Anatomy of the Shoulder Joint | Bones, Ligaments, and Muscles

[00:00] In this video we're taking a look at the anatomy of the shoulder joint. We're going to look at every bone involved, every ligament that holds the joint together. We'll take a look at the four muscles of the rotator cuff and what each of those does as well as talk a little about the movement of the joint. It's a complicated joint with a lot going on, but we're going to take it step by step drawing it out in the diagram and there'll be a chance to practice as well. So by the end of this video you're going to

[00:20] know the ins and outs of the shoulder joint. So without further ado, let's jump to the lightboard and get started. Because there's so many structures, we're going to break this up into two diagrams. And for our first diagram, we're just going to look at the anterior side of the shoulder joint. So we're sort of looking at the shoulder like this. So we're going to start here with two bones. That's going to be the sternum right here as well as the clavicle.

[00:40] calyclol is your collarbone and the sternum is right here in the middle. We have the scapula, which is this irregular shaped bone. It's your shoulder blade and it's got several parts to it. Three important parts that we're going to label on the scapula. The first is the coracoid process and that's going to be this piece of the bone that's sort of sticking out here. It sticks out anteriorly or towards the front and we have the acromion, which is going to wrap around.

[01:00] right here and then sort of come up against the clavicle at that spot. And then here on the lateral side of the clavicle we've got this sort of indented part or concave part right here and that's going to be the glenoid fossa or the glenoid cavity. So again in the scapula we have the coracoid process right here, we have the acromion here and the glenoid cavity. A lot of names so far

[01:20] but once we look at the ligaments we'll find that they're actually pretty easy to name if you know the different parts of the scapula as well as the other bones involved here. So next I'm going to draw the humerus. The humerus is the main bone in your arm right here. It's got a head at the top which makes the ball of this ball and socket joint. So this is the head of the humerus and it's going to articulate with the glenoid cavity right.

[01:40] here and these make up the ball and the socket of this ball and socket joint. Of all the types of joints the ball and socket gives the most degrees of freedom in its movement. So if you think about your shoulder you can kind of move your shoulder up and down and back and forward and sort of make circles with it. That's because of the nature of this ball and socket joint. Now that allows a lot of extra freedom but that also means that it's going to

[02:00] need a little bit of extra support compared to maybe some other joints to keep it in place where it needs to be and that's where all the ligaments are going to come in. But before we get to that it is a synovial joint which means there's going to be articular cartilage at both sides of this joint where it articulates or moves across each other. So right here we've got articular cartilage on the head of the humerus and we're going to have articular

[02:20] cartilage as well on the glenoid fossa or glenoid cavity. So these two surfaces are going to have the smooth articular cartilage and of course that is to reduce the friction between those two bones so they don't wear on each other. Being a synovial joint there will also be synovial fluid between those two. It's sort of a lubricant that's going to help prevent that articular cartilage from breaking down and further reduce

[02:40] the friction between those two bones. All synovial joints have a joint capsule, we'll get to that in a minute. So that's all the bones involved. There's really three main bones here, right? The scapula, the clavicle, and the humerus, and three important parts of the scapula so far, the coracoid, acromion, and glenoid cavity. Now to hold all this together, we need the ligaments. So we're gonna start with the ligament between the acromion and the clavicle, and that's just gonna be

[03:00] dense fibers or dense connective tissue, and that's gonna be a fibrous joint. And fibrous joints don't have much movement. So there's not gonna be much movement between the acromion and the clavicle. The purpose of this is sort of just to hold those two bones together. Now I love the naming conventions on these. They make really long words, but they're easy to remember if you remember the two parts of this. Here we have the acromion and the clavicle. So this ligament joint is called the acromion.

[03:20] right here is going to be the acromoclavicular ligament. All you're doing is taking the two bones or parts of the bone in terms of the scapula and putting those two words together to make one really long word. One quick naming convention, coraco is always going to come first if the coracoid is involved and the clavicle is always going to come last if the clavicle is involved. If you remember those two rules

[03:40] the rest of it flows pretty easily. So coracoid will always be first in the naming and then clavicle will always be last in the naming. The next ligament on the diagram isn't really part of this joint per se, but it's the ligament between the sternum and the clavicle. So follow our naming convention, put the two words together. Clavicle always comes last. So this is going to be the sternoclavicular joint.

