Scapula

Scapula

Before I say anything, look at this:

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There’s the scapula with the humerus (bone of the arm) attached to it… what does it look like?

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Come on, I’m sure all of you saw this. I’ll be damned if you can’t swing around your scapula as a battle axe.

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Anyways, coming back to the scapula, I think we can all appreciate the reason why the scapula is called the shoulder blade. Because we can swing our scapula to victory it resembles the blade of an axe when put onto our own humerus.

Now that we’ve gotten that very important information (and gif) out of the way, let’s dive into the scapula.

Positional Anatomy

Where is the scapula? Given that we know it is the shoulder blade, we know it s somewhere near the shoulder joint.

The scapula, a triangular, flat bone, lies on the posterolateral aspect of the thorax, overlying the 2nd to 7th ribs.

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Notice where the scapula overlies in the diagram above – right over the 2nd to 7th vertebrae.

We’ve seen how the scapula looks from the back; what about the front?

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So the scapula is located in the posterolateral aspect of the thorax, but notice it is still able to articulate with the humerus. 

The answer lies in its superior view:

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Notice how the scapula is angled: it is obliquely angled so that the main body is still on the posterolateral aspect, but its most lateral end is located lateral to the thorax, where it can articulate with the humerus.

And just for completeness, here’s an X-Ray of the scapula (and by extension, the shoulder joint).

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Structure of the Scapula

The scapula is a triangular, flat bone, as already mentioned above. We can figure out what surfaces, borders and angles it has simply by examining a triangle. Let’s try it:

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So we can easily see that the scapula has 3 sides (as do typical triangles), 3 borders (as do typical triangles) and 2 surfaces, one facing anteriorly and one facing posteriorly

Let’s begin with the surfaces:

Surfaces of the Scapula:

The scapula has two surfaces, one facing anteriorly and one facing posteriorly.

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The one facing anteriorly, is facing towards the ribs and costal cartilages, and is thus called the costal surface.

The one facing posteriorly is simply named the posterior surface, simply because it is very closely related to the skin (we say it is a very cutaneous bone).

Posterior Surface:

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The posterior surface protrudes outwards from the body, and thus it is convex. 

Look at the image to appreciate this:

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Notice there is a slight arch pushing out towards the posterior surface.

Using this same image, let us also appreciate the presence of the large and labeled, spine of scapula and acromion. They essentially divide the posterior surface into two uneven fossae:

1) Supraspinous Fossa: The supraspinous fossa is the much smaller, superiorly placed fossa located above the spine of the scapula.

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The supraspinous fossa is smooth and concave, and becomes broader/larger moving towards the medial/vertebral border. You can see this in the diagram below.

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important to understand is that even though the posterior surface of the scapula is convex. the supraspinous fossa is concave. Look at the view,”Lateral View” in the diagram above and see if you can appreciate that.

The supraspinous fossa is related to the supraspinatus muscle (supra… fossa = supra… muscle), which originates from the medial 2/3 of the supraspinous fossa, and insersts into an area called the superior facet of the greater tubercle of the humerus. Here it is below:

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2) Infraspinous Fossa: This is the much larger fossa of the posterior surface of the scapula, located below the spine of the scapula.

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Its centre forms a very prominent convexity, and this is the reason the posterior surface of the scapula is described as being convex.

Near to the vertebral/medial border, there is a shallow concavity located superiorly, labelled in the diagram above.

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Near the axillary/lateral border, there is a deep groove running from superior to inferior, stopping about 2.5cm above the inferior angle of the scapula. This deep groove runs right parallel to a ridge, that provides attachment to a fibrous septum that separates 2 groups of muscles, the infraspinatus muscle from the teres minor and teres major muscles, all of whom will be discussed later. Along the middle of this ridge, there is a very small groove for the scapular circumflex vessels, through which the named vessels pass. Look at the diagrams above and below to see where the scapular circumflex vessels are.

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Again, the muscle related to this fossa is the infraspinatus muscle (infra–fossa = infra–muscle). The infraspinatus muscle again, originates from the medial 2/3 of the infraspinous fossa, and inserts into a region known as the middle facet of the greater tubercle of the humerus. Here it is below:

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Note that for both the infraspinatus and supraspinatus, even though the muscle originates from the medial 2/3 of the fossa, it still covers the entire fossa as it runs across it towards the humerus.

Now let us quickly revisit the spine of scapula and acromion.

Spine of Scapula and Acromion:

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The spine of the scapula is a thick, prominent plate of bone that crosses obliquely the medial 4/5 of the posterior surface of the scapula. By doing this, it reserves the upper 1/5 for the supraspinous fossa, and the lower 4/5 for the infraspinous fossa.

