Spondylolisthesis refers to a process in which one vertebra slips on top of the other and what we usually see is the vertebrae slipping on top of the other in the lower back or the lower segments. That being said, it is important to remember there are different types of spondylolisthesis. There are actually six types of spondylolisthesis - or the reasons for that slippage to occur.
Before we talk about the specifics of it, the basic theory is that there are structures that are opposing that slippage that are maintaining one vertebra properly aligned on another. Something happens to those structures, allowing that slippage. So if we think about it that way, everything else begins to make sense. So trauma to any of the structures and fracture of that point will allow slippage forward. There is a very specific trauma - or a specific lysis, or in other words “breaking away of the structures” - which is called pars interarticularis and that happens due to additional stress to that structure, allowing that slippage to happen. Of course, other sources can be developmental and those people that develop those abnormal connections allow that slippage to happen from birth and as they develop. Also, another source is pathologic. Pathologic can be due to infection or pathologic due to a tumor in the bone, weakening that bone and causing the fracture.
In thinking about all of these, we have to remember that slippage - regardless of the source - will frequently develop into very similar symptoms. Of course, the severity of the symptoms will depend on the severity of the slippage. So what we have to do is look at the slippage and grade it. The way we look at the slippage is we get x-rays. Those are the easiest and best way to look at spondylolisthesis. Spondylolisthesis is looked at by looking at both the lateral (or view from the side), anterior (looking directly on), and oblique views. The oblique views, specifically, are used to look at these structures - the pars interarticularis - to see if there is an actual dissociation or lysis of that structure. Also, when looking at the lateral views, we are looking at how much slippage there is of one vertebra on another. We grade it by percentage of the slip and there are four grades. Each one is essentially 25% of the slip, so this would be grade one (just 25%), 25-50% would be grade two, and so on and so forth all the way to the condition called spondyloptosis, where one vertebra slips completely off another.
In looking at spondylolisthesis, as you would imagine, something allows that slippage to happen, so we must think about instability. It doesn't present a problem, necessarily, at the beginning, meaning grade one is unlikely to be unstable. But anything beyond that, especially with variable symptoms based on position of the body or exertion, can become unstable. The way we evaluate for instability is by using what's called "flexion-extension films" and those are x-rays that look from the side. What happens is that the patient flexes forward first and the snapshot is taken and then flexes backwards. That way we can actually see if there is a change in the position of that vertebra on top of another and we can measure what translation of one vertebra on another is. We have set thresholds to allow some motion before we consider it unstable.
Now, in talking about the symptoms, symptoms usually tend to be muscle spasms, muscle tightness. There can be tightness in the buttocks. Tightness in the hamstrings is very common because that stabilizes the pelvis and really counteracts the forces that have changed because of that slippage. If the slippage is large enough, we can actually see a step-off. If we run our hand on the spine, we actually feel a bump and that bump is that slipped vertebra forward. So, that can be an indication to consider spondylolisthesis. If spondylolisthesis slips far enough, it can actually affect the foramen. Now, the foramen is an opening between the vertebrae that allows the passage of the nerve. If this vertebrae slides forward far enough, it can actually compress/affect this nerve one way or another and that can cause what we call "radicular pain" - or pain shooting down the leg and affecting the nerve and the muscles that are supported by that nerve. That can very important, obviously, depending on the degree of that slippage.