis that predisposes certain persons to stress fracture.
Plain film oblique views demonstrate a radiolucent area through the neck of the “Scottie dog”. Wedging and hypoplasia of the fifth lumbar vertebra in association with spondylolysis may create a radiographic appearance of spondylolisthesis when true slippage is not present. In cases of unilateral spondylolysis, hypertrophy and reactive sclerosis as well as fracture of the contralateral pedicle and lamina may be detected as a physiologic response to the presence of an unstable neural arch.
Scintigraphy can be a helpful diagnostic method in cases of low back pain of obscure cause in which a definite pars interarticularis defect cannot be verified. Increased uptake of bone-seeking radiopharmaceutical agents may indicate increasing stress in the pars interarticularis with or without a definite fracture. It has been suggested that scintigraphy allows the detection of more recently acquired and symptomatic spondylolyses, as such defects will accumulate the bone-seeking radiotracer, whereas older and non-symptomatic spondylolyses will not. Single photon emission computed tomography (SPECT) is more reliable than standard bone scintigraphy in the assessment of spondylolysis and related conditions.

EDITOR'S COMMENTS:
-SPECT is essential in the setting of back pain with normal planar scintigraphy; and can be extremely helpful in localizing an abnormality when one is present on the planar images. At NNMC, we routinely perform SPECT imaging on all patients presenting for evaluation of back pain.