Spondylolisthesis is  the slippage of one bone over other, in the spine.  This is very common in gymnasts and athletes, due to repeated hyperextension motion.  One vertebra slips over the other due to the deficiency of their posterior connection called pars interarticularis.  The classification of spondylolisthesis is according to the amount of slip.  Grade 1 is upto 25% slip, grade 2 , 25% to 50%, grade 3 to 50% to 75%, grade 4 to 75% to 100% and grade 5, is also known as spondyloptosis, is slip more than 100% and vertebra coming in front of each other.
Ethology of spondylolisthesis is 1.  Congenital, pars interarticular is defect present since birth 2.  Traumatic, most common in gymnasts or repeated trauma in adults 3.  Degenerative, happens with age due to damage of facet joints and loosening of capsule 4.  Tumours/ infections 5.  Iatrogenic, due to surgical damage of pars.

Spondylolisthesis causes low back pain and later numbness and pains in the legs, back pain increases with hyper extension of the back, the pain in the legs and numbness increases with standing and walking, called neurogenic claudication.  Spondylolisthesis can be easily diagnose by plain X-RAY but neurological compression is detected by MRI.  

Just the pars defect can be treated by procedure to repair the pars by stabilising it and adding bone graft around the defect. pars can be fixed with wires or pedicel screws and rod.  If the intervening disc is also degenerated then, spinal fusion is a good option to cure backache comprehensively.  
Back pain can be effectively treated by medicines, epidural injection and physiotherapy but failure of conservative trial is the indication for surgery.
Surgical Treatments Of Spondylolisthesis, Comprises of in situ fixation and fusion or reduction of the slip and instrumented spinal fusion using special reduction pedicel screws and technique along with inter body cages and bone grafting.