Video: Deltoid muscle level

Do you know what one of the most common shoulder injuries is? Any guesses what's happened to this baseball player? People who overexert their shoulder muscles can cause a rotator cuff injury. You've probably heard of it before, but do you know what it is? Do you know what muscles are part of the rotator cuff and how they support the shoulder? Let's discover them in this tutorial on the cross-section at the deltoid muscle level.

We'll see these muscles from a different perspective and identify bones, ligaments, and muscles involved in the shoulder joint. Before we begin, let's quickly go over what we'll talk about today.

So, we'll begin by reviewing the planes of the body, and ultimately, discover what plane we’re actually working with when looking at the cross-sectional images. We'll then move on to identify the bones in this area of our cross-section followed by the ligaments that help strengthen the shoulder joint. Finally, we'll look at the muscles in the area and see their relationship to each other in this cross-sectional plane.

Okay, so let's start with a review of the body planes.

So, body planes are imaginary planes or flat surfaces that cut through and section the body while it's in the anatomical position. There are three cardinal body planes, and the first is the plane that we can see highlighted in this image. This is the coronal plane. The coronal plane is a vertical plane that divides the body into anterior and posterior parts.

So, now, what we can see highlighted is another vertical plane, but this time dividing the body into left and right parts, and this is called the sagittal plane.

The third and final body plane is the transverse plane, which we can now see highlighted in green. And this is a horizontal plane which divides the body into superior and inferior portions. Transverse planes are the planes we make cross-sectional cuts in, so this is what's happened to the body when we look at cross-sectional images, and, of course, that's what we'll be focusing on in this tutorial.

In this video, we'll be looking at some structures that can be seen at this level in the transverse or cross-sectional plane, and these cross-sectional images are similar to what can be seen in CT and MRI scans. Just as with those scans in cross-sectional anatomy, we're looking at the images as if we're standing at the patient's feet looking superiorly towards the patient's head. Also, it should be noted that the patient is always lying on their back; therefore, this side of the image is the right-handed side of the patient's body and this is the left-hand side, and here's the anterior or the front of the body and at the bottom of the screen is the posterior aspect where the patient's back is. Keep these positions in mind as this will help you stay oriented throughout the tutorial.

So, let's start identifying structures. First up are the bones.

The most obvious bone we can see in this cross-section is highlighted now in green. So, as you can see, we have a round, ball-like structure within the shoulder joint. Do you know what this could be? That's right. It's the humerus.

The humerus is a long bone in the arm of the upper limb and at its superior end is the head of the humerus and it’s this rounded portion that we can see in the cross-sectional image. So our cross section was made at about this level so the part of the humerus we can see in this image on the left is this part here and this rounded part of the humerus, which is the head of the humerus, is what articulates with the scapula to form the shoulder joint, also known as the glenohumeral joint. And if you look closely, you can even see the smooth articular cartilage of the head of the humerus right here.

The thin green line we can see around the humerus is a group of ligaments called the glenohumeral ligaments, and these ligaments help to stabilize the shoulder or glenohumeral joint contributing to the fibrous capsule.

Speaking of the scapula, we can now see parts of it highlighted in green, and the scapula is commonly known as the shoulder blade and forms the posterior part of the shoulder girdle. And this flat bone overlies the second to seventh ribs on the posterolateral aspect of the thorax. Towards its superolateral end, the scapula extends a bit more anteriorly to form the shoulder joint by articulating with the head of the humerus. The scapula also provides muscle attachment for several muscles of the arm and the shoulder that are located on the anterior side of the thoracic cage and arm, which is why we see it protrude towards the top of the cross-sectional image.

There are two specific features of the scapula that we can see in this image, and the first is this anterior projection which is called the coracoid process. In this image on the right, you can see the coracoid process is found on the superior end of the scapula and that it projects anteriorly, and this process is where muscles such as the pectoralis minor attach.

The second feature we can see is this more posterior portion, and this is the spine of the scapula. In this image on the right, we can see the spine of the scapula on the posterior aspect of the scapula. The spine of the scapula also serves as an attachment point for muscles such as the trapezius and deltoid muscles and divides the posterior aspect of the scapula into two fossae for muscles we will identify later on.

So those are all the bones and the bony features that we'll be looking at today. Next stop, we'll look at the muscles located in the area of the deltoid cross-section.

So, let's start off with an easy one – the one named in the title of this tutorial.

Highlighted now in green is the deltoid muscle. So, this muscle is the most superficial muscle of the shoulder, and in this image on the right, we can see the deltoid on the anterior surface of the shoulder wrapping around to the lateral aspect and also on the posterior side. With this in mind, we can actually look at the deltoid as having three parts – a clavicular part found anteriorly which attaches to the lateral third of the clavicle bone, the acromial part found laterally which attaches to the acromion of the scapula, and finally, a scapular spinal part which as you might have guessed is found here along the posterior aspect attaching to the spine of the scapula.

