Spondylolisthesis is a condition in which one vertebral body slips forward over the one below (subluxation). This may be due to a congenital issue (present at birth), trauma to the spine or it can be a result of degenerative and aging changes to the spine.

A spondylolysis is a term to describe a gap or a nonunion of a portion of the spine known as the pars interarticularis. This gap may be congenital or it may result from trauma to the spine. Spondylolisthesis can also occur when the joints connecting the different components of the spine become weaker, and allow one bone to slip on the other; in these cases a defect of the pars interarticularis does not exist.

When spondylolisthesis occurs, the nerves passing through the central canal may become compressed and the nerves leaving through the neural foramen at the level of the slip may become compressed as well. Also, a patient with spondylolysis has a higher likelihood of suffering from a slip of the spine.

Spondylolisthesis is graded depending on the amount of slippage of one bone on the other. Grade I refers to a slippage of one to 24%. Grade II refers to a slip of 25 to 49%. Grade III is a slip of 50 to 74%. Grade IV is a slip of 75 to 99%. Grade V is a slip of 100% or greater.