This word is derived from the Latin 'spondy' meaning 'spine' and 'listhesis' meaning 'slip'. Spondylolisthesis is therefore a deformity of the spine where one vertebral body has slipped forward or backward on the other. Anterolisthesis describes a forward slip and retrolisthesis describes a backward slip.

Figures 1 and 2 show examples of anterolisthesis of L4 on L5. The grading of listhesis is on how far the slip has progressed expressed as a percentage of the whole length of the vertebral body. You can see from the diagram on the left how this affects the nerve root behind the slip. There is nothing special about the ways in which spondylolisthesis causes back and leg pain. If you have read Causes of Back and Neck Pain page and the Radiculopathy page, the very same mechanisms are responsible. Disc degeneration as a result of the slip or disc degeneration as the cause of the slip doesn't matter – the same mechanisms cause pain. The nerve gets trapped or compressed as the slip progresses, and put on stretch the greater the grade of slip causing radicular pain. The facet joints behind the slip suffer even more than in the presence of simple degeneration and normal lordosis is lost.

The causes of spondylosthesis are well defined. Simple degeneration in a disc which has deflated can always produce a very minor listhesis (either antero or retro) as a result of losing tension in the ALL and PLL and there thus being a bit of slack in the system. Many people with higher grades of listhesis have what is known as a pars defect. The posterior bony elements of the spine protect the spinal cord as we discovered in The Spine - Basic Anatomy and Mechanics page. One of the ways in which the spinal cord is protected is a bony structure called the pars interarticularis which is a bony strut between a facet joint above and the one below. It acts as a check-reign to prevent one vertebral body slipping forward on the one below. In some people it is defective, and this is the pars defect.

Figure 3 shows an x-ray where the label 'A' is sitting in the defect, and you can clearly see how this allows the slip. This is typically a developmental phenomena, not a congenital one. In other words, no-one is born with this but during growth it occurs and usually appears from the age of 6-12. Males are twice as affected as females. Most do not cause symptoms, and if they do usually it is in adult life as a result of disc degeneration in between the slip. There is a rarer congenital spondylosthesis called a dysplastic spondylolisthesis where the defect in the posterior elements is more pronounced and the sacrum is rounded and malformed – these can be dangerous ones which sometimes involve the whole of the body of L5 'falling off' the sacrum – a condition called spondyloptosis.

The old-fashioned method of treating painful spondylisthesis was simply to fuse it in the slipped position with no attempt made to reduce the slip to its normal anatomical position. This is called a 'fusion in situ'. Certainly, risks are involved in reducing a slip to its normal anatomical position but there is very little doubt nowadays that better results are achieved by reducing the slip 'as much as is safe for that individual patient' and then performing a stabilisation procedure to keep it there.

Patients who have a significant spondylolisthesis cannot have a total disc replacement and are only suitable to be reconstructed using fusion technologies – usually from in front (anterior) and behind (posterior).