Scapula

The scapula, also called the shoulder blade, is a relatively large, flat, triangular-shaped shoulder girdle bone. It is found in the upper thoracic region at the back of the trunk lying posterior to the thoracic cage. The scapula directly articulates with the humerus and clavicle, forming the glenohumeral (shoulder) and acromioclavicular joints. 

The scapula serves as an attachment site for many muscles that form the shoulder and arm. The intrinsic muscles attached to the scapula include the rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis), as well as the teres major muscle. 

The extrinsic muscles that are connected to the landmarks of the scapula are the triceps brachii, biceps brachii and deltoid, pectoralis minor, and latissimus dorsi, as well as the levator scapulae, trapezius, rhomboids, serratus anterior, coracobrachialis, and omohyoid. These muscles not only move the arm but also contribute to stabilizing the scapula.

Overall, the scapula has two surfaces and three margins that meet at three angles. Additionally, it presents three processes (acromion, scapular spine, and coracoid process).

 

Surfaces of scapula

The scapula has two surfaces - anterior (ventral or costal) surface and posterior (dorsal) surface.

The anterior surface is also known as the ventral or costal surface. It faces the posterior thoracic wall and is slightly concave. This surface is smoother than the posterior surface, as most of it is taken up by a large, shallow concavity named the subscapular fossa. The subscapularis muscle fills the fossa. 

  • The anterior surface also presents several oblique lines known as oblique ridges. They cross the subscapular fossa from the superolateral to the inferomedial aspect of the subscapular fossa. The oblique ridges are formed by the intermuscular tendons of the subscapularis muscle.

The posterior surface is also called the dorsal surface. It appears convex and is divided into two parts by a protruding ridge known as the spine of the scapula. It transverse the dorsal surface and passes mediolaterally across it, separating it into two uneven portions. 

  • The smaller elongated concavity above the scapular spine is known as the supraspinous fossa. It serves as an origin site for the supraspinatus muscle, and the mentioned muscle fills the fossa.
  • Inferior to the scapular spine is another hollow space called the infraspinous fossa. It is much larger than the supraspinous fossa. Opposite to the supraspinous fossa, it is weakly concave. This fossa serves as an origin site for the infraspinatus muscle.
  • Both fossae connect at an indentation found lateral to the root of the scapular spine. It is called the spinoglenoid notch.
  • The scapular spine terminates as a bony projection called the acromion. It forms the summit of the shoulder. It is a large, somewhat triangular or oblong process that at first projects laterally and then curves forward and upward. It overhangs the glenoid cavity. The acromion serves as an attachment site for several muscles, including the trapezius and deltoid.

 

Margins of scapula

As a triangular-shaped bone, the scapula has three margins - superior, medial, and lateral.

The superior margin is also called the cranial margin, as it is the closest border to the skull. This border is the shortest and most irregular of all three margins. 

  • The superior border is marked by a deep, semicircular indentation called the scapular notch. It is found on its lateral aspect and is partly formed by the base of the coracoid process. This notch is usually converted into a foramen by the superior transverse scapular ligament that stretches across it. This foramen serves as a passage for the suprascapular nerve and variably for the suprascapular vessels (they can alternatively pass above the ligament).
  • From the lateral aspect of the superior border arises a blunt, finger-like bony projection known as the coracoid process. It is directed forward, upward, and laterally.

The medial margin is also called the vertebral border, as it is faced against the vertebrae and spine. It is the longest and straightest margin, extending from the superior angle to the inferior angle. The medial margin serves as an attachment site for several muscles, including the serratus anterior, levator scapulae, rhomboid minor, and rhomboid major muscles.

The lateral margin is also known as the axillary border, as it is directed against the armpit. It is the thickest margin. It starts right below the glenoid cavity, then inclines obliquely downward and backward and terminates by meeting the medial margin and forming the inferior angle. The lateral margin is an origin site for the teres major muscle.

 

Angles of scapula

As mentioned previously, the scapula has three angles - superior, inferior, and lateral.

The superior angle is the upper angle formed by the junction of the superior and medial margins of the bone. It is located approximately at the level of the second or third thoracic vertebrae (T2 - T3). The superior angle is covered by the trapezius muscle. Also, the posterior surface of the superior angle gives attachment to some fibers of the levator scapulae muscle. 

The inferior angle is the lower angle and also the lowest portion of the scapula. It is formed by the intersection of the medial and lateral margins. This angle is covered by the latissimus dorsi muscle. The anterior and posterior aspects of the inferior angle serve as attachment sites to the serratus anterior and teres major muscles. Also, the inferior angle gives attachment to a few fibers of the latissimus dorsi muscle.

The lateral angle is the thickest angle and is also known as the glenoid angle or the head of the scapula. It bears the glenoid cavity on its articular surface. The glenoid cavity articulates with the head of the humerus, forming the glenohumeral (shoulder) joint. Above and below the glenoid cavity are two small projections:

Medial to the glenoid cavity is a slightly narrowed region called the neck of the scapula. The coracoid process lies above it.