Why do some people’s SI joints get stuck, when others’ don’t?

I’ve had a bunch of people contact me for advice on painful SI joints recently… all of whom have said their SI joints don’t lock up.

I think this is so interesting, because for one thing, my recommendations to them tend to be a little bit different.

But also, it really got me thinking… why, exactly, do some people’s joints not lock, whereas I was plagued by the constant sensation of locking (and then not being able to fully move my leg on that side) for years?


Well, to answer this question, let’s first re-establish some terminology.  You may know this info already, but since I never know if a particular post is going to be the first post someone lands on from a search engine, I like to make sure all my posts are comprehensive.

The SI joint is formed by the connection of the sacrum (the base of the spine) and the two ilia (the hip bones).


The sacroiliac joint

An SI joint can become “stuck” when the ilium, or hip bone, on one side rotates backward and becomes “jammed” against the sacrum.  This isn’t something that shows up on an MRI or x-ray, because really, it only moves a millimeter or two.  We still do not have great ways of testing or diagnosing this (more on that in later posts).

But the first time it happened to me, it was very clear that something was wrong, because I had this crazy pain in my lower back on one side, and I was having trouble lifting the leg on that side.

Desperate, I went and saw my chiropractor the next day and he explained that my SI joint was stuck.  With just a few clicks of his Activator tool, managed to “unstick” it– it seemed simple.  But the problem came back repeatedly until I figured out everything I talk about on this blog.

So, the question I’m addressing in this post is: why did this happen to me over and over for five years, when other people can be in as much pain or more, and yet never have the joint lock?

I can think of two main factors.

Factor #1: The shape of each individual person’s SI joint.  

This is really the biggest factor, and it has so much to do with who even gets sacroiliac joint dysfunction in the first place, and how long it takes to heal.

Bone Shape

As I learned in my Kinesiology class, all of the joints in the body range on a scale from stability to mobility.   Some joints are not meant to move at all; some are meant to move a lot.

The amount of motion the sacroiliac joint is meant to have is still a subject of debate.  However, part of the reason it’s been so hard to study is that the degree of motion it has can range widely from person to person.  It has to do with the shape of the sacrum and the two ilia, and how they fit together.

Women, generally, have bone shapes that fit together in a way that gives their SI joints more freedom to move.   This is why women tend to experience SI joint dysfunction at higher rates than men.

But also, some individuals (such as me) tend to be born with joints that are hypermobile.  It’s just a genetic trait that we have bone shapes that don’t fit together in ways that are super stable.  So when we injure the ligaments that are meant to hold the joints in place, we are subject to developing way more problems than someone else who has a more stable bone shape, because we don’t have as much built-in stability in the shape of our joints.

Having hypermobile joints has affected me in many ways throughout my life– my SI joints are not my only hypermobile joints (more on that later as well).

Here’s a post I wrote about how it affected my decision not to get prolotherapy for my SI joints.

Joint Surface

Here’s another factor which can contribute to how stable or unstable someone’s SI joint is.  This is not something that I completely understand, but I’m okay with that, because people with way more qualifications than me are still trying to sort it out.

What we know so far:

Part of what also gives the Si joint stability is the actual surfaces of the sacrum and the ilium, where they come together.  Each surface of the joint has a certain texture to it– bumps and ridges– that are meant to fit together with the surface of the opposite bone in in the joint.  So, where the sacrum has a bump, the ilium will have a dip (and vice versa).

The video on the SI joint that I linked to from Coordinated Health does a pretty good job of illustrating this (screenshot below).  The narrator describes these interlocking surfaces almost like a lock and key:


The black drawing shoes the surfaces lined up the proper way– the red drawing shows how it can create pain when they are not lined up.

I have read in various articles that the intensity of these bumps and grooves can increase over a person’s lifetime.  So, by the time someone is elderly, their joint might not move very much (or might not move at all).  The bumps and grooves have become so pronounced that motion is significantly decreased (or impossible).  So I would actually expect that new cases of SI joint dysfunction would occur less often among the elderly (at least, the kind due to sprained ligaments.  This does not rule out other sources of pain, such as arthritis).

Factor #2: Which ligaments the person actually injured

There are many ligaments which are meant to hold the SI joint in place:


As you can see, each ligament attaches to a different point on both the sacrum and the ilium, and so provides stability at a certain position and angle.  (And therefore, when you sprain that ligament, you are also decreasing the stability of the joint at a certain position and angle).

I have not read much on this, but I would expect that exactly which ligament (or ligaments) the person initially sprained would then play a role on where, and how their joint is unstable.

I do think bone shape and joint surface are probably the more significant factors, however.  And out of those two, I’d put my money on actual bone shape making the biggest difference.

So why do people whose joints don’t lock still have pain?

If your SI joints are causing you a lot of pain, but they aren’t locking, this probably means they are hypermobile.  This means that the two ilia are moving more than they are supposed to (beyond their normal range of motion) relative to the sacrum.

This can still be its own pattern of dysfunction that creates pain and inflammation.  It just means that, due to whatever factors are at play in your body (again, I think the bone shape that you’re born with is the biggest one) your hip bone doesn’t actually get wedged against the sacrum if and when it rotates backwards.

Instead, your hip bones are probably rotating forward and backward too much, relative to the sacrum– more than your joints were intended to allow.

So, if you tell me your joints don’t lock but they really hurt, I believe you.  Having hypermobile joints can still be painful, whether or not they get stuck.

Whew!  Okay, that’s all for now.  I always enjoy writing these super science-y posts, so I hope you were able to follow along!

Any questions, feel free to comment or email me at sunlightinwinter12@gmail.com


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