What is Pars Defect. Pars defect is a painful condition that affects the lower spine. It is actually a stress fracture that occurs in the region of the vertebrae called pars interarticularis (or just pars).The pars interarticularis acts as a bony bridge between joints, and helps to keep the vertebra in place.ars defect is believed to be caused by repetitive strain to the lower back or lumbar section of the spine, and typically affects the fifth and fourth lumbar vertebrae.