[04:00] sternoclavicular ligament. Up next we have a ligament between the coracoid and the acromion. So this will be the coracoacromial ligament. And I find these not too hard to remember because basically every part that we've labeled here there's going to be a ligament between those parts if they're close to each other and then you just name them based on the parts. Up next we have a ligament between

[04:20] the coracoid and the clavicle. So of course that would be the coracoclavicular ligament. So that's four ligaments so far. Now let's actually move to the glenohumeral joint. We say shoulder joint, oftentimes we're talking about like all of the stuff involved right here, but the glenohumeral joint is specifically talking about where the ball and the socket is and what's involved there.

[04:40] about like the acromioclavicular ligament that's not really directly involved with the movement of this joint right here. So now we're just going to look at the glenohumeral joint, the ball and socket joint of the shoulder. Now being a synovial joint there's going to be a joint capsule that's going to do a couple things. It's going to sort of stabilize the joint a little bit. It's also going to hold in all of the synovial fluid.

[05:00] that's lubricating the joint. So here's our joint capsule and it's going to just completely wrap around all of that articular cartilage and make this sort of capsule to hold in all that fluid. Now the joint capsule itself isn't all that strong so it provides some support but it's definitely not going to be enough. Similarly to the knee right we have the ACL and the PCL and all these ligaments that sort of that give it extra

[05:20] extra structure because the joint capsule isn't strong enough to provide all the support that it needs. So same thing here, here's a few more ligaments we need to take a look at. There's also going to be a ligament between the humerus and the coracoid process right here. So I'm going to draw that one in. That's going to be the, follow the same naming convention, the coraco-humoral ligament. Okay, there's three more ligaments that we're going to draw in here.

[05:40] they're going to connect the humerus to the glenoid cavity. And we've kind of already got that drawn in. So these are really sort of an extension or a thickened part of the joint capsule that's going to provide some extra support. I'm going to draw them in right here. And as they're drawing them in, think about the naming convention that we would have here. They're between the humerus and really the glenoid fossa, glenoid cavity. So we're just going to call them the glenoid cavity.

[06:00] glenohumeral ligaments. There are three of them and we can just name them using superior, middle and inferior. So the superior glenohumeral ligament, the middle, and then the inferior glenohumeral ligament. You're a complicated little guy. Alright, quick recap of this. We've got three bones involved, the clavicle, the scapula, and the humerus.

[06:20] has a few important parts. That's the acromion, the coracoid, and the glenoid fossa. We've got a whole bunch of ligaments. We've got the acromoclavicular ligament, the sternoclavicular ligament, the coracoacromial ligament, the coracoclavicular ligament. We have the joint capsule that's going to surround the head of the humerus and the glenoid cavity. That's going to contain synovial fluid. There's also

[06:40] articular cartilage on the head and then the fossa. And then we have the coracohumoral ligament as well as the three glenohumoral ligaments that are going to provide extra support. We're about to move to a second diagram to look at the rotator cuff and all that it does, but at the end of the video if you want to review this I'm going to have a blank version of this diagram that you can go through and try to label all the parts to it. All right now let's take a look

[07:00] at the rotator cuff muscles. To really see where these muscles are located, we need to look at the anterior and the posterior parts of this. So let's go ahead and draw in our bones. Of course, we've got the clavicle, we've got the scapula. We're going to draw in a few parts here. On the anterior part, we've already seen this. We've got the main body of the scapula. We've got the coracoid. We've got the acromion sticking out over here. We haven't seen the posterior side of the

[07:20] the scapula yet. There's gonna be one additional structure that we need to draw in here and that's gonna be the spine of the scapula. That's this part right here. It kind of goes down sort of the middle or across the middle sort of like a spine and it sort of forms into the acromion right here. And then of course we have the humerus which is going to articulate with the glenoid fossa of the scapula. Alright, rotator cuff.

