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Although the shoulder is made up of only two bones – the clavicle and the scapula – shoulder bony landmarks are numerous. The shoulder’s composition, as well as that of each bone, is not that simple. Shoulder bony landmarks are located both anteriorly and posteriorly. The scapula, in particular, holds many of the shoulder bony landmarks, which can change with the scapula’s movement.
In this article, we’ll describe and illustrate all the shoulder bony landmarks and explain the morphology and physiology of the bones of the shoulder. We’ll cover the scapula movements such as scapular rotation, protraction, and retraction. We’ll also look at how scapula movements influence the position of the shoulder bony landmarks.
The body of the scapula is covered by muscle, but the bone features many protrusions that are visible on the surface. The main scapula bony landmarks are the spine, medial border, and inferior angle of the scapula. Of these three, the most notable bony landmark is the spine of the scapula, which is tucked in between the trapezius and deltoid muscles.
The main scapula bony landmarks: the spine, medial border, and inferior angle.
The three parts of the spine of the scapula: the trigonum, body, and acromion process.
As seen in the picture above, the spine of the scapula is further divided into three distinct parts. Colored in green is its medial end, called the trigonum of the spine. This bony triangle marks the point where the spine meets the medial border. The body of the spine follows a straight line towards its lateral end which is called the acromion process.
While the medial and middle parts of the spine are only visible posteriorly and mark the borders of the back muscles, the acromion process is visible both from the back and the front. The following picture illustrates how the acromion process protrudes forward to make a hook-like socket for the lateral end of the clavicle. The acromion process closes the bony framework of the shoulder.
The acromion process makes a socket for the lateral end of the clavicle.
To illustrate why it’s so important to know all these bony landmarks, let’s take a closer look at the deltoid muscle. It’s composed of three parts that originate from three separate locations. The posterior head (3) of the deltoid muscle originates from the spine of the scapula, its lateral head (2) – from the acromion process. The anterior head (1) of the deltoid muscle originates from the clavicle.
In other words, this muscle originates from three prominently visible shoulder bony landmarks. Knowing them helps you create a realistic deltoid muscle. This is just one example why bony landmarks are super helpful for artists!
The three heads of the deltoid muscle originate from well-visible shoulder bony landmarks.
Let’s move on to the medial border of the scapula. The medial border of the scapula extends downwards from the trigonum of the spine and is visible in the surface forms of the back. It ends in the inferior angle of the scapula, which is in most cases hidden underneath the latissimus dorsi muscle. Unless a person is exceptionally lean or the scapula is retracted, the inferior angle of the scapula can rarely be seen on the surface.
The medial border and the inferior angle of the scapula.
Most shapes visible on the surface of the back are formed by muscles, yet all the muscles are attached to the bones. If you see the bony landmarks of the scapula, you also see the borders of the back muscles. When a person is muscular, shoulder bony landmarks look like depressions between the muscles, and if the person is lean, they protrude.
Bony landmarks are depressions on muscular backs, but they protrude on lean backs.
Although it’s not technically a bony landmark, it’s also good to know about the coracoid process. The coracoid process is a protuberance on the scapula’s anterior side, right next to the acromion process. It can be felt when palpated, but it’s never visible on the surface. Nevertheless, it is an essential point where the Biceps brachii, Coracobrachialis, and Pectoralis minor muscles attach.
Location of the coracoid process relative to the acromion process.
The coracoid process visible on a live model.
Now that we know the shape of the scapula, let’s look at its position relative to the other parts of the back. When the arms are relaxed, the medial borders of scapulae are never parallel – the upper parts are a bit closer together than the lower parts.
Similarly, the spines of the scapulae are not perpendicular to the spine of the back. Instead, they go upwards with the trigonum end being lower and the acromion process end always higher.
The location and position of scapulae on the back.
The height of each scapula is about the same as the distance between the inferior angles of the scapulae when the arms are at rest.
Scapula movements are various, and they include scapular protraction, scapular retraction, and scapular rotation. Scapula movements are very diverse because the scapula is a free-floating bone – it is not connected to the thorax with any joints. That also means that when the arms are moving the space between the scapulae may change considerably. Shoulder blades move a lot together with the upper limb.
Since there are a lot of muscles attached to the shoulder blade, scapula movements also move all of the attached muscles! That means studying and understanding all the scapula movements and their range of motion enables you to create realistic human figures in motion.
Bony landmarks make the most robust and reliable anatomical roadmap of the human body. While the muscles and fat can differ from person to person, the bony landmarks don’t change much. But in the case of the scapula, they do vary with motion. When you understand the movements, these shifts in the bony landmarks become predictable.
Scapular protraction and scapular retraction are possible because the shapes of the thorax and the scapulae match. Thorax can be roughly viewed as this big spherical egg shape, and the form of the scapulae corresponds to it. The anterior surface of the scapulae is curved in a way that fits the thoracic surface perfectly, enabling scapular protraction and scapular retraction.
