We performed a prospective roentgenographic study to determine the
incidence of spondylolysis, spondylolisthesis, or both, in 500 unselected
first-grade children from 1955 through 1957. The families of the children
with spondylolysis were followed in a similar manner. The incidence of
spondylolysis at the age of six years was 4.4 per cent and increased to 6
per cent in adulthood. The degree of spondylolisthesis was as much as 28
per cent, and progression of the olisthesis was unusual. The data support
the hypothesis that the spondylolytic defect is the result of a defect in
the cartilaginous anlage of a vertebra. There is a hereditary
pre-disposition to the defect and a strong association with spina bifida
occulta. Progression of a slip was unlikely after adolescence and the slip
was never symptomatic in the population that we studied.