by Alex Quintana

Take a deepest breath you’ve taken all day.

Bring your hand in front of you so you can see it.

Close your hand into a fist.

Then, slowly open it, fingers together.

Lastly, go ahead and start closing the fingers again, keep them close together, really together and stop half-way through.

Right here in front of you - is a shape and size of your own sacrum. Voilá!

THE NAME

If my sources are to be believed, the English term ‘sacrum’ was introduced to the world of anatomy in the mid-eighteenth century as a shortened version of the Latin ‘os sacrum’ - ‘the holy bone’.

Why holy?

Well, there is a lot of speculation about this.

Some say that, as a centre of reproductive power, the sacrum is a holy bone, a part of our root chakra. Others cite historical records describing the sacral bone being used as part of rituals involving animal sacrifice - their significance is not fully understood. Frequently, when a corpse is burned, the sacral bone will just not burn, seemingly indestructible. This adds to the mystique, seems magical, even.

And then, some people destroy all these nice theories by saying that that the name is a result of mistranslation… and it should really be named ‘strong bone’ — nothing holy about it. Spoilsports!

SACRUM ARTICULATIONS

For those who like visualising things, the sacrum is wide at the top and narrow at the bottom. This triangular shape is… I want to say ‘snuggled’… but no, there is nothing cosy about it. It’s quite literally ‘wedged' between:

The lowest part of the lumbar spine - L5 vertebrae
The two ilium bones, i.e., the large bones at the side of your hips; and
The coccyx, also known as tailbone, located at the very end of the spine (more on that truly under-appreciated bone and its importance later).

Fun fact: the female sacrum weighs around 2.5 ounces (approx. 65 grams) whilst the male sacrum tends to be a little heavier at 3 ounces (approx. 85 grams). The size of our bodies are usually different. A heavier man would put more load onto his sacrum. The body’s mass always corresponds to function - therefore the more load the bone is subjected to, the more strong it will get. This explains, by the way, why weight training is so good for our bone strength and density.

Located in between such strong neighbours, sacrum is sensitive to weight. How?

The more weight we carry - and I am not talking about grocery shopping here, but the extra ‘cushioning' that we accumulate in our bodies over the years :-) — the harder the sacrum’s life becomes. The extra weight puts an additional downward pressure on it. The months or years of such extra pressure might translate into tension between the L5, the hips and the coccyx. Just writing about it makes my lower back feel a little uneasy. Can my willpower keep me away from another biscuit? Deep breaths…

SACRUM STRUCTURE

The sacrum is composed of 5 segments (also known as S1-S5) which start their life as properly separated vertebrae and gradually fuse together. The fusion process starts as early as 8 weeks following conception, and continues until the age of 25. Some say that the fusion speeds up greatly from the age of 18 onwards. Once we reach 25, our sacrum is fused and we are done growing (hopefully only in a physical sense though).

However, and this is a really interesting bit about the sacrum, the intervertebral discs that were originally between the sacral segments are still there, and they stay there for the long haul, well into our elderly years. We call these formations intervertebral disk material. Each 1.5 mm layer is flattened of course, but still there, cushioning, shock-absorbing and thus making the sacrum the strongest bone in a body. How cool is that?

Of course, our bodies are different, and in some rare cases we see people with 4 or 6 sacral segments in our clinic.

Fun fact - the level of sacral segment fusion is one of the factors used in forensic investigation in order to establish the age of a body found in suspect circumstances.

IMPORTANT CONNECTIONS - SYMPHYSIS PUBIS

The pubic symphysis is the cartilage connecting the two sides of the pelvis — the one that becomes high on hormones in pregnancy, softening and allowing the pelvis to open during birth. It is an important factor in sacral health and movement, despite not being “directly” connected to the sacrum.

The symphysis pubis gradually grows harder and harder as we grow older (something else to look forward to, for those of us who are too old to die young). With passing time, its composition changes from cartilage (read: soft, bouncy, pliable) to bone (read: hard and allowing very little movement). That influences the sacrum greatly because it stops being able to enjoy the full range of movement it once had. We shouldn’t despair though. On average, we have until our 70s before this change is fully completed. Interestingly, and as it’s true of every other joint in the body, keeping it moving can buy you a few extra years. And if you are really going for it, i.e. being able to do splits at the ripe age of 70, your symphysis pubis may never harden. Well done!

IMPORTANT CONNECTIONS - SACROILIAC JOINT

The sacroiliac joints are cute L-shaped things with a bad rap. You might be wondering, why?