The anterior surface of the scapula is curved to match the surface of the thorax.
Scapular protraction happens when the arm moves forwards: the scapula and all the muscles attached to it move forward along with the upper limb. Scapular retraction is the opposite of scapular protraction – the scapula and all the attached muscles move backward along with the rest of the arm.
Left: scapular retraction, arm and the scapula moves backward; right: scapular protraction, arm and the scapula moves forward.
During scapular protraction, the space between the scapulae becomes wider. In contrast, during scapular retraction, it narrows as the scapulae come closer to each other, traveling along the oval surface of the thorax.
Scapular rotation happens when you lift your arm: the movement is created by the Scapular muscles, Latissimus dorsi, Teres major, and Serratus anterior. Yet, scapular rotation only happens when you start raising your arm above shoulder level. Any movements below shoulder level do not involve the scapular rotation. Why is that?
Scapular rotation happens when the arm is raised above shoulder level and the humerus locks with the acromion process.
The mechanism of scapular rotation is simple: below shoulder level, the humerus moves freely in its socket. However, it locks with the acromion process on the scapula when it’s raised at shoulder level. When moved above shoulder level, it continues its upwards movement together with the scapula, thus rotating it.
The position and the angle of the medial border of the scapula are good indicators of the degree of scapular rotation.
Check out our video about the scapula anatomy here. In it Uldis Zarins explains the topography, morphology and physiology of the scapula and also talks about scapular protraction, retraction, and rotation.
The clavicle connects with the scapula completing the bony framework of the shoulder. It also connects with the sternum, the only point where the upper limb connects with the trunk of the skeleton. The bony landmarks of the clavicle are points of attachment for the Pectoralis major, Deltoid, Trapezius, Sternocleidomastoid, and Serratus anterior muscles.
The bony landmarks of the clavicle can be split into three parts:
The lateral end of the clavicle is called the acromial end – there, the clavicle connects with the scapula at the acromion process.
The place where the clavicle meets the scapula is called the scapulo-clavicular joint. There, the acromion process wraps around the end of the clavicle. The clavicle doesn’t go to the very end of the shoulder. Instead, the shoulder is finished by the acromion process, which closes the whole structure of the shoulder girdle and makes it robust.
The red line marks the sterno-clavicular joint: the only place, where the arm connects to the rest of the body.
The medial end of the clavicle is also called its sternal end because it connects to the sternum. There, it creates the sterno-clavicular joint, the only connection between the upper limb and the skeleton’s trunk. Since the scapula is a free-floating bone that only connects with the humerus and the clavicle, it doesn’t have a direct connection with the thorax.
The clavicle finds its location between the sternum’s manubrium and the scapula’s acromion process. Its location also influences its form: the sternal end of the clavicle is rounder, while the acromial end and the shaft of the clavicle are flat. That’s because the flat acromial end of the clavicle connects with the equally flat acromion process.
Morphology of the clavicle: complex forms to simple shapes.
The clavicle is shaped like an “S” when viewed from the top. Both clavicles together create the Cupid’s bow, as shown in the picture below.
When viewed from above, both clavicles form the Cupid’s bow.
In the frontal view, the clavicle is straight. Both clavicles create either a perfectly straight line or a “V” shape when viewed together. In the case of the “V” shape, the acromial end of the clavicle sticks out and makes a little bump on the shoulder. When the clavicles form a straight line, there is no such bump.
When viewed from the front, both clavicles can form either a straight line or a V shape.
Various muscles surround the clavicle: the Sternocleidomastoid and Trapezius muscles attach to it superiorly. The Deltoid and Pectoralis major muscles attach to it inferiorly. Depending on how trained the Pectoralis major muscle is, the visibility of the clavicle changes. If the Pectoralis major muscle is thicker, the clavicle becomes less visible.
If the Pectoralis major muscle is thicker, the clavicle becomes less visible.
There are two spots where the clavicle will always be visible regardless of muscle thickness. Superiorly, it’s the L-triangle, between the Sternocleidomastoid and Trapezius muscles. Inferiorly, it’s the Deltopectoral triangle. This triangular notch is bordered by the clavicle and the Deltoid and Pectoralis major muscles. It can almost be seen as a gap under the bridge above.
The clavicle is never hidden by muscle at the Deltopectoral triangle.
It’s also important to note that the distal end of the clavicle can only be seen when the arm is resting. As you lift the arm, the scapulo-clavicular joint moves towards the back and the distal end of the clavicle becomes invisible from the front.
As the arm gets lifted, the distal end of the clavicle becomes invisible.
We’ve also prepared a video that explains the clavicle anatomy. Watch it here! In it Uldis Zarins explains the topography, morphology and physiology of the clavicle. The form of the clavicle is relatively complex, Uldis uses simplified visuals to help you understand this bone better.
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Hey, Anatomy For Sculptors team here
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