Today on the Doc on the Run podcast, we’re talking about a broken sesamoid versus a bipartite sesamoid. What’s the difference?
Now, if you have pain in your foot and you go to see the doctor, they might tell you that you have an issue with one of these two little bones right here. They’re two little bones about size of beans sitting underneath your big toe joint. Now these are like teeny tiny little versions of your kneecap. They’re actually embedded within a tendon on the bottom of the joint, and they are called the sesamoid bones.
When you get sesamoiditis, or you have a broken sesamoid, or you have pain under there, here’s the difference you’ve got when we look at the bottom of your foot and if you want to comment about my terrible drawings, go right ahead. This is not my forte. I’m not an artist. I’m a podiatrist. I help runners get better. I help runners run, but I’m not an artist.
The two sesamoid bones are sitting here under the big toe joint about like that. Now, when you have pain, it’s obviously somewhere in this area under the big toe joint that on the bottom of your foot, where you’ll have pain and so when we get x-rays of your foot, we can actually see those little bones.
Interestingly, a lot of people that have fractures and other things wrong with their foot, they’ll always ask what are those two little things right there? And those are the sesamoid bones and so you can have a broken sesamoid, which means it’s broken, it’s cracked, it’s split, it’s crushed, whatever. Or you can have this thing called a bipartite sesamoid. What is that? Well, “Oh, my handwriting is the only thing worse than my drawings.” By the way, but you comment on that as well. If you want me to work on that, let me know but good luck. I’ve had terrible handwriting my whole life.
So the first thing is when you think about what is bipartite mean? Well, bi means two pieces and if you think about the word partition, it means what the state of being divided. It means being into separate pieces or more. So when you have a bipartite sesamoid, it just means you have a sesamoid that’s in two pieces. Well, how does that happen if it’s not broken? Because sometimes when you get an x-ray, when you look at this same bone, you’ll see a thing that looks like this, where one sesamoid looks normal and the other one looks like it’s in two pieces and you think, “Oh man, I must have broken that.” Well, not necessarily.
Sometimes you’ll get one where you have what looks very similar. Let’s say one of them looks normal. And one of them looks like this and one of them looks like this. Well, this is an easy one. So when you look at these two, what we’re really looking at here is that as physicians, we say, “Okay.” Look, if you’re looking at the sesamoid bones and one of the sesamoid bones that’s in two pieces is really two round pieces. We know that those did not break, why? Well, there’s a separate growth center in two halves of the sesamoid bones.
As you grow and develop, they actually fuse together to form one whole sesamoid bone like this. And so if you have a normal sesamoid, it’s all round and it’s in one piece like this, when you have a bipartite sesamoid what happens is those two pieces actually fused or did not fuse. They grew separately and they developed into two completely separate parts, but they’re normal tissue connects these two things. So you have something, whether it’s strands of scar tissue or collagen, basically that holds them together. Part of the fibrous joint capsule can hold them together. Part of the flexor tending can hold together.
A bunch of stuff, hold them together, but they’re not moving, they’re not loose and they’re not going to move apart on you. And it’s pretty normal. You just have some irritation. Maybe you have some irritation of the soft tissue that’s holding them together. When you have pain in the sesamoids, you have a bipartite sesamoid.
A fracture is different, it’s broken and what happens? Well, instead of it being two solid things, you actually have a fracture that starts to develop like this. You’ve got a sesamoid bone, that’s one piece, not two. And then you start to get a stress fracture. You’re applying too much force to the bone and it starts to get irritated aggravated, and it begins to form a crack and then eventually it’ll form a real crack and maybe you’ll see a line across it like that on the x-ray at some point. If you ignore it, you keep running on it, you don’t do anything about it, you keep applying the same stresses to it.
Then the crack will actually come apart and then you’ll have a real crack. And if you keep pulling on it by walking on it, it’ll start to move apart. And so what happens is the bone breaks and you now have, what’s obviously two halves of one bone and when we look on the x-ray, the way we know that, and the way we can be sure is that you have a thick cortical bone or the wall of the bone is thick all the way around. Like if you take these bipartite pieces, for example, the wall is uniformly thick all the way around when we look at it on the x-ray.
But when you have a fracture, the part of the cortex that was intact before this part, it’s uniformly thick all the way around here, but where it cracks its much thinner. So this line is actually a little bit inaccurate and that it would be faded where you start to develop the fracture line and then when it splits apart, because you don’t have a whole lot of bone in there like you do, when you’re looking through the wall of the bone, then it’s very faded, it’s it looks foggy, it doesn’t look as clear as the wall around here.
So when you have this very clear, thick cortical bone around these two pieces, you know it’s two pieces, but this is different. So a fracture is much worse. A fracture is more significant and something you really need to pay attention to, but hopefully this helps you understand a little bit about the differences between a broken sesamoid and a bipartite sesamoid.
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