Welcome to another blog series. In this blog series we are going to be talking about the lumbar spine. We are going to talk about the bones, discs, ligaments, muscles, nerves and blood supply to this area of the body. Not all of these areas get talked about. Every two weeks you will get another blog post educating you on the different parts of your low back.
Let’s talk about the humble lumbar spine (L-spine). This is the lowest part of the spine, your low back. It is below the cervical and thoracic regions. The lumbar spine consists of 5 vertebrae and they are numbered sequentially from the top down, L1 vertebrae, L2 vertebrae… L5 vertebrae. The L5 vertebrae sits on top of the sacrum. The first sacral vertebrae is called the S1 vertebrae. Whereas the L1 vertebrae sits below the T12, 12th thoracic vertebrae.
Taken from: https://www.physio-pedia.com
There are many specific features that make the lumbar spine unique from the other parts of the spine. Let’s start with the vertebrae themselves. Looking at the vertebrae, the first thing that people notice is the size of the vertebral body, it is much larger than other areas of the spine. When someone stands up or sits, approximately half their body weight is above the L2 level, our center of mass. This means that the lumbar vertebrae can take a great amount of pressure and loading. The vertebral body is built a little different to allow this amount of weight to pass through without causing injury. The outside of the vertebral body is cortical bone. This type of bone is very hard and strong. However it is also brittle. If the vertebral body was hollow and made of cortical bone it would be similar to a cardboard box. The vertebrae would be able to withstand a lot of force but once that force capacity is reached, it would fold in, on itself, like a cardboard box. To stop that from happening, the vertebral body has cancellous bone inside. This cancellous bone is a softer, spongier bone that helps to absorb the load on the vertebrae. The cancellous bone is put in there perfectly matching the outside dimensions of the vertebrae. The cancellous bone is in the form of trabeculae. Trabeculae are lines of bone that help to transmit the forces. There are vertical and horizontal trabeculae. The vertical trabeculae help to stabilize the ‘cardboard box’ of cortical bone to vertical forces. However, if there is a vertical load and a horizontal force, for example lifting and twisting, the vertebrae can collapse. This is where the horizontal trabeculae come into play. Both the vertical and the horizontal trabeculae act as a stabilizing grid with in the cortical bone vertebral body to give it amazing compressive strength as well as strength with forces shearing across the body as well. The body is built so amazingly well.
Coming off the back/posterior aspect of the vertebral body, are the pedicles. There are two pedicles. The pedicles are short and stout, it is not very common to break a pedicle. The pedicles travel posteriorly for about 1 cm or so, then they slowly start to angle towards each other and they meet in the middle. Where the pedicles angle, the name changes as well, they are now called lamina. The lamina meet in the middle and form the spinous process. The pedicles come out of the vertebral body about ⅓ of the way down. Something else happens where the pedicles turn into the lamina. There are 3 other bony parts that emerge. The superior articular facet, the inferior articular facet and the transverse process. These three bony growths occur on both, the left and right sides of the pedicle/lamina intersection.
Let’s discuss these three bony prominences. The superior articular facet attaches to the inferior articular facet of the vertebrae above. They are complementary, that is the superior facet faces anterior, superior and slightly lateral. While the inferior facet faces, posterior, inferior and slightly medial. The facet joints, more commonly known as zygophopheal joints, or z joints, are one the main factors that controls the motion of the lumbar spine. The orientation of the z joint facets is something that is different than anywhere else in the spine. The z joints in the cervical spine, neck, are planar joints, on average at about 45° above the horizontal plane. While the z joints in the thoracic spine, mid back, are quite planar as well but this time in the coronal place, close to being straight up and down. However, the z joints in the lumbar spine are different.
They can be either ‘J’ or ‘C’ shaped. This may get a little complicated in the next few lines… The ‘J’ shaped joints are mainly in the sagittal plane, front to back, with the little hook being in the anterior part of the joint in the coronal plane, side to side.
The ‘C’ shaped joints tend to fit in the middle of the sagittal (side-to-side) and coronal (front-to-back) planes. Looking down from the top, the joint looks to be in a ‘C’ shape.
Taken from: https://www.innova-pain.com
To make things even more complicated, the z joint shapes may be different side-to-side of the same vertebrae! This is called tropism. A tropism can really muck things up when trying to help someone with a stiff low back. You are trying to mobilize/ manipulate the z joints in the lumbar spine but they always feel stiff, that could be because the plane of the joints, the imaginary line that passes through the joint that separates the front and back halves of the joint, can be different from the joint above and the joint below. Tropisms are diagnosed via MRI or CT scans. The finds of the scan are listed with side note, “...the left L3-4 facet is a tropism compared to the rest of the z joints in the lumbar spine…”. Tropisms rarely cause issues, they are just found and help to explain things.
