Spondylolisthesis is the slipping of all or part of one vertebra forward on another vertebra. The most common type of spondylolisthesis is isthmic spondylolisthesis and it is commonly seen in children and young adults. It is most common in children 5-8 years old and it occurs in about 5% of the population. For gymnasts, it is thought that the slippage occurs as a result of repetitive hyperextension activities (bending backwards) which causes a shear stress at the pars interarticularis.
What is spondylolysis?
Spondylolysis is a stress fracture or defect of the pars interarticularis in a vertebra. A stress fracture can occur at the pars interarticularis due to repetitive movements of extension and rotation which leads to an increase in shear forces in the lumbar spine. As shear forces increase, pressures on the facet joints increase. These forces are then transmited to the pars interarticularis. Since the pars interarticularis is small, it is unable to absorb repetitive shock which leads to the stress fracture.
What is a vertebra?
Your vertebrae are the bones in your back which make up your spinal column or back bone. There are 7 cervical vertebrae (1), 12 thoracic vertebrae (2), and 5 lumbar vertebrae (3) in your spinal column. The function of your vertebrae are to give support to your skeleton, protect your spinal cord and to provide movement through the many muscular attachments and joints formed between each individual vertebra. Both spondylolysis and spondylolisthesis usually occur at the 5th lumbar vertebra. Less common is at the 4th lumbar vertebra.
What is the pars interarticularis?
The pars interarticularis is the part of the vertebra which is between the superior and inferior facets.
Can spondylolisthesis and spondylolysis occur together?
Yes, spondylolisthesis and spondylolysis can occur together. But, having one doesn't mean you'll develop the other one. Usually, a spondylolysis will occur before a spondylolisthesis. But once vertebral growth is complete, it is rare to see a spondylolisthesis progress to the next grade.
Grades of spondylolisthesis:
Grade I - < 25% slippage of one vertebra on top of another vertebra.
Grade II - 25-50% slippage