This production: Isthmic Spondylolysthesis (Stress Fracture of the Back Portion of the Vertebra) Video–describes how the back section of the vertebra develops a stress fracture in both sides of the bony portion called the pars interarticularis. Isthmic spondylolisthesis is the slipping of a vertebra because of a fracture and is a problem that typically occurs with a fracture in the bone that connects the upper and the lower facets. The most common area for this to occur is L5-S1. In this video, you will understand how this portion of the vertebra acts as a door stop and when it is fractured, how the body of the vertebra can slide forward. This forward slide can cause pain in the back and eventually leg pain if the nerve gets caught in the slip.