The root of the spine is the most medial area of the scapula from which the spine begins, and this corresponds to a region on the vertebral or medial border that is at the same level as the tip of the spinous process of T3. 

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As it moves laterally, it gradually becomes higher and higher until it overlies the shoulder joint. At this point, its most lateral region is known as the acromion, a flat plate of bone that articulates with the clavicle at its lateral end, to form the acromioclavicular joint. It’s superior surface gives attachment to some fibres of the deltoid muscle and trapezius muscle. It also gives attachment to a ligament, the coracoacromial ligament.

The spine of scapula possesses 3 borders:

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Anterior Border: This border is faced anteriorly towards the posterior surface of the bone, and is attached to it.

Lateral Border: This border is the thickest and broadest, and is connected to the acromion superiorly, and to the scapula inferiorly. It forms the medial boundary of the great scapular notch, which connects the supraspinous fossa and infraspinous fossa. The Great scapular notch allows the suprascapular artery and nerve to pass through. It is made into a great scapular foramen by a ligament known as the inferior transverse ligament. It is depicted below.

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Posterior Border: The posterior border is known as the crest of the spine, and contains, in its middle region, a thickening known as the deltoid tubercle. This tubercle contains 2 lips; the upper of which provides insertion for the trapezius muscle, and the lower of which provides insertion for the deltoid muscle.

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Costal/Anterior Surface

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The costal surface of the scapula is concave anteriorly, and forms one large fossa, the subscapular fossa. This fossa gives attachment to a very large, broad muscle, the subscapularis muscle. This muscle originates from the medial 2/3 of the subscapular fossa (notice a pattern here? All the muscles are from the medial 2/3 of their respective fossa), and inserts into the lesser tubercle of the humerus. Here it is below, looking like someone lodged an axe in somebody.

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There are 3 obliquely running ridges that run across the medial 2/3 of the subscapular fossa, all which give attachment to tendons of the subscapularis muscle.

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Furthermore, the body of the scapula in this region is indeed very thin, and is sometimes even semi-transparent, with fibrous tissue running in the middle of the bone. Look how interesting this looks above.

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There’s one last thing. The subscapular fossa itself has its own angle, the subscapular angle, depicted above, where the bone appears bent on itself, as the part of the scapula functioning as the shoulder joint (glenoid cavity) bends anteriorly to reach the shoulder. This bending forwards occurs at a line, (the blue line) that is at a right angle to the direction the glenoid cavity faces (red line). This subscapular angle gives great strength to the arch of the scapula.

Borders of the Scapula

As already discussed, there are 3 borders of the scapula.

Medial Border of Scapula

The medial border is called the vertebral border, and is thus named because it lies parallel to the vertebrae. It runs from the inferior angle all the way up to the superior angle. As it runs parallel to the vertebrae, it is only separated from the spinous processes of the vertebrae by 5cm.

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The medial border is important because it gives insertion to 4 muscles.

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  1. Serratus Anterior: Has a long insertion that runs along the entire medial border on the costal surface.
  2. Levator Scapulae: Attaches to the upper part of the medial border on the posterior surface, as well as the superior angle.
  3. Rhomboid Minor: Attaches to the medial border on the posterior surface at the level of the root of the spine. 
  4. Rhomboid Major: Attaches to the middle and lower part of the medial border on the posterior surface.

Lateral Border of Scapula

The lateral border is also known as the axillary border, and this is because this border points towards the axilla, or arm pit region. It is actually the thickest of the three borders. Directly above its superiormost part, leads to the lateral angle of the scapula, also known as the head of the scapula, closely related to the neck of the scapula. The head of the scapula, or lateral angle, holds the glenoid cavity, and the neck runs along the outside of the head.

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The lateral border begins at a rough tubercle known as the infraglenoid tubercle, located below the glenoid cavity, and extends obliquely and inferiorly towards the inferior angle of the scapula. The infraglenoid tubercle and glenoid cavity are explained below.

It gives origin to 4 muscles.

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Long Head of Triceps Brachii: The infraglenoid tubercle itself gives origin to the long head of triceps brachii, the highest point of the lateral border, both on the costal surface and posterior surface.

Subscapularis: The most medial part of the lateral border on the costal surface is still able to afford origin to some fibres of the subscapularis.

Teres Minor: This muscle is attached to the upper part of the lateral border of the posterior surface of the scapula.

Teres Major: This muscle is attached to the lower part of the lateral border on the posterior surface of the scapula.

