In this video we're going to be discussing the biomechanics of the temporomandibular joint and we're going to begin by discussing the process of mouth opening. Now over here on the top right both of these pictures here are depicting mouth opening. But there's two different phases of mouth opening.
There's an early phase, which is what we're going to look at first, and then the blurred out one over here, late phase, we'll be looking at that on the next slide. And it's important to know that there's two phases because very different things are happening in each phase. So if we look at mouth opening, what is the major osteokinematic movement?
Well, if you open your mouth, the mandible moves down. So we would say it's mandibular depression. Now this type of depression is different than let's say scapular depression.
When we look at scapular depression, that is a simple downward glide, a downward translation. This is not a simple downward translation or a simple depression. It's actually a combination of two movements. So mandibular depression equals bilateral downward condylar rotation and bilateral anterior condylar translation.
Now just to clue you in on this, the first part here, the downward condylar rotation, this is occurring in the early phase and the anterior condylar translation is occurring in the late phase. So rotation early phase, translation late phase. Okay, more details on this coming in just a minute. Now it's also worth mentioning that the only mastication muscle that's involved in mandibular depression is the lateral pterygoid muscle. And this is also assisted by gravity.
So gravity and the lateral pterygoids facilitate mandibular depression. Another important fact here is that for any symmetric movement of the mandible, that's mandibular depression, elevation, protrusion, retrusion, you have to have identical actions at both TMJs. So they're both doing the same thing at the same time, left and right.
You can't just move about the left TMJ. without having the same movement at the right TMJ. So all the stuff we're about to talk about here is occurring bilaterally. It's occurring at both temporomandibular joints. And that being said, let's now go into the early phase of mouth opening.
Remember I said that mandibular depression involved both downward condylar rotation and anterior condylar translation. Well, in the early phase, the rotation is much greater than the translation. Okay, so let's see how that works.
So the mandibular condyle is going to roll anteriorly about the articular disc within the mandibular fossa. So right here is the mandibular condyle, up top is the mandibular fossa, and of course within the joint cavity we have the articular disc. That separates the cavity into a superior part up here and an inferior part down here. So this condylar rotation is actually occurring really within the lower or inferior area. joint cavity space right here.
And you could almost think of the mandibular condyle articulating directly with the inferior portion of the articular disc. In any case, when you get mandibular depression, you initially get the mandibular condyle here rolling anteriorly. And what that produces is net downward rotation of the mandible. So if you imagine the mandible extends all the way out here.
And so here's the front of it where you've got the teeth right. Well initially that would rotate downward like this. Okay so anterior roll within the joint itself produces a net downward rotation of the mandible.
And again this is occurring at both TMJs. So if we look within this inferior joint cavity we see that we're getting anterior roll of the mandibular condyle. And this is occurring on both the left and right sides.
And then if we imagine the mandible comes down here, right, curves around anteriorly, maybe we have the teeth over here, if we have bilateral anterior roll of this condyle, then we're going to get net downward rotation of the mandible. Okay, now we do have a little bit of anterior translation of the mandible in the early phase, but the downward rotation far outweighs the anterior translation. Okay. This is very different than what we see in the late phase of mouth opening, where now we have much more anterior translation of the mandible than downward rotation, although there still is some downward rotation.
Now also notice that the anterior translation of the mandible, it's not just anterior. We also have a little bit of inferior movement as well, but for simplicity we're just going to call it anterior translation. So if you look here within the joint capsule, you see that the mandibular condyle has now slid anteriorly. All right, so when we look at this anterior slide of both mandibular condyles, it almost looks like they're dislocating from the mandibular fossa.
I mean these condyles are pretty far away from these concavities, right? But then we remember that the mandibular condyle is actually articulating not directly with the fossa but actually within this inferior joint cavity. So that's okay for it to be this far away from the mandibular fossa.
However what this does is it puts tension on the articular disc. And the more that you open your mouth in late phase, the more tension you put on the articular disc. And so finally, at full mouth opening, so at end range of mandibular depression, you have the maximum tension on the articular disc.
And two things occur. Number one, at full mouth opening, the intermediate region of the disc, which is right here, is going to reside between the mandibular condyle and this bony protrusion called the articular articular tubercle or the articular eminence. Okay, so that's the intermediate region right there. And then also at full mouth opening, we're putting maximum stretch on this superior retrodiscal tissue, which is the posterior most aspect of the disc. And recall that it actually attaches in part on the posterior aspect of the TMJ joint cavity, but also on the posterior aspect of the mandibular fossa.
Okay. So the bottom line here is that the early phase of mouth opening involves mostly downward mandibular rotation, whereas the late phase of mouth opening mainly involves anterior mandibular translation with a little bit of inferior translation as well. So what about mouth closing?
Well, if the major osteokinematic movement of mouth opening was mandibular depression, then that must mean that the osteokinematic movement of mouth closing should be mandibular elevation. And the nice thing here is that for the biomechanics of mandibular elevation, it's really just an exact reversal of what we just saw. So the early phase would actually be the translation and it would be in the opposite direction. So we'd actually see, instead of anterior and inferior, we would actually see posterior and superior translation, but mostly posterior. Posterior condylar translation.
And then the late phase of closing would be the rotation. But again, it wouldn't be downward mandibular rotation. It would be upward mandibular rotation. Okay, so closing is just a reversal of what we saw with opening.
Thank you for all your support. Be sure to check out my Instagram for cool science and not science stuff.