"You are as strong as your backbone is" is a common old adage.  The backbone must not only be mobile but also strong to support our body and protect the spinal cord.  Stability is vital to allow movements between the bony parts of the spine, to carry loads, to provide posture and form to the body and to protect the spinal cords and nerves. Instability can produce pain, loss of function and also loss of strength. Degeneration  of  disc  or  the joints in the spine or break of the posterior arch (Lysis) can lead to abnormal forward movement of one vertebral bone over the other (spondylolisthesis).  This will lead to pain in the back and later to nerve problems due to pressure on the spinal cord.

Simple measures like weight reduction, exercises and a support by the use of a lumbar belt will alleviate pain in the majority of patients but frank instability often needs surgery to stabilise the spine.  Bone grafts are frequently used to achieve spinal fusion and a variety of interpedicular instruments are used to achieve stability and aid global fusion. Surgery for instability needs precision in technique and high requirement in instrumentation. The advances in spinal surgery like the Image Guidance system have increased the safety and accuracy of these procedures. 

Frank instability causes neurological deficit and slippage of one vertebra over the other. (Spondylolysthesis).  The spine has been stabilised by the use of posterior instrumentation and postero lumbar interbody fusion (PLIF).

Instability has been caused by the removal of the disc leading to severe pain and neurological deficit to the patient.  Spine was stabilised by the use of interpedicular screw ad interbody fusion by titanium cages.