[07:40] is it exactly? It's a set of four muscles that have some special functions with the shoulder joint. Like a lot of muscles, they're going to cause movement, but they're also going to be support muscles, structural support that's going to hold that joint in place in addition to those other ligaments that we saw. Our first muscle that we have is the infraspinatus muscle. Infraspinatus

[08:00] the spine. So this is literally the muscle under the spine of the scapula. The infraspinatus muscle, which is on the posterior or the backside of the scapula, it's going to pull on the humerus, rotating the shoulder back like this. Next, just sort of underneath that or inferior to that is going to be the Terry's minor muscle. You can see that muscle right here and you can also see it peeking out right here.

[08:20] here on our anterior side of the diagram. The teres minor is a synergist with the infraspinatus, meaning that they're going to have the same function, laterally rotating the humerus. So if we have the infraspinatus below the spine, we must have the supraspinatus above the spine. We can also see that in our anterior diagram back over here. If you look at the supraspinatus, its origin is here above

[08:40] of the spine of the scapula and it's going to follow all the way over here and have an insertion in this part of the humerus. When it contracts it's going to work to lift the humerus. That's going to be a synergist with the deltoid muscle. When the deltoid muscle contracts it's also going to work to lift the humerus like this. Now a torn rotator cuff is a common injury that happens. It can be a sports injury.

[09:00] But it can also happen in just kind of regular strenuous use of the shoulder. The torn rotator cuff could be any of the four rotator cuff muscles, but the most common one is the supraspinatus and it'll tear somewhere here in this dense connective tissue. When that happens it can be painful, you can lose stability and also some movement in that joint. Now I said there's four muscles so there's one more involved and that's going to be the subscapularis.

[09:20] scapularis, sub-meaning under and scapularis meaning scapula. It's sort of under the scapula or it's on the anterior side of the scapula. So here's that subscapularis that's going to originate here on the medial side of the scapula and its insertion is going to be here on the head of the humerus. You see kind of a ridge right here in the connective tissue. We'll talk about that in just a second.

[09:40] Subscapularis being on the front is going to work very similarly as the deltoid. When it contracts it's going to pull on the humerus and try to rotate it medially or rotate it anteriorly. Subscapularis and pectoralis major are going to be synergists working to rotate the humerus forward. Those are the four rotator cuff muscles. We have the infraspinatus and the teres minor.

[10:00] which are going to work to rotate the shoulder back. We have the supraspinatus which along with the deltoids is going to work to abduct the humerus or extend the shoulder joint. And we have the subscapularis which is going to work in conjunction with the pectoralis to rotate the shoulder forward. Now that we have this drawn let's go over the three primary functions of the rotator cuff muscles. The first is stabilization. Now we kind of

[10:20] talked about this a little bit already, but those four muscles are going to wrap around the humerus on the top and the two sides, sort of just having like this extra strap on there holding it in place. So structure and stabilization is number one. The second is called concavity compression. So this one's a little bit harder to explain, but whenever the deltoids pull up on this, without

[10:40] Without the rotator cuff muscles, that's going to lift the head of the humerus upward. Without the rotator cuff muscles, that's going to lift the head of the humerus upward. And that would actually make it a less efficient movement. It would make the deltoids a less efficient lever because instead of just rotating it in place, it's going to be lifting up on it. And we don't really want that upward lift.

[11:00] of the humerus. We want it to be held in place and in fact we want it to kind of push in a little bit and that's going to make that rotate a little bit easier. So we call that concavity compression. The rotator cuff muscle is whenever we lift our arm like this it's going to pull the head of the humerus medially so it's going to make that rotation a little bit more easy and efficient.

[11:20] pulls the head in so that we can lift it easier. And the third function is just movement which we talked about already. We have abducting the arm which the supraspinatus does and then we have rotation which the infraspinatus, teres minor, and subscapularis do. It's terminology y'all, it's a lot. Okay next we've got one more muscle I want to draw in here and that's going to be the biceps.

[11:40] biceps brachii. This is sort of involved in the shoulder joint. It's not one of the main muscles you think about when you think about moving the shoulder joint, but it's directly involved in the shoulder joint and anatomically speaking its location at least. Without studying this stuff I would think that biceps would have an origin probably at the top of the humerus. It's going to work to flex the arm.