Now where were we… oh yes, the facet joints. Only one more thing left to say about the facet joints. It is the little piece of bone between the facets on the same vertebrae, the pars interarticularis. This little piece of bone has to be strong. The pars is mainly made out of cortical bone, see above. When people are flexible, not quite Cirque du Soleil flexible, and they bend backwards a lot, the inferior aspect of the superior facet can come crashing down on this point, pars interarticularis. If this happens repeated number of times in a short period or a fairly consistent number of times over a longer period, this part of the bone can start to break down and people can have a stress fracture here! This is spondylolysis (big word of the blog), spond-i-low-lie-sis. It means just that, breaking of the bone, specifically the pars interarticularis. This can have some pretty dramatic effects over time. One example, is that the vertebrae with the fracture MAY start to slip forward over time and cause what is called a spondylolisthesis, spond-i-low-lie-thesis. A couple of the bad things with a spondylolisthesis is that the spinal canal can get quite a bit smaller and this can lead to possibly needing spinal surgery in the future.
Taken from: https://orthoinfo.aaos.org
Let’s stop about something more pleasant than spinal surgery, shall we? The other bony point that sticks out from that point is the transverse process. The transverse process sticks out of the side of the vertebrae. It is slightly tilted backwards from the coronal plane. The transverse process is thought to act as a lever to move the vertebrae. The muscles that attach to the transverse process are psoas and quadratus lumborum, to name a couple. In addition, there are multiple ligaments that attach to the transverse processes, think of them as “guy wires” that help to stabilize the lower lumbar vertebrae by attaching them to the other vertebrae and the pelvis.
Leaving the pedicle/ lamina intersection, heading posteriorly, backwards, we go along the lamina. Both laminae head towards the midline and attach. This is a very important attachment. This attachment of the lamina closes the circle around the spinal cord, creating the bony spinal canal or spinal foramen in the lumbar vertebrae. This bony arch protects the spinal cord from injury and provides a very stable surface for the spinous process to attach to. If an injury does occur, or sometimes simply with aging, the lamina start to get thicker. This can cause the spinal canal to get smaller, creating what is called spinal stenosis. People with spinal stenosis tend to dislike extension, for example, standing up straight or bending backwards. They find that standing up straight may hurt their back, send pain down their legs or make their legs feel very weak. Or all of the above! Generally people with spinal stenosis like to be slightly bent forward, in a little bit of flexion. For example, they cannot stand up straight and walk around to go grocery shopping. However, if they get a shopping cart and are able to lean on the handles, they can walk around 2 or 3 large grocery stores or a shopping mall, without any issues! If someone gets severe spinal stenosis, you may end up going and getting a laminectomy from a neurosurgeon. The basics of the surgery is that the neurosurgeon snips off the lamina, just behind where the facet joints/ transverse process are located, on both sides. This, in theory, opens up the spinal canal so there is less pressure on the spinal cord/ nerves. The lamina and the spinous process are then removed and the client is beautifully sewed up. My experience working with people that have had a laminectomy, at least the first few days after surgery, is that most of them do quite well. There is still pain, after all they just had surgery, but their pain has changed, they can stand up straight, there is less leg pain, they can walk further… However all is not always good. Even though it is a routine surgery, some people still pass away during or after the surgery. There is an inherent risk of simply having surgery. It should be the last opinion.
Taken from: https://www.spineuniverse.com
Lastly, on the vertebrae, but definitely not least, is the spinous process. The spinous process is short and stout. The lumbar spinous process has a single tip that points straight back. Unlike the thoracic vertebrae, whose spinous process points inferior and is usually level with the vertebrae below or cervical spinous process is usually bifid, two tips. Like the transverse process, the spinal process is a long lever off the vertebrae to control the motion of the vertebrae. Some muscles attach to the spinous process. In addition, more commonly ligaments attach to here, for example, the interspinous ligament, supraspinous ligament and the thoracolumbar fascia. These ligaments all play a role to limiting the movement of the vertebrae. The L5 spinous process is one of the shortest and a little more difficult feel for when palpating the vertebrae on a client. A simple trick to make sure you are on the L5 spinous process is the find the clients PSIS, posterior superior iliac spine, go to the middle between them and that should be the S2 level, go up superiorly about 1 cm, that is S1 and go superior again 1-2 cm. That should be the L5 vertebrae, to make sure ask the person to tilt their pelvis a little bit. The L5 spinous process should disappear under your fingers.
That is a deep dive into the bony part of the around the L-spine. Next post will be about what most people think cause pain in the low back... the disc