Anteriorly and towards the midline is a large muscle of the chest and this is the pectoralis major muscle, now highlighted in green. Specifically, what we can see in the cross-section is part of the clavicular head of the pectoralis major.

Moving deeper, we come across a small muscle that is associated with the clavicle as well, and this is the subclavius muscle. The subclavius travels from the inferior surface of the clavicle, the subclavian groove, to the junction of the first rib and the first costal cartilage.

Now, we'll move on to the posterior aspect of the image. Don't forget that this means the bottom of our screen as our patient is lying on their back, and the most superficial muscle on the posterior side is the trapezius muscle. The trapezius is a large muscle spanning much of the back and we only see it towards the midline in the cross-section because we're at the level of its inferior portion. And remember that in the cross-section, we identified the spine of the scapula.

Deep to the trapezius is the rhomboid major muscle. This muscle inserts onto the medial border of the scapula inferior to the spine of the scapula and we can see it inserting onto the medial border here in this cross-sectional image. Also attached to the medial border of the scapula is the serratus anterior muscle, and this muscle wraps around the thoracic cage and actually originates from ribs one to nine, as we can see in this image on the right. In the cross-sectional image, we can follow the muscle anteriorly towards the top of the image where its costal origin is.

Moving laterally, we see a large belly of a muscle filling the subscapular fossa of the scapula and this is the subscapularis muscle. So, in this image on the right, we're looking at the shoulder girdle from an anterior view with the thoracic cage removed and the muscle fills the subscapular fossa and travels towards the humerus as a rotator cuff muscle.

The remaining two muscles that we're going to be looking at today are found on either side of the spine of the scapula, and the first one we'll look at is the more anterior muscle. So, this cross section is just superior enough for us to begin to see the supraspinatus muscle which is what is now highlighted in green. The supraspinatus muscle which originates superior to the spine of the scapula in the supraspinous fossa travels laterally towards the humerus, and in the cross-sectional image, we're not superior enough to see the lateral attachment.

And last but not least, the infraspinatus muscle. The infraspinatus is located inferior to the spine of the scapula occupying the infraspinous fossa and we can see a large part of it towards the posterior aspect of this image. In this image on the right, we can see the insertion point of the infraspinatus on the humerus. And looking back at the cross-section, we can see its attachment point as well.

So, if one muscle is above the spine of the scapula and one muscle is below, why can we see both in this horizontal cross-sectional image? Well, the spine of the scapula is not a horizontal feature. Because it is on an angle, we can see the muscle above – the supraspinatus – and the muscle below – the infraspinatus – in the same horizontal cross-section.

Okay, so those are all instructors we need to identify at this cross-sectional level. So, do you remember that rotator cuff tear we were talking about at the beginning of the tutorial? Let's have a look at that now that we know a little bit more about the muscles in this cross-section.

So, do you now know what the four muscles that make up the rotator cuff are? We saw three of them in the cross-section. One is the supraspinatus highlighted now on the cross-section, second is the infraspinatus, and the third muscle that we saw from the rotator cuff is the subscapularis.

So, the fourth and final muscle which is too inferior to see in this cross section is the teres minor, and in this cross-section, you can see how much muscle is part of the rotator cuff and why it's so important for shoulder stability and movement.

So, rotator cuff injuries can result from a single incident or from repeated stress on muscles or their tendons and people with rotator cuff injuries will feel pain in the shoulder, often a dull, deep ache, and will often have weakness in the same arm. Depending on the severity of the tear, various treatments are available from rest, ice, and physical therapy to surgery.

And with that, you're now an expert on the cross-section at the level of the deltoid.

Before I let you go, let's quickly review what we looked at today.

So, we started out by looking at the bones in this region, the first being the head of the humerus. Next, we identified a group of ligaments called the glenohumeral ligaments which help to keep the head of the humerus in the glenoid fossa. The other bone that we saw was the scapula and all of its parts are highlighted now. We identified two specific features – the spine of the scapula and the coracoid process. And then we moved on to the muscles, beginning with the large deltoid which wraps around the humerus superficially, and still superficially but anterior, we found the pectoralis major muscle, and deep to that, the subclavius muscle.

We then moved to the posterior part of the image and found the most superficial muscle was the trapezius, and deep to that but still medial is the rhomboid major muscle. Attaching to the medial border of the scapula and wrapping anteriorly around the ribs, we identified the serratus anterior. Filling the subscapular fossa, one of the rotator cuff muscles is the subscapularis. We also found the supraspinatus close to the spine of the scapula and the infraspinatus on the other side of the spine of the scapula, and both are also rotator cuff muscles. We then looked at what a rotator cuff injury involves including naming the fourth rotator cuff muscle – the teres minor.

And that brings us to the end of the tutorial. Thanks for watching. Happy studying!