[12:00] that would think it's pulling on that, but it's actually pulling on the scapula. So let's take a look at how that works. Here we have the two heads of the biceps brachii. This medial one is going to extend all the way up and connect to the coracoid process. So whenever you're flexing your biceps like this, that muscle is actually pulling up on the coracoid. And the other head of the biceps brachii is where it gets really weird in my opinion.

[12:20] So the tendon of the other head of the biceps brachii extends up and then under the tendons of the subscapularis. It's kind of like there's a little tunnel right here. The head of this is going to thread up through there, and then it's going to make sort of a 90 degree turn, and then it's going to connect back over here on the scapula. So again, whatever you flex that elbow joint using your biceps brachii.

[12:40] brachii, that tendon is threaded up under the subscapularis tendons and then cuts medially all the way over to the scapula right here. And those are the two origins of the biceps brachii. They're both on the scapula, one right here on the coracoid, and then the other back here on the scapula. Now if you look this stuff up online, sometimes you'll see something called the transverse ligament.

[13:00] And I originally had this on the drawing as this extra little ligament here that kind of crosses over right there and that the biceps brachii kind of goes under it first before going up under the subscapularis. But then I did a little bit of digging and I found a research study where they had dissected I think like 14 cadavers maybe and then done a bunch of MRIs and stuff to see if there's actually a distinct transverse ligament.

[13:20] besides just the extension of the fibers of the subscapularis. And it turns out, from what they found, there is no separate transverse ligament. But I thought this was fascinating just because when you look up diagrams, sometimes it's there, sometimes it's not. But apparently, best as I can tell, transverse ligament, it's not its own distinct thing. It's really just the extension of the subscapularis.

[13:40] tendon that the biceps brachii tendon is threaded up through. Alright, quick recap. We've got the infraspinatus and teres minor that is going to rotate the shoulder back. We have the supraspinatus, which along with the deltoid muscle is going to abduct the arm or raise the humerus. On the anterior or front side we have the subscapularis, which along with the pectoralis major is going to work to rotate the

[14:00] the humerus or the shoulder forward or anteriorly. A common injury is a torn rotator cuff that most often happens in the supraspinatus tendons right here. Those rotator cuff muscles have four main functions. That's stabilization of the joint, concavity, compression. Whenever you abduct the humerus, it kind of pulls the humeral head in or medially. And then for movement, which we just talked about.

[14:20] out. Alright now here's a blank diagram of all of the ligaments and bones and parts of the bone in the shoulder joint. Take a minute, pause the video, see if you can go through and name every ligament and every bone and every part of the bone in this diagram. Showing the answers in five, four, three, two, and... Alright we have the humerus.

[14:40] In the case of the dermis, the scapula, the parts of the scapula, including the acromion and the coracoid, we have the clavicle, we have the sternum, and then we have all of those ligaments. We've got the acromioclavicular ligament, number one. We've got number two, the sternoclavicular. Number three is the coracoacromial. Number four is the coracoclavicular.

[15:00] Number five is the joint capsule, number six is the coracohumeral ligament, and then number seven is the inferior, superior, and middle glenohumeral ligament. And then finally here's a blank diagram of the rotator cuff muscles. So take a minute, pause the video, see if you can name all the rotator cuff muscles as well as the function of the rotator cuff.

[15:20] The answer is showing in 5, 4, 3, 2, and we have the supraspinatus, the infraspinatus, and the teres minor. We've got the subscapularis, those are the four rotator cuff muscles. We have the biceps brachii here, which we included because it's sort of anatomically relevant to the shoulder joint. We've got the transverse brachii.

[15:40] ligament here which isn't a distinct thing, it's really just the subscapularis tendon. And the functions of the rotator cuff include stabilization of the joint, concavity, compression, whenever you abduct the shoulder, as well as movement which includes abduction for the supraspinatus and then rotation. Subscapularis posterior rotation and infraspinatus and teres minor anterior.

[16:00] rotation. Look, I'm going to be posting a bunch more anatomy videos. So if you're studying anatomy, click the subscribe button to follow along for some more anatomy content. And then up here is another video that you might be interested in for learning anatomy. Alright, thanks for watching. Catch you in the next video.