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 The lumbar spine begins where the thorax ends. There are five vertebrae (usually) without ribs below the thoracic area and connecting to the sacrum.
So in a sense, the spine sits on the back side of a ring and that ring is a see-saw (teeter totter) balanced on the hips.
Muscles from the ribs to the front of the pelvis (especially the rectus abdominus or 6-pack) pull up the front of the pelvis. When the pelvis is pulled up in front the pelvis levels and the lumbar lordosis gets less. That feels good. Loose abdominal muscles let the front of the pelvis fall and the back is forced to arch. The tilt leans vertebral body L-5 forward. To level off the lumbar spine must extend backward. That does two things we really care about.
1) It forces the facet joints on the back together with great force and disallows the movement they need to get lubrication and sustenance (joint surfaces need movement to get supplies - fed).
2) It shifts the vertical line of weight bearing over the facets rather than over the vertebral bodies. The bodies are built to take weight. The facets are designed to steer.
Here are are lumbar three vertebrae seen on end looking down the spinal canal. The spinal canal sides are bounded by the pedicles.
Did you notice, looking at the side view, that beneath each pedicle is a space - a window (Latin: window = foramen, and windows = foramena - sorry there are no Foramena XP). These windows, sorry, these foramena are open, and through them pass nerves at each level.
Now see those facet joints that bound the foramena? They’re joints. Like other joints, they can be sprained. And like arthritic knobby finger joints they too can get knobby.
If you had a really knobby finger joint you couldn’t get that ring on, right? Here a really knobby facet joint can impinge (protrude, pressing into) the foramena and pinch nerves or at least making for less space for anything else to more easily pinch those nerves. They can also bulge into the spinal canal and make the canal too small (tight) for comfort. That’s called spinal too-tight-for-comfortness (Latin = stenosis). Latin to the rescue this time. Spinal stenosis. Much easier.
The pedicles are the two stout arms that join the posterior elements to the vertebral body. Two per level, left & right.
Notice that pedicles jut out from the top half of the vertebral bodies.
About those pedicles. They span the front big load bearing stuff to connect to the posterior stabilizing steering stuff. In so doing they go between all the important things. Therefore pedicles are an important and strong way for surgeons to attach mechanically to the strongest part of the spine from the back side (the easiest way to the spine).
If the surgeon can site (through the bone of the posterior elements) the canal of the pedicle then a hole for a screw can be made to attach to the vertebral body by way of the pedicle. But how do you see through bone? As Ali-G would describe it: “Tekmology”.
The pedicles are the two stout arms that join the posterior elements to the vertebral body. Two per level, left & right. Notice that pedicles jut out from the top half of the vertebral bodies.
Here is a pedicle screw being shown as guided by a preformed hole traversing the center of the pedicle to get to the vertebral body. The inset shows the surgeons visual angle of that pedicle, a tube on end target is down the center of that tube.
Vertebral screws can be put into the body of the vertebrae without going through the pedicles, but this requires either opening the chest, or opening the abdomen or both. For very severe deformities the complete removal of disc structure (the whole works, right down to endplates) creates significant space and allows substantial corrections that pushing, twisting or leaning on would never allow. While there, generating that ability to be corrected, anterior fixation may be attached to actually hold the correction. Something hooks to bone, as seen here, then rods or cables or plates span several vertebrae or as the circumstances require.
The screw type seen here has a screw head that can accept a rod which then spans multiple levels to hold shape. There are other kinds as well.
There is a zone- a region - of the posterior elements that gets a special name. It does so because it is a trouble maker. Just below the superior facets and the transverse processes, which makes it also just below the pedicle attachment to the posterior elements - it goobers.
Goobers?. Sometimes it just cracks. But sometimes it just never forms. Not rare it forms but stays as cartilage. You see the big bones often have a start in cartilage then convert to bone. That conversion happens in several places at once and the conversion spreads out and the bone zones link up. become continuous. Usually. But, not always. This is a spot in which about 15% of the population has less than complete cartilage conversion to bone. There are places where much higher rates prevail.
Now if you stand in lordosis (low back arched inward, letting belly hang loose) then the pressure here can complete the separation. That separation is called spondylolysis. Why? So that people without teeth can’t say it. Hate that.
Kidding. The region we are referring to is the zona interarticularis or pars interarticularis. (Latin: the zone between the two joints). There’s a whole section on this.
The disc
The disc is not a hockey puck. It isn’t a solid uninteresting slice of something. It has a rather complex architecture. The end flat surface of each vertebral body is called the end plate. It is pretty hard bone, harder denser bone than the rest of the vertebral body. This end plate was once cartilage and is where and how the vertebral body grew. Once that cartilage converts to bone, it becomes quite tough.
To protect the cartilage growth plate, the capsule is a multi-ply fiber structure - not unlike a radial tire in structure that embraces the entire disc perimeter. It spans over the end plates to get to the original bone. That’s how it protects the growth plates. But it then attaches more strongly to the closed bone converted plates at the time growth ends. The capsule is tough, very tough.
Immediately inside the capsule is less organized filler stuff, which has some water content. Kind of like wet felt. At the center (nucleus) is a pulpy gel called the nucleus pulposis.
So looking at the disc, it is kind of like a jelly donut. What happens when you stamp on a jelly donut? Well the jelly squirts out. The criss cross of capsule fibers dodging the pedicle makes for the weakest spot, and so that’s where many rupture happen. Like a tire blow out. It can bulge (a bulging disc) or it can just plain blow (ruptured disc). The extrusion of gel goo into the spinal canal is greater in scope of course than a bulge (which is still contained).
So what’s the big deal if it blows? It’s just goo.
Chemistry. That goo does not belong there and is reacted to. Also, no longer being where it belongs and is nurtured, it quickly dries out and goes from being goo to being very much like cooked lobster. Rubbery and tough. During the drying out phase the volume of the rupture shrinks. If that is enough shrinkage to stop jabbing nerves then people say they “healed” as long as more goo doesn’t come out the same hole. The dry goo may actually serve as a patch, but it takes a long time for that to get fibrous and secure enough to try trampoline.
When surgeons “remove a ruptured disc”, they do not remove the disc. They remove the extruded stuff that went into the spinal canal or foramen. They also enter the rupture hole and pull out what may later come out the same hole or else “rerupture” will be heard - and felt.
So, what’s a “slipped disc?” It is a word used in wrong context so often that it has no meaning at all. If someone actually were to use it correctly the listeners would not get it it right. It is often used as a synonym for bulging or ruptured disc. But there is a condition in which slippage occurs through the disc, yet the word is directed at the displaced bone structure.
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