Last Updated on November 27, 2022
The scapula, also known as the shoulder blade is a triangular, flat bone of shoulder girdle that articulates with the head of the humerus at the glenohumeral joint which is popularly called the shoulder joint, and with the lateral end of the clavicle at the acromioclavicular joint. In doing so, it connects the upper limb to the trunk. The following diagram would explain the relation of the shoulder blade to the thorax.
The scapula provides attachment for a number of muscles of the arm and shoulder.
The scapula has got two surfaces, three borders, three angles and three processes. Scapula serves as a site for attachment for many important muscles around the shoulder.
The Surfaces of Scapula
Scapula has got two surfaces – costal surface that abuts the thorax and dorsal surface which can be felt when we palpate the bone from behind.
The Costal surface
The costal surface of the scapula is concave and is directed medially and forwards.
A concave depression over most of its surface is called the subscapular fossa. The subscapularis muscle, one of the rotator cuff muscles, originates from this side.
Coracoid process is a hook-like projection originating from the superolateral surface of the costal scapula which lies just underneath the clavicle.
The costal surface is marked by three longitudinal ridges. Another thick ridge adjoins the lateral border. This part of the bone is almost rod-like. It acts as a lever for the action of the serratus anterior in the overhead abduction of the arm
The Dorsal Surface
This surface gives attachment to the spine of the scapula which divides the surface into a smaller supraspinous fossa and a larger infraspinous fossa. The two fossae are connected by the spinoglenoid notch, situated lateral to the root of the spine.
Acromion is the projection of the spine that arches over the glenohumeral joint and articulates with the clavicle.
Borders, Angles, and Processes
The scapula has three borders, three angles and three processes. [See diagram]
- Superior border is thin and short. It has a suprascapular notch near the root of the coracoid process.
- Lateral border is thick and presents the infraglenoid tubercle at the upper end.
- Medial border is thin extends from the superior angle to the inferior angle.
The superior angle is covered by the trapezius. The inferior angle is covered by the latissimus dorsi and moves forward around the chest when the arm is abducted. The lateral angle is broad and bears the glenoid cavity which is directed forwards, laterally and slightly upwards. Glenoid cavity articulates with the head of the humerus to form shoulder joint.
The spinous process or spine of scapula is a triangular plate of bone with three borders and two surfaces. It divides the dorsal surface of the scapula into the supraspinous and infraspinous fossae. Its posterior border is called the crest of the spine. The crest has upper and lower lips.
The acromion process has two borders, medial and lateral, two surfaces, superior and inferior, and a facet for the clavicle.
Coracoids process is directed forwards and slightly laterally and plays an important role in shoulder stabilization by providing attachments to ligaments and muscles
How to Determine Side of Scapula?
- The lateral angle is large and bears the glenoid cavity.
- The dorsal surface is convex and is divided by the triangular spine into the supraspinous and infraspinous fossae.
- The costal surface is concave to fit on the convex chest wall.
- The lateral thickest border runs from the glenoid cavity above to the inferior angle below.
Muscles and Ligaments of Scapula
Various muscles and ligaments attach on scapula. These stabilize and move the bone. Movement of scapula helps in movements of the shoulder joint.
Muscles of Scapula
It arises from the medial two-thirds of the subscapular fossa.
Serratus anterior muscle is inserted along the medial border of the costal surface-one digitations to the superior angle, two digitations to the medial border, and five digitations to the inferior angle.
It arises from the medial two-thirds of the supraspinous fossa (including the upper surface of the spine).
arises from the medial two-thirds of the infraspinous fossa (including the lower surface of the spine).
Deltoid muscle arises from the lower border of the crest of the spine and from the lateral border of the acromion.
Trapezius muscle is inserted into the upper border of the crest of the spine and into the medial border of the acromion.
The long head of the biceps brachii arises from the supraglenoid tubercle. The short head from the lateral part of the tip of the coracoid process.
The coracobrachialis arises from the medial part of the tip of the coracoid process.
Pectoralis minor is inserted into the medial border and superior surface of the coracoid process.
The long head of the triceps arises from the infraglenoid tubercle.
Teres minor arises from the upper two-thirds of the rough strip on the dorsal surface along the lateral border.
It arises from the lower one-third of the rough strip on the lateral aspect of the lateral border.
is inserted along the dorsal aspect of the medial border, from the superior angle up to the root of the spine.
It is inserted into the medial border (dorsal aspect) opposite the root of the spine of the scapula.
Rhomboideus major is inserted into the medial border (dorsal aspect) between the root of the spine and the inferior angle.
The inferior belly of the omohyoid arises from the upper border near the suprascapular notch.
[Learn more about shoulder muscles]
Ligaments of Scapula
The margin of the glenoid cavity gives attachment to the capsule of the shoulder joint and to the glenoid labrum.
The margin of the facet on the medial aspect of the acromion gives attachment to the capsule of the acromioclavicular joint.
The coracoacromial ligament extends between the coracoid process and the acromion. It is attached to the lateral border of the coracoid process, and the medial side of the tip of the acromion process.
This ligament protects the head of the humerus from dislocating during motion.
It is attached to the root of the coracoid process.
It is attached to the coracoid process. The trapezoid part attaches to the superior aspect and the conoid part near the root.
It is a ligament that bridges across the suprascapular notch. and converts it into a foramen. The suprascapular nerve passes through the foramen and the suprascapular artery is above the ligament.
It bridges the spino-glenoid notch. The suprascapular vessels and nerve pass deep to it.
Scapula participates in shoulder motion. In shoulder abduction, the scapulohumeral movement is in the ratio of 2:1. A disturbance in this rhythm results in scapular dyskinesis.
Winging of Scapula
Scapula needs to be kept close to the posterior chest wall by muscle forces. A break in forces as in paralysis of the serratus anterior cause ‘winging’ of the scapula. The medial border of the bone becomes unduly prominent, and the arm cannot be abducted.
Winging may also be caused by trapezius muscle paralysis.
Scapula bone is frequently injured in trauma especially motor vehicle injuries.
A developmental anomaly called where the medial border is concave.
Ossification of Scapula
At birth, the body and spine of the scapula form one ossified mass, with the coracoid, acromion, glenoid, and inferior angle all being cartilaginous.
- At 3 to 18 months, a center of ossification appears at the mid-coracoid.
- At 7 to 10 years, the center for the coracoid base, and the upper third of the glenoid, appear.
- Sometimes called a subcoracoid bone and it joins the rest of the coracoid at 14 to 16 years.
- For the the medial apex of the coracoid ]
- Two or three acromial centers form at the age of 14 to 16 years, coalesce at 19 years, and fuse to the spine between 20 and 25 years of age.
- Failure of fusion with the persistence of one ossification center past 25 years of age creates what is known as an os acromiale.
- The glenoid fossa ossifies from four sources:
- The coracoid base (including the upper third of the glenoid)
- The deep portion of the coracoid process,
- The body,
- The lower pole
- Between 8 and 13 years of age, the glenoid border starts ossifying
- At the inferior angle of the scapula, an ossification center appears at 15 and fuses at 20 years of age
- The vertebral border center appears from 16 to 18 and fuses by 25 years of age.
The ossification centers may be asymmetric, and thus comparison films may not be helpful.