You know the dimples that you can see in some people’s lower back area? They are incredibly cute, in my personal opinion (as luck would have it, I haven’t been blessed with these, which leaves me forever admiring them in other people). In the clinic, these dimples are helpful if you are looking for the 1st sacral segments. They are just where the dimples are. Genius! This is a life saver on anatomy exams, especially if you are young and bushy-tailed, like I was back in 2014.

And the bad rap? Most of us will only learn about the sacroiliac joints when they become the main suspect in a lower back pain investigation. Tension or imbalance of the sacroiliac joints can be quite a nuisance. But I am getting ahead of myself.

Let’s start from the location. The name, as usual in anatomy, explains what connects to what. The sacroiliac joints link the 1st, 2nd, and top part of the 3rd segments of the sacrum to the iliac bones. This is sometimes poetically called ‘the wings of the pelvis.’

IMPORTANT CONNECTIONS - THE COCCYX

Most people think that the coccyx - also known as tailbone - does next to nothing and is of little use. But when you look closer, you might discover this little fellow is hugely under-appreciated and deserves more attention than it gets.

The coccyx consists of 4 fused vertebrae, which complete the length of our spine. It extends from the bottom of the sacrum towards the anus. It actually ends very close to the anus - about half an inch, or 1.5cm. If it’s positioned as it should, i.e. without being forced into a Frankensteinian twist or bend caused by a fall or an accident, then everything is okay in the world. If, however, you are unlucky enough to fall on your bum with some force, the coccyx may move out of alignment, and that might cause considerable interference with the motility of the urogenital organs. As such, it can contribute to a lot of very, and I mean it, very unpleasant symptoms.

Not many people know that we all start out with a tail in utero. In most cases, by week 6 of our embryonic adventure, the tail is absorbed by the body and we never see it again. Very rarely though, some of us are born with a tail that has not absorbed fully, meaning it’s there, sticking out from our bums. This was the unfortunate cause of death of many innocent children in the Middle Ages, who were quickly labelled as the devil’s offspring and killed instantly after birth, along with their mothers for good witch-hunting measure.

Although extremely rare, some midwives closer to our times were still taught to break it off, quickly and preferably out of sight, to save the poor child a whole lot of prejudice. Unfortunately, when snapped off the segments of the coccyx can be lost, and this may impair freedom of movement in the sacrum. Our bodies are so weird sometimes, huh?

IMPORTANT CONNECTIONS - THE CRANIUM

Wait, wait, wait. Cranium - that’s the head, right? How on earth is it connected to sacrum?

I am glad you asked, because this is a very good question! One of my favourites, actually. To explain this important connection, I need to explain quickly the movement of the sacrum according to craniosacral therapy theory.

When I first learned anatomy, I was told that there are 3 possible movements of the sacrum — flexion, or nutation; extension, or counter-nutation, and shifting from front to back. Since then I have learned that the sacrum is capable of much more fluidity, but let’s stick to the classical list of movements for now.

The superior and anterior rotation (upward and forward) is the movement called nutation after the Latin word which means ‘to nod’ (once I learned about this translation, everything became clear all of a sudden). In this motion, the top of the sacrum comes into the true pelvis, i.e. birth canal. In some circles this movement is called flexion of the sacrum.
The posterior and inferior rotation (backward and downward) movement is the opposite motion and it is called counter-nutation or extension of the sacrum.
Slight movement along the sagittal plane.

The axis of the rotation is transverse at the level of the 2nd sacral segment. This is where it gets really interesting. The 2nd sacral segment happens to be the location where the spinal dura attaches. The spinal dura pulls the sacrum into flexion, and therefore creates a link between the sacrum and the occiput, where it also attaches. It is fascinating to visualise the movement of the sacrum influencing what happens in the cranium via this connection through the dura. The implication is that craniosacral and fascial therapy can positively influence the cranial bones through contact with the sacrum, and vice versa. It is fascinating to observe these connections in our clinic.

SACRUM FUNCTION

As the strongest bone in the body, reinforced by the remaining layers of intervertebral disk material, the sacrum provides strength, shock-absorption, and stability to the spine. It also houses the spinal nerves and articulates with the hips. But as I hope my description above made clear, the role of the sacrum extends far away from its immediate neighbours. Treating the sacrum can have a profound effect on the cranial bones, too.

Sources:

Andrew Biel, Trail Guide to the Body, 2013

Frank Netter, Atlas of Human Anatomy, 2015

Hugh Milne, The Heart of Listening. The Visionary Approach to Craniosacral Work, 1995