Superior Border of the Scapula

The superior border of the scapula extends from the lateral angle to a process known as the coracoid process.

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You can see this in the diagram above. We will get to what the coracoid process is in less than half a minute (less if you’re a speed-reader), but for now, focus on that little notch, the suprascapular notch. This notch becomes a foramen by the work of a ligament known as the superior transverse suprascapular ligament. It is labelled “SSL” in the diagram below:

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This ligament converts the previously open suprascapular notch into a suprascapular foramen. This allows the passage of the suprascapular nerve (But NOT the corresponding artery). Compare this with the great scapular notch, which allows both the suprascapular artery and suprascapular nerve to pass.

Let us see how exactly this artery and nerve travel. The suprascapular nerve arises from the brachial plexus, travels through the suprascapular notch, then curves around as it travels through the great scapular notch. It innervates both the supraspinatus and infraspinatus muscles. 

The suprascapular artery however, a branch of the thyrocervical trunk, passes directly into the supraspinous fossa, then travels through the great scapular foramen towards the infraspinous fossa.

Furthermore, the superior border gives attachment to 1 muscle, the omohyoid muscle.

The omohyoid muscle originates from the upper surface of the scapula, very close to the suprascapular notch, and inserts into the inferior surface of the hyoid bone. 

Now that we’ve spoken about this, let us finally visit the coracoid process.

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The coracoid process is a hook shaped process that, together with the acromion, stabilize the scapula by forming joints with the clavicle, and attaching several muscles. The root of the coracoid process begins at the upper part of the neck of the clavicle, and it originally projects superiorly and medially, before abruptly changing its direction and projecting horizontally and laterally.

It gives origin to 2 muscles, insertion to 1 muscle, and attachment of 3 ligaments.

Gives Origin to: Coracobrachialis and short head of biceps brachii at apex of coracoid process.

Gives insertion to: Pectoralis Minor

Attaches: Coracoclavicular Ligament, Coracoacromial Ligament, Coracohumeral Ligament (All 3 of these will be explained under a different topic, the Shoulder Joint).

Angles of the Scapula

As discussed already, there are 3 angles of the scapula:

Inferior Angle

The inferior angle is the inferiormost part of the scapula, and is formed by the fusion of the medial and lateral borders of the scapula. It is located at the level of T7 (recall that the scapula runs from T2-T7, thus, this makes sense). Anteriorly, it provides the insertion for some fibres of the serratus anterior muscle. The serratus anterior actually makes sure that the scapula is pulled anteriorly towards the ribs. If the serratus anterior is paralyzed, as in damage to the serratus anterior, or damage to its nerve, the long thoracic nerve, then the scapula rebounds backwards, giving a condition known as winged scapula, where the inferior angle and medial border of scapula protrude outwards.

On its posterior surface, it is completely covered by the teres major muscle. It’s tip, or apex, sometimes also attaches the lattisimus dorsi muscle.

Superior Angle

This angle is formed by the junction of the superior border and medial border. It is located approximately at the level of T2.

It is thin, smooth and rounded, and as mentioned above, it gives attachment to some fibres of the levator scapulae muscle. If you think about it, the levator is an elevator, and lifts the scapula upwards, and thus must be attached to the superior angle, as well as to the uppermost part of the medial border.

Lateral Angle

The lateral angle of the scapula is also referred to as the head of the scapula. It contains one of its most important structures, the glenoid cavity.

The glenoid cavity is a pyriform (pear-shaped) recess that is responsible for forming the shoulder joint with the head of the humerus.

It is basically a shallow concavity that the head of the humerus attempts to fit into. However, it is too small to support the entire head of humerus, and only about 1/3 of the humerus fits into the glenoid cavity.

Directly above and below the glenoid cavity is the supraglenoid tubercle and infraglenoid tubercle.

The supraglenoid tubercle, shown above, is a rough tubercle located directly above the glenoid cavity, and provides attachment to the long head of biceps brachii.

The infraglenoid tubercle, shown above, gives origin to the triceps brachii muscle. Thus, supra…tubercle, gives origin to the muscle that is higher in the anatomical position, the biceps brachii. The infra… tubercle, gives origin to the muscle that is lower in the anatomical position, the triceps brachii. 


And that guys, is the scapula! I will explain all the ligaments in the scapula in greater detail when we look at the joints of the body. But as usual, I hope this helps!

Here are some extra resources for you guys:

VIDEOS:

Scapula 3D Anatomy

Scapula and Clavicle

ANIMATIONS:

http://anatomyzone.com/3d_atlas/musculoskeletal/upper-limb/scapula/

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