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Origins, insertions, innervation and functions of the rotator cuff muscles.
The shoulder joint is the most flexible joint in the human body allowing us to engage in a variety of activities such as swimming, tennis, and gymnastics. However, increased mobility means decreased joint stability and makes the shoulder joint particularly susceptible to dislocation. We have various ways of reducing the risk of dislocation of the shoulder joint, one of which is through the muscles that we’re going to be talking about today – the muscles of the rotator cuff.
Before we begin, I’d like to give you a quick overview of what we’re going to be talking about in this tutorial. So, firstly, we’re going to be looking at the bones that make up the shoulder joint as they form the bony framework that our muscles attach to, and then I will give you a little introduction to the muscles of the rotator cuff as a whole before we go on to look at each muscle individually discussing their origin, insertion, function and innervation. And finally, we’ll conclude our tutorial with some clinical notes about dislocation of the shoulder joint.
Okay, so let’s get started with the bones of the shoulder joint.
So as I mentioned earlier, the shoulder joint is the most mobile joint in the human body and this is down to the fact that it’s a synovial ball-and-socket joint which we can see here from an anterior perspective. The ball component of this joint is formed by the head of the humerus which is a feature of the long bone in our arm, the humerus, whereas the socket component is formed by the glenoid cavity which is part of the scapula or the shoulder blade. And as such, the shoulder joint is also known as the glenohumeral joint.
Okay now that we’re familiar with the bony framework that our muscles attach to, let’s move on and have a look at the muscles of the rotator cuff.
So before we get on to the details, let’s orientate ourselves, and here we have our lovely Kenhub volunteer. Let’s call her Annie, and we can see here from both a posterior perspective and an anterior perspective. So, if we remove any skin, connective tissue, the neurovasculature and some surrounding musculature – thank you Annie! – We can see her rotator cuff muscles.
Posteriorly, we have the supraspinatus, the infraspinatus and the teres minor, and anteriorly we have the subscapularis.
So, now, we’re going to get Annie to stand side on for us and we’re going to remove some of the surrounding soft tissues and the humerus, and here we can see all four rotator cuff muscles. And again, we can see the supraspinatus, the infraspinatus and the teres minor posteriorly, and we have the subscapularis anteriorly.
A usual tool to help you remember the muscles of the rotator cuff is the phrase “The head of the humerus SITS in the glenoid cavity” with SITS being a handy little mnemonic for you to use here with S standing for supraspinatus, I standing for infraspinatus, T standing for teres minor, and S standing for subscapularis.
Okay, so now that we know what we’re looking at, just let me give you some information about these muscles as a whole.
So, generally, the muscles of the rotator cuff originate from the scapula and they insert at the humerus which is at this point over here. As for their functions, these muscles work together to reduce the risk of dislocation by stabilizing the shoulder joint. Essentially, they help keep the head of the humerus in its correct position within the glenoid cavity and also as the name suggests, the rotator cuff muscles play a major role in medial and lateral rotation of the arm at the shoulder joint.
So now that we’re familiar with the rotator cuff muscles as a whole, let’s look at each muscle individually, and we’re going to be discussing their origin, their insertion, their function and their innervation.
So the first muscle that we’re going to be talking about is the supraspinatus which we can see here highlighted in green. And as the name suggests, this muscle originates from the supraspinous fossa of the scapula which is a shallow depression in the body of the scapula above its spine. The thing goes on to insert here at the greater tubercle of the humerus. So when the supraspinatus contracts, it initiates abduction of the arm at the shoulder joint and as a rotator cuff muscle, it also stabilizes the shoulder joint. And, of course, in order for this muscle to perform its functions, it requires innervation, and the supraspinatus is innervated by the suprascapular nerve which arises from the superior trunk of the brachial plexus.
Inferior to the spine of the scapula, we find the infraspinatus muscle, and this muscle originates from the infraspinous fossa of the scapula, and like the supraspinatus, it inserts over here at the greater tubercle of the humerus. As for its functions, the infraspinatus is responsible for lateral rotation of the arm at the shoulder joint and it also stabilizes the shoulder joint. And like the supraspinatus, this muscle is innervated by the suprascapular nerve which we can see here highlighted in green.
So the next muscle of the rotator cuff that we’re going to be talking about is the teres minor muscle, and this muscle originates from the infraspinous fossa and the lateral border of the scapula. It then runs as you can see obliquely upwards and laterally to insert at the greater tubercle of the humerus. Contraction of this muscle results in lateral rotation of the arm at the shoulder joint and, of course, it also stabilizes the shoulder joint. As for its innervation, the teres minor is innervated by the axillary nerve which we can see here highlighted in green.
Okay, so in order to see the last muscle of the rotator cuff, we need to change to an anterior view of the shoulder joint, and over here, we can see the subscapularis muscle which originates from the subscapular fossa. Unlike its buddies, this muscle inserts at the lesser tubercle of the humerus as we can see here. The subscapularis is responsible for medial rotation of the arm at the shoulder joint and it also stabilizes the shoulder joint. When it comes to the innervation of the subscapularis muscle, this muscle is innervated by the upper and lower subscapular nerves which are branches of the posterior cord of the brachial plexus.
Okay so as we come towards the ends of our tutorial, we should now be feeling comfortable with the muscles of the rotator cuff. Let’s move on to discuss some clinical notes relating to the dislocation of the shoulder joint.
So as I have said at the start of our tutorial, the shoulder joint is the most flexible joint of the human body which means that it’s particularly vulnerable to dislocation. And because we have ways to reduce the risk of dislocation such as our rotator cuff muscles, it requires a pretty strong force to pop this joint out of its socket, therefore, it’s normally caused by a fall or a collision with another person or an object.
So most people dislocate their shoulder while playing a contact sports such as rugby or while undertaking sports that have a potential for frequent or high impact falls such as gymnastics. However, in older individuals, it can also be caused by falling on outstretched hands and most shoulder dislocations involved anterior displacement of the humeral head, which we can see here in this x-ray. So, over here, we can see the head of the humerus and behind it, we can see the glenoid cavity of the scapula.
So, symptoms of dislocated shoulder include intense pain, inability to move the joint, and visible deformation of the shoulder joint. And in terms treatment, we can use a procedure called reduction where the doctor rotates the arm until it goes back into its socket and some patients damage soft tissues when they dislocate their shoulder and may need surgery to repair them.
So, recovery requires rest and pain medication as well as physiotherapy to improve strength and mobility and it can take up to three to four months to recover fully from a dislocated shoulder.
Okay so before we bring our tutorial to a close, let’s quickly summarize what we’ve learned today.
So we started our tutorial with talking about the bones of the shoulder joint which are the humerus and the scapula and then we talked about the muscles of the rotator cuff as a whole before looking at each muscle individually discussing their origin, insertion, function and innervation. And finally, we finished up our tutorial with some clinical notes about dislocation of the shoulder joint.
So that brings us to the end of our tutorial on the muscles of the rotator cuff. I hope you enjoyed it. Thanks for watching and happy studying.