The Sacrum
In previous posts, we've examined the back as a whole, the individual bones of the back (ie, the vertebrae), and the hip bones that lie either side of the lower spine. What connects the back with the hip bone is called the sacrum. Let's look closer at the sacrum today.
The sacrum sits at the bottom of the spine. The 24 or so 'free' vertebrae (the individual bones of the spine) sit one atop the other, with the very last one (lumbar vertebra #5, ie, L5) sitting on top of the sacrum. The sacrum, thus, takes the weight of the entire spine and it's appendages (ie, two arms and a head). That's a lot of weight to support! On either side of the sacrum is a hip bone (ie, called an 'innominate', which we looked at last month). The hip bones are stitched tightly to the sacrum via numerous very strong ligaments. And thus the weight of the upper body is transferred through the sacrum to each hip bone to each leg to each foot to the ground.
The sacrum sits at the bottom of the spine. The 24 or so 'free' vertebrae (the individual bones of the spine) sit one atop the other, with the very last one (lumbar vertebra #5, ie, L5) sitting on top of the sacrum. The sacrum, thus, takes the weight of the entire spine and it's appendages (ie, two arms and a head). That's a lot of weight to support! On either side of the sacrum is a hip bone (ie, called an 'innominate', which we looked at last month). The hip bones are stitched tightly to the sacrum via numerous very strong ligaments. And thus the weight of the upper body is transferred through the sacrum to each hip bone to each leg to each foot to the ground.
The sacrum is a strange-looking bone (aren't they all?!). It has an interesting development, which continues after birth. When born, a baby doesn't have a sacrum, per se. Rather, she has five separate sacral vertebrae (much like the vertebrae above in the lower back, ribcage, and neck regions). Over time, and by the time she's in her 20s or early 30s, those five sacral vertebrae will have fused into one bone, the sacrum. Neat, eh?
If you look at the sacrum from a bird's eye view, you'd see a hole penetrating the back part of the bone. It's a canal. In fact, it's part of the spinal canal - the passageway for the spinal cord. While technically our spinal cords end around the second lumbar (ie, lower back) vertebrae, for our purposes it's fair to say that the end of the spinal cord extends all the way down and into the spinal canal of the sacrum.
If you look at the sacrum from a bird's eye view, you'd see a hole penetrating the back part of the bone. It's a canal. In fact, it's part of the spinal canal - the passageway for the spinal cord. While technically our spinal cords end around the second lumbar (ie, lower back) vertebrae, for our purposes it's fair to say that the end of the spinal cord extends all the way down and into the spinal canal of the sacrum.
If you look at the sacrum from the front, you'll see 4 or 5 paired holes (called "foramen") on either side of it's midline. These holes allow nerves to come out of the sacrum (from the spinal cord) and go onto their various destinations, innervating all sorts of tissue (ie, muscles, skin, organs, etc). Similarly, if you look at the sacrum from the back, you'll see matching 4 or 5 paired holes to what you saw on the front side. And these holes on the backside also allow spinal nerves to exit the sacrum and innervate muscles and skin and other tissues of the lower back and buttocks area.
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Unlike most bones in the body, the sacrum does not serve as an attachment point for several muscles. (You might recall from a previous couple of posts that the innominate (ie, the hip bone) serves as an attachment for over 30 different muscles! And the scapula (ie, the shoulder blade) serves as an attachment for 17 different muscles. Not so with the sacrum. Nope, it's an attachment point for only a couple of main muscles: the piriformis arises from the front surface of the sacrum, and the spinal erectors (those muscles that run up and down alongside your spine) arise from the back surface of the sacrum.
If a bone doesn't have many muscles attaching to it, it's fair to assume that the bone isn't meant to move very much. And that is precisely the case with the sacrum. If you look at the sacrum from it's side, it is capable of being moved in a fashion similar to it nodding it's head (imagine it having a head) forward and backwards. If you view it from the front, the sacrum is capable of being turned to the left or the right (as if it were shoulder-checking while driving). Also looking at it from the front, it is capable of being moved such that it leans over to the left or right ("I'm a little saucer, pour me out..."). But the movement of the sacrum is passive, in that muscles attaching to the sacrum aren't purposely making it move (we don't want it to move, very much). Rather, the sacrum moves in response to forces acting on it. The weight of the spine and upper body loading on the top of the sacrum can cause the sacrum to nod, or turn or tilt, or a bit of all three. The hip bones on either side of the sacrum can also cause the sacrum to nod, turn, tilt or all three.
If a bone doesn't have many muscles attaching to it, it's fair to assume that the bone isn't meant to move very much. And that is precisely the case with the sacrum. If you look at the sacrum from it's side, it is capable of being moved in a fashion similar to it nodding it's head (imagine it having a head) forward and backwards. If you view it from the front, the sacrum is capable of being turned to the left or the right (as if it were shoulder-checking while driving). Also looking at it from the front, it is capable of being moved such that it leans over to the left or right ("I'm a little saucer, pour me out..."). But the movement of the sacrum is passive, in that muscles attaching to the sacrum aren't purposely making it move (we don't want it to move, very much). Rather, the sacrum moves in response to forces acting on it. The weight of the spine and upper body loading on the top of the sacrum can cause the sacrum to nod, or turn or tilt, or a bit of all three. The hip bones on either side of the sacrum can also cause the sacrum to nod, turn, tilt or all three.
A little movement of the sacrum is okay. Is allowed. But too much movement, or a lack of movement (ie, "My SI joint is stuck.") can lead to issues. Pain, guarding, altered movement, compensation, etc. We're a touch limited in the massage room with helping getting that sacrum back to where it wants to be. But we do our best by softening the muscles that act on the lower back, the legs, the hips so as to hopefully offload some of the strain that is being experienced by the sacrum.