WebDec 14, 2017 · Pars defect describes a crack or fracture in your pars interarticularis which is a small connecting bone in your lower spine. A defect in the pars interarticularis is a common cause of lower back pain and can result in pain when walking and stiffness in the lumbar region. A pars defect can also cause radiated pain in the buttock, thighs, or the ...