The spinal column has three joints that work together at each motion segment: the intervertebral disc, and two facet joints, one on each side. The pars is a bridge of bone connecting the facet joints.
At L5-S1 and to a lesser extent at L4-5, repetitive extension of the spine can cause a stress fracture in the pars. This commonly happens in active teenagers. Most acute pars fractures don’t completely heal.
If there is a pars fracture, the facet joints are no longer connected to each other. The disc bears an increased load and is subject to disc height loss and spinal listhesis, or slipping forward.
Patients can be very active with pars defects. There are examples of successful professional athletes with this condition.
Patients seek surgical treatment primarily when the nerves become pinched as the